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通过Android源代码分析startActivity()过程(上)

Cundong
 Cundong
发布于 2016/04/05 21:34
字数 9576
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#通过Android源代码分析startActivity()过程(上)


一 概述

在Android中,我们去调用startActivit()来启动一个Activity,经过复杂的代码跳转后,最终是由ActivityManagerService(简称ams,下同)来真正的打开一个应用的Activity的。

由于ams运行在system_server进程中,所以普通的app进程与ams进行通信,就涉及到跨进程通信的问题,这种情况下的跨进程,Android采用的Binder机制,通过AIDL来实现,如果对这个不了解,可以参考前一篇关于AIDL的博文,如果你对这个了解了,下面我们将要分析到的代码逻辑,你会觉得似曾相识。

在startActivit()的整个过程,可以分为两部分来理解:

  • app进程去调用位于system_server进程中的ams

startActivit()会调用ams去启动app的入口Activity,如果ams发现这个app进程未启动,则会由于zygote进程fork出一个新的进程出来,然后在启动这个 app进程的ActivityThread main()方法。

  • 位于system_server进程中的ams去调用app进程

新启动的app进程,在其ActivityThread main()方法种,会将自己和 system_server进程中的ams绑定在一起,这样ams就会在特定的时机去回调app进程中Activity的生命周期方法。

二 UML图

如果理解了Android的Binder机制,我们就会知道,app进程去调用system_server进程中的ams,是通过Binder调用实现的,所以app进程中必然会保留一个ams的IBinder,也就是一个代理。

同样,system_server进程中的ams去调用app进程,同样也会在ams进程中保留一个app进程的IBinder,也就是一个代理。

通过阅读源码,我们可以参照考前一篇关于AIDL的博文中的UML图,画出这两部分的UML图,这也是一个典型的代理模式,我们把这两部分的UML通前一篇文章中的UML图比较一下,架构基本一模一样。

1.app进程调用ams过程中用到的类UML图

app进程调用ams过程中用到的类UML图

其中,ActivityManagerProxy是ActivityManagerService在普通app进程中的代理,app进程不会直接跟ActivityManagerService交互,而是通过其留在app进程中的代理ActivityManagerProxy来与ActivityManagerService通信。

2.ams调用app进程中用到的类UML图

ams调用app进程中用到的类UML图

其中,ApplicationThreadProxy是普通app进程放在ActivityManagerService所在的system_server进程的代理对象,ActivityManagerService也不会直接与app进程交互,而是通过 其留在ActivityManagerService进程的代理对象ApplicationThreadProxy来与应用进程通信。

三 先看app进程调用ams过程的代码

整个跳转逻辑大概可以分拆为以下几个步骤:

  1. Activity#startActivity
  2. Activity#startActivityForResult
  3. Instrumentation#execStartActivity
  4. ActivityManagerNative#getDefault().startActivity
  5. ActivityManagerProxy#startActivity
  6. ActivityMangerService#startActivity
  7. ActivityMangerService#startActivityAsUser
  8. ActivityStackSupervisor#startActivityMayWait
  9. ActivityStackSupervisorr#startActivityLocked
  10. ActivityStackSupervisorr#startActivityUncheckedLocked
  11. ActivityStack#startActivityLocked
  12. ActivityStackSupervisor#resumeTopActivitiesLocked
  13. ActivityStack#resumeTopActivityLocked
  14. ActivityStack#resumeTopActivityInnerLocked
  15. ActivityStackSupervisor#startSpecificActivityLocked
  16. ActivityManagerService#startProcessLocked
  17. Process#start

Activity#startActivity

调用startActivity(Intent intent)打开一个Activity,或者在Launcher上点击应用图标,最终都会调用Activity#startActivity()接口:

/**
     * Same as {@link #startActivity(Intent, Bundle)} with no options
     * specified.
     *
     * @param intent The intent to start.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see {@link #startActivity(Intent, Bundle)}
     * @see #startActivityForResult
     */
    @Override
    public void startActivity(Intent intent) {
        this.startActivity(intent, null);
    }

Activity#startActivityForResult

最终会走到:

  public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) {
        //只要没有用到ActivityGroup,mParent就为空
        if (mParent == null) {
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
            if (ar != null) {
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }
            if (requestCode >= 0) {
                // If this start is requesting a result, we can avoid making
                // the activity visible until the result is received.  Setting
                // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
                // activity hidden during this time, to avoid flickering.
                // This can only be done when a result is requested because
                // that guarantees we will get information back when the
                // activity is finished, no matter what happens to it.
                mStartedActivity = true;
            }

            cancelInputsAndStartExitTransition(options);
            // TODO Consider clearing/flushing other event sources and events for child windows.
        } else {
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                // Note we want to go through this method for compatibility with
                // existing applications that may have overridden it.
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }

因为当前未使用ActivityGroup,所以mParent == null,会走下面这个逻辑:

Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);

            if (ar != null) {
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }

看到这里,出现了一个新类:Instrumentation,他是干什么的呢?

Instrumentation#execStartActivity

它是一个Activity的辅助类,躲在界面后面,帮助Activity实现真正的业务逻辑,例如上面的startActivity()跳来跳去,最终还是来到了Instrumentation的execStartActivity()方法,并且执行完execStartActivity方法之后,还接着调用了sendActivityResult方法(ActivityThread类的方法,我们后面会介绍),从而会触发onActivityResult()接口的回调。

我们继续跟踪到Instrumentation#execStartActivity()内部:

 public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        Uri referrer = target != null ? target.onProvideReferrer() : null;
        if (referrer != null) {
            intent.putExtra(Intent.EXTRA_REFERRER, referrer);
        }
        
        //1.第一步,迭代Activity,判断是否合法
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    final ActivityMonitor am = mActivityMonitors.get(i);
                    if (am.match(who, null, intent)) {
                        am.mHits++;
                        if (am.isBlocking()) {
                            return requestCode >= 0 ? am.getResult() : null;
                        }
                        break;
                    }
                }
            }
        }
        
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess();
            
            //2.调用ams,真正的去startActivity
            int result = ActivityManagerNative.getDefault()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, options);
                        
            //3.根据返回值抛异常
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }

这个方法做了3件事:

  1. 遍历mActivityMonitors,看目标Activity是否存在,如果不存在直接退出
  2. 调用ActivityManagerNative.getDefault() 的startActivity()
  3. 通过checkStartActivityResult()来检测操作结果

第1、3两个步骤就不细说了,我们主要看ActivityManagerNative.getDefault()的startActivity()方法。

ActivityManagerNative#getDefault().startActivity

先看看ActivityManagerNative.getDefault()是个什么鬼吧:

 static public IActivityManager getDefault() {
        return gDefault.get();
    }

返回的是gDefault.get(),那么gDefault又是个什么呢:

private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
        protected IActivityManager create() {
            IBinder b = ServiceManager.getService("activity");
            if (false) {
                Log.v("ActivityManager", "default service binder = " + b);
            }
            IActivityManager am = asInterface(b);
            if (false) {
                Log.v("ActivityManager", "default service = " + am);
            }
            return am;
        }
    };

原来,是从ServiceManager中拿到一个Service的IBinder,它在ServiceManager中,key值为"activity",也就是传说中ams的IBinder,并且用asInterface()将其转换了一下,通过前面那篇介绍AIDL原理的博文,我们知道这个asInterface()内部其实就是做一个判断,当你调用的接口和调用者属于同一个进程的时候,就当做普通引用来使用接口,当被调用者和调用者不属于同一个进程的时候,就会转化成远程代理。

ActivityManagerProxy#startActivity

我们打开asInterface的代码,果然是这样的:

static public IActivityManager asInterface(IBinder obj) {
        if (obj == null) {
            return null;
        }
        
        //queryLocalInterface()就是通过接口描述来查询它是否一个本地接口
        IActivityManager in =
            (IActivityManager)obj.queryLocalInterface(descriptor);
        if (in != null) {
            return in;
        }

        return new ActivityManagerProxy(obj);
    }

写到这里,我再总结一下ActivityManagerNative.getDefault()是个什么?它就是身处system_server进程的ActivityManagerService位于app进程的一个代理,名为:ActivityManagerProxy。

在饶了这么多路之后,我们终于看懂了,startActivity()的过程实际上最后是调用ActivityManagerProxy的startActivity()方法,ActivityManagerProxy的startActivity()方法最终还是通过Binder调用,调用到ActivityMangerService的startActivity()方法中。

好吧,我们再去ActivityMangerService的startActivity()方法中去吧。

ActivityMangerService#startActivity

ActivityMangerService#startActivity:

@Override
    public final int startActivity(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle options) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
            resultWho, requestCode, startFlags, profilerInfo, options,
            UserHandle.getCallingUserId());
    }

ActivityMangerService#startActivityAsUser

ActivityMangerService#startActivityAsUser:

@Override
    public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle options, int userId) {
        enforceNotIsolatedCaller("startActivity");
        userId = handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId,
                false, ALLOW_FULL_ONLY, "startActivity", null);
        // TODO: Switch to user app stacks here.
        return mStackSupervisor.startActivityMayWait(caller, -1, callingPackage, intent,
                resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
                profilerInfo, null, null, options, false, userId, null, null);
    }

ActivityMangerService#startActivityAsUser中会走到ActivityStackSupervisor#startActivityMayWait中,ActivityStackSupervisor又是什么鬼?

通过类名字就能看出来,他是负责管理Activity栈的一个管理类。

ActivityStackSupervisor#startActivityMayWait

ActivityStackSupervisorr#startActivityMayWait:

final int startActivityMayWait(IApplicationThread caller, int callingUid,
            String callingPackage, Intent intent, String resolvedType,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            IBinder resultTo, String resultWho, int requestCode, int startFlags,
            ProfilerInfo profilerInfo, WaitResult outResult, Configuration config,
            Bundle options, boolean ignoreTargetSecurity, int userId,
            IActivityContainer iContainer, TaskRecord inTask) {
        // Refuse possible leaked file descriptors
        if (intent != null && intent.hasFileDescriptors()) {
            throw new IllegalArgumentException("File descriptors passed in Intent");
        }
        boolean componentSpecified = intent.getComponent() != null;

        // Don't modify the client's object!
        intent = new Intent(intent);

        // Collect information about the target of the Intent.
        ActivityInfo aInfo =
                resolveActivity(intent, resolvedType, startFlags, profilerInfo, userId);

        ActivityContainer container = (ActivityContainer)iContainer;
        synchronized (mService) {
            if (container != null && container.mParentActivity != null &&
                    container.mParentActivity.state != RESUMED) {
                // Cannot start a child activity if the parent is not resumed.
                return ActivityManager.START_CANCELED;
            }
            final int realCallingPid = Binder.getCallingPid();
            final int realCallingUid = Binder.getCallingUid();
            int callingPid;
            if (callingUid >= 0) {
                callingPid = -1;
            } else if (caller == null) {
                callingPid = realCallingPid;
                callingUid = realCallingUid;
            } else {
                callingPid = callingUid = -1;
            }

            final ActivityStack stack;
            if (container == null || container.mStack.isOnHomeDisplay()) {
                stack = mFocusedStack;
            } else {
                stack = container.mStack;
            }
            stack.mConfigWillChange = config != null && mService.mConfiguration.diff(config) != 0;
            if (DEBUG_CONFIGURATION) Slog.v(TAG_CONFIGURATION,
                    "Starting activity when config will change = " + stack.mConfigWillChange);

            final long origId = Binder.clearCallingIdentity();

            if (aInfo != null &&
                    (aInfo.applicationInfo.privateFlags
                            &ApplicationInfo.PRIVATE_FLAG_CANT_SAVE_STATE) != 0) {
                // This may be a heavy-weight process!  Check to see if we already
                // have another, different heavy-weight process running.
                if (aInfo.processName.equals(aInfo.applicationInfo.packageName)) {
                    if (mService.mHeavyWeightProcess != null &&
                            (mService.mHeavyWeightProcess.info.uid != aInfo.applicationInfo.uid ||
                            !mService.mHeavyWeightProcess.processName.equals(aInfo.processName))) {
                        int appCallingUid = callingUid;
                        if (caller != null) {
                            ProcessRecord callerApp = mService.getRecordForAppLocked(caller);
                            if (callerApp != null) {
                                appCallingUid = callerApp.info.uid;
                            } else {
                                Slog.w(TAG, "Unable to find app for caller " + caller
                                      + " (pid=" + callingPid + ") when starting: "
                                      + intent.toString());
                                ActivityOptions.abort(options);
                                return ActivityManager.START_PERMISSION_DENIED;
                            }
                        }

