Mybatis——SqlSession对象创建过程分析

摘要

主要讲解SqlSession对象创建过程和源码分析。sqlSessionFactory创建SqlSession对象的过程进行了源码分析，最后返回的SqlSession中包含有两个重要的对象，分别是configuration与executor。configuration对象在创建SqlSessionFactory的时候就已经被创建出来了，用来保存全局配置文件与SQL 映射文件中的信息，executor是一个执行器对象。

SqlSession对象创建源码分析

private SqlSessionFactory getSqlSessionFactory() throws IOException {
    String resource = "mybatis-config.xml";
    InputStream is = Resources.getResourceAsStream(resource);

    return new SqlSessionFactoryBuilder().build(is);
}

@Test
public void testMyBatis3Simple() throws IOException {
   SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
   // 将断点打在下面代码的前面
   SqlSession sqlSession = sqlSessionFactory.openSession();
}

1.首先会跳到DefaultSqlSessionFactory类中的openSession()方法中。

// ====== DefaultSqlSessionFactory 类中的方法 ======
@Override
public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }
在上面这个方法中调用了另一个方法：
openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit)

  // ====== DefaultSqlSessionFactory 类中的方法 ======

  private SqlSession openSessionFromDataSource(ExecutorType execType, 
                              TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      // 获取配置环境中的一些信息，用于创建事务
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      /**
      * ExecutorType 是一个枚举类型，默认是SIMPLE
      * 根据ExecutorType 创建一个Executor 对象
      * 因为Executor 对象比较重要，下面来分析一下Executor 对象创建的过程
      */
      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();
    }
  }

2.configuration.newExecutor(tx, execType)创建过程如下:

  // ====== Configuration 类中的方法 ======

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    /**
    * 根据executorType 类型创建对应的Executor 
    * BatchExecutor：批量执行器
    * ReuseExecutor：会执行预处理的执行器
    * SimpleExecutor：简单的执行器
    */
    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);
    }
    /**
    * 如果开启了二级缓存，则使用CachingExecutor 来包装executor，
    * 在查询之前都会先查询缓存中是否有对应的数据
    * 包装的过程使用了装饰者模式，装饰者模式可参考博文：
    * http://blog.csdn.net/codejas/article/details/79112824
    */
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    // 最后使用每个拦截器重新包装executor 并返回
    executor = (Executor) interceptorChain.pluginAll(executor);
    // executor 对象创建完成并返回
    return executor;
  }

3.Executor 对象创建完成后，会接着执行
openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit)方法。

  // ====== DefaultSqlSessionFactory 类中的方法 ======

  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);
      /**
       * configuration 对象在创建SqlSessionFactory 对象的时候就已经创建了
       * Executor 对象创建完成后，使用executor与configuration 对象来创建DefaultSqlSession
       * 返回DefaultSqlSession 对象
       */
      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();
    }
  }

到这里会接着向上一步返回，SqlSession对象创建的过程也就结束了。