（二）MyBatis源码阅读：SqlSession分析

一、核心流程

以下代码便是MyBatis的核心流程，我们从该代码出发分析MyBatis的源码。

 @Testpublic void test2() throws Exception{// 1.获取配置文件InputStream in = Resources.getResourceAsStream("mybatis-config.xml");// 2.加载解析配置文件并获取SqlSessionFactory对象SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);// 3.根据SqlSessionFactory对象获取SqlSession对象SqlSession sqlSession = factory.openSession();// 4.通过SqlSession中提供的 API方法来操作数据库UserMapper mapper = sqlSession.getMapper(UserMapper.class);List<User> list = mapper.selectUserList();for (User user : list) {System.out.println(user);}// 5.关闭会话sqlSession.close();}

二、SqlSessionFactory创建流程

（一）SqlSessionFactoryBuilder

SqlSessionFactoryBuilder调用了XMLConfigBuilder 的parse方法进行了解析。我们接下来去看XMLConfigBuilder类。

  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {try {// 用于解析 mybatis-config.xml，同时创建了 Configuration 对象 >>XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);// 解析XML，最终返回一个 DefaultSqlSessionFactory >>return build(parser.parse());} catch (Exception e) {throw ExceptionFactory.wrapException("Error building SqlSession.", e);} finally {ErrorContext.instance().reset();try {inputStream.close();} catch (IOException e) {// Intentionally ignore. Prefer previous error.}}}

（二）XMLConfigBuilder

构造方法：

  private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {super(new Configuration()); // 完成了Configuration的初始化ErrorContext.instance().resource("SQL Mapper Configuration");this.configuration.setVariables(props); // 设置对应的Properties属性this.parsed = false; // 设置 是否解析的标志为 falsethis.environment = environment; // 初始化environmentthis.parser = parser; // 初始化 解析器}

我们去看看configuration里面干了什么，里面主要为各个类取了别名。 

parseConfiguration方法：

parseConfiguration方法就是对各个具体的方法进行了封装，我们任意找几个方法分析即可。

private void parseConfiguration(XNode root) {try {//issue #117 read properties first// 对于全局配置文件各种标签的解析propertiesElement(root.evalNode("properties"));// 解析 settings 标签Properties settings = settingsAsProperties(root.evalNode("settings"));// 读取文件loadCustomVfs(settings);// 日志设置loadCustomLogImpl(settings);// 类型别名typeAliasesElement(root.evalNode("typeAliases"));// 插件pluginElement(root.evalNode("plugins"));// 用于创建对象objectFactoryElement(root.evalNode("objectFactory"));// 用于对对象进行加工objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));// 反射工具箱reflectorFactoryElement(root.evalNode("reflectorFactory"));// settings 子标签赋值，默认值就是在这里提供的 >>settingsElement(settings);// read it after objectFactory and objectWrapperFactory issue #631// 创建了数据源 >>environmentsElement(root.evalNode("environments"));databaseIdProviderElement(root.evalNode("databaseIdProvider"));typeHandlerElement(root.evalNode("typeHandlers"));// 解析引用的Mapper映射器mapperElement(root.evalNode("mappers"));} catch (Exception e) {throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);}}

propertiesElement方法：

private void propertiesElement(XNode context) throws Exception {if (context != null) {// 创建了一个 Properties 对象，后面可以用到Properties defaults = context.getChildrenAsProperties();String resource = context.getStringAttribute("resource");String url = context.getStringAttribute("url");if (resource != null && url != null) {// url 和 resource 不能同时存在throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other.");}// 加载resource或者url属性中指定的 properties 文件if (resource != null) {defaults.putAll(Resources.getResourceAsProperties(resource));} else if (url != null) {defaults.putAll(Resources.getUrlAsProperties(url));}Properties vars = configuration.getVariables();if (vars != null) {// 和 Configuration中的 variables 属性合并defaults.putAll(vars);}// 更新对应的属性信息parser.setVariables(defaults);configuration.setVariables(defaults);}}

settingsAsProperties方法：

  private Properties settingsAsProperties(XNode context) {if (context == null) {return new Properties();}// 获取settings节点下的所有的子节点Properties props = context.getChildrenAsProperties();// Check that all settings are known to the configuration classMetaClass metaConfig = MetaClass.forClass(Configuration.class, localReflectorFactory);for (Object key : props.keySet()) {//if (!metaConfig.hasSetter(String.valueOf(key))) {throw new BuilderException("The setting " + key + " is not known.  Make sure you spelled it correctly (case sensitive).");}}return props;}

mapperElement方法：

调用 Configuration的addMapper方法，将接口放入MapperRegistry中。MapperRegistry是接口与接口工程的map键值对，便于之后创建对应的接口。

