Mybatis之拦截器Interceptor

原创
2019/06/15 22:57
阅读数 680

    使用mybatis时用PageHelper进行分页,用到了PageInterceptor,借此了解下mybatis的interceptor。Mybatis的版本是3.4.6,MybatisHelper的版本是5.1.3。

1、PageInterceptor

    先上一段代码,如下List-1:

    List-1

@Test
public void testPage() {
    PageHelper.startPage(2, 3);
    List<Person> all = personMapper.findAll();
    PageInfo<Person> personPageInfo = new PageInfo<>(all,3);
    log.info(all.toString());
    int pageNum = personPageInfo.getPageNum();
    int pageSize = personPageInfo.getSize();
    int pages = personPageInfo.getPages();
    log.info("pageNum:"+pageNum+" size:"+ pageSize +" pages:"+ pages);
}

    List-1中,查询所有的Person,不过分页查询,注意使用PageHelper后,得到的List类型的all是com.github.pagehelper.Page,它继承了JDK的ArrayList,如下List-2所示,我在使用时一开始也以为是JDK的List实现,直到在看PageInfo时出现一些情况,深入了解后才发现是PageHelper继承的ArrayList:

    List-2

public class Page<E> extends ArrayList<E> implements Closeable {
    private static final long serialVersionUID = 1L;
    private int pageNum;
    private int pageSize;
    private int startRow;
    private int endRow;
    private long total;
    private int pages;
    private boolean count;
    private Boolean reasonable;
    private Boolean pageSizeZero;
    private String countColumn;
    private String orderBy;
    private boolean orderByOnly;
......

    mybatis的Interceptor如下List-3所示,PageInterceptor实现了这个接口:

    List-3

public interface Interceptor {

  Object intercept(Invocation invocation) throws Throwable;

  Object plugin(Object target);

  void setProperties(Properties properties);

}

    来看PageInterceptor如何实现intercept接口的,如下List-4

    List-4

@Override
public Object intercept(Invocation invocation) throws Throwable {
    try {
        Object[] args = invocation.getArgs();
        MappedStatement ms = (MappedStatement) args[0];
        Object parameter = args[1];
        RowBounds rowBounds = (RowBounds) args[2];
        ResultHandler resultHandler = (ResultHandler) args[3];
        Executor executor = (Executor) invocation.getTarget();
        CacheKey cacheKey;
        BoundSql boundSql;
        //由于逻辑关系,只会进入一次
        if(args.length == 4){
            //4 个参数时
            boundSql = ms.getBoundSql(parameter);
            cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
        } else {
            //6 个参数时
            cacheKey = (CacheKey) args[4];
            boundSql = (BoundSql) args[5];
        }
        List resultList;
        //调用方法判断是否需要进行分页,如果不需要,直接返回结果
        if (!dialect.skip(ms, parameter, rowBounds)) {
            //反射获取动态参数
            String msId = ms.getId();
            Configuration configuration = ms.getConfiguration();
            Map<String, Object> additionalParameters = (Map<String, Object>) additionalParametersField.get(boundSql);
            //判断是否需要进行 count 查询
            if (dialect.beforeCount(ms, parameter, rowBounds)) {
                String countMsId = msId + countSuffix;
                Long count;
                //先判断是否存在手写的 count 查询
                MappedStatement countMs = getExistedMappedStatement(configuration, countMsId);
                if(countMs != null){
                    count = executeManualCount(executor, countMs, parameter, boundSql, resultHandler);
                } else {
                    countMs = msCountMap.get(countMsId);
                    //自动创建
                    if (countMs == null) {
                        //根据当前的 ms 创建一个返回值为 Long 类型的 ms
                        countMs = MSUtils.newCountMappedStatement(ms, countMsId);
                        msCountMap.put(countMsId, countMs);
                    }
                    count = executeAutoCount(executor, countMs, parameter, boundSql, rowBounds, resultHandler);
                }
                //处理查询总数
                //返回 true 时继续分页查询,false 时直接返回
                if (!dialect.afterCount(count, parameter, rowBounds)) {
                    //当查询总数为 0 时,直接返回空的结果
                    return dialect.afterPage(new ArrayList(), parameter, rowBounds);
                }
            }
            //判断是否需要进行分页查询
            if (dialect.beforePage(ms, parameter, rowBounds)) {
                //生成分页的缓存 key
                CacheKey pageKey = cacheKey;
                //处理参数对象
                parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey);
                //调用方言获取分页 sql
                String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);
                BoundSql pageBoundSql = new BoundSql(configuration, pageSql, boundSql.getParameterMappings(), parameter);
                //设置动态参数
                for (String key : additionalParameters.keySet()) {
                    pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
                }
                //执行分页查询
                resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
            } else {
                //不执行分页的情况下,也不执行内存分页
                resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
            }
        } else {
            //rowBounds用参数值,不使用分页插件处理时,仍然支持默认的内存分页
            resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
        }
        return dialect.afterPage(resultList, parameter, rowBounds);
    } finally {
        dialect.afterAll();
    }
}

