mybatis源码阅读(二):mybatis初始化上

原创
2018/05/03 18:33
阅读数 430

1.初始化入口

//Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互
private static SqlSessionFactory getSessionFactory() {
    SqlSessionFactory sessionFactory = null;
    String resource = "configuration.xml";
    try {
        sessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader(resource));
    } catch (IOException e) {
        e.printStackTrace();
    }
    return sessionFactory;
}

那么,我们就先从SqlSessionFactoryBuilder入手, 咱们先看看源码是怎么实现的

SqlSessionFactoryBuilder

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
  try {
    // 读取配置文件
    XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
    //解析配置得到Configuration对象,创建DefaultSqlSessionFactory对象
    return build(parser.parse());
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error building SqlSession.", e);
  } finally {
    //关闭读取配置文件的输入流对象
    ErrorContext.instance().reset();
    try {
      inputStream.close();
    } catch (IOException e) {
      // Intentionally ignore. Prefer previous error.
    }
  }
}

XMLConfigBuilder

XMLConfigBuilder是BaseBuilder的众多子类之一,核心字段如下

//表示是否已经解析过了
private boolean parsed;
//用于解析配置文件的对象
private final XPathParser parser;
//配置文件中表示<environment>的名称  默认读取default属性
private String environment;
// 负责和创建Reflector对象
private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();

XMLConfigBuilder.parse()方法是解析mybatis-config.xml配置文件的如。它调用parseConfiguration()方法实现整个解析过程。具体实现如下:

/**
   * 解析配置文件的入口
   * @return
   */
public Configuration parse() {
  if (parsed) {
    throw new BuilderException("Each XMLConfigBuilder can only be used once.");
  }
  parsed = true;
  parseConfiguration(parser.evalNode("/configuration"));
  return configuration;
}

  /**
   * 对配置文件每个节点具体的解析过程
   * configuration节点为根节点。
   * 在configuration节点之下,我们可以配置11  个子节点,
   * 分别为:properties、settings、typeAliases、plugins、objectFactory、objectWrapperFactory、reflectorFactory、
   * environments、databaseIdProvider、typeHandlers、mappers。
   * @param root 根节点
   */
private void parseConfiguration(XNode root) {
  try {
      // 解析properties节点
    propertiesElement(root.evalNode("properties"));
      //解析settings节点
    Properties settings = settingsAsProperties(root.evalNode("settings"));
    loadCustomVfs(settings);//设置vfsImpl字段
      //解析typeAliases节点
    typeAliasesElement(root.evalNode("typeAliases"));
      //解析plugins节点
    pluginElement(root.evalNode("plugins"));
      //解析objectFactory节点
    objectFactoryElement(root.evalNode("objectFactory"));
      //解析objectWrapperFactory节点
    objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      //解析reflectorFactory节点
    reflectorFactoryElement(root.evalNode("reflectorFactory"));
    settingsElement(settings);
      //解析environments节点
    environmentsElement(root.evalNode("environments"));
      //解析databaseIdProvider节点
    databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      //解析typeHandlers节点
    typeHandlerElement(root.evalNode("typeHandlers"));
      //解析mappers节点
    mapperElement(root.evalNode("mappers"));
  } catch (Exception e) {
    throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
  }
}

propertiesElement()方法会解析配置文件中的properties节点并形成Java.util.Properties对象,之后将改对象设置到XpathParse和Configguration的variables字段中,占位符就是用Properties中的信息替换的,具体实现如下:

/**
   * 解析properties的具体方法
   * @param context
   * @throws Exception
   */
private void propertiesElement(XNode context) throws Exception {
  if (context != null) {
      // 将子节点的 name 以及value属性set进properties对象
      // 这儿可以注意一下顺序,xml配置优先, 外部指定properties配置其次
       Properties defaults = context.getChildrenAsProperties();
      // 获取properties节点上 resource属性的值
      String resource = context.getStringAttribute("resource");
      // 获取properties节点上 url属性的值, resource和url不能同时配置
      String url = context.getStringAttribute("url");
    if (resource != null && url != null) {//url 和resource不能同时配置
      throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other.");
    }
      // 把解析出的properties文件set进Properties对象
    if (resource != null) {
      defaults.putAll(Resources.getResourceAsProperties(resource));
    } else if (url != null) {
      defaults.putAll(Resources.getUrlAsProperties(url));
    }
      // 将configuration对象中已配置的Properties属性与刚刚解析的融合
      // configuration这个对象会装载所解析mybatis配置文件的所有节点元素,以后也会频频提到这个对象
    Properties vars = configuration.getVariables();
    if (vars != null) {
      defaults.putAll(vars);
    }
      // 把装有解析配置propertis对象set进解析器, 因为后面可能会用到
    parser.setVariables(defaults);
      // set进configuration对象
    configuration.setVariables(defaults);
  }
}

