目录

• 一 SqlSession的创建
• • 1.1 获取environments配置元素
  • 1.2 获取事务工厂
  • 1.3 获取执行器Executor
  • 1.4 构建DefaultSqlSession

• 官网：mybatis – MyBatis 3 | 简介
• 参考书籍：《通用源码阅读指导书：MyBatis源码详解》 易哥
• 参考文章：Mybatis源码解析
• CSDN：Mybatis源码分析_长安不及十里的博客-CSDN博客
• 掘金：稀土掘金
• 语雀文档地址：Mybatis 源码分析
• 个人博客地址：mybatis – 疯狂的程序员

使用 MyBatis 的主要 Java 接口就是 SqlSession。你可以通过这个接口来执行命令，获取映射器实例和管理事务。在介绍 SqlSession 接口之前，我们先来了解如何获取一个 SqlSession 实例。SqlSessions 是由 SqlSessionFactory 实例创建的。SqlSessionFactory 对象包含创建 SqlSession 实例的各种方法。而 SqlSessionFactory 本身是由 SqlSessionFactoryBuilder 创建的，它可以从 XML、注解或 Java 配置代码来创建 SqlSessionFactory。

一 SqlSession的创建

@Testvoid contextLoads() {// 第一阶段：MyBatis的初始化阶段String resource = "mybatis-config.xml";// 得到配置文件的输入流InputStream inputStream = null;try {inputStream = Resources.getResourceAsStream(resource);} catch (IOException e) {e.printStackTrace();}// 得到SqlSessionFactorySqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);// 第二阶段：数据读写阶段try (SqlSession session = sqlSessionFactory.openSession()) {// 找到接口对应的实现UserMapper userMapper = session.getMapper(UserMapper.class);// 组建查询参数User userParam = new User();userParam.setSchoolname("Sunny School");// 调用接口展开数据库操作List userList =  userMapper.queryAllByLimit(userParam);// 打印查询结果for (User user : userList) {System.out.println("name : " + user.getName() + " ;  email : " + user.getEmail());}}}

• 我们来看看SqlSession的创建是如何创建的，在上两篇文件中完成配置文件的解析返回SqlSessionFactory

          // 第二阶段：数据读写阶段try (SqlSession session = sqlSessionFactory.openSession()) {

• 可以通过调式看出，我们的SqlSessionFactory的实现是DefaultSqlSessionFactory

DefaultSqlSessionFactory

@Overridepublic SqlSession openSession() {return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);}public enum ExecutorType {SIMPLE, // 为每个语句创建新的预处理语句REUSE,  // 复用BATCH   // 执行批量操作
}/*** 从数据源中获取SqlSession对象* @param execType 执行器类型* @param level 事务隔离级别* @param autoCommit 是否自动提交事务* @return SqlSession对象*/private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {Transaction tx = null;try {// 找出要使用的指定环境final Environment environment = configuration.getEnvironment();// 从环境中获取事务工厂final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);// 从事务工厂中生产事务tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);// 创建执行器final Executor executor = configuration.newExecutor(tx, execType);// 创建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();}}

主要包含以下几个步骤：

1. 首先从configuration获取Environment对象，里面主要包含了DataSource和TransactionFactory对象
2. 创建TransactionFactory对象
3. 创建Transaction对象
4. 从configuration获取Executor
5. 构造DefaultSqlSession对象

1.1 获取environments配置元素

//配置environment环境
/** 事务配置 type= JDBC、MANAGED *  1.JDBC:这个配置直接简单使用了JDBC的提交和回滚设置。它依赖于从数据源得到的连接来管理事务范围。*  2.MANAGED:这个配置几乎没做什么。它从来不提交或回滚一个连接。*//** 数据源类型：type = UNPOOLED、POOLED、JNDI *  1.UNPOOLED：这个数据源的实现是每次被请求时简单打开和关闭连接。*  2.POOLED：这是JDBC连接对象的数据源连接池的实现。 *  3.JNDI：这个数据源的实现是为了使用如Spring或应用服务器这类的容器*///默认连接事务隔离级别 