                        IIntentSender target = mService.getIntentSenderLocked(
                                ActivityManager.INTENT_SENDER_ACTIVITY, "android",
                                appCallingUid, userId, null, null, 0, new Intent[] { intent },
                                new String[] { resolvedType }, PendingIntent.FLAG_CANCEL_CURRENT
                                | PendingIntent.FLAG_ONE_SHOT, null);

                        Intent newIntent = new Intent();
                        if (requestCode >= 0) {
                            // Caller is requesting a result.
                            newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_HAS_RESULT, true);
                        }
                        newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_INTENT,
                                new IntentSender(target));
                        if (mService.mHeavyWeightProcess.activities.size() > 0) {
                            ActivityRecord hist = mService.mHeavyWeightProcess.activities.get(0);
                            newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_CUR_APP,
                                    hist.packageName);
                            newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_CUR_TASK,
                                    hist.task.taskId);
                        }
                        newIntent.putExtra(HeavyWeightSwitcherActivity.KEY_NEW_APP,
                                aInfo.packageName);
                        newIntent.setFlags(intent.getFlags());
                        newIntent.setClassName("android",
                                HeavyWeightSwitcherActivity.class.getName());
                        intent = newIntent;
                        resolvedType = null;
                        caller = null;
                        callingUid = Binder.getCallingUid();
                        callingPid = Binder.getCallingPid();
                        componentSpecified = true;
                        try {
                            ResolveInfo rInfo =
                                AppGlobals.getPackageManager().resolveIntent(
                                        intent, null,
                                        PackageManager.MATCH_DEFAULT_ONLY
                                        | ActivityManagerService.STOCK_PM_FLAGS, userId);
                            aInfo = rInfo != null ? rInfo.activityInfo : null;
                            aInfo = mService.getActivityInfoForUser(aInfo, userId);
                        } catch (RemoteException e) {
                            aInfo = null;
                        }
                    }
                }
            }

            int res = startActivityLocked(caller, intent, resolvedType, aInfo,
                    voiceSession, voiceInteractor, resultTo, resultWho,
                    requestCode, callingPid, callingUid, callingPackage,
                    realCallingPid, realCallingUid, startFlags, options, ignoreTargetSecurity,
                    componentSpecified, null, container, inTask);

            Binder.restoreCallingIdentity(origId);

            if (stack.mConfigWillChange) {
                // If the caller also wants to switch to a new configuration,
                // do so now.  This allows a clean switch, as we are waiting
                // for the current activity to pause (so we will not destroy
                // it), and have not yet started the next activity.
                mService.enforceCallingPermission(android.Manifest.permission.CHANGE_CONFIGURATION,
                        "updateConfiguration()");
                stack.mConfigWillChange = false;
                if (DEBUG_CONFIGURATION) Slog.v(TAG_CONFIGURATION,
                        "Updating to new configuration after starting activity.");
                mService.updateConfigurationLocked(config, null, false, false);
            }

            if (outResult != null) {
                outResult.result = res;
                if (res == ActivityManager.START_SUCCESS) {
                    mWaitingActivityLaunched.add(outResult);
                    do {
                        try {
                            mService.wait();
                        } catch (InterruptedException e) {
                        }
                    } while (!outResult.timeout && outResult.who == null);
                } else if (res == ActivityManager.START_TASK_TO_FRONT) {
                    ActivityRecord r = stack.topRunningActivityLocked(null);
                    if (r.nowVisible && r.state == RESUMED) {
                        outResult.timeout = false;
                        outResult.who = new ComponentName(r.info.packageName, r.info.name);
                        outResult.totalTime = 0;
                        outResult.thisTime = 0;
                    } else {
                        outResult.thisTime = SystemClock.uptimeMillis();
                        mWaitingActivityVisible.add(outResult);
                        do {
                            try {
                                mService.wait();
                            } catch (InterruptedException e) {
                            }
                        } while (!outResult.timeout && outResult.who == null);
                    }
                }
            }

            return res;
        }
    }

ActivityStackSupervisorr#startActivityLocked

跳转到:ActivityStackSupervisorr#startActivityLocked():

 final int startActivityLocked(IApplicationThread caller,
            Intent intent, String resolvedType, ActivityInfo aInfo,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            IBinder resultTo, String resultWho, int requestCode,
            int callingPid, int callingUid, String callingPackage,
            int realCallingPid, int realCallingUid, int startFlags, Bundle options,
            boolean ignoreTargetSecurity, boolean componentSpecified, ActivityRecord[] outActivity,
            ActivityContainer container, TaskRecord inTask) {
        int err = ActivityManager.START_SUCCESS;

        ProcessRecord callerApp = null;
        if (caller != null) {
            callerApp = mService.getRecordForAppLocked(caller);
            if (callerApp != null) {
                callingPid = callerApp.pid;
                callingUid = callerApp.info.uid;
            } else {
                Slog.w(TAG, "Unable to find app for caller " + caller
                      + " (pid=" + callingPid + ") when starting: "
                      + intent.toString());
                err = ActivityManager.START_PERMISSION_DENIED;
            }
        }

        final int userId = aInfo != null ? UserHandle.getUserId(aInfo.applicationInfo.uid) : 0;

        if (err == ActivityManager.START_SUCCESS) {
            Slog.i(TAG, "START u" + userId + " {" + intent.toShortString(true, true, true, false)
                    + "} from uid " + callingUid
                    + " on display " + (container == null ? (mFocusedStack == null ?
                            Display.DEFAULT_DISPLAY : mFocusedStack.mDisplayId) :
                            (container.mActivityDisplay == null ? Display.DEFAULT_DISPLAY :
                                    container.mActivityDisplay.mDisplayId)));
        }

        ActivityRecord sourceRecord = null;
        ActivityRecord resultRecord = null;
        if (resultTo != null) {
            sourceRecord = isInAnyStackLocked(resultTo);
            if (DEBUG_RESULTS) Slog.v(TAG_RESULTS,
                    "Will send result to " + resultTo + " " + sourceRecord);
            if (sourceRecord != null) {
                if (requestCode >= 0 && !sourceRecord.finishing) {
                    resultRecord = sourceRecord;
                }
            }
        }

        final int launchFlags = intent.getFlags();

        if ((launchFlags & Intent.FLAG_ACTIVITY_FORWARD_RESULT) != 0 && sourceRecord != null) {
            // Transfer the result target from the source activity to the new
            // one being started, including any failures.
            if (requestCode >= 0) {
                ActivityOptions.abort(options);
                return ActivityManager.START_FORWARD_AND_REQUEST_CONFLICT;
            }
            resultRecord = sourceRecord.resultTo;
            if (resultRecord != null && !resultRecord.isInStackLocked()) {
                resultRecord = null;
            }
            resultWho = sourceRecord.resultWho;
            requestCode = sourceRecord.requestCode;
            sourceRecord.resultTo = null;
            if (resultRecord != null) {
                resultRecord.removeResultsLocked(sourceRecord, resultWho, requestCode);
            }
            if (sourceRecord.launchedFromUid == callingUid) {
                // The new activity is being launched from the same uid as the previous
                // activity in the flow, and asking to forward its result back to the
                // previous.  In this case the activity is serving as a trampoline between
                // the two, so we also want to update its launchedFromPackage to be the
                // same as the previous activity.  Note that this is safe, since we know
                // these two packages come from the same uid; the caller could just as
                // well have supplied that same package name itself.  This specifially
                // deals with the case of an intent picker/chooser being launched in the app
                // flow to redirect to an activity picked by the user, where we want the final
                // activity to consider it to have been launched by the previous app activity.
                callingPackage = sourceRecord.launchedFromPackage;
            }
        }

        if (err == ActivityManager.START_SUCCESS && intent.getComponent() == null) {
            // We couldn't find a class that can handle the given Intent.
            // That's the end of that!
            err = ActivityManager.START_INTENT_NOT_RESOLVED;
        }

        if (err == ActivityManager.START_SUCCESS && aInfo == null) {
            // We couldn't find the specific class specified in the Intent.
            // Also the end of the line.
            err = ActivityManager.START_CLASS_NOT_FOUND;
        }

        if (err == ActivityManager.START_SUCCESS
                && !isCurrentProfileLocked(userId)
                && (aInfo.flags & FLAG_SHOW_FOR_ALL_USERS) == 0) {
            // Trying to launch a background activity that doesn't show for all users.
            err = ActivityManager.START_NOT_CURRENT_USER_ACTIVITY;
        }

        if (err == ActivityManager.START_SUCCESS && sourceRecord != null
                && sourceRecord.task.voiceSession != null) {
            // If this activity is being launched as part of a voice session, we need
            // to ensure that it is safe to do so.  If the upcoming activity will also
            // be part of the voice session, we can only launch it if it has explicitly
            // said it supports the VOICE category, or it is a part of the calling app.
            if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) == 0
                    && sourceRecord.info.applicationInfo.uid != aInfo.applicationInfo.uid) {
                try {
                    intent.addCategory(Intent.CATEGORY_VOICE);
                    if (!AppGlobals.getPackageManager().activitySupportsIntent(
                            intent.getComponent(), intent, resolvedType)) {
                        Slog.w(TAG,
                                "Activity being started in current voice task does not support voice: "
                                + intent);
                        err = ActivityManager.START_NOT_VOICE_COMPATIBLE;
                    }
                } catch (RemoteException e) {
                    Slog.w(TAG, "Failure checking voice capabilities", e);
                    err = ActivityManager.START_NOT_VOICE_COMPATIBLE;
                }
            }
        }

        if (err == ActivityManager.START_SUCCESS && voiceSession != null) {
            // If the caller is starting a new voice session, just make sure the target
            // is actually allowing it to run this way.
            try {
                if (!AppGlobals.getPackageManager().activitySupportsIntent(intent.getComponent(),
                        intent, resolvedType)) {
                    Slog.w(TAG,
                            "Activity being started in new voice task does not support: "
                            + intent);
                    err = ActivityManager.START_NOT_VOICE_COMPATIBLE;
                }
            } catch (RemoteException e) {
                Slog.w(TAG, "Failure checking voice capabilities", e);
                err = ActivityManager.START_NOT_VOICE_COMPATIBLE;
            }
        }

        final ActivityStack resultStack = resultRecord == null ? null : resultRecord.task.stack;

        if (err != ActivityManager.START_SUCCESS) {
            if (resultRecord != null) {
                resultStack.sendActivityResultLocked(-1,
                    resultRecord, resultWho, requestCode,
                    Activity.RESULT_CANCELED, null);
            }
            ActivityOptions.abort(options);
            return err;
        }

        boolean abort = false;

        final int startAnyPerm = mService.checkPermission(
                START_ANY_ACTIVITY, callingPid, callingUid);

        if (startAnyPerm != PERMISSION_GRANTED) {
            final int componentRestriction = getComponentRestrictionForCallingPackage(
                    aInfo, callingPackage, callingPid, callingUid, ignoreTargetSecurity);
            final int actionRestriction = getActionRestrictionForCallingPackage(
                    intent.getAction(), callingPackage, callingPid, callingUid);

