MyBatis原理分析

MyBatis工作流程简述
原生MyBatis原理分析
- 初始化工作
- - 解析配置文件
  - 配置类方式
- 执行SQL
- - SqlSession API方式
  - 接口方式

真正掌握一个框架源码分析是少不了的~

在讲解整合Spring的原理之前理解原生的MyBatis执行原理是非常有必要的

MyBatis工作流程简述

传统工作模式：

public static void main(String[] args) {InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = factory.openSession();String name = "tom";List<User> list = sqlSession.selectList("com.demo.mapper.UserMapper.getUserByName",params);
}

创建SqlSessionFactoryBuilder对象，调用build(inputstream)方法读取并解析配置文件，返回SqlSessionFactory对象
由SqlSessionFactory创建SqlSession 对象，没有手动设置的话事务默认开启
调用SqlSession中的api，传入Statement Id和参数，内部进行复杂的处理，最后调用jdbc执行SQL语句，封装结果返回。

使用Mapper接口：
由于面向接口编程的趋势，MyBatis也实现了通过接口调用mapper配置文件中的SQL语句

public static void main(String[] args) {//前三步都相同InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = factory.openSession();//这里不再调用SqlSession 的api，而是获得了接口对象，调用接口中的方法。UserMapper mapper = sqlSession.getMapper(UserMapper.class);List<User> list = mapper.getUserByName("tom");
}

原生MyBatis原理分析

初始化工作

解析配置文件

InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
//这一行代码正是初始化工作的开始。
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);

进入源码分析：

// 1.我们最初调用的build
public SqlSessionFactory build(InputStream inputStream) {//调用了重载方法return build(inputStream, null, null);}// 2.调用的重载方法
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {try {//  XMLConfigBuilder是专门解析mybatis的配置文件的类XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);//这里又调用了一个重载方法。parser.parse()的返回值是Configuration对象return build(parser.parse());} catch (Exception e) {throw ExceptionFactory.wrapException("Error building SqlSession.", e);} //省略部分代码}

下面进入对配置文件解析部分：

首先对Configuration对象进行介绍：

Configuration对象的结构和xml配置文件的对象几乎相同。

回顾一下xml中的配置标签有哪些：

properties（属性），settings（设置），typeAliases（类型别名），typeHandlers（类型处理器），objectFactory（对象工厂），mappers（映射器）等

Configuration也有对应的对象属性来封装它们：
（图片来自：https://blog.csdn.net/luanlouis/article/details/37744073）
也就是说，初始化配置文件信息的本质就是创建Configuration对象，将解析的xml数据封装到Configuration内部的属性中。

//在创建XMLConfigBuilder时，它的构造方法中解析器XPathParser已经读取了配置文件
//3. 进入XMLConfigBuilder 中的 parse()方法。
public Configuration parse() {if (parsed) {throw new BuilderException("Each XMLConfigBuilder can only be used once.");}parsed = true;//parser是XPathParser解析器对象，读取节点内数据，<configuration>是MyBatis配置文件中的顶层标签parseConfiguration(parser.evalNode("/configuration"));//最后返回的是Configuration 对象return configuration;
}//4. 进入parseConfiguration方法
//此方法中读取了各个标签内容并封装到Configuration中的属性中。
private void parseConfiguration(XNode root) {try {//issue #117 read properties firstpropertiesElement(root.evalNode("properties"));Properties settings = settingsAsProperties(root.evalNode("settings"));loadCustomVfs(settings);loadCustomLogImpl(settings);typeAliasesElement(root.evalNode("typeAliases"));pluginElement(root.evalNode("plugins"));objectFactoryElement(root.evalNode("objectFactory"));objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));reflectorFactoryElement(root.evalNode("reflectorFactory"));settingsElement(settings);// read it after objectFactory and objectWrapperFactory issue #631environmentsElement(root.evalNode("environments"));databaseIdProviderElement(root.evalNode("databaseIdProvider"));typeHandlerElement(root.evalNode("typeHandlers"));mapperElement(root.evalNode("mappers"));} catch (Exception e) {throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);}
}

到此对xml配置文件的解析就结束了（下文会对部分解析做详细介绍），回到步骤 2. 中调用的重载build方法。

// 5. 调用的重载方法
public SqlSessionFactory build(Configuration config) {//创建了DefaultSqlSessionFactory对象，传入Configuration对象。return new DefaultSqlSessionFactory(config);}

配置类方式

发散一下思路，既然解析xml是对Configuration中的属性进行复制，那么我们同样可以在一个类中创建Configuration对象，手动设置其中属性的值来达到配置的效果。

执行SQL

先简单介绍SqlSession：

SqlSession是一个接口，它有两个实现类：DefaultSqlSession（默认）和SqlSessionManager（弃用，不做介绍）
SqlSession是MyBatis中用于和数据库交互的顶层类，通常将它与ThreadLocal绑定，一个会话使用一个SqlSession，并且在使用完毕后需要close。

SqlSession中的两个最重要的参数，configuration与初始化时的相同，Executor为执行器，

Executor：

Executor也是一个接口，他有三个常用的实现类BatchExecutor（重用语句并执行批量更新），ReuseExecutor（重用预处理语句prepared statements），SimpleExecutor（普通的执行器，默认）。

SqlSession API方式

继续分析，初始化完毕后，我们就要执行SQL了：

		SqlSession sqlSession = factory.openSession();String name = "tom";List<User> list = sqlSession.selectList("com.demo.mapper.UserMapper.getUserByName",params);

