MyBatis3.3.0文檔學習總結（四） --Executor

1：openSessionFromDataSource
通過openSession()最終調用的是 openSessionFromDataSource，源碼如下：

  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      final Executor executor = configuration.newExecutor(tx, execType);
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

說明：ExecutorType（執行器類型）類型有 SIMPLE（默認）,REUSE,BATCH。

     TransactionIsolationLevel  隔離級別5種 

      NONE(Connection.TRANSACTION_NONE),

      READ_COMMITTED(Connection.TRANSACTION_READ_COMMITTED),

      READ_UNCOMMITTED(Connection.TRANSACTION_READ_UNCOMMITTED),

      REPEATABLE_READ(Connection.TRANSACTION_REPEATABLE_READ),

      SERIALIZABLE(Connection.TRANSACTION_SERIALIZABLE);

通過源碼看出首先是從configuration獲取environment，接着創建Transaction，最後通過 configuration.newExecutor創建Executor。

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    //根據executorType創建不同的Executor對象
    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;
  }

根據executorType創建對應的Executor，從源碼可以看出他有BatchExecutor、ReuseExecutor、CachingExecutor、SimpleExecutor。那麼Executor是做什麼的呢？

2：Executor
Executor是接口，是對於Statement的封裝，我們看下Executor,他是真正執行sql的地方。

public interface Executor {
  ResultHandler NO_RESULT_HANDLER = null;
  int update(MappedStatement ms, Object parameter) throws SQLException;
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
  List<BatchResult> flushStatements() throws SQLException;
  void commit(boolean required) throws SQLException;
  void rollback(boolean required) throws SQLException;
 CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
  boolean isCached(MappedStatement ms, CacheKey key);
  void clearLocalCache();
  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
  Transaction getTransaction();
  void close(boolean forceRollback);
  boolean isClosed();
  void setExecutorWrapper(Executor executor);
}

上面源碼我可以看到Executor接口定義了update 、query、commit、rollback等方法.我們拿其中的SimpleExecutor類裏面的doQuery方法看下源碼.

  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

我們看看StatementHandler 的一個實現類 PreparedStatementHandler（這也是我們最常用的，封裝的是PreparedStatement）, 看看它使怎麼去處理的：

   @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
  }

以上是sql底層執行的基本流程，說的直白一點就是所以sql底層都交給了Excutor。