讀mybatis源碼之六：執行器Executor之基類BaseExecutor

        在sqlsession中主要是執行都是通過executor來處理的，具體執行邏輯，大部分在BaseExecutor裏面，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語句，參數等

CacheKey ：緩存key，通過mapperID,sql、參數等產生緩存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;
  }

可以看見：

1、先查本地緩存，沒有再去查數據庫，注意這裏的本地緩存是同一個session內的緩存，也就是同一個opensession內。

2、通過configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT來看，可以設置參數，將本地緩存去掉，不使用本地緩存。

 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;
  }

本地緩存先佔位，後更新。

使用不同的執行器去做查詢動作，具體的doQuery由不同的執行去具體實現

      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);
  }

清除本地緩存，做更新操作，具體更新操作在具體執行器中。

各大執行器主要需要寫自己的：

 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;