settings节点下的配饰是mybatis的全局性配置,修改的是configuration对象的属性,具体说明参考官方文档

/**
 * settings标签就是设置configuration对象的各种属性,
 * 具体属性说明可以参考mybatis官方文档
 * @param props
 * @throws Exception
 */
private void settingsElement(Properties props) throws Exception {
  configuration.setAutoMappingBehavior(AutoMappingBehavior.valueOf(props.getProperty("autoMappingBehavior", "PARTIAL")));
  configuration.setAutoMappingUnknownColumnBehavior(AutoMappingUnknownColumnBehavior.valueOf(props.getProperty("autoMappingUnknownColumnBehavior", "NONE")));
  configuration.setCacheEnabled(booleanValueOf(props.getProperty("cacheEnabled"), true));
  configuration.setProxyFactory((ProxyFactory) createInstance(props.getProperty("proxyFactory")));
  configuration.setLazyLoadingEnabled(booleanValueOf(props.getProperty("lazyLoadingEnabled"), false));
  configuration.setAggressiveLazyLoading(booleanValueOf(props.getProperty("aggressiveLazyLoading"), false));
  configuration.setMultipleResultSetsEnabled(booleanValueOf(props.getProperty("multipleResultSetsEnabled"), true));
  configuration.setUseColumnLabel(booleanValueOf(props.getProperty("useColumnLabel"), true));
  configuration.setUseGeneratedKeys(booleanValueOf(props.getProperty("useGeneratedKeys"), false));
  configuration.setDefaultExecutorType(ExecutorType.valueOf(props.getProperty("defaultExecutorType", "SIMPLE")));
  configuration.setDefaultStatementTimeout(integerValueOf(props.getProperty("defaultStatementTimeout"), null));
  configuration.setDefaultFetchSize(integerValueOf(props.getProperty("defaultFetchSize"), null));
  configuration.setMapUnderscoreToCamelCase(booleanValueOf(props.getProperty("mapUnderscoreToCamelCase"), false));
  configuration.setSafeRowBoundsEnabled(booleanValueOf(props.getProperty("safeRowBoundsEnabled"), false));
  configuration.setLocalCacheScope(LocalCacheScope.valueOf(props.getProperty("localCacheScope", "SESSION")));
  configuration.setJdbcTypeForNull(JdbcType.valueOf(props.getProperty("jdbcTypeForNull", "OTHER")));
  configuration.setLazyLoadTriggerMethods(stringSetValueOf(props.getProperty("lazyLoadTriggerMethods"), "equals,clone,hashCode,toString"));
  configuration.setSafeResultHandlerEnabled(booleanValueOf(props.getProperty("safeResultHandlerEnabled"), true));
  configuration.setDefaultScriptingLanguage(resolveClass(props.getProperty("defaultScriptingLanguage")));
  @SuppressWarnings("unchecked")
  Class<? extends TypeHandler> typeHandler = (Class<? extends TypeHandler>)resolveClass(props.getProperty("defaultEnumTypeHandler"));
  configuration.setDefaultEnumTypeHandler(typeHandler);
  configuration.setCallSettersOnNulls(booleanValueOf(props.getProperty("callSettersOnNulls"), false));
  configuration.setUseActualParamName(booleanValueOf(props.getProperty("useActualParamName"), true));
  configuration.setReturnInstanceForEmptyRow(booleanValueOf(props.getProperty("returnInstanceForEmptyRow"), false));
  configuration.setLogPrefix(props.getProperty("logPrefix"));
  @SuppressWarnings("unchecked")
  Class<? extends Log> logImpl = (Class<? extends Log>)resolveClass(props.getProperty("logImpl"));
  configuration.setLogImpl(logImpl);
  configuration.setConfigurationFactory(resolveClass(props.getProperty("configurationFactory")));
}

environments元素节点主要配置数据库事物,数据源。可以配置多个environment子节点,假如我们系统的开发环境和正式环境所用的数据库不一样(这是肯定的), 那么可以设置两个environment, 两个id分别对应开发环境(development)和正式环境(final),那么通过配置environments的default属性就能选择对应的environment了, 例如,我将environments的deault属性的值配置为development, 那么就会选择dev的environment。具体实现如下