    我们来分析下List-4的内容,"if (!dialect.skip(ms, parameter, rowBounds)) {"判断是否需要进行分页,这个dialect就是PageHelper,如下List-5,有兴趣可以看下"pageParams.getPage(parameterObject, rowBounds);",

    List-5

public class PageHelper extends PageMethod implements Dialect {
    private PageParams pageParams;
    private PageAutoDialect autoDialect;

    @Override
    public boolean skip(MappedStatement ms, Object parameterObject, RowBounds rowBounds) {
        if(ms.getId().endsWith(MSUtils.COUNT)){
            throw new RuntimeException("在系统中发现了多个分页插件,请检查系统配置!");
        }
        Page page = pageParams.getPage(parameterObject, rowBounds);
        if (page == null) {
            return true;
        } else {
            //设置默认的 count 列
            if(StringUtil.isEmpty(page.getCountColumn())){
                page.setCountColumn(pageParams.getCountColumn());
            }
            autoDialect.initDelegateDialect(ms);
            return false;
        }
    }
...

    List-4中如果不需要分页,则直接调用executor的query方法。需要分页情况下,首先会看是否需要进行count查询——List-4中的"if (dialect.beforeCount(ms, parameter, rowBounds))",dialect.beforeCount的最终实现是在AbstractHelperDialect的beforeCount方法,如下List-6,getLocalPage()调用PageHelper.getLocalPage(),得到com.github.pagehelper.Page——isOrderByOnly默认返还false,isCount默认返还true。

    List-6

public abstract class AbstractHelperDialect extends AbstractDialect implements Constant {
...

    @Override
    public boolean beforeCount(MappedStatement ms, Object parameterObject, RowBounds rowBounds) {
        Page page = getLocalPage();
        return !page.isOrderByOnly() && page.isCount();
    }
...

    List-4中,需要count查询后,判断手写的count已存在,不存在则调用mybatis的builder等工具类构造,之后进行count查询,得到结果后调用dialect的afterCount方法,实现在AbstractHelperDialect的afterCount方法,如下List-7,getLocalPage()获取PageInfo中的Page,之后设置total。在分页的时候,我们会对PageHelper进行pageSize的设置,所以只要count的结果大于0,就会返还true。

    List-7

@Override
public boolean afterCount(long count, Object parameterObject, RowBounds rowBounds) {
    Page page = getLocalPage();
    page.setTotal(count);
    if (rowBounds instanceof PageRowBounds) {
        ((PageRowBounds) rowBounds).setTotal(count);
    }
    //pageSize < 0 的时候,不执行分页查询
    //pageSize = 0 的时候,还需要执行后续查询,但是不会分页
    if (page.getPageSize() < 0) {
        return false;
    }
    return count > 0;
}

    List-4中查询到count,且大于0,则继续后续,先判断是否需要进行分页查询——List-4中的"dialect.beforePage(ms, parameter, rowBounds)",实现是在AbstractHelperDialect中,这里不再深入这点,只要我们的pageSize设置大于0,该方法默认是返还true的。

    进行分页查询,会调用AbstractHelperDialect的getPageSql方法获取数据库执行的sql,以mysql为例子,AbstractHelperDialect在调用子类MySqlDialect的getPageSql方法,如下List-8,会在sql的最后加上limit语句。AbstractHelperDialect的子类有很多种,对应不同的数据库,这里使用了模板设计模式。之后再将这个sql交给executor执行,达到分页的操作。有些人说mybatis的分页查询插件底层上是全部查出到内存之后进行切分,但是我看到的是通过limit进行分页,没有什么问题的,控制台打印的sql也是带有limit的。