            if (componentRestriction == ACTIVITY_RESTRICTION_PERMISSION
                    || actionRestriction == ACTIVITY_RESTRICTION_PERMISSION) {
                if (resultRecord != null) {
                    resultStack.sendActivityResultLocked(-1,
                            resultRecord, resultWho, requestCode,
                            Activity.RESULT_CANCELED, null);
                }
                String msg;
                if (actionRestriction == ACTIVITY_RESTRICTION_PERMISSION) {
                    msg = "Permission Denial: starting " + intent.toString()
                            + " from " + callerApp + " (pid=" + callingPid
                            + ", uid=" + callingUid + ")" + " with revoked permission "
                            + ACTION_TO_RUNTIME_PERMISSION.get(intent.getAction());
                } else if (!aInfo.exported) {
                    msg = "Permission Denial: starting " + intent.toString()
                            + " from " + callerApp + " (pid=" + callingPid
                            + ", uid=" + callingUid + ")"
                            + " not exported from uid " + aInfo.applicationInfo.uid;
                } else {
                    msg = "Permission Denial: starting " + intent.toString()
                            + " from " + callerApp + " (pid=" + callingPid
                            + ", uid=" + callingUid + ")"
                            + " requires " + aInfo.permission;
                }
                Slog.w(TAG, msg);
                throw new SecurityException(msg);
            }

            if (actionRestriction == ACTIVITY_RESTRICTION_APPOP) {
                String message = "Appop Denial: starting " + intent.toString()
                        + " from " + callerApp + " (pid=" + callingPid
                        + ", uid=" + callingUid + ")"
                        + " requires " + AppOpsManager.permissionToOp(
                                ACTION_TO_RUNTIME_PERMISSION.get(intent.getAction()));
                Slog.w(TAG, message);
                abort = true;
            } else if (componentRestriction == ACTIVITY_RESTRICTION_APPOP) {
                String message = "Appop Denial: starting " + intent.toString()
                        + " from " + callerApp + " (pid=" + callingPid
                        + ", uid=" + callingUid + ")"
                        + " requires appop " + AppOpsManager.permissionToOp(aInfo.permission);
                Slog.w(TAG, message);
                abort = true;
            }
        }

        abort |= !mService.mIntentFirewall.checkStartActivity(intent, callingUid,
                callingPid, resolvedType, aInfo.applicationInfo);

        if (mService.mController != null) {
            try {
                // The Intent we give to the watcher has the extra data
                // stripped off, since it can contain private information.
                Intent watchIntent = intent.cloneFilter();
                abort |= !mService.mController.activityStarting(watchIntent,
                        aInfo.applicationInfo.packageName);
            } catch (RemoteException e) {
                mService.mController = null;
            }
        }

        if (abort) {
            if (resultRecord != null) {
                resultStack.sendActivityResultLocked(-1, resultRecord, resultWho, requestCode,
                        Activity.RESULT_CANCELED, null);
            }
            // We pretend to the caller that it was really started, but
            // they will just get a cancel result.
            ActivityOptions.abort(options);
            return ActivityManager.START_SUCCESS;
        }

        ActivityRecord r = new ActivityRecord(mService, callerApp, callingUid, callingPackage,
                intent, resolvedType, aInfo, mService.mConfiguration, resultRecord, resultWho,
                requestCode, componentSpecified, voiceSession != null, this, container, options);
        if (outActivity != null) {
            outActivity[0] = r;
        }

        if (r.appTimeTracker == null && sourceRecord != null) {
            // If the caller didn't specify an explicit time tracker, we want to continue
            // tracking under any it has.
            r.appTimeTracker = sourceRecord.appTimeTracker;
        }

        final ActivityStack stack = mFocusedStack;
        if (voiceSession == null && (stack.mResumedActivity == null
                || stack.mResumedActivity.info.applicationInfo.uid != callingUid)) {
            if (!mService.checkAppSwitchAllowedLocked(callingPid, callingUid,
                    realCallingPid, realCallingUid, "Activity start")) {
                PendingActivityLaunch pal =
                        new PendingActivityLaunch(r, sourceRecord, startFlags, stack);
                mPendingActivityLaunches.add(pal);
                ActivityOptions.abort(options);
                return ActivityManager.START_SWITCHES_CANCELED;
            }
        }

        if (mService.mDidAppSwitch) {
            // This is the second allowed switch since we stopped switches,
            // so now just generally allow switches.  Use case: user presses
            // home (switches disabled, switch to home, mDidAppSwitch now true);
            // user taps a home icon (coming from home so allowed, we hit here
            // and now allow anyone to switch again).
            mService.mAppSwitchesAllowedTime = 0;
        } else {
            mService.mDidAppSwitch = true;
        }

        doPendingActivityLaunchesLocked(false);

        err = startActivityUncheckedLocked(r, sourceRecord, voiceSession, voiceInteractor,
                startFlags, true, options, inTask);

        if (err < 0) {
            // If someone asked to have the keyguard dismissed on the next
            // activity start, but we are not actually doing an activity
            // switch...  just dismiss the keyguard now, because we
            // probably want to see whatever is behind it.
            notifyActivityDrawnForKeyguard();
        }
        return err;
    }

ActivityStackSupervisorr#startActivityUncheckedLocked

继续跳到:ActivityStackSupervisorr#startActivityUncheckedLocked:

final int startActivityUncheckedLocked(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor, int startFlags,
            boolean doResume, Bundle options, TaskRecord inTask) {
        final Intent intent = r.intent;
        final int callingUid = r.launchedFromUid;

        // In some flows in to this function, we retrieve the task record and hold on to it
        // without a lock before calling back in to here...  so the task at this point may
        // not actually be in recents.  Check for that, and if it isn't in recents just
        // consider it invalid.
        if (inTask != null && !inTask.inRecents) {
            Slog.w(TAG, "Starting activity in task not in recents: " + inTask);
            inTask = null;
        }

        final boolean launchSingleTop = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TOP;
        final boolean launchSingleInstance = r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE;
        final boolean launchSingleTask = r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK;

        int launchFlags = intent.getFlags();
        if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_DOCUMENT) != 0 &&
                (launchSingleInstance || launchSingleTask)) {
            // We have a conflict between the Intent and the Activity manifest, manifest wins.
            Slog.i(TAG, "Ignoring FLAG_ACTIVITY_NEW_DOCUMENT, launchMode is " +
                    "\"singleInstance\" or \"singleTask\"");
            launchFlags &=
                    ~(Intent.FLAG_ACTIVITY_NEW_DOCUMENT | Intent.FLAG_ACTIVITY_MULTIPLE_TASK);
        } else {
            switch (r.info.documentLaunchMode) {
                case ActivityInfo.DOCUMENT_LAUNCH_NONE:
                    break;
                case ActivityInfo.DOCUMENT_LAUNCH_INTO_EXISTING:
                    launchFlags |= Intent.FLAG_ACTIVITY_NEW_DOCUMENT;
                    break;
                case ActivityInfo.DOCUMENT_LAUNCH_ALWAYS:
                    launchFlags |= Intent.FLAG_ACTIVITY_NEW_DOCUMENT;
                    break;
                case ActivityInfo.DOCUMENT_LAUNCH_NEVER:
                    launchFlags &= ~Intent.FLAG_ACTIVITY_MULTIPLE_TASK;
                    break;
            }
        }

        final boolean launchTaskBehind = r.mLaunchTaskBehind
                && !launchSingleTask && !launchSingleInstance
                && (launchFlags & Intent.FLAG_ACTIVITY_NEW_DOCUMENT) != 0;

        if (r.resultTo != null && (launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0
                && r.resultTo.task.stack != null) {
            // For whatever reason this activity is being launched into a new
            // task...  yet the caller has requested a result back.  Well, that
            // is pretty messed up, so instead immediately send back a cancel
            // and let the new task continue launched as normal without a
            // dependency on its originator.
            Slog.w(TAG, "Activity is launching as a new task, so cancelling activity result.");
            r.resultTo.task.stack.sendActivityResultLocked(-1,
                    r.resultTo, r.resultWho, r.requestCode,
                    Activity.RESULT_CANCELED, null);
            r.resultTo = null;
        }

        if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_DOCUMENT) != 0 && r.resultTo == null) {
            launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
        }

        // If we are actually going to launch in to a new task, there are some cases where
        // we further want to do multiple task.
        if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
            if (launchTaskBehind
                    || r.info.documentLaunchMode == ActivityInfo.DOCUMENT_LAUNCH_ALWAYS) {
                launchFlags |= Intent.FLAG_ACTIVITY_MULTIPLE_TASK;
            }
        }

        // We'll invoke onUserLeaving before onPause only if the launching
        // activity did not explicitly state that this is an automated launch.
        mUserLeaving = (launchFlags & Intent.FLAG_ACTIVITY_NO_USER_ACTION) == 0;
        if (DEBUG_USER_LEAVING) Slog.v(TAG_USER_LEAVING,
                "startActivity() => mUserLeaving=" + mUserLeaving);

        // If the caller has asked not to resume at this point, we make note
        // of this in the record so that we can skip it when trying to find
        // the top running activity.
        if (!doResume) {
            r.delayedResume = true;
        }

        ActivityRecord notTop =
                (launchFlags & Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP) != 0 ? r : null;

        // If the onlyIfNeeded flag is set, then we can do this if the activity
        // being launched is the same as the one making the call...  or, as
        // a special case, if we do not know the caller then we count the
        // current top activity as the caller.
        if ((startFlags&ActivityManager.START_FLAG_ONLY_IF_NEEDED) != 0) {
            ActivityRecord checkedCaller = sourceRecord;
            if (checkedCaller == null) {
                checkedCaller = mFocusedStack.topRunningNonDelayedActivityLocked(notTop);
            }
            if (!checkedCaller.realActivity.equals(r.realActivity)) {
                // Caller is not the same as launcher, so always needed.
                startFlags &= ~ActivityManager.START_FLAG_ONLY_IF_NEEDED;
            }
        }

        boolean addingToTask = false;
        TaskRecord reuseTask = null;

        // If the caller is not coming from another activity, but has given us an
        // explicit task into which they would like us to launch the new activity,
        // then let's see about doing that.
        if (sourceRecord == null && inTask != null && inTask.stack != null) {
            final Intent baseIntent = inTask.getBaseIntent();
            final ActivityRecord root = inTask.getRootActivity();
            if (baseIntent == null) {
                ActivityOptions.abort(options);
                throw new IllegalArgumentException("Launching into task without base intent: "
                        + inTask);
            }

            // If this task is empty, then we are adding the first activity -- it
            // determines the root, and must be launching as a NEW_TASK.
            if (launchSingleInstance || launchSingleTask) {
                if (!baseIntent.getComponent().equals(r.intent.getComponent())) {
                    ActivityOptions.abort(options);
                    throw new IllegalArgumentException("Trying to launch singleInstance/Task "
                            + r + " into different task " + inTask);
                }
                if (root != null) {
                    ActivityOptions.abort(options);
                    throw new IllegalArgumentException("Caller with inTask " + inTask
                            + " has root " + root + " but target is singleInstance/Task");
                }
            }

            // If task is empty, then adopt the interesting intent launch flags in to the
            // activity being started.
            if (root == null) {
                final int flagsOfInterest = Intent.FLAG_ACTIVITY_NEW_TASK
                        | Intent.FLAG_ACTIVITY_MULTIPLE_TASK | Intent.FLAG_ACTIVITY_NEW_DOCUMENT
                        | Intent.FLAG_ACTIVITY_RETAIN_IN_RECENTS;
                launchFlags = (launchFlags&~flagsOfInterest)
                        | (baseIntent.getFlags()&flagsOfInterest);
                intent.setFlags(launchFlags);
                inTask.setIntent(r);
                addingToTask = true;

            // If the task is not empty and the caller is asking to start it as the root
            // of a new task, then we don't actually want to start this on the task.  We
            // will bring the task to the front, and possibly give it a new intent.
            } else if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
                addingToTask = false;