/**
   * 解析enviroments元素节点的方法
   * @param context
   * @throws Exception
   */
private void environmentsElement(XNode context) throws Exception {
  if (context != null) {
    if (environment == null) {
      //获取 <environments default="development"> 中的default值
      environment = context.getStringAttribute("default");
    }
    // 循环environments的子节点
    for (XNode child : context.getChildren()) {
      // 获取 <environment id="development"> z中的id
      String id = child.getStringAttribute("id");
      if (isSpecifiedEnvironment(id)) {//根据由environments的default属性去选择对应的enviroment
        // 事物 mybatis有两种:JDBC 和 MANAGED, 配置为JDBC则直接使用JDBC的事务,配置为MANAGED则是将事务托管给容器
        // <transactionManager type="JDBC"/>
        TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
        //enviroment节点下面就是dataSource节点了,解析dataSource节点
        DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
        DataSource dataSource = dsFactory.getDataSource();
        Environment.Builder environmentBuilder = new Environment.Builder(id)
            .transactionFactory(txFactory)
            .dataSource(dataSource);
        // 将dataSource设置进configuration对象
        configuration.setEnvironment(environmentBuilder.build());
      }
    }
  }
}

typeAliases节点主要用来设置别名,其实这是挺好用的一个功能, 通过配置别名,我们不用再指定完整的包名

/**
 * 解析typeAliases 节点
 * <typeAliases>
 *     <!--<package name="com.lpf.entity"></package>-->
 *     <typeAlias alias="UserEntity" type="com.lpf.entity.User"/>
 * </typeAliases>
 * @param parent
 */
private void typeAliasesElement(XNode parent) {
  if (parent != null) {
    for (XNode child : parent.getChildren()) {
      //如果子节点是package, 那么就获取package节点的name属性, mybatis会扫描指定的package
      if ("package".equals(child.getName())) {
        String typeAliasPackage = child.getStringAttribute("name");
        //TypeAliasRegistry 负责管理别名, 这儿就是通过TypeAliasRegistry 进行别名注册
        configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
      } else {
          //如果子节点是typeAlias节点,那么就获取alias属性和type的属性值
        String alias = child.getStringAttribute("alias");
        String type = child.getStringAttribute("type");
        try {
          Class<?> clazz = Resources.classForName(type);
          if (alias == null) {
            typeAliasRegistry.registerAlias(clazz);
          } else {
            typeAliasRegistry.registerAlias(alias, clazz);
          }
        } catch (ClassNotFoundException e) {
          throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
        }
      }
    }
  }
}

具体的别名注册类

public class TypeAliasRegistry {

  // 别名通过一个HashMap来实现, key为别名, value就是别名对应的类型(class对象)
  private final Map<String, Class<?>> TYPE_ALIASES = new HashMap<String, Class<?>>();

  /**
   * mybatis默认为我们注册的别名
   */
  public TypeAliasRegistry() {
    registerAlias("string", String.class);

    registerAlias("byte", Byte.class);
    registerAlias("long", Long.class);
    registerAlias("short", Short.class);
    registerAlias("int", Integer.class);
    registerAlias("integer", Integer.class);
    registerAlias("double", Double.class);
    registerAlias("float", Float.class);
    registerAlias("boolean", Boolean.class);

    registerAlias("byte[]", Byte[].class);
    registerAlias("long[]", Long[].class);
    registerAlias("short[]", Short[].class);
    registerAlias("int[]", Integer[].class);
    registerAlias("integer[]", Integer[].class);
    registerAlias("double[]", Double[].class);
    registerAlias("float[]", Float[].class);
    registerAlias("boolean[]", Boolean[].class);

    registerAlias("_byte", byte.class);
    registerAlias("_long", long.class);
    registerAlias("_short", short.class);
    registerAlias("_int", int.class);
    registerAlias("_integer", int.class);
    registerAlias("_double", double.class);
    registerAlias("_float", float.class);
    registerAlias("_boolean", boolean.class);

    registerAlias("_byte[]", byte[].class);
    registerAlias("_long[]", long[].class);
    registerAlias("_short[]", short[].class);
    registerAlias("_int[]", int[].class);
    registerAlias("_integer[]", int[].class);
    registerAlias("_double[]", double[].class);
    registerAlias("_float[]", float[].class);
    registerAlias("_boolean[]", boolean[].class);

