Mybatis源码分析二SqlSession和executor

博客摘抄自： https://www.cnblogs.com/jeffen/p/6277602.html

通过上一篇文章的分析我们，我初步了解了它是如何创建sessionFactory的（地址：Mybatis源码分析之SqlSessionFactory（一）），

今天我们分析下Mybatis如何创建SqlSession（ sessionFactory.openSession()）和Excutor到底做了什么事情

 

1.  追踪 sqlSessionFactory.openSession() 做了什么处理，

　　openSession -> interface SqlSessionFactory.openSession -> 真正实现类 org.apache.ibatis.session.defaults.DefaultSqlSessionFactory#openSession()

　　源码 ： 

　　

@Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

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. Execuotor

　　

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。

 　　org.apache.ibatis.executor.Executor

　　

public interface Executor {
  ResultHandler NO_RESULT_HANDLER = null;
  int update(MappedStatement ms, Object parameter) throws SQLException;
   List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
   List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
   Cursor queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
  List 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，他的实现类如下图

 

　

我们拿其中的SimpleExecutor类里面的doQuery方法看下源码

 

@Override
  public  List 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);
//StatementHandler封装了Statement, 让 StatementHandler 去处理
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.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  List query(Statement statement, ResultHandler resultHandler) throws SQLException {
    String sql = boundSql.getSql();
    //这个和jdbc一样执行sql
    statement.execute(sql);
    //结果交给了ResultSetHandler 去处理
    return resultSetHandler.handleResultSets(statement);
  }

　　

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，我们将在下一讲中分析上一层的调用，也就是Excutor的上层。

 　　

我们继续回到openSessionFromDataSource上面来，通过上面的executor的创建最终是创建

  new DefaultSqlSession(configuration, executor, autoCommit);到此我们已经完成了SqlSession的创建,后面我们将分析sqlSession.getMapper 也就是Excutor的上层调用。