            } else {
                addingToTask = true;
            }

            reuseTask = inTask;
        } else {
            inTask = null;
        }

        if (inTask == null) {
            if (sourceRecord == null) {
                // This activity is not being started from another...  in this
                // case we -always- start a new task.
                if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) == 0 && inTask == null) {
                    Slog.w(TAG, "startActivity called from non-Activity context; forcing " +
                            "Intent.FLAG_ACTIVITY_NEW_TASK for: " + intent);
                    launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
                }
            } else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {
                // The original activity who is starting us is running as a single
                // instance...  this new activity it is starting must go on its
                // own task.
                launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
            } else if (launchSingleInstance || launchSingleTask) {
                // The activity being started is a single instance...  it always
                // gets launched into its own task.
                launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
            }
        }

        ActivityInfo newTaskInfo = null;
        Intent newTaskIntent = null;
        ActivityStack sourceStack;
        if (sourceRecord != null) {
            if (sourceRecord.finishing) {
                // If the source is finishing, we can't further count it as our source.  This
                // is because the task it is associated with may now be empty and on its way out,
                // so we don't want to blindly throw it in to that task.  Instead we will take
                // the NEW_TASK flow and try to find a task for it. But save the task information
                // so it can be used when creating the new task.
                if ((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
                    Slog.w(TAG, "startActivity called from finishing " + sourceRecord
                            + "; forcing " + "Intent.FLAG_ACTIVITY_NEW_TASK for: " + intent);
                    launchFlags |= Intent.FLAG_ACTIVITY_NEW_TASK;
                    newTaskInfo = sourceRecord.info;
                    newTaskIntent = sourceRecord.task.intent;
                }
                sourceRecord = null;
                sourceStack = null;
            } else {
                sourceStack = sourceRecord.task.stack;
            }
        } else {
            sourceStack = null;
        }

        boolean movedHome = false;
        ActivityStack targetStack;

        intent.setFlags(launchFlags);
        final boolean noAnimation = (launchFlags & Intent.FLAG_ACTIVITY_NO_ANIMATION) != 0;

        // We may want to try to place the new activity in to an existing task.  We always
        // do this if the target activity is singleTask or singleInstance; we will also do
        // this if NEW_TASK has been requested, and there is not an additional qualifier telling
        // us to still place it in a new task: multi task, always doc mode, or being asked to
        // launch this as a new task behind the current one.
        if (((launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&
                (launchFlags & Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)
                || launchSingleInstance || launchSingleTask) {
            // If bring to front is requested, and no result is requested and we have not
            // been given an explicit task to launch in to, and
            // we can find a task that was started with this same
            // component, then instead of launching bring that one to the front.
            if (inTask == null && r.resultTo == null) {
                // See if there is a task to bring to the front.  If this is
                // a SINGLE_INSTANCE activity, there can be one and only one
                // instance of it in the history, and it is always in its own
                // unique task, so we do a special search.
                ActivityRecord intentActivity = !launchSingleInstance ?
                        findTaskLocked(r) : findActivityLocked(intent, r.info);
                if (intentActivity != null) {
                    // When the flags NEW_TASK and CLEAR_TASK are set, then the task gets reused
                    // but still needs to be a lock task mode violation since the task gets
                    // cleared out and the device would otherwise leave the locked task.
                    if (isLockTaskModeViolation(intentActivity.task,
                            (launchFlags & (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK))
                            == (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK))) {
                        showLockTaskToast();
                        Slog.e(TAG, "startActivityUnchecked: Attempt to violate Lock Task Mode");
                        return ActivityManager.START_RETURN_LOCK_TASK_MODE_VIOLATION;
                    }
                    if (r.task == null) {
                        r.task = intentActivity.task;
                    }
                    if (intentActivity.task.intent == null) {
                        // This task was started because of movement of
                        // the activity based on affinity...  now that we
                        // are actually launching it, we can assign the
                        // base intent.
                        intentActivity.task.setIntent(r);
                    }
                    targetStack = intentActivity.task.stack;
                    targetStack.mLastPausedActivity = null;
                    // If the target task is not in the front, then we need
                    // to bring it to the front...  except...  well, with
                    // SINGLE_TASK_LAUNCH it's not entirely clear.  We'd like
                    // to have the same behavior as if a new instance was
                    // being started, which means not bringing it to the front
                    // if the caller is not itself in the front.
                    final ActivityStack focusStack = getFocusedStack();
                    ActivityRecord curTop = (focusStack == null)
                            ? null : focusStack.topRunningNonDelayedActivityLocked(notTop);
                    boolean movedToFront = false;
                    if (curTop != null && (curTop.task != intentActivity.task ||
                            curTop.task != focusStack.topTask())) {
                        r.intent.addFlags(Intent.FLAG_ACTIVITY_BROUGHT_TO_FRONT);
                        if (sourceRecord == null || (sourceStack.topActivity() != null &&
                                sourceStack.topActivity().task == sourceRecord.task)) {
                            // We really do want to push this one into the user's face, right now.
                            if (launchTaskBehind && sourceRecord != null) {
                                intentActivity.setTaskToAffiliateWith(sourceRecord.task);
                            }
                            movedHome = true;
                            targetStack.moveTaskToFrontLocked(intentActivity.task, noAnimation,
                                    options, r.appTimeTracker, "bringingFoundTaskToFront");
                            movedToFront = true;
                            if ((launchFlags &
                                    (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_TASK_ON_HOME))
                                    == (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_TASK_ON_HOME)) {
                                // Caller wants to appear on home activity.
                                intentActivity.task.setTaskToReturnTo(HOME_ACTIVITY_TYPE);
                            }
                            options = null;
                        }
                    }
                    if (!movedToFront) {
                        if (DEBUG_TASKS) Slog.d(TAG_TASKS, "Bring to front target: " + targetStack
                                + " from " + intentActivity);
                        targetStack.moveToFront("intentActivityFound");
                    }

                    // If the caller has requested that the target task be
                    // reset, then do so.
                    if ((launchFlags&Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED) != 0) {
                        intentActivity = targetStack.resetTaskIfNeededLocked(intentActivity, r);
                    }
                    if ((startFlags & ActivityManager.START_FLAG_ONLY_IF_NEEDED) != 0) {
                        // We don't need to start a new activity, and
                        // the client said not to do anything if that
                        // is the case, so this is it!  And for paranoia, make
                        // sure we have correctly resumed the top activity.
                        if (doResume) {
                            resumeTopActivitiesLocked(targetStack, null, options);

                            // Make sure to notify Keyguard as well if we are not running an app
                            // transition later.
                            if (!movedToFront) {
                                notifyActivityDrawnForKeyguard();
                            }
                        } else {
                            ActivityOptions.abort(options);
                        }
                        return ActivityManager.START_RETURN_INTENT_TO_CALLER;
                    }
                    if ((launchFlags & (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK))
                            == (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK)) {
                        // The caller has requested to completely replace any
                        // existing task with its new activity.  Well that should
                        // not be too hard...
                        reuseTask = intentActivity.task;
                        reuseTask.performClearTaskLocked();
                        reuseTask.setIntent(r);
                    } else if ((launchFlags & FLAG_ACTIVITY_CLEAR_TOP) != 0
                            || launchSingleInstance || launchSingleTask) {
                        // In this situation we want to remove all activities
                        // from the task up to the one being started.  In most
                        // cases this means we are resetting the task to its
                        // initial state.
                        ActivityRecord top =
                                intentActivity.task.performClearTaskLocked(r, launchFlags);
                        if (top != null) {
                            if (top.frontOfTask) {
                                // Activity aliases may mean we use different
                                // intents for the top activity, so make sure
                                // the task now has the identity of the new
                                // intent.
                                top.task.setIntent(r);
                            }
                            ActivityStack.logStartActivity(EventLogTags.AM_NEW_INTENT,
                                    r, top.task);
                            top.deliverNewIntentLocked(callingUid, r.intent, r.launchedFromPackage);
                        } else {
                            // A special case: we need to start the activity because it is not
                            // currently running, and the caller has asked to clear the current
                            // task to have this activity at the top.
                            addingToTask = true;
                            // Now pretend like this activity is being started by the top of its
                            // task, so it is put in the right place.
                            sourceRecord = intentActivity;
                            TaskRecord task = sourceRecord.task;
                            if (task != null && task.stack == null) {
                                // Target stack got cleared when we all activities were removed
                                // above. Go ahead and reset it.
                                targetStack = computeStackFocus(sourceRecord, false /* newTask */);
                                targetStack.addTask(
                                        task, !launchTaskBehind /* toTop */, false /* moving */);
                            }

                        }
                    } else if (r.realActivity.equals(intentActivity.task.realActivity)) {
                        // In this case the top activity on the task is the
                        // same as the one being launched, so we take that
                        // as a request to bring the task to the foreground.
                        // If the top activity in the task is the root
                        // activity, deliver this new intent to it if it
                        // desires.
                        if (((launchFlags&Intent.FLAG_ACTIVITY_SINGLE_TOP) != 0 || launchSingleTop)
                                && intentActivity.realActivity.equals(r.realActivity)) {
                            ActivityStack.logStartActivity(EventLogTags.AM_NEW_INTENT, r,
                                    intentActivity.task);
                            if (intentActivity.frontOfTask) {
                                intentActivity.task.setIntent(r);
                            }
                            intentActivity.deliverNewIntentLocked(callingUid, r.intent,
                                    r.launchedFromPackage);
                        } else if (!r.intent.filterEquals(intentActivity.task.intent)) {
                            // In this case we are launching the root activity
                            // of the task, but with a different intent.  We
                            // should start a new instance on top.
                            addingToTask = true;
                            sourceRecord = intentActivity;
                        }
                    } else if ((launchFlags&Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED) == 0) {
                        // In this case an activity is being launched in to an
                        // existing task, without resetting that task.  This
                        // is typically the situation of launching an activity
                        // from a notification or shortcut.  We want to place
                        // the new activity on top of the current task.
                        addingToTask = true;
                        sourceRecord = intentActivity;
                    } else if (!intentActivity.task.rootWasReset) {
                        // In this case we are launching in to an existing task
                        // that has not yet been started from its front door.
                        // The current task has been brought to the front.
                        // Ideally, we'd probably like to place this new task
                        // at the bottom of its stack, but that's a little hard
                        // to do with the current organization of the code so
                        // for now we'll just drop it.
                        intentActivity.task.setIntent(r);
                    }
                    if (!addingToTask && reuseTask == null) {
                        // We didn't do anything...  but it was needed (a.k.a., client
                        // don't use that intent!)  And for paranoia, make
                        // sure we have correctly resumed the top activity.
                        if (doResume) {
                            targetStack.resumeTopActivityLocked(null, options);
                            if (!movedToFront) {
                                // Make sure to notify Keyguard as well if we are not running an app
                                // transition later.
                                notifyActivityDrawnForKeyguard();
                            }
                        } else {
                            ActivityOptions.abort(options);
                        }
                        return ActivityManager.START_TASK_TO_FRONT;
                    }
                }
            }
        }

        //String uri = r.intent.toURI();
        //Intent intent2 = new Intent(uri);
        //Slog.i(TAG, "Given intent: " + r.intent);
        //Slog.i(TAG, "URI is: " + uri);
        //Slog.i(TAG, "To intent: " + intent2);

        if (r.packageName != null) {
            // If the activity being launched is the same as the one currently
            // at the top, then we need to check if it should only be launched
            // once.
            ActivityStack topStack = mFocusedStack;
            ActivityRecord top = topStack.topRunningNonDelayedActivityLocked(notTop);
            if (top != null && r.resultTo == null) {
                if (top.realActivity.equals(r.realActivity) && top.userId == r.userId) {
                    if (top.app != null && top.app.thread != null) {
                        if ((launchFlags & Intent.FLAG_ACTIVITY_SINGLE_TOP) != 0
                            || launchSingleTop || launchSingleTask) {
                            ActivityStack.logStartActivity(EventLogTags.AM_NEW_INTENT, top,
                                    top.task);
                            // For paranoia, make sure we have correctly
                            // resumed the top activity.
                            topStack.mLastPausedActivity = null;
                            if (doResume) {
                                resumeTopActivitiesLocked();
                            }
                            ActivityOptions.abort(options);
                            if ((startFlags&ActivityManager.START_FLAG_ONLY_IF_NEEDED) != 0) {
                                // We don't need to start a new activity, and
                                // the client said not to do anything if that
                                // is the case, so this is it!
                                return ActivityManager.START_RETURN_INTENT_TO_CALLER;
                            }
                            top.deliverNewIntentLocked(callingUid, r.intent, r.launchedFromPackage);
                            return ActivityManager.START_DELIVERED_TO_TOP;
                        }
                    }
                }
            }