    registerAlias("date", Date.class);
    registerAlias("decimal", BigDecimal.class);
    registerAlias("bigdecimal", BigDecimal.class);
    registerAlias("biginteger", BigInteger.class);
    registerAlias("object", Object.class);

    registerAlias("date[]", Date[].class);
    registerAlias("decimal[]", BigDecimal[].class);
    registerAlias("bigdecimal[]", BigDecimal[].class);
    registerAlias("biginteger[]", BigInteger[].class);
    registerAlias("object[]", Object[].class);

    registerAlias("map", Map.class);
    registerAlias("hashmap", HashMap.class);
    registerAlias("list", List.class);
    registerAlias("arraylist", ArrayList.class);
    registerAlias("collection", Collection.class);
    registerAlias("iterator", Iterator.class);

    registerAlias("ResultSet", ResultSet.class);
  }

  /**
   * 处理别名, 直接从保存有别名的hashMap中取出即可
   */
  @SuppressWarnings("unchecked")
  // throws class cast exception as well if types cannot be assigned
  public <T> Class<T> resolveAlias(String string) {
    try {
      if (string == null) {
        return null;
      }
      // issue #748
      String key = string.toLowerCase(Locale.ENGLISH);
      Class<T> value;
      if (TYPE_ALIASES.containsKey(key)) {
        value = (Class<T>) TYPE_ALIASES.get(key);
      } else {
        value = (Class<T>) Resources.classForName(string);
      }
      return value;
    } catch (ClassNotFoundException e) {
      throw new TypeException("Could not resolve type alias '" + string + "'.  Cause: " + e, e);
    }
  }


    /**
     * 配置文件中配置为package的时候,扫描包下的Javabean ,然后自动注册别名
     * 默认会使用 Bean 的首字母小写的非限定类名来作为它的别名
     * 也可在javabean 加上注解@Alias 来自定义别名, 例如: @Alias(user)
     */
  public void registerAliases(String packageName){
    registerAliases(packageName, Object.class);
  }

  public void registerAliases(String packageName, Class<?> superType){
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> typeSet = resolverUtil.getClasses();
    for(Class<?> type : typeSet){
      // Ignore inner classes and interfaces (including package-info.java)
      // Skip also inner classes. See issue #6
      if (!type.isAnonymousClass() && !type.isInterface() && !type.isMemberClass()) {
        registerAlias(type);
      }
    }
  }

  public void registerAlias(Class<?> type) {
    String alias = type.getSimpleName();
    Alias aliasAnnotation = type.getAnnotation(Alias.class);
    if (aliasAnnotation != null) {
      alias = aliasAnnotation.value();
    } 
    registerAlias(alias, type);
  }

    //向hashMap中注册别名
  public void registerAlias(String alias, Class<?> value) {
    if (alias == null) {
      throw new TypeException("The parameter alias cannot be null");
    }
    // issue #748
    String key = alias.toLowerCase(Locale.ENGLISH);
    if (TYPE_ALIASES.containsKey(key) && TYPE_ALIASES.get(key) != null && !TYPE_ALIASES.get(key).equals(value)) {
      throw new TypeException("The alias '" + alias + "' is already mapped to the value '" + TYPE_ALIASES.get(key).getName() + "'.");
    }
    TYPE_ALIASES.put(key, value);
  }

  public void registerAlias(String alias, String value) {
    try {
      registerAlias(alias, Resources.classForName(value));
    } catch (ClassNotFoundException e) {
      throw new TypeException("Error registering type alias "+alias+" for "+value+". Cause: " + e, e);
    }
  }

    /**
     * 获取保存别名的HashMap, Configuration对象持有对TypeAliasRegistry的引用,
     * 因此,如果需要,我们可以通过Configuration对象获取
     */
  public Map<String, Class<?>> getTypeAliases() {
    return Collections.unmodifiableMap(TYPE_ALIASES);
  }

