CachingExecutor 和 BaseExecutor 里的 query

最新推荐文章于 2024-09-04 22:57:20 发布

原创最新推荐文章于 2024-09-04 22:57:20 发布 · 416 阅读

0 ·

CC 4.0 BY-SA版权

Mybatis 源码专栏收录该内容

12 篇文章

订阅专栏

本文详细解析了MyBatis框架中二级缓存的工作原理和实现机制，介绍了如何通过CachingExecutor处理SQL查询，利用缓存提高数据访问效率。

CachingExecutor

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
	//获取sql语句信息，包括占位符，参数等信息
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    //拼装缓存的key值
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }
  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
	//从MappedStatement中获取二级缓存
    Cache cache = ms.getCache();
    //如果配置了二级缓存
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);//从二级缓存中获取数据
        if (list == null) {
          //二级缓存为空，才会调用BaseExecutor.query
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

BaseExecutor

上面的 query 会到下面这个方法来

@Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {//检查当前executor是否关闭
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {//非嵌套查询，并且FlushCache配置为true，则需要清空一级缓存
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;//查询层次加一
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;//从缓存拿
      if (list != null) {
    	 //针对调用存储过程的结果处理
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
    	 //缓存未命中，从数据库加载数据
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }


    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {//延迟加载处理
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {//如果当前sql的一级缓存配置为STATEMENT，查询完既清空一集缓存
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }