sqlSession简单介绍

• 拿到SqlSessionFactory对象后，会调用SqlSessionFactory的openSesison方法，这个方法会创建一个Sql执行器（Executor），这个Sql执行器会代理你配置的拦截器方法。
• 获得上面的Sql执行器后，会创建一个SqlSession（默认使用DefaultSqlSession）,这个SqlSession中也包含了Configration对象，所以通过SqlSession也能拿到全局配置；
• 获得SqlSession对象后就能执行各种CRUD方法了。
  

获得sqlSession对象源码分析

/**
 * 通过sqlSessionFactory.openSession进行获取sqlSession对象
 * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSession()
 */
public SqlSession openSession() {
	return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}

/**
 * 通过数据源去获取SqlSession 
 * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(ExecutorType, TransactionIsolationLevel, boolean)
 */
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level,
		boolean autoCommit) {
	Transaction tx = null;
	try {
		// 获取环境变量
		final Environment environment = configuration.getEnvironment();
		// 获取事务工厂
		final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
		// 获取一个事务
		tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
		// 获取执行器，这边获得的执行器已经代理拦截器的功能
		final Executor executor = configuration.newExecutor(tx, execType);
		// 根据获取的执行器创建SqlSession
		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();
	}
}

/**
 * 获取执行器
 * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(ExecutorType, TransactionIsolationLevel, boolean)
 */
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
	// 默认使用SIMPLE的执行器
	executorType = executorType == null ? defaultExecutorType : executorType;
	executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
	Executor executor;

	if (ExecutorType.BATCH == executorType) {
		// 批量的执行器
		executor = new BatchExecutor(this, transaction);
	} else if (ExecutorType.REUSE == executorType) {
		// 可重复使用的执行器
		executor = new ReuseExecutor(this, transaction);
	} else {
		// 简单的sql执行器
		executor = new SimpleExecutor(this, transaction);
	}

	// 判断Mybatis的全局配置文件是否开启二级缓存
	if (cacheEnabled) {
		// 开启缓存，吧executor包装为CachingExecutor
		executor = new CachingExecutor(executor);
	}
	
	// 插件的调用：责任链模式
	executor = (Executor) interceptorChain.pluginAll(executor);
	return executor;
}

SQL执行流程，以查询为例

/**
 * 查询的入口方法
 * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(String, Object)
 */
public <T> T selectOne(String statement, Object parameter) {
	// Popular vote was to return null on 0 results and throw exception on too many.
	// 查询数据
	List<T> list = this.<T>selectList(statement, parameter);

	// 长度为1，拿第一个
	if (list.size() == 1) {
		return list.get(0);
	} else if (list.size() > 1) { 
		// 长度大于一，抛异常
		throw new TooManyResultsException(
				"Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
	} else {
		// 没有拿到返回null
		return null;
	}
}

/**
 * 查询数据
 * 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(String, Object, RowBounds)
 */
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
	try {
		// 通过statement去全局配置文件中获取MappedStatement（得到mapper中增删改查的节点）
		MappedStatement ms = configuration.getMappedStatement(statement);

		// 通过执行器去执行SQL
		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();
	}
}

/**
 * 执行查询操作的准备工作
 * 源码位置：org.apache.ibatis.executor.CachingExecutor.query(MappedStatement, Object, RowBounds, ResultHandler)
 */
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds,
		ResultHandler resultHandler) throws SQLException {

	// 通过参数进行sql解析
	BoundSql boundSql = ms.getBoundSql(parameterObject);
	CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
	return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

/**
 * 执行查询操作的准备工作
 * 源码位置：org.apache.ibatis.executor.CachingExecutor.query(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql)
 */
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds,
		ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
	// 判断sql是否开启了缓存 <cache></cache>
	Cache cache = ms.getCache();
	
	// 有缓存
	if (cache != null) {
		// 判断是否需要刷新缓存
		flushCacheIfRequired(ms);
		if (ms.isUseCache() && resultHandler == null) {
			ensureNoOutParams(ms, boundSql);
			@SuppressWarnings("unchecked")

			// 去二级缓存中获取（装饰者模式）
			List<E> list = (List<E>) tcm.getObject(cache, key);

			// 二级缓存没有找到
			if (list == null) {

				// 查询数据，并放入缓存
				list = delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
				tcm.putObject(cache, key, list); // issue #578 and #116
			}
			return list;
		}
	}
	// 查询数据
	return delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

/**
 * 一级缓存查询的调用
 * 源码位置：org.apache.ibatis.executor.BaseExecutor.query(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql)
 */
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler,
		CacheKey key, BoundSql boundSql) throws SQLException {
	ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());

	// 已经关闭了，抛异常
	if (closed) {
		throw new ExecutorException("Executor was closed.");
	}

	// 清空本地缓存
	if (queryStack == 0 && ms.isFlushCacheRequired()) {
		clearLocalCache();
	}
	List<E> list;
	try {
		// 从一级缓存中获取数据
		queryStack++;
		list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
		if (list != null) {
			// 缓存里面有，进行处理
			handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
		} else {
			// 缓存没有，进行查询
			list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
		}
	} finally {
		queryStack--;
	}
	if (queryStack == 0) {
		for (DeferredLoad deferredLoad : deferredLoads) {
			deferredLoad.load();
		}
		// issue #601
		deferredLoads.clear();
		if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
			// issue #482
			clearLocalCache();
		}
	}
	return list;
}

/**
 * 在数据库中查询
 * 源码位置：org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql)
 */
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds,
		ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
	List<E> list;
	localCache.putObject(key, EXECUTION_PLACEHOLDER);
	try {
		// 去数据库查询
		list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
	} finally {
		localCache.removeObject(key);
	}

	// 一级缓存进行缓存
	localCache.putObject(key, list);
	if (ms.getStatementType() == StatementType.CALLABLE) {
		localOutputParameterCache.putObject(key, parameter);
	}
	return list;
}

/**
 * 查询逻辑
 * 源码位置：org.apache.ibatis.executor.SimpleExecutor.doQuery(MappedStatement, Object, RowBounds, ResultHandler, BoundSql)
 */
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler,
		BoundSql boundSql) throws SQLException {
	Statement stmt = null;
	try {
		// 得到整体的配置对象
		Configuration configuration = ms.getConfiguration();

		// 内部封装了ParameterHandler和ResultSetHandler
		StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds,
				resultHandler, boundSql);
		stmt = prepareStatement(handler, ms.getStatementLog());

		// 执行查询
		return handler.<E>query(stmt, resultHandler);
	} finally {
		closeStatement(stmt);
	}
}

/**
 * 执行查询语句
 * 源码位置：org.apache.ibatis.executor.statement.SimpleStatementHandler.query(Statement, ResultHandler)
 */
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
	// 得到要执行的sql
	String sql = boundSql.getSql();

	// 执行sql
	statement.execute(sql);

	// 处理结果集
	return resultSetHandler.<E>handleResultSets(statement);
}

结束语

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