        } else {
            if (r.resultTo != null && r.resultTo.task.stack != null) {
                r.resultTo.task.stack.sendActivityResultLocked(-1, r.resultTo, r.resultWho,
                        r.requestCode, Activity.RESULT_CANCELED, null);
            }
            ActivityOptions.abort(options);
            return ActivityManager.START_CLASS_NOT_FOUND;
        }

        boolean newTask = false;
        boolean keepCurTransition = false;

        TaskRecord taskToAffiliate = launchTaskBehind && sourceRecord != null ?
                sourceRecord.task : null;

        // Should this be considered a new task?
        if (r.resultTo == null && inTask == null && !addingToTask
                && (launchFlags & Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {
            newTask = true;
            targetStack = computeStackFocus(r, newTask);
            targetStack.moveToFront("startingNewTask");

            if (reuseTask == null) {
                r.setTask(targetStack.createTaskRecord(getNextTaskId(),
                        newTaskInfo != null ? newTaskInfo : r.info,
                        newTaskIntent != null ? newTaskIntent : intent,
                        voiceSession, voiceInteractor, !launchTaskBehind /* toTop */),
                        taskToAffiliate);
                if (DEBUG_TASKS) Slog.v(TAG_TASKS,
                        "Starting new activity " + r + " in new task " + r.task);
            } else {
                r.setTask(reuseTask, taskToAffiliate);
            }
            if (isLockTaskModeViolation(r.task)) {
                Slog.e(TAG, "Attempted Lock Task Mode violation r=" + r);
                return ActivityManager.START_RETURN_LOCK_TASK_MODE_VIOLATION;
            }
            if (!movedHome) {
                if ((launchFlags &
                        (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_TASK_ON_HOME))
                        == (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_TASK_ON_HOME)) {
                    // Caller wants to appear on home activity, so before starting
                    // their own activity we will bring home to the front.
                    r.task.setTaskToReturnTo(HOME_ACTIVITY_TYPE);
                }
            }
        } else if (sourceRecord != null) {
            final TaskRecord sourceTask = sourceRecord.task;
            if (isLockTaskModeViolation(sourceTask)) {
                Slog.e(TAG, "Attempted Lock Task Mode violation r=" + r);
                return ActivityManager.START_RETURN_LOCK_TASK_MODE_VIOLATION;
            }
            targetStack = sourceTask.stack;
            targetStack.moveToFront("sourceStackToFront");
            final TaskRecord topTask = targetStack.topTask();
            if (topTask != sourceTask) {
                targetStack.moveTaskToFrontLocked(sourceTask, noAnimation, options,
                        r.appTimeTracker, "sourceTaskToFront");
            }
            if (!addingToTask && (launchFlags&Intent.FLAG_ACTIVITY_CLEAR_TOP) != 0) {
                // In this case, we are adding the activity to an existing
                // task, but the caller has asked to clear that task if the
                // activity is already running.
                ActivityRecord top = sourceTask.performClearTaskLocked(r, launchFlags);
                keepCurTransition = true;
                if (top != null) {
                    ActivityStack.logStartActivity(EventLogTags.AM_NEW_INTENT, r, top.task);
                    top.deliverNewIntentLocked(callingUid, r.intent, r.launchedFromPackage);
                    // For paranoia, make sure we have correctly
                    // resumed the top activity.
                    targetStack.mLastPausedActivity = null;
                    if (doResume) {
                        targetStack.resumeTopActivityLocked(null);
                    }
                    ActivityOptions.abort(options);
                    return ActivityManager.START_DELIVERED_TO_TOP;
                }
            } else if (!addingToTask &&
                    (launchFlags&Intent.FLAG_ACTIVITY_REORDER_TO_FRONT) != 0) {
                // In this case, we are launching an activity in our own task
                // that may already be running somewhere in the history, and
                // we want to shuffle it to the front of the stack if so.
                final ActivityRecord top = sourceTask.findActivityInHistoryLocked(r);
                if (top != null) {
                    final TaskRecord task = top.task;
                    task.moveActivityToFrontLocked(top);
                    ActivityStack.logStartActivity(EventLogTags.AM_NEW_INTENT, r, task);
                    top.updateOptionsLocked(options);
                    top.deliverNewIntentLocked(callingUid, r.intent, r.launchedFromPackage);
                    targetStack.mLastPausedActivity = null;
                    if (doResume) {
                        targetStack.resumeTopActivityLocked(null);
                    }
                    return ActivityManager.START_DELIVERED_TO_TOP;
                }
            }
            // An existing activity is starting this new activity, so we want
            // to keep the new one in the same task as the one that is starting
            // it.
            r.setTask(sourceTask, null);
            if (DEBUG_TASKS) Slog.v(TAG_TASKS, "Starting new activity " + r
                    + " in existing task " + r.task + " from source " + sourceRecord);

        } else if (inTask != null) {
            // The caller is asking that the new activity be started in an explicit
            // task it has provided to us.
            if (isLockTaskModeViolation(inTask)) {
                Slog.e(TAG, "Attempted Lock Task Mode violation r=" + r);
                return ActivityManager.START_RETURN_LOCK_TASK_MODE_VIOLATION;
            }
            targetStack = inTask.stack;
            targetStack.moveTaskToFrontLocked(inTask, noAnimation, options, r.appTimeTracker,
                    "inTaskToFront");

            // Check whether we should actually launch the new activity in to the task,
            // or just reuse the current activity on top.
            ActivityRecord top = inTask.getTopActivity();
            if (top != null && top.realActivity.equals(r.realActivity) && top.userId == r.userId) {
                if ((launchFlags & Intent.FLAG_ACTIVITY_SINGLE_TOP) != 0
                        || launchSingleTop || launchSingleTask) {
                    ActivityStack.logStartActivity(EventLogTags.AM_NEW_INTENT, top, top.task);
                    if ((startFlags&ActivityManager.START_FLAG_ONLY_IF_NEEDED) != 0) {
                        // We don't need to start a new activity, and
                        // the client said not to do anything if that
                        // is the case, so this is it!
                        return ActivityManager.START_RETURN_INTENT_TO_CALLER;
                    }
                    top.deliverNewIntentLocked(callingUid, r.intent, r.launchedFromPackage);
                    return ActivityManager.START_DELIVERED_TO_TOP;
                }
            }

            if (!addingToTask) {
                // We don't actually want to have this activity added to the task, so just
                // stop here but still tell the caller that we consumed the intent.
                ActivityOptions.abort(options);
                return ActivityManager.START_TASK_TO_FRONT;
            }

            r.setTask(inTask, null);
            if (DEBUG_TASKS) Slog.v(TAG_TASKS, "Starting new activity " + r
                    + " in explicit task " + r.task);

        } else {
            // This not being started from an existing activity, and not part
            // of a new task...  just put it in the top task, though these days
            // this case should never happen.
            targetStack = computeStackFocus(r, newTask);
            targetStack.moveToFront("addingToTopTask");
            ActivityRecord prev = targetStack.topActivity();
            r.setTask(prev != null ? prev.task : targetStack.createTaskRecord(getNextTaskId(),
                            r.info, intent, null, null, true), null);
            mWindowManager.moveTaskToTop(r.task.taskId);
            if (DEBUG_TASKS) Slog.v(TAG_TASKS, "Starting new activity " + r
                    + " in new guessed " + r.task);
        }

        mService.grantUriPermissionFromIntentLocked(callingUid, r.packageName,
                intent, r.getUriPermissionsLocked(), r.userId);

        if (sourceRecord != null && sourceRecord.isRecentsActivity()) {
            r.task.setTaskToReturnTo(RECENTS_ACTIVITY_TYPE);
        }
        if (newTask) {
            EventLog.writeEvent(EventLogTags.AM_CREATE_TASK, r.userId, r.task.taskId);
        }
        ActivityStack.logStartActivity(EventLogTags.AM_CREATE_ACTIVITY, r, r.task);
        targetStack.mLastPausedActivity = null;
        targetStack.startActivityLocked(r, newTask, doResume, keepCurTransition, options);
        if (!launchTaskBehind) {
            // Don't set focus on an activity that's going to the back.
            mService.setFocusedActivityLocked(r, "startedActivity");
        }
        return ActivityManager.START_SUCCESS;
    }

ActivityStack#startActivityLocked

又跳进了ActivityStack#startActivityLocked():


 final void startActivityLocked(ActivityRecord r, boolean newTask,
            boolean doResume, boolean keepCurTransition, Bundle options) {
        TaskRecord rTask = r.task;
        final int taskId = rTask.taskId;
        // mLaunchTaskBehind tasks get placed at the back of the task stack.
        if (!r.mLaunchTaskBehind && (taskForIdLocked(taskId) == null || newTask)) {
            // Last activity in task had been removed or ActivityManagerService is reusing task.
            // Insert or replace.
            // Might not even be in.
            insertTaskAtTop(rTask, r);
            mWindowManager.moveTaskToTop(taskId);
        }
        TaskRecord task = null;
        if (!newTask) {
            // If starting in an existing task, find where that is...
            boolean startIt = true;
            for (int taskNdx = mTaskHistory.size() - 1; taskNdx >= 0; --taskNdx) {
                task = mTaskHistory.get(taskNdx);
                if (task.getTopActivity() == null) {
                    // All activities in task are finishing.
                    continue;
                }
                if (task == r.task) {
                    // Here it is!  Now, if this is not yet visible to the
                    // user, then just add it without starting; it will
                    // get started when the user navigates back to it.
                    if (!startIt) {
                        if (DEBUG_ADD_REMOVE) Slog.i(TAG, "Adding activity " + r + " to task "
                                + task, new RuntimeException("here").fillInStackTrace());
                        task.addActivityToTop(r);
                        r.putInHistory();
                        mWindowManager.addAppToken(task.mActivities.indexOf(r), r.appToken,
                                r.task.taskId, mStackId, r.info.screenOrientation, r.fullscreen,
                                (r.info.flags & ActivityInfo.FLAG_SHOW_FOR_ALL_USERS) != 0,
                                r.userId, r.info.configChanges, task.voiceSession != null,
                                r.mLaunchTaskBehind);
                        if (VALIDATE_TOKENS) {
                            validateAppTokensLocked();
                        }
                        ActivityOptions.abort(options);
                        return;
                    }
                    break;
                } else if (task.numFullscreen > 0) {
                    startIt = false;
                }
            }
        }

        // Place a new activity at top of stack, so it is next to interact
        // with the user.