}

typeHandlers节点的解析和typeAlianses节点的解析类似

/**
   * 解析typeHandlers节点
   * 无论是 MyBatis 在预处理语句(PreparedStatement)中设置一个参数时,
   * 还是从结果集中取出一个值时,都会用类型处理器将获取的值以合适的方式转换成 Java 类型。
   * Mybatis默认为我们实现了许多TypeHandler, 当我们没有配置指定TypeHandler时,
   * Mybatis会根据参数或者返回结果的不同,默认为我们选择合适的TypeHandler处理。
   * @param parent
   * @throws Exception
   */
private void typeHandlerElement(XNode parent) throws Exception {
  if (parent != null) {
    for (XNode child : parent.getChildren()) {
        //子节点为package时,获取其name属性的值,然后自动扫描package下的自定义typeHandler
      if ("package".equals(child.getName())) {
        String typeHandlerPackage = child.getStringAttribute("name");
        typeHandlerRegistry.register(typeHandlerPackage);
      } else {
          //子节点为typeHandler时, 可以指定javaType属性, 也可以指定jdbcType, 也可两者都指定
          //javaType 是指定java类型
          //jdbcType 是指定jdbc类型(数据库类型: 如varchar)
        String javaTypeName = child.getStringAttribute("javaType");
        String jdbcTypeName = child.getStringAttribute("jdbcType");
          //handler就是我们配置的typeHandler
        String handlerTypeName = child.getStringAttribute("handler");
        Class<?> javaTypeClass = resolveClass(javaTypeName);
          //JdbcType是一个枚举类型,resolveJdbcType方法是在获取枚举类型的值
        JdbcType jdbcType = resolveJdbcType(jdbcTypeName);
        Class<?> typeHandlerClass = resolveClass(handlerTypeName);
          //注册typeHandler, typeHandler通过TypeHandlerRegistry这个类管理
        if (javaTypeClass != null) {
          if (jdbcType == null) {
            typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);
          } else {
            typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);
          }
        } else {
          typeHandlerRegistry.register(typeHandlerClass);
        }
      }
    }
  }
}

插件是mybatis提供的扩展机制,用户可以通过添加自定义插件在SQL语句执行的过程中某一环节进行拦截,mybatis中的自定义插件只需实现Interceptor接口,并通过注解指定拦截的方法签名,这个后面具体介绍。

/**
   * 解析plugins标签
   * mybatis中的plugin其实就是个interceptor,
   * 它可以拦截Executor 、ParameterHandler 、ResultSetHandler 、StatementHandler 的部分方法,处理我们自己的逻辑。
   * @param parent
   * @throws Exception
   */
private void pluginElement(XNode parent) throws Exception {
  if (parent != null) {
    for (XNode child : parent.getChildren()) {
      String interceptor = child.getStringAttribute("interceptor");
      Properties properties = child.getChildrenAsProperties();
      // 我们在定义一个interceptor的时候,需要去实现Interceptor
      Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
      interceptorInstance.setProperties(properties);
      // 向configuration对象中注册拦截器
      configuration.addInterceptor(interceptorInstance);
    }
  }
}

mybatis初始化时,出了加载mybatis-config.xml的全局配置文件,还会加载全部的映射配置文件,即mappers节点配置的mapper.

/**
   * 解析mapper文件,mapper可以理解为dao的实现
   * @param parent
   * @throws Exception
   */
private void mapperElement(XNode parent) throws Exception {
  if (parent != null) {
    for (XNode child : parent.getChildren()) {
        //如果mappers节点的子节点是package, 那么就扫描package下的文件, 注入进configuration
      if ("package".equals(child.getName())) {
        String mapperPackage = child.getStringAttribute("name");
        configuration.addMappers(mapperPackage);
      } else {
        String resource = child.getStringAttribute("resource");
        String url = child.getStringAttribute("url");
        String mapperClass = child.getStringAttribute("class");
          //resource, url, class 三选一
        if (resource != null && url == null && mapperClass == null) {
          ErrorContext.instance().resource(resource);
          InputStream inputStream = Resources.getResourceAsStream(resource);
            //mapper映射文件都是通过XMLMapperBuilder解析
          XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
          mapperParser.parse();
        } else if (resource == null && url != null && mapperClass == null) {
          ErrorContext.instance().resource(url);
          InputStream inputStream = Resources.getUrlAsStream(url);
          //同上
          XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
          mapperParser.parse();
        } else if (resource == null && url == null && mapperClass != null) {
          Class<?> mapperInterface = Resources.classForName(mapperClass);
          configuration.addMapper(mapperInterface);
        } else {
          throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
        }
      }
    }
  }
}

mybatis初始化过程中对mybatis-config.xml配置文件的解析过程到这吧,下一个就叫啥mapper配置文件的解析过程。

展开阅读全文
打赏
0
2 收藏
分享
加载中
更多评论
打赏
0 评论
2 收藏
0
分享
返回顶部
顶部