MyBatis3.3.0文档学习总结（四） --Executor

1：openSessionFromDataSource
通过openSession()最终调用的是 openSessionFromDataSource，源码如下：

  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();    }  }

说明：ExecutorType（执行器类型）类型有 SIMPLE（默认）,REUSE,BATCH。

     TransactionIsolationLevel  隔离级别5种       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);

通过源码看出首先是从configuration获取environment，接着创建Transaction，最后通过 configuration.newExecutor创建Executor。

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

根据executorType创建对应的Executor，从源码可以看出他有BatchExecutor、ReuseExecutor、CachingExecutor、SimpleExecutor。那么Executor是做什么的呢？

2：Executor
Executor是接口，是对于Statement的封装，我们看下Executor,他是真正执行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);}

上面源码我可以看到Executor接口定义了update 、query、commit、rollback等方法.我们拿其中的SimpleExecutor类里面的doQuery方法看下源码.

  @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);    }  }

我们看看StatementHandler 的一个实现类 PreparedStatementHandler（这也是我们最常用的，封装的是PreparedStatement）, 看看它使怎么去处理的：

   @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底层执行的基本流程，说的直白一点就是所以sql底层都交给了Excutor。