读mybatis源码之六：执行器Executor之基类BaseExecutor

        在sqlsession中主要是执行都是通过executor来处理的，具体执行逻辑，大部分在BaseExecutor里面，使用的模板模式，

        一、query：

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
 }

BoundSql ：sql语句，参数等

CacheKey ：缓存key，通过mapperID,sql、参数等产生缓存key

 @SuppressWarnings("unchecked")
  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) throw new ExecutorException("Executor was closed.");
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      deferredLoads.clear(); // issue #601
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        clearLocalCache(); // issue #482
      }
    }
    return list;
  }

可以看见：

1、先查本地缓存，没有再去查数据库，注意这里的本地缓存是同一个session内的缓存，也就是同一个opensession内。

2、通过configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT来看，可以设置参数，将本地缓存去掉，不使用本地缓存。

 private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

本地缓存先占位，后更新。

使用不同的执行器去做查询动作，具体的doQuery由不同的执行去具体实现

      二、update

 public int update(MappedStatement ms, Object parameter) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
    if (closed) throw new ExecutorException("Executor was closed.");
    clearLocalCache();
    return doUpdate(ms, parameter);
  }

清除本地缓存，做更新操作，具体更新操作在具体执行器中。

各大执行器主要需要写自己的：

 protected abstract int doUpdate(MappedStatement ms, Object parameter)
      throws SQLException;
  protected abstract List<BatchResult> doFlushStatements(boolean isRollback)
      throws SQLException;
  protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
      throws SQLException;