        // If we are not placing the new activity frontmost, we do not want
        // to deliver the onUserLeaving callback to the actual frontmost
        // activity
        if (task == r.task && mTaskHistory.indexOf(task) != (mTaskHistory.size() - 1)) {
            mStackSupervisor.mUserLeaving = false;
            if (DEBUG_USER_LEAVING) Slog.v(TAG_USER_LEAVING,
                    "startActivity() behind front, mUserLeaving=false");
        }

        task = r.task;

        // Slot the activity into the history stack and proceed
        if (DEBUG_ADD_REMOVE) Slog.i(TAG, "Adding activity " + r + " to stack to task " + task,
                new RuntimeException("here").fillInStackTrace());
        task.addActivityToTop(r);
        task.setFrontOfTask();

        r.putInHistory();
        if (!isHomeStack() || numActivities() > 0) {
            // We want to show the starting preview window if we are
            // switching to a new task, or the next activity's process is
            // not currently running.
            boolean showStartingIcon = newTask;
            ProcessRecord proc = r.app;
            if (proc == null) {
                proc = mService.mProcessNames.get(r.processName, r.info.applicationInfo.uid);
            }
            if (proc == null || proc.thread == null) {
                showStartingIcon = true;
            }
            if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION,
                    "Prepare open transition: starting " + r);
            if ((r.intent.getFlags() & Intent.FLAG_ACTIVITY_NO_ANIMATION) != 0) {
                mWindowManager.prepareAppTransition(AppTransition.TRANSIT_NONE, keepCurTransition);
                mNoAnimActivities.add(r);
            } else {
                mWindowManager.prepareAppTransition(newTask
                        ? r.mLaunchTaskBehind
                                ? AppTransition.TRANSIT_TASK_OPEN_BEHIND
                                : AppTransition.TRANSIT_TASK_OPEN
                        : AppTransition.TRANSIT_ACTIVITY_OPEN, keepCurTransition);
                mNoAnimActivities.remove(r);
            }
            mWindowManager.addAppToken(task.mActivities.indexOf(r),
                    r.appToken, r.task.taskId, mStackId, r.info.screenOrientation, r.fullscreen,
                    (r.info.flags & ActivityInfo.FLAG_SHOW_FOR_ALL_USERS) != 0, r.userId,
                    r.info.configChanges, task.voiceSession != null, r.mLaunchTaskBehind);
            boolean doShow = true;
            if (newTask) {
                // Even though this activity is starting fresh, we still need
                // to reset it to make sure we apply affinities to move any
                // existing activities from other tasks in to it.
                // If the caller has requested that the target task be
                // reset, then do so.
                if ((r.intent.getFlags() & Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED) != 0) {
                    resetTaskIfNeededLocked(r, r);
                    doShow = topRunningNonDelayedActivityLocked(null) == r;
                }
            } else if (options != null && new ActivityOptions(options).getAnimationType()
                    == ActivityOptions.ANIM_SCENE_TRANSITION) {
                doShow = false;
            }
            if (r.mLaunchTaskBehind) {
                // Don't do a starting window for mLaunchTaskBehind. More importantly make sure we
                // tell WindowManager that r is visible even though it is at the back of the stack.
                mWindowManager.setAppVisibility(r.appToken, true);
                ensureActivitiesVisibleLocked(null, 0);
            } else if (SHOW_APP_STARTING_PREVIEW && doShow) {
                // Figure out if we are transitioning from another activity that is
                // "has the same starting icon" as the next one.  This allows the
                // window manager to keep the previous window it had previously
                // created, if it still had one.
                ActivityRecord prev = mResumedActivity;
                if (prev != null) {
                    // We don't want to reuse the previous starting preview if:
                    // (1) The current activity is in a different task.
                    if (prev.task != r.task) {
                        prev = null;
                    }
                    // (2) The current activity is already displayed.
                    else if (prev.nowVisible) {
                        prev = null;
                    }
                }
                mWindowManager.setAppStartingWindow(
                        r.appToken, r.packageName, r.theme,
                        mService.compatibilityInfoForPackageLocked(
                                r.info.applicationInfo), r.nonLocalizedLabel,
                        r.labelRes, r.icon, r.logo, r.windowFlags,
                        prev != null ? prev.appToken : null, showStartingIcon);
                r.mStartingWindowShown = true;
            }
        } else {
            // If this is the first activity, don't do any fancy animations,
            // because there is nothing for it to animate on top of.
            mWindowManager.addAppToken(task.mActivities.indexOf(r), r.appToken,
                    r.task.taskId, mStackId, r.info.screenOrientation, r.fullscreen,
                    (r.info.flags & ActivityInfo.FLAG_SHOW_FOR_ALL_USERS) != 0, r.userId,
                    r.info.configChanges, task.voiceSession != null, r.mLaunchTaskBehind);
            ActivityOptions.abort(options);
            options = null;
        }
        if (VALIDATE_TOKENS) {
            validateAppTokensLocked();
        }

        if (doResume) {
            mStackSupervisor.resumeTopActivitiesLocked(this, r, options);
        }
    }

ActivityStackSupervisor#resumeTopActivitiesLocked

startActivityLocked中会调用mStackSupervisor#resumeTopActivitiesLocked()来恢复栈顶Activity。

mStackSupervisor#resumeTopActivitiesLocked():

boolean resumeTopActivitiesLocked(ActivityStack targetStack, ActivityRecord target,
            Bundle targetOptions) {
        if (targetStack == null) {
            targetStack = mFocusedStack;
        }
        // Do targetStack first.
        boolean result = false;
        if (isFrontStack(targetStack)) {
            result = targetStack.resumeTopActivityLocked(target, targetOptions);
        }

        for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
            final ArrayList<ActivityStack> stacks = mActivityDisplays.valueAt(displayNdx).mStacks;
            for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx) {
                final ActivityStack stack = stacks.get(stackNdx);
                if (stack == targetStack) {
                    // Already started above.
                    continue;
                }
                if (isFrontStack(stack)) {
                    stack.resumeTopActivityLocked(null);
                }
            }
        }
        return result;
    }

ActivityStack#resumeTopActivityLocked

继续跳转到ActivityStack#resumeTopActivityLocked:

final boolean resumeTopActivityLocked(ActivityRecord prev, Bundle options) {
        if (mStackSupervisor.inResumeTopActivity) {
            // Don't even start recursing.
            return false;
        }
        
        boolean result = false;
        try {
            // Protect against recursion.
            mStackSupervisor.inResumeTopActivity = true;
            if (mService.mLockScreenShown == ActivityManagerService.LOCK_SCREEN_LEAVING) {
                mService.mLockScreenShown = ActivityManagerService.LOCK_SCREEN_HIDDEN;
                mService.updateSleepIfNeededLocked();
            }
            result = resumeTopActivityInnerLocked(prev, options);
        } finally {
            mStackSupervisor.inResumeTopActivity = false;
        }
        return result;
    }

ActivityStack#resumeTopActivityInnerLocked

继续跳转到ActivityStack#resumeTopActivityInnerLocked:

private boolean resumeTopActivityInnerLocked(ActivityRecord prev, Bundle options) {
        if (DEBUG_LOCKSCREEN) mService.logLockScreen("");

        if (!mService.mBooting && !mService.mBooted) {
            // Not ready yet!
            return false;
        }

        ActivityRecord parent = mActivityContainer.mParentActivity;
        if ((parent != null && parent.state != ActivityState.RESUMED) ||
                !mActivityContainer.isAttachedLocked()) {
            // Do not resume this stack if its parent is not resumed.
            // TODO: If in a loop, make sure that parent stack resumeTopActivity is called 1st.
            return false;
        }

        cancelInitializingActivities();

        // Find the first activity that is not finishing.
        final ActivityRecord next = topRunningActivityLocked(null);

        // Remember how we'll process this pause/resume situation, and ensure
        // that the state is reset however we wind up proceeding.
        final boolean userLeaving = mStackSupervisor.mUserLeaving;
        mStackSupervisor.mUserLeaving = false;

        final TaskRecord prevTask = prev != null ? prev.task : null;
        if (next == null) {
            // There are no more activities!
            final String reason = "noMoreActivities";
            if (!mFullscreen) {
                // Try to move focus to the next visible stack with a running activity if this
                // stack is not covering the entire screen.
                final ActivityStack stack = getNextVisibleStackLocked();
                if (adjustFocusToNextVisibleStackLocked(stack, reason)) {
                    return mStackSupervisor.resumeTopActivitiesLocked(stack, prev, null);
                }
            }
            // Let's just start up the Launcher...
            ActivityOptions.abort(options);
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: No more activities go home");
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            // Only resume home if on home display
            final int returnTaskType = prevTask == null || !prevTask.isOverHomeStack() ?
                    HOME_ACTIVITY_TYPE : prevTask.getTaskToReturnTo();
            return isOnHomeDisplay() &&
                    mStackSupervisor.resumeHomeStackTask(returnTaskType, prev, reason);
        }

        next.delayedResume = false;

        // If the top activity is the resumed one, nothing to do.
        if (mResumedActivity == next && next.state == ActivityState.RESUMED &&
                    mStackSupervisor.allResumedActivitiesComplete()) {
            // Make sure we have executed any pending transitions, since there
            // should be nothing left to do at this point.
            mWindowManager.executeAppTransition();
            mNoAnimActivities.clear();
            ActivityOptions.abort(options);
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Top activity resumed " + next);
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return false;
        }

        final TaskRecord nextTask = next.task;
        if (prevTask != null && prevTask.stack == this &&
                prevTask.isOverHomeStack() && prev.finishing && prev.frontOfTask) {
            if (DEBUG_STACK)  mStackSupervisor.validateTopActivitiesLocked();
            if (prevTask == nextTask) {
                prevTask.setFrontOfTask();
            } else if (prevTask != topTask()) {
                // This task is going away but it was supposed to return to the home stack.
                // Now the task above it has to return to the home task instead.
                final int taskNdx = mTaskHistory.indexOf(prevTask) + 1;
                mTaskHistory.get(taskNdx).setTaskToReturnTo(HOME_ACTIVITY_TYPE);
            } else if (!isOnHomeDisplay()) {
                return false;
            } else if (!isHomeStack()){
                if (DEBUG_STATES) Slog.d(TAG_STATES,
                        "resumeTopActivityLocked: Launching home next");
                final int returnTaskType = prevTask == null || !prevTask.isOverHomeStack() ?
                        HOME_ACTIVITY_TYPE : prevTask.getTaskToReturnTo();
                return isOnHomeDisplay() &&
                        mStackSupervisor.resumeHomeStackTask(returnTaskType, prev, "prevFinished");
            }
        }

        // If we are sleeping, and there is no resumed activity, and the top
        // activity is paused, well that is the state we want.
        if (mService.isSleepingOrShuttingDown()
                && mLastPausedActivity == next
                && mStackSupervisor.allPausedActivitiesComplete()) {
            // Make sure we have executed any pending transitions, since there
            // should be nothing left to do at this point.
            mWindowManager.executeAppTransition();
            mNoAnimActivities.clear();
            ActivityOptions.abort(options);
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Going to sleep and all paused");
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return false;
        }

        // Make sure that the user who owns this activity is started.  If not,
        // we will just leave it as is because someone should be bringing
        // another user's activities to the top of the stack.
        if (mService.mStartedUsers.get(next.userId) == null) {
            Slog.w(TAG, "Skipping resume of top activity " + next
                    + ": user " + next.userId + " is stopped");
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return false;
        }

        // The activity may be waiting for stop, but that is no longer
        // appropriate for it.
        mStackSupervisor.mStoppingActivities.remove(next);
        mStackSupervisor.mGoingToSleepActivities.remove(next);
        next.sleeping = false;
        mStackSupervisor.mWaitingVisibleActivities.remove(next);

        if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Resuming " + next);

        // If we are currently pausing an activity, then don't do anything
        // until that is done.
        if (!mStackSupervisor.allPausedActivitiesComplete()) {
            if (DEBUG_SWITCH || DEBUG_PAUSE || DEBUG_STATES) Slog.v(TAG_PAUSE,
                    "resumeTopActivityLocked: Skip resume: some activity pausing.");
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return false;
        }

        // Okay we are now going to start a switch, to 'next'.  We may first
        // have to pause the current activity, but this is an important point
        // where we have decided to go to 'next' so keep track of that.
        // XXX "App Redirected" dialog is getting too many false positives
        // at this point, so turn off for now.
        if (false) {
            if (mLastStartedActivity != null && !mLastStartedActivity.finishing) {
                long now = SystemClock.uptimeMillis();
                final boolean inTime = mLastStartedActivity.startTime != 0
                        && (mLastStartedActivity.startTime + START_WARN_TIME) >= now;
                final int lastUid = mLastStartedActivity.info.applicationInfo.uid;
                final int nextUid = next.info.applicationInfo.uid;
                if (inTime && lastUid != nextUid
                        && lastUid != next.launchedFromUid
                        && mService.checkPermission(
                                android.Manifest.permission.STOP_APP_SWITCHES,
                                -1, next.launchedFromUid)
                        != PackageManager.PERMISSION_GRANTED) {
                    mService.showLaunchWarningLocked(mLastStartedActivity, next);
                } else {
                    next.startTime = now;
                    mLastStartedActivity = next;
                }
            } else {
                next.startTime = SystemClock.uptimeMillis();
                mLastStartedActivity = next;
            }
        }

        mStackSupervisor.setLaunchSource(next.info.applicationInfo.uid);