• 解析我们配置文件中的environment配置元素，具体解析过程请参考前面的文章。

XMLConfigBuilder

// 解析我们配置文件中的environment配置元素
private void environmentsElement(XNode context) throws Exception {if (context != null) {if (environment == null) {// 获取 default 属性environment = context.getStringAttribute("default");}for (XNode child : context.getChildren()) {// 获取 id 属性String id = child.getStringAttribute("id");/** 检测当前 environment 节点的 id 与其父节点 environments 的属性 default * 内容是否一致，一致则返回 true，否则返回 false* 将其default属性值与子元素environment的id属性值相等的子元素设置为当前使用的Environment对象*/if (isSpecifiedEnvironment(id)) {// 将environment中的transactionManager标签转换为TransactionFactory对象TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));// 将environment中的dataSource标签转换为DataSourceFactory对象DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));// 创建 DataSource 对象DataSource dataSource = dsFactory.getDataSource();Environment.Builder environmentBuilder = new Environment.Builder(id).transactionFactory(txFactory).dataSource(dataSource);// 构建 Environment 对象，并设置到 configuration 中configuration.setEnvironment(environmentBuilder.build());}}}
}

1.2 获取事务工厂

DefaultSqlSessionFactory

 private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {if (environment == null || environment.getTransactionFactory() == null) {// 委托事务工厂return new ManagedTransactionFactory();}// 我们配置的事务工厂JdbcTransactionFactoryreturn environment.getTransactionFactory();}

• JdbcTransaction由JDBC进行事务管理

JdbcTransaction

// 由JDBC进行事务管理
public class JdbcTransaction implements Transaction {private static final Log log = LogFactory.getLog(JdbcTransaction.class);// 数据库连接protected Connection connection;// 数据源protected DataSource dataSource;// 事务隔离级别protected TransactionIsolationLevel level;// 是否自动提交事务protected boolean autoCommit;public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {dataSource = ds;level = desiredLevel;autoCommit = desiredAutoCommit;}public JdbcTransaction(Connection connection) {this.connection = connection;}@Overridepublic Connection getConnection() throws SQLException {if (connection == null) {openConnection();}return connection;}/*** 提交事务* @throws SQLException*/@Overridepublic void commit() throws SQLException {// 连接存在且不会自动提交事务if (connection != null && !connection.getAutoCommit()) {if (log.isDebugEnabled()) {log.debug("Committing JDBC Connection [" + connection + "]");}// 调用connection对象的方法提交事务connection.commit();}}/*** 回滚事务* @throws SQLException*/@Overridepublic void rollback() throws SQLException {if (connection != null && !connection.getAutoCommit()) {if (log.isDebugEnabled()) {log.debug("Rolling back JDBC Connection [" + connection + "]");}connection.rollback();}}@Overridepublic void close() throws SQLException {if (connection != null) {resetAutoCommit();if (log.isDebugEnabled()) {log.debug("Closing JDBC Connection [" + connection + "]");}connection.close();}}protected void setDesiredAutoCommit(boolean desiredAutoCommit) {try {if (connection.getAutoCommit() != desiredAutoCommit) {if (log.isDebugEnabled()) {log.debug("Setting autocommit to " + desiredAutoCommit + " on JDBC Connection [" + connection + "]");}connection.setAutoCommit(desiredAutoCommit);}} catch (SQLException e) {// Only a very poorly implemented driver would fail here,// and there's not much we can do about that.throw new TransactionException("Error configuring AutoCommit.  "+ "Your driver may not support getAutoCommit() or setAutoCommit(). "+ "Requested setting: " + desiredAutoCommit + ".  Cause: " + e, e);}}protected void resetAutoCommit() {try {if (!connection.getAutoCommit()) {// MyBatis does not call commit/rollback on a connection if just selects were performed.// Some databases start transactions with select statements// and they mandate a commit/rollback before closing the connection.// A workaround is setting the autocommit to true before closing the connection.// Sybase throws an exception here.if (log.isDebugEnabled()) {log.debug("Resetting autocommit to true on JDBC Connection [" + connection + "]");}connection.setAutoCommit(true);}} catch (SQLException e) {if (log.isDebugEnabled()) {log.debug("Error resetting autocommit to true "+ "before closing the connection.  Cause: " + e);}}}protected void openConnection() throws SQLException {if (log.isDebugEnabled()) {log.debug("Opening JDBC Connection");}connection = dataSource.getConnection();if (level != null) {connection.setTransactionIsolation(level.getLevel());}setDesiredAutoCommit(autoCommit);}@Overridepublic Integer getTimeout() throws SQLException {return null;}}