    List-8

@Override
public String getPageSql(String sql, Page page, CacheKey pageKey) {
    StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);
    sqlBuilder.append(sql);
    if (page.getStartRow() == 0) {
        sqlBuilder.append(" LIMIT ? ");
    } else {
        sqlBuilder.append(" LIMIT ?, ? ");
    }
    pageKey.update(page.getPageSize());
    return sqlBuilder.toString();
}

    List-4中,分页查询到结果后,调用"dialect.afterPage(resultList, parameter, rowBounds)",即AbstractHelperDialect的afterPage方法,如下List-9,从PageHelper中得到Page,之后将分页查询的结果放入到Page中。

    List-9

@Override
public Object afterPage(List pageList, Object parameterObject, RowBounds rowBounds) {
    Page page = getLocalPage();
    if (page == null) {
        return pageList;
    }
    page.addAll(pageList);
    if (!page.isCount()) {
        page.setTotal(-1);
    } else if ((page.getPageSizeZero() != null && page.getPageSizeZero()) && page.getPageSize() == 0) {
        page.setTotal(pageList.size());
    } else if(page.isOrderByOnly()){
        page.setTotal(pageList.size());
    }
    return page;
}

    List-4中最后的finally块调用dialect.afterAll(),我们来看下实现,如下的PageHelper的afterAll(),清理各项清理。

    List-10

@Override
public void afterAll() {
    //这个方法即使不分页也会被执行,所以要判断 null
    AbstractHelperDialect delegate = autoDialect.getDelegate();
    if (delegate != null) {
        delegate.afterAll();
        autoDialect.clearDelegate();
    }
    clearPage();
}

    最终,这个PageInterceptor的intercept方法返还的是自己定义的Page。

2、PageHelper

    来看看PageHelper,它的父类PageMethod,如下的List-10,使用ThreadLocal来存储Page,如果熟悉JDK的ThreadLocal那么,对其要注意的点,在PageInterceptor的使用时也要注意,这里不再深究。

    List-10

public abstract class PageMethod {
    protected static final ThreadLocal<Page> LOCAL_PAGE = new ThreadLocal<Page>();
    protected static boolean DEFAULT_COUNT = true;

    /**
     * 设置 Page 参数
     *
     * @param page
     */
    protected static void setLocalPage(Page page) {
        LOCAL_PAGE.set(page);
    }

    /**
     * 获取 Page 参数
     *
     * @return
     */
    public static <T> Page<T> getLocalPage() {
        return LOCAL_PAGE.get();
    }

    /**
     * 移除本地变量
     */
    public static void clearPage() {
        LOCAL_PAGE.remove();
    }
...

3、在哪里调用Interceptor

    什么时候调用Interceptor呢,来看mybatis的Configuration的几个方法,这几个方法最后都调用了interceptorChain,如List-12所示,使用责任链模式,调用plugin方法。

    List-11

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
  ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
  parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
  return parameterHandler;
}

public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
    ResultHandler resultHandler, BoundSql boundSql) {
  ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
  resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
  return resultSetHandler;
}

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
  StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
  statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
  return statementHandler;
}

public Executor newExecutor(Transaction transaction) {
  return newExecutor(transaction, defaultExecutorType);
}

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
  executorType = executorType == null ? defaultExecutorType : executorType;
  executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
  Executor executor;
  if (ExecutorType.BATCH == executorType) {
    executor = new BatchExecutor(this, transaction);
  } else if (ExecutorType.REUSE == executorType) {
    executor = new ReuseExecutor(this, transaction);
  } else {
    executor = new SimpleExecutor(this, transaction);
  }
  if (cacheEnabled) {
    executor = new CachingExecutor(executor);
  }
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}

    List-12

public class InterceptorChain {

  private final List<Interceptor> interceptors = new ArrayList<Interceptor>();

  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }
  
  public List<Interceptor> getInterceptors() {
    return Collections.unmodifiableList(interceptors);
  }
}

    思考,插件还可以做其它什么呢,我们可以用来进行sql性能耗时的统计,或者对更新操作统一的加上更新者、时间等。

Reference

  1. 源码
  2. https://blog.csdn.net/top_code/article/details/55657776
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