        // We need to start pausing the current activity so the top one
        // can be resumed...
        boolean dontWaitForPause = (next.info.flags&ActivityInfo.FLAG_RESUME_WHILE_PAUSING) != 0;
        boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, true, dontWaitForPause);
        if (mResumedActivity != null) {
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Pausing " + mResumedActivity);
            pausing |= startPausingLocked(userLeaving, false, true, dontWaitForPause);
        }
        if (pausing) {
            if (DEBUG_SWITCH || DEBUG_STATES) Slog.v(TAG_STATES,
                    "resumeTopActivityLocked: Skip resume: need to start pausing");
            // At this point we want to put the upcoming activity's process
            // at the top of the LRU list, since we know we will be needing it
            // very soon and it would be a waste to let it get killed if it
            // happens to be sitting towards the end.
            if (next.app != null && next.app.thread != null) {
                mService.updateLruProcessLocked(next.app, true, null);
            }
            if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
            return true;
        }

        // If the most recent activity was noHistory but was only stopped rather
        // than stopped+finished because the device went to sleep, we need to make
        // sure to finish it as we're making a new activity topmost.
        if (mService.isSleeping() && mLastNoHistoryActivity != null &&
                !mLastNoHistoryActivity.finishing) {
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "no-history finish of " + mLastNoHistoryActivity + " on new resume");
            requestFinishActivityLocked(mLastNoHistoryActivity.appToken, Activity.RESULT_CANCELED,
                    null, "resume-no-history", false);
            mLastNoHistoryActivity = null;
        }

        if (prev != null && prev != next) {
            if (!mStackSupervisor.mWaitingVisibleActivities.contains(prev)
                    && next != null && !next.nowVisible) {
                mStackSupervisor.mWaitingVisibleActivities.add(prev);
                if (DEBUG_SWITCH) Slog.v(TAG_SWITCH,
                        "Resuming top, waiting visible to hide: " + prev);
            } else {
                // The next activity is already visible, so hide the previous
                // activity's windows right now so we can show the new one ASAP.
                // We only do this if the previous is finishing, which should mean
                // it is on top of the one being resumed so hiding it quickly
                // is good.  Otherwise, we want to do the normal route of allowing
                // the resumed activity to be shown so we can decide if the
                // previous should actually be hidden depending on whether the
                // new one is found to be full-screen or not.
                if (prev.finishing) {
                    mWindowManager.setAppVisibility(prev.appToken, false);
                    if (DEBUG_SWITCH) Slog.v(TAG_SWITCH,
                            "Not waiting for visible to hide: " + prev + ", waitingVisible="
                            + mStackSupervisor.mWaitingVisibleActivities.contains(prev)
                            + ", nowVisible=" + next.nowVisible);
                } else {
                    if (DEBUG_SWITCH) Slog.v(TAG_SWITCH,
                            "Previous already visible but still waiting to hide: " + prev
                            + ", waitingVisible="
                            + mStackSupervisor.mWaitingVisibleActivities.contains(prev)
                            + ", nowVisible=" + next.nowVisible);
                }
            }
        }

        // Launching this app's activity, make sure the app is no longer
        // considered stopped.
        try {
            AppGlobals.getPackageManager().setPackageStoppedState(
                    next.packageName, false, next.userId); /* TODO: Verify if correct userid */
        } catch (RemoteException e1) {
        } catch (IllegalArgumentException e) {
            Slog.w(TAG, "Failed trying to unstop package "
                    + next.packageName + ": " + e);
        }

        // We are starting up the next activity, so tell the window manager
        // that the previous one will be hidden soon.  This way it can know
        // to ignore it when computing the desired screen orientation.
        boolean anim = true;
        if (prev != null) {
            if (prev.finishing) {
                if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION,
                        "Prepare close transition: prev=" + prev);
                if (mNoAnimActivities.contains(prev)) {
                    anim = false;
                    mWindowManager.prepareAppTransition(AppTransition.TRANSIT_NONE, false);
                } else {
                    mWindowManager.prepareAppTransition(prev.task == next.task
                            ? AppTransition.TRANSIT_ACTIVITY_CLOSE
                            : AppTransition.TRANSIT_TASK_CLOSE, false);
                }
                mWindowManager.setAppWillBeHidden(prev.appToken);
                mWindowManager.setAppVisibility(prev.appToken, false);
            } else {
                if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION,
                        "Prepare open transition: prev=" + prev);
                if (mNoAnimActivities.contains(next)) {
                    anim = false;
                    mWindowManager.prepareAppTransition(AppTransition.TRANSIT_NONE, false);
                } else {
                    mWindowManager.prepareAppTransition(prev.task == next.task
                            ? AppTransition.TRANSIT_ACTIVITY_OPEN
                            : next.mLaunchTaskBehind
                                    ? AppTransition.TRANSIT_TASK_OPEN_BEHIND
                                    : AppTransition.TRANSIT_TASK_OPEN, false);
                }
            }
            if (false) {
                mWindowManager.setAppWillBeHidden(prev.appToken);
                mWindowManager.setAppVisibility(prev.appToken, false);
            }
        } else {
            if (DEBUG_TRANSITION) Slog.v(TAG_TRANSITION, "Prepare open transition: no previous");
            if (mNoAnimActivities.contains(next)) {
                anim = false;
                mWindowManager.prepareAppTransition(AppTransition.TRANSIT_NONE, false);
            } else {
                mWindowManager.prepareAppTransition(AppTransition.TRANSIT_ACTIVITY_OPEN, false);
            }
        }

        Bundle resumeAnimOptions = null;
        if (anim) {
            ActivityOptions opts = next.getOptionsForTargetActivityLocked();
            if (opts != null) {
                resumeAnimOptions = opts.toBundle();
            }
            next.applyOptionsLocked();
        } else {
            next.clearOptionsLocked();
        }

        ActivityStack lastStack = mStackSupervisor.getLastStack();
        if (next.app != null && next.app.thread != null) {
            if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Resume running: " + next);

            // This activity is now becoming visible.
            mWindowManager.setAppVisibility(next.appToken, true);

            // schedule launch ticks to collect information about slow apps.
            next.startLaunchTickingLocked();

            ActivityRecord lastResumedActivity =
                    lastStack == null ? null :lastStack.mResumedActivity;
            ActivityState lastState = next.state;

            mService.updateCpuStats();

            if (DEBUG_STATES) Slog.v(TAG_STATES, "Moving to RESUMED: " + next + " (in existing)");
            next.state = ActivityState.RESUMED;
            mResumedActivity = next;
            next.task.touchActiveTime();
            mRecentTasks.addLocked(next.task);
            mService.updateLruProcessLocked(next.app, true, null);
            updateLRUListLocked(next);
            mService.updateOomAdjLocked();

            // Have the window manager re-evaluate the orientation of
            // the screen based on the new activity order.
            boolean notUpdated = true;
            if (mStackSupervisor.isFrontStack(this)) {
                Configuration config = mWindowManager.updateOrientationFromAppTokens(
                        mService.mConfiguration,
                        next.mayFreezeScreenLocked(next.app) ? next.appToken : null);
                if (config != null) {
                    next.frozenBeforeDestroy = true;
                }
                notUpdated = !mService.updateConfigurationLocked(config, next, false, false);
            }

            if (notUpdated) {
                // The configuration update wasn't able to keep the existing
                // instance of the activity, and instead started a new one.
                // We should be all done, but let's just make sure our activity
                // is still at the top and schedule another run if something
                // weird happened.
                ActivityRecord nextNext = topRunningActivityLocked(null);
                if (DEBUG_SWITCH || DEBUG_STATES) Slog.i(TAG_STATES,
                        "Activity config changed during resume: " + next
                        + ", new next: " + nextNext);
                if (nextNext != next) {
                    // Do over!
                    mStackSupervisor.scheduleResumeTopActivities();
                }
                if (mStackSupervisor.reportResumedActivityLocked(next)) {
                    mNoAnimActivities.clear();
                    if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
                    return true;
                }
                if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
                return false;
            }

            try {
                // Deliver all pending results.
                ArrayList<ResultInfo> a = next.results;
                if (a != null) {
                    final int N = a.size();
                    if (!next.finishing && N > 0) {
                        if (DEBUG_RESULTS) Slog.v(TAG_RESULTS,
                                "Delivering results to " + next + ": " + a);
                        next.app.thread.scheduleSendResult(next.appToken, a);
                    }
                }

                if (next.newIntents != null) {
                    next.app.thread.scheduleNewIntent(next.newIntents, next.appToken);
                }

                EventLog.writeEvent(EventLogTags.AM_RESUME_ACTIVITY, next.userId,
                        System.identityHashCode(next), next.task.taskId, next.shortComponentName);

                next.sleeping = false;
                mService.showAskCompatModeDialogLocked(next);
                next.app.pendingUiClean = true;
                next.app.forceProcessStateUpTo(mService.mTopProcessState);
                next.clearOptionsLocked();
                next.app.thread.scheduleResumeActivity(next.appToken, next.app.repProcState,
                        mService.isNextTransitionForward(), resumeAnimOptions);

                mStackSupervisor.checkReadyForSleepLocked();

                if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Resumed " + next);
            } catch (Exception e) {
                // Whoops, need to restart this activity!
                if (DEBUG_STATES) Slog.v(TAG_STATES, "Resume failed; resetting state to "
                        + lastState + ": " + next);
                next.state = lastState;
                if (lastStack != null) {
                    lastStack.mResumedActivity = lastResumedActivity;
                }
                Slog.i(TAG, "Restarting because process died: " + next);
                if (!next.hasBeenLaunched) {
                    next.hasBeenLaunched = true;
                } else  if (SHOW_APP_STARTING_PREVIEW && lastStack != null &&
                        mStackSupervisor.isFrontStack(lastStack)) {
                    mWindowManager.setAppStartingWindow(
                            next.appToken, next.packageName, next.theme,
                            mService.compatibilityInfoForPackageLocked(next.info.applicationInfo),
                            next.nonLocalizedLabel, next.labelRes, next.icon, next.logo,
                            next.windowFlags, null, true);
                }
                mStackSupervisor.startSpecificActivityLocked(next, true, false);
                if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
                return true;
            }

            // From this point on, if something goes wrong there is no way
            // to recover the activity.
            try {
                next.visible = true;
                completeResumeLocked(next);
            } catch (Exception e) {
                // If any exception gets thrown, toss away this
                // activity and try the next one.
                Slog.w(TAG, "Exception thrown during resume of " + next, e);
                requestFinishActivityLocked(next.appToken, Activity.RESULT_CANCELED, null,
                        "resume-exception", true);
                if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
                return true;
            }
            next.stopped = false;

        } else {
            // Whoops, need to restart this activity!
            if (!next.hasBeenLaunched) {
                next.hasBeenLaunched = true;
            } else {
                if (SHOW_APP_STARTING_PREVIEW) {
                    mWindowManager.setAppStartingWindow(
                            next.appToken, next.packageName, next.theme,
                            mService.compatibilityInfoForPackageLocked(
                                    next.info.applicationInfo),
                            next.nonLocalizedLabel,
                            next.labelRes, next.icon, next.logo, next.windowFlags,
                            null, true);
                }
                if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Restarting: " + next);
            }
            if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Restarting " + next);
            mStackSupervisor.startSpecificActivityLocked(next, true, true);
        }

        if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
        return true;
    }