JdbcTransaction主要维护了一个默认autoCommit为false的Connection对象，对事物的提交，回滚，关闭等都是接见通过Connection完成的。

1.3 获取执行器Executor

/*** 创建一个执行器* @param transaction 事务* @param executorType 数据库操作类型* @return 执行器*/public Executor newExecutor(Transaction transaction, ExecutorType executorType) {executorType = executorType == null ? defaultExecutorType : executorType;executorType = executorType == null ? ExecutorType.SIMPLE : executorType;Executor executor;// 根据数据操作类型创建实际执行器if (ExecutorType.BATCH == executorType) {// 批处理执行器executor = new BatchExecutor(this, transaction);} else if (ExecutorType.REUSE == executorType) {// 可以重用执行器executor = new ReuseExecutor(this, transaction);} else {//一个简单的执行器executor = new SimpleExecutor(this, transaction);}// 根据配置文件中settings节点cacheEnabled配置项确定是否启用缓存if (cacheEnabled) { // 如果配置启用缓存// 使用CachingExecutor装饰实际执行器executor = new CachingExecutor(executor);}// 为执行器增加拦截器（插件），以启用各个拦截器的功能executor = (Executor) interceptorChain.pluginAll(executor);return executor;}

• 执行器的类型，我们来看看执行器的接口信息

Executor

public interface Executor {ResultHandler NO_RESULT_HANDLER = null;// 数据更新操作，其中数据的增加、删除、更新均可由该方法实现int update(MappedStatement ms, Object parameter) throws SQLException;// 数据查询操作，返回结果为列表形式 List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;// 数据查询操作，返回结果为列表形式/*** 执行查询操作* @param ms 映射语句对象* @param parameter 参数对象* @param rowBounds 翻页限制* @param resultHandler 结果处理器* @param  输出结果类型* @return 查询结果* @throws SQLException*/ List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;// 数据查询操作，返回结果为游标形式 Cursor queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;// 清理缓存List flushStatements() throws SQLException;// 提交事务void commit(boolean required) throws SQLException;// 回滚事务void rollback(boolean required) throws SQLException;// 创建当前查询的缓存键值CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);// 本地缓存是否有指定值boolean isCached(MappedStatement ms, CacheKey key);// 清理本地缓存void clearLocalCache();// 懒加载void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class targetType);// 获取事务Transaction getTransaction();// 关闭执行器void close(boolean forceRollback);// 判断执行器是否关闭boolean isClosed();// 设置执行器包装void setExecutorWrapper(Executor executor);}

executor包含了Configuration和刚刚创建的Transaction，默认的执行器为SimpleExecutor，如果开启了二级缓存(默认开启)，则CachingExecutor会包装SimpleExecutor，然后依次调用拦截器的plugin方法返回一个被代理过的Executor对象，记住这个地方，后面Sql语句具体的执行是交给执行器来进行处理的。

1.4 构建DefaultSqlSession

 return new DefaultSqlSession(configuration, executor, autoCommit);

public class DefaultSqlSession implements SqlSession {// 配置信息private final Configuration configuration;// 执行器private final Executor executor;// 是否自动提交private final boolean autoCommit;// 缓存是否已经被污染private boolean dirty;// 游标列表private List> cursorList;public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {this.configuration = configuration;this.executor = executor;this.dirty = false;this.autoCommit = autoCommit;}}

SqlSession的所有查询接口最后都归结位Exector的方法调用。后面文章我们来分析其调用流程。