MyBatis3.3.0文档学习总结（三续） --sqlSession深入源码

工厂接口（sqlSessionFactory）如何创建sqlsession会话接口.

   public static SqlSession openSession() {  
            return getSqlSessionFactory().openSession();  
        }  

openSession() 方法调用package org.apache.ibatis.session.defaults ，DefaultSqlSessionFactory类的openSession方法。

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

调用同一个类下的openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);方法。从这我们就可以猜到这里设定了Session的SQL执行器的类型、事务类型和是否自动提交。而这些设置信息都是由配置文件configuration决定的。这样我们就知道了配置信息是如何决定了sqlSession的功能。

  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();
    }
  }

返回了DefaultSqlSession(configuration,executor,autoCommit)的 实例，这是DefaultSqlSession类的构造方法。

补充：package org.apache.ibatis.session;下ExecutorType.class源码。

public enum ExecutorType {
  SIMPLE, REUSE, BATCH
}

package org.apache.ibatis.session; 下的Configuration.class类，设置defaultExecutorType的值是“SIMPLE”

protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;

我们已经拿到了一个SqlSession的实例，也就是说我们已经明白了1.的执行步骤，现在我们来看2.3.的执行过程。

 1.     SqlSession sqlSession = MyBatisSqlSessionFactory.openSession();   
 2.  StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);  
 3.  return studentMapper.findAllStudents();  

sqlsession.getMapper(StudentMapper.class)方法，调用了package org.apache.ibatis.session.defaults;下的DefaultSqlSession的getMapper(xxx.class)方法。源码如下:

  @Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

调用了configuration实例的

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

调用了package org.apache.ibatis.binding;下的mapperRegistry实例的：

  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

通过knownMappers获取一个MapperProxyFactory。knowMappers代码如下：

private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

然后newInstance，调用package org.apache.ibatis.binding;下的MapperProxyFactory 类的newInstance(SqlSession sqlSession)方法。再调用此类下的newInstance(MapperProxy mapperProxy)方法，通过动态代理，我们就可以使用接口了。 （这个我也不懂。。。），

 private final Class<T> mapperInterface;
 @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

得到了mapper实例。