MapperRegistry就是记录接口与工厂的关系。

 addMapper方法调用了解析器的parse方法。

（三）XMLMapperBuilder

parse方法

public void parse() {// 总体上做了两件事情，对于语句的注册和接口的注册if (!configuration.isResourceLoaded(resource)) {// 1、具体增删改查标签的解析。// 一个标签一个MappedStatement。 >>configurationElement(parser.evalNode("/mapper"));configuration.addLoadedResource(resource);// 2、把namespace（接口类型）和工厂类绑定起来，放到一个map。// 一个namespace 一个 MapperProxyFactory >>bindMapperForNamespace();}parsePendingResultMaps();parsePendingCacheRefs();parsePendingStatements();}

（四）创建流程图

三、SqlSession 创建流程

通过DefaultSqlSessionFactory的openSession方法来创建对应的SqlSession。在这个DefaultSqlSession对象中包括了Configuration和Executor对象

 private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {Transaction tx = null;try {final Environment environment = configuration.getEnvironment();// 获取事务工厂final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);// 创建事务tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);// 根据事务工厂和默认的执行器类型，创建执行器 >>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();}}

执行的具体流程如下：

四、获取代理对象流程

DefaultSqlSession调用 MapperRegistry的getMapper方法，该方法拿到对应的代理工厂，创建对应的代理对象。

进入newInstance方法，最终我们在代码中发现代理对象是通过JDK动态代理创建，返回的代理对象。而且里面也传递了一个实现了InvocationHandler接口的触发管理类。

 执行的具体流程如下：

五、SQL语句执行流程

由于所有的Mapper都是JDK动态代理对象，所以任意的方法都是执行触发管理类MapperProxy的invoke()方法。

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {// toString hashCode equals getClass等方法，无需走到执行SQL的流程if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, args);} else {// 提升获取 mapperMethod 的效率，到 MapperMethodInvoker（内部接口） 的 invoke// 普通方法会走到 PlainMethodInvoker（内部类） 的 invokereturn cachedInvoker(method).invoke(proxy, method, args, sqlSession);}} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}
}

然后进入到PlainMethodInvoker的invoke方法

    @Overridepublic Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {// SQL执行的真正起点return mapperMethod.execute(sqlSession, args);}

进入mapperMethod.execute()方法。在这一步，根据不同的type（INSERT、UPDATE、DELETE、SELECT）和返回类型：（1）调用convertArgsToSqlCommandParam()将方法参数转换为SQL的参数。（2）调用sqlSession的insert()、update()、delete()、selectOne ()方法。我们以查询为例，会走到selectOne()方法。

  public Object execute(SqlSession sqlSession, Object[] args) {Object result;switch (command.getType()) { // 根据SQL语句的类型调用SqlSession对应的方法case INSERT: {// 通过 ParamNameResolver 处理args[] 数组 将用户传入的实参和指定参数名称关联起来Object param = method.convertArgsToSqlCommandParam(args);// sqlSession.insert(command.getName(), param) 调用SqlSession的insert方法// rowCountResult 方法会根据 method 字段中记录的方法的返回值类型对结果进行转换result = rowCountResult(sqlSession.insert(command.getName(), param));break;}case UPDATE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));break;}case DELETE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));break;}case SELECT:if (method.returnsVoid() && method.hasResultHandler()) {// 返回值为空 且 ResultSet通过 ResultHandler处理的方法executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {result = executeForMap(sqlSession, args);} else if (method.returnsCursor()) {result = executeForCursor(sqlSession, args);} else {// 返回值为 单一对象的方法Object param = method.convertArgsToSqlCommandParam(args);// 普通 select 语句的执行入口 >>result = sqlSession.selectOne(command.getName(), param);if (method.returnsOptional()&& (result == null || !method.getReturnType().equals(result.getClass()))) {result = Optional.ofNullable(result);}}break;case FLUSH:result = sqlSession.flushStatements();break;default:throw new BindingException("Unknown execution method for: " + command.getName());}if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {throw new BindingException("Mapper method '" + command.getName()+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");}return result;}

