转载自  mybatis源码阅读(一)：SqlSession和SqlSessionFactory 

一、接口定义

    听名字就知道这里使用了工厂方法模式，SqlSessionFactory负责创建SqlSession对象。其中开发人员最常用的就是DefaultSqlSession

（1）SqlSession接口定义

public interface SqlSession extends Closeable {// 泛型方法，参数表示使用的查询SQL语句，返回值为查询的结果对象<T> T selectOne(String statement);// 第二个参数表示需要用户传入的实参，也就是SQL语句绑定的实参<T> T selectOne(String statement, Object parameter);// 查询结果集有多条记录，会封装成结果对象列表返回<E> List<E> selectList(String statement);<E> List<E> selectList(String statement, Object parameter);// 第三个参数用于限制解析结果集的范围<E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds);/****  selectMap 方法的原理和参数都与selectList方法类型，但结果集会被映射成Map对象返回*  其中mapKey参数指定了结果集哪一列作为map的key,其他参数同上*/<K, V> Map<K, V> selectMap(String statement, String mapKey);<K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey);<K, V> Map<K, V> selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds);// 返回值是游标对象，参数同上<T> Cursor<T> selectCursor(String statement);<T> Cursor<T> selectCursor(String statement, Object parameter);<T> Cursor<T> selectCursor(String statement, Object parameter, RowBounds rowBounds);// 查询的结果对象将由此处指定的handler对象处理，其余参数同上void select(String statement, Object parameter, ResultHandler handler);void select(String statement, ResultHandler handler);void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler);// 执行insert语句int insert(String statement);int insert(String statement, Object parameter);// 执行update语句int update(String statement);int update(String statement, Object parameter);// 执行deleteint delete(String statement);int delete(String statement, Object parameter);// 提交事务void commit();void commit(boolean force);// 事务回滚void rollback();void rollback(boolean force);// 将请求刷新到数据库List<BatchResult> flushStatements();// 关闭当前session@Overridevoid close();// 清空session 缓存void clearCache();// 获取Configuration 对象Configuration getConfiguration();// 获取type 对象的Mapper对象<T> T getMapper(Class<T> type);// 获取该Sqlsession 对象的数据库连接Connection getConnection();
}

SqlSession数据库的C、R、U、D及事务处理接口，你懂的。

（2）SqlSessionFactory

public interface SqlSessionFactory {SqlSession openSession();SqlSession openSession(boolean autoCommit);SqlSession openSession(Connection connection);SqlSession openSession(TransactionIsolationLevel level);SqlSession openSession(ExecutorType execType);SqlSession openSession(ExecutorType execType, boolean autoCommit);SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);SqlSession openSession(ExecutorType execType, Connection connection);Configuration getConfiguration();
}

这个大家也都懂的

SqlSession实现类：DefaultSqlSession和SqlSessionManager

SqlSessionFactory实现类：DefaultSqlSessionFactory和SqlSessionManager

（3）DefaultSqlSession

@Override
public int update(String statement, Object parameter) {try {dirty = true;MappedStatement ms = configuration.getMappedStatement(statement);return executor.update(ms, wrapCollection(parameter));} catch (Exception e) {throw ExceptionFactory.wrapException("Error updating database.  Cause: " + e, e);} finally {ErrorContext.instance().reset();}
}
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {MappedStatement ms = configuration.getMappedStatement(statement);return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);} catch (Exception e) {throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);} finally {ErrorContext.instance().reset();}
}

这里主要看这两个方法，因为delete和insert最终执行掉用的都是update方法，查询就更不用说了。从代码上看都是从configuration对象中获取MappedStatement 对象 然后把事情交给小弟Executor去执行，这里用了很典型的策略设计模式，这个关于Executor 后面介绍。

（4）DefaultSqlSessionFactory

/*** 通过数据源获取数据库连接，并创建Executor以及DefaultSqlSession对象*/
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();}
}/***  用户提供数据库连接对象，使用该数据库连接对象创建Executor和DefaultSqlSession对象*/
private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {try {boolean autoCommit;try {autoCommit = connection.getAutoCommit();} catch (SQLException e) {// Failover to true, as most poor drivers// or databases won't support transactionsautoCommit = true;}      final Environment environment = configuration.getEnvironment();final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);final Transaction tx = transactionFactory.newTransaction(connection);final Executor executor = configuration.newExecutor(tx, execType);return new DefaultSqlSession(configuration, executor, autoCommit);} catch (Exception e) {throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);} finally {ErrorContext.instance().reset();}
}

DefaultSqlSessionFactory主要提供了两种创建DefaultSqlSession对象的方式，一种是通过数据源获取数据库连接，并创建Executor以及DefaultSqlSession对象，另一种是用户提供数据库连接对象，使用该数据库连接对象创建Executor和DefaultSqlSession对象。

（5）SqlSessionManager

SqlSessionManager同时实现SqlSession接口和SqlSessionFactory接口，也就是同时提供了创建SqlSession对象以及SqlSession对象操作数据库的功能。SqlSessionManager与DefaultSqlSessionFactory的主要不同点是SqlSessionManager提供了两种模式，一种是和DefaultSqlSessionFactory的行为相同，同一线程每次访问数据库就都会创建新的DefaultSqlSession，第二种是通过ThreadLocal变量记录当前线程的SqlSession对象，避免同一线程多次创建SqlSession对象。至于使用动态代理的目的，是为了通过拦截器InvocationHandler，增强目标target的方法调用。

private final SqlSessionFactory sqlSessionFactory;
// SqlSession的代理对象 会使用JDK的动态代理方式实现
private final SqlSession sqlSessionProxy;/*** ThreadLocal 变量，记录一个与当前线程绑定的SqlSession对象* localSqlSession 中记录的SqlSession对象的代理对象，在SqlSessionManager初始化的时候*/
private final ThreadLocal<SqlSession> localSqlSession = new ThreadLocal<SqlSession>();
/*** SqlSessionManager 的私有构造方法*/
private SqlSessionManager(SqlSessionFactory sqlSessionFactory) {this.sqlSessionFactory = sqlSessionFactory;// 使用动态代理生成SqlSession的代理对象this.sqlSessionProxy = (SqlSession) Proxy.newProxyInstance(SqlSessionFactory.class.getClassLoader(),new Class[]{SqlSession.class},new SqlSessionInterceptor());
}/*** 通过newInstance方法创建SqlSessionManager对象* @param reader* @return*/public static SqlSessionManager newInstance(Reader reader) {return new SqlSessionManager(new SqlSessionFactoryBuilder().build(reader, null, null));}
// 内部类
private class SqlSessionInterceptor implements InvocationHandler {public SqlSessionInterceptor() {// Prevent Synthetic Access}@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {// 获取当前线程绑定的SqlSession对象final SqlSession sqlSession = SqlSessionManager.this.localSqlSession.get();if (sqlSession != null) {// 第二种模式try {return method.invoke(sqlSession, args);} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}} else {// 第一种模式 创建新的SqlSessionfinal SqlSession autoSqlSession = openSession();try {final Object result = method.invoke(autoSqlSession, args);autoSqlSession.commit();return result;} catch (Throwable t) {autoSqlSession.rollback();throw ExceptionUtil.unwrapThrowable(t);} finally {autoSqlSession.close();}}}
}