这个方法比较重要的逻辑在这里:

ActivityStack lastStack = mStackSupervisor.getLastStack();
    
    //这个逻辑说明当前APP进程已经被启动过
    if (next.app != null && next.app.thread != null) {
            if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Resume running: " + next);

            // This activity is now becoming visible.
            mWindowManager.setAppVisibility(next.appToken, true);

	    ...
	}
	else{
		...
		mStackSupervisor.startSpecificActivityLocked(next, true, true);
	}

next.app.thread != null说明当前进程的ActivityThread已经被启动了,不需要zygote进程再重新fork进程出来,只需要拿到栈顶,展现在用户面前。 否则,就要调用到StackSupervisor.startSpecificActivityLocked

ActivityStackSupervisor#startSpecificActivityLocked

mStackSupervisor#startSpecificActivityLocked:

 void startSpecificActivityLocked(ActivityRecord r,
            boolean andResume, boolean checkConfig) {
        // Is this activity's application already running?
        ProcessRecord app = mService.getProcessRecordLocked(r.processName,
                r.info.applicationInfo.uid, true);

        r.task.stack.setLaunchTime(r);

        if (app != null && app.thread != null) {
            try {
                if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
                        || !"android".equals(r.info.packageName)) {
                    // Don't add this if it is a platform component that is marked
                    // to run in multiple processes, because this is actually
                    // part of the framework so doesn't make sense to track as a
                    // separate apk in the process.
                    app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode,
                            mService.mProcessStats);
                }
                realStartActivityLocked(r, app, andResume, checkConfig);
                return;
            } catch (RemoteException e) {
                Slog.w(TAG, "Exception when starting activity "
                        + r.intent.getComponent().flattenToShortString(), e);
            }

            // If a dead object exception was thrown -- fall through to
            // restart the application.
        }

        mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
                "activity", r.intent.getComponent(), false, false, true);
    }

从我们上面那个步骤进到这个方法,app.thread必然是null,所以会走到下面这个逻辑中去:

//mService就是ActivityManagerService
mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
                "activity", r.intent.getComponent(), false, false, true);

ActivityManagerService#startProcessLocked

再来看ActivityManagerService#startProcessLocked:

private final void startProcessLocked(ProcessRecord app, String hostingType,
            String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
        long startTime = SystemClock.elapsedRealtime();
        if (app.pid > 0 && app.pid != MY_PID) {
            checkTime(startTime, "startProcess: removing from pids map");
            synchronized (mPidsSelfLocked) {
                mPidsSelfLocked.remove(app.pid);
                mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
            }
            checkTime(startTime, "startProcess: done removing from pids map");
            app.setPid(0);
        }

        if (DEBUG_PROCESSES && mProcessesOnHold.contains(app)) Slog.v(TAG_PROCESSES,
                "startProcessLocked removing on hold: " + app);
        mProcessesOnHold.remove(app);

        checkTime(startTime, "startProcess: starting to update cpu stats");
        updateCpuStats();
        checkTime(startTime, "startProcess: done updating cpu stats");

        try {
            try {
                if (AppGlobals.getPackageManager().isPackageFrozen(app.info.packageName)) {
                    // This is caught below as if we had failed to fork zygote
                    throw new RuntimeException("Package " + app.info.packageName + " is frozen!");
                }
            } catch (RemoteException e) {
                throw e.rethrowAsRuntimeException();
            }

            int uid = app.uid;
            int[] gids = null;
            int mountExternal = Zygote.MOUNT_EXTERNAL_NONE;
            if (!app.isolated) {
                int[] permGids = null;
                try {
                    checkTime(startTime, "startProcess: getting gids from package manager");
                    final IPackageManager pm = AppGlobals.getPackageManager();
                    permGids = pm.getPackageGids(app.info.packageName, app.userId);
                    MountServiceInternal mountServiceInternal = LocalServices.getService(
                            MountServiceInternal.class);
                    mountExternal = mountServiceInternal.getExternalStorageMountMode(uid,
                            app.info.packageName);
                } catch (RemoteException e) {
                    throw e.rethrowAsRuntimeException();
                }

                /*
                 * Add shared application and profile GIDs so applications can share some
                 * resources like shared libraries and access user-wide resources
                 */
                if (ArrayUtils.isEmpty(permGids)) {
                    gids = new int[2];
                } else {
                    gids = new int[permGids.length + 2];
                    System.arraycopy(permGids, 0, gids, 2, permGids.length);
                }
                gids[0] = UserHandle.getSharedAppGid(UserHandle.getAppId(uid));
                gids[1] = UserHandle.getUserGid(UserHandle.getUserId(uid));
            }
            checkTime(startTime, "startProcess: building args");
            if (mFactoryTest != FactoryTest.FACTORY_TEST_OFF) {
                if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
                        && mTopComponent != null
                        && app.processName.equals(mTopComponent.getPackageName())) {
                    uid = 0;
                }
                if (mFactoryTest == FactoryTest.FACTORY_TEST_HIGH_LEVEL
                        && (app.info.flags&ApplicationInfo.FLAG_FACTORY_TEST) != 0) {
                    uid = 0;
                }
            }
            int debugFlags = 0;
            if ((app.info.flags & ApplicationInfo.FLAG_DEBUGGABLE) != 0) {
                debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER;
                // Also turn on CheckJNI for debuggable apps. It's quite
                // awkward to turn on otherwise.
                debugFlags |= Zygote.DEBUG_ENABLE_CHECKJNI;
            }
            // Run the app in safe mode if its manifest requests so or the
            // system is booted in safe mode.
            if ((app.info.flags & ApplicationInfo.FLAG_VM_SAFE_MODE) != 0 ||
                mSafeMode == true) {
                debugFlags |= Zygote.DEBUG_ENABLE_SAFEMODE;
            }
            if ("1".equals(SystemProperties.get("debug.checkjni"))) {
                debugFlags |= Zygote.DEBUG_ENABLE_CHECKJNI;
            }
            String genDebugInfoProperty = SystemProperties.get("debug.generate-debug-info");
            if ("true".equals(genDebugInfoProperty)) {
                debugFlags |= Zygote.DEBUG_GENERATE_DEBUG_INFO;
            }
            if ("1".equals(SystemProperties.get("debug.jni.logging"))) {
                debugFlags |= Zygote.DEBUG_ENABLE_JNI_LOGGING;
            }
            if ("1".equals(SystemProperties.get("debug.assert"))) {
                debugFlags |= Zygote.DEBUG_ENABLE_ASSERT;
            }
            if (mNativeDebuggingApp != null && mNativeDebuggingApp.equals(app.processName)) {
                // Enable all debug flags required by the native debugger.
                debugFlags |= Zygote.DEBUG_ALWAYS_JIT;          // Don't interpret anything
                debugFlags |= Zygote.DEBUG_GENERATE_DEBUG_INFO; // Generate debug info
                debugFlags |= Zygote.DEBUG_NATIVE_DEBUGGABLE;   // Disbale optimizations
                mNativeDebuggingApp = null;
            }

            String requiredAbi = (abiOverride != null) ? abiOverride : app.info.primaryCpuAbi;
            if (requiredAbi == null) {
                requiredAbi = Build.SUPPORTED_ABIS[0];
            }

            String instructionSet = null;
            if (app.info.primaryCpuAbi != null) {
                instructionSet = VMRuntime.getInstructionSet(app.info.primaryCpuAbi);
            }

            app.gids = gids;
            app.requiredAbi = requiredAbi;
            app.instructionSet = instructionSet;

            // Start the process.  It will either succeed and return a result containing
            // the PID of the new process, or else throw a RuntimeException.
            boolean isActivityProcess = (entryPoint == null);
            if (entryPoint == null) entryPoint = "android.app.ActivityThread";
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "Start proc: " +
                    app.processName);
            checkTime(startTime, "startProcess: asking zygote to start proc");
            Process.ProcessStartResult startResult = Process.start(entryPoint,
                    app.processName, uid, uid, gids, debugFlags, mountExternal,
                    app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
                    app.info.dataDir, entryPointArgs);
            checkTime(startTime, "startProcess: returned from zygote!");
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

            if (app.isolated) {
                mBatteryStatsService.addIsolatedUid(app.uid, app.info.uid);
            }
            mBatteryStatsService.noteProcessStart(app.processName, app.info.uid);
            checkTime(startTime, "startProcess: done updating battery stats");

            EventLog.writeEvent(EventLogTags.AM_PROC_START,
                    UserHandle.getUserId(uid), startResult.pid, uid,
                    app.processName, hostingType,
                    hostingNameStr != null ? hostingNameStr : "");

            if (app.persistent) {
                Watchdog.getInstance().processStarted(app.processName, startResult.pid);
            }

            checkTime(startTime, "startProcess: building log message");
            StringBuilder buf = mStringBuilder;
            buf.setLength(0);
            buf.append("Start proc ");
            buf.append(startResult.pid);
            buf.append(':');
            buf.append(app.processName);
            buf.append('/');
            UserHandle.formatUid(buf, uid);
            if (!isActivityProcess) {
                buf.append(" [");
                buf.append(entryPoint);
                buf.append("]");
            }
            buf.append(" for ");
            buf.append(hostingType);
            if (hostingNameStr != null) {
                buf.append(" ");
                buf.append(hostingNameStr);
            }
            Slog.i(TAG, buf.toString());
            app.setPid(startResult.pid);
            app.usingWrapper = startResult.usingWrapper;
            app.removed = false;
            app.killed = false;
            app.killedByAm = false;
            checkTime(startTime, "startProcess: starting to update pids map");
            synchronized (mPidsSelfLocked) {
                this.mPidsSelfLocked.put(startResult.pid, app);
                if (isActivityProcess) {
                    Message msg = mHandler.obtainMessage(PROC_START_TIMEOUT_MSG);
                    msg.obj = app;
                    mHandler.sendMessageDelayed(msg, startResult.usingWrapper
                            ? PROC_START_TIMEOUT_WITH_WRAPPER : PROC_START_TIMEOUT);
                }
            }
            checkTime(startTime, "startProcess: done updating pids map");
        } catch (RuntimeException e) {
            // XXX do better error recovery.
            app.setPid(0);
            mBatteryStatsService.noteProcessFinish(app.processName, app.info.uid);
            if (app.isolated) {
                mBatteryStatsService.removeIsolatedUid(app.uid, app.info.uid);
            }
            Slog.e(TAG, "Failure starting process " + app.processName, e);
        }
    }

Process#start

这里面会调用到Process#start:

Process.ProcessStartResult startResult = Process.start(entryPoint,
                    app.processName, uid, uid, gids, debugFlags, mountExternal,
                    app.info.targetSdkVersion, app.info.seinfo, requiredAbi, instructionSet,
                    app.info.dataDir, entryPointArgs);

从类名、方法名就能看出,这是用来启动进程的。

看一看Process#start的代码,果然调用了startViaZygote(),通过Zygote fork了一个新的进程出来:

 public static final ProcessStartResult start(final String processClass,
                                  final String niceName,
                                  int uid, int gid, int[] gids,
                                  int debugFlags, int mountExternal,
                                  int targetSdkVersion,
                                  String seInfo,
                                  String abi,
                                  String instructionSet,
                                  String appDataDir,
                                  String[] zygoteArgs) {
        try {
            return startViaZygote(processClass, niceName, uid, gid, gids,
                    debugFlags, mountExternal, targetSdkVersion, seInfo,
                    abi, instructionSet, appDataDir, zygoteArgs);
        } catch (ZygoteStartFailedEx ex) {
            Log.e(LOG_TAG,
                    "Starting VM process through Zygote failed");
            throw new RuntimeException(
                    "Starting VM process through Zygote failed", ex);
        }
    }

写到这里,目标app进程就已经启动起来了(如果早就被启动过了就会回到栈顶),接下来就是执行目标app进程ActivityThread中main()方法,开启主线程消息循环,同时将app进程通AMS绑定在一起了,下一篇blog将分析这部分代码。

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