这里来到了对外的接口的默认实现类DefaultSqlSession。selectOne()最终也是调用了selectList()。

  @Overridepublic <T> T selectOne(String statement, Object parameter) {// 来到了 DefaultSqlSession// Popular vote was to return null on 0 results and throw exception on too many.List<T> list = this.selectList(statement, parameter);if (list.size() == 1) {return list.get(0);} else if (list.size() > 1) {throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());} else {return null;}}

在SelectList()中，我们先根据command name（Statement ID）从Configuration中拿到MappedStatement。ms里面有xml中增删改查标签配置的所有属性，包括id、statementType、sqlSource、useCache、入参、出参等等。然后执行了Executor的query()方法。Executor是第二步openSession的时候创建的，创建了执行器基本类型之后，依次执行了二级缓存装饰，和插件包装。

  @Overridepublic <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {MappedStatement ms = configuration.getMappedStatement(statement);// 如果 cacheEnabled = true（默认），Executor会被 CachingExecutor装饰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();}}

BaseExecutor.query方法

 @Overridepublic <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)throws SQLException {Cache cache = ms.getCache();// cache 对象是在哪里创建的？  XMLMapperBuilder类 xmlconfigurationElement()// 由 <cache> 标签决定if (cache != null) {// flushCache="true" 清空一级二级缓存 >>flushCacheIfRequired(ms);if (ms.isUseCache() && resultHandler == null) {ensureNoOutParams(ms, boundSql);// 获取二级缓存// 缓存通过 TransactionalCacheManager、TransactionalCache 管理@SuppressWarnings("unchecked")List<E> list = (List<E>) tcm.getObject(cache, key);if (list == null) {list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);// 写入二级缓存tcm.putObject(cache, key, list); // issue #578 and #116}return list;}}// 走到 SimpleExecutor | ReuseExecutor | BatchExecutorreturn delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);}public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {// 异常体系之 ErrorContextErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());if (closed) {throw new ExecutorException("Executor was closed.");}if (queryStack == 0 && ms.isFlushCacheRequired()) {// flushCache="true"时，即使是查询，也清空一级缓存clearLocalCache();}List<E> list;try {// 防止递归查询重复处理缓存queryStack++;// 查询一级缓存// ResultHandler 和 ResultSetHandler的区别list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;if (list != null) {handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);} else {// 真正的查询流程list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);}} finally {queryStack--;}if (queryStack == 0) {for (DeferredLoad deferredLoad : deferredLoads) {deferredLoad.load();}// issue #601deferredLoads.clear();if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {// issue #482clearLocalCache();}}return list;}

  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {List<E> list;// 先占位localCache.putObject(key, EXECUTION_PLACEHOLDER);try {// 三种 Executor 的区别，看doUpdate// 默认Simplelist = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);} finally {// 移除占位符localCache.removeObject(key);}// 写入一级缓存localCache.putObject(key, list);if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter);}return list;}

我们到具体的实现类SimpleExecutor中查看doQuery方法

  @Overridepublic <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {Statement stmt = null;try {Configuration configuration = ms.getConfiguration();// 注意，已经来到SQL处理的关键对象 StatementHandler >>StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);// 获取一个 Statement对象stmt = prepareStatement(handler, ms.getStatementLog());// 执行查询return handler.query(stmt, resultHandler);} finally {// 用完就关闭closeStatement(stmt);}}

在configuration.newStatementHandler()中，new一个StatementHandler，先得到RoutingStatementHandler。RoutingStatementHandler里面没有任何的实现，是用来创建基本的StatementHandler的。这里会根据MappedStatement里面的statementType决定StatementHandler的类型。默认是PREPARED（STATEMENT、PREPARED、CALLABLE）。

 

 

StatementHandler里面包含了处理参数的ParameterHandler和处理结果集的ResultSetHandler。

这两个对象都是在上面new的时候创建的。

 

 进入到PreparedStatementHandler中处理

  @Overridepublic <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {PreparedStatement ps = (PreparedStatement) statement;// 到了JDBC的流程ps.execute();// 处理结果集return resultSetHandler.handleResultSets(ps);}

 执行的具体流程如下：