获得sqlSession

//6. 进入openSession方法。public SqlSession openSession() {//getDefaultExecutorType()传递的是SimpleExecutorreturn openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);}//7. 进入openSessionFromDataSource。
//ExecutorType 为Executor的类型，TransactionIsolationLevel为事务隔离级别，autoCommit是否开启事务
//openSession的多个重载方法可以指定获得的SeqSession的Executor类型和事务的处理
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);//根据参数创建指定类型的Executorfinal Executor executor = configuration.newExecutor(tx, execType);//返回的是DefaultSqlSessionreturn 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sqlsession中的api

//8.进入selectList方法，多个重载方法。
public <E> List<E> selectList(String statement) {return this.selectList(statement, null);
}
public <E> List<E> selectList(String statement, Object parameter) {return this.selectList(statement, parameter, RowBounds.DEFAULT);
}public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {try {//根据传入的全限定名+方法名从映射的Map中取出MappedStatement对象MappedStatement ms = configuration.getMappedStatement(statement);//调用Executor中的方法处理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();}}

介绍一下MappedStatement ：

作用： MappedStatement与Mapper配置文件中的一个select/update/insert/delete节点相对应。mapper中配置的标签都被封装到了此对象中，主要用途是描述一条SQL语句。
**初始化过程：**回顾刚开始介绍的加载配置文件的过程中，会对mybatis-config.xml中的各个标签都进行解析，其中有 mappers标签用来引入mapper.xml文件或者配置mapper接口的目录。

 <select id="getUser" resultType="user" >select * from user where id=#{id}</select>

这样的一个select标签会在初始化配置文件时被解析封装成一个MappedStatement对象，然后存储在Configuration对象的mappedStatements属性中，mappedStatements 是一个HashMap，存储时key = 全限定类名 + 方法名，value = 对应的MappedStatement对象。

在configuration中对应的属性为

Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")

在XMLConfigBuilder中的处理：

  private void parseConfiguration(XNode root) {try {// 省略其他标签的处理mapperElement(root.evalNode("mappers"));} catch (Exception e) {throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);}}

继续源码中的步骤，进入 executor.query()

//此方法在SimpleExecutor的父类BaseExecutor中实现
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {//根据传入的参数动态获得SQL语句，最后返回用BoundSql对象表示BoundSql boundSql = ms.getBoundSql(parameter);//为本次查询创建缓存的KeyCacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);return query(ms, parameter, rowBounds, resultHandler, key, boundSql);}//进入query的重载方法中
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());if (closed) {throw new ExecutorException("Executor was closed.");}if (queryStack == 0 && ms.isFlushCacheRequired()) {clearLocalCache();}List<E> list;try {queryStack++;list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;if (list != null) {handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);} else {// 如果缓存中没有本次查找的值，那么从数据库中查询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;}//从数据库查询
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 {// 查询的方法list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);} finally {localCache.removeObject(key);}// 将查询结果放入缓存localCache.putObject(key, list);if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter);}return list;}// SimpleExecutor中实现父类的doQuery抽象方法
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {Statement stmt = null;try {Configuration configuration = ms.getConfiguration();// 传入参数创建StatementHanlder对象来执行查询StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);// 创建jdbc中的statement对象stmt = prepareStatement(handler, ms.getStatementLog());// StatementHandler进行处理return handler.query(stmt, resultHandler);} finally {closeStatement(stmt);}}// 创建Statement的方法
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {Statement stmt;//条代码中的getConnection方法经过重重调用最后会调用openConnection方法，从连接池中获得连接。Connection connection = getConnection(statementLog);stmt = handler.prepare(connection, transaction.getTimeout());handler.parameterize(stmt);return stmt;}
//从连接池获得连接的方法
protected void openConnection() throws SQLException {if (log.isDebugEnabled()) {log.debug("Opening JDBC Connection");}//从连接池获得连接connection = dataSource.getConnection();if (level != null) {connection.setTransactionIsolation(level.getLevel());}setDesiredAutoCommit(autoCommit);}//进入StatementHandler进行处理的query，StatementHandler中默认的是PreparedStatementHandler
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {PreparedStatement ps = (PreparedStatement) statement;//原生jdbc的执行ps.execute();//处理结果返回。return resultSetHandler.handleResultSets(ps);}

接口方式

回顾一下写法：

public static void main(String[] args) {//前三步都相同InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = factory.openSession();//这里不再调用SqlSession 的api，而是获得了接口对象，调用接口中的方法。UserMapper mapper = sqlSession.getMapper(UserMapper.class);List<User> list = mapper.getUserByName("tom");
}

思考一个问题，通常的Mapper接口我们都没有实现的方法却可以使用，是为什么呢？答案很简单 动态代理

开始之前介绍一下MyBatis初始化时对接口的处理：MapperRegistry是Configuration中的一个属性，它内部维护一个HashMap用于存放mapper接口的工厂类，每个接口对应一个工厂类。mappers中可以配置接口的包路径，或者某个具体的接口类。

<!-- 将包内的映射器接口实现全部注册为映射器 -->
<mappers><mapper class="com.demo.mapper.UserMapper"/><package name="com.demo.mapper"/>
</mappers>

当解析mappers标签时，它会判断解析到的是mapper配置文件时，会再将对应配置文件中的增删改查标签一一封装成MappedStatement对象，存入mappedStatements中。（上文介绍了）
当判断解析到接口时，会创建此接口对应的MapperProxyFactory对象，存入HashMap中，key = 接口的字节码对象，value = 此接口对应的MapperProxyFactory对象。

//MapperRegistry类
public class MapperRegistry {private final Configuration config;//这个类中维护一个HashMap存放MapperProxyFactoryprivate final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();//解析到接口时添加接口工厂类的方法public <T> void addMapper(Class<T> type) {if (type.isInterface()) {if (hasMapper(type)) {throw new BindingException("Type " + type + " is already known to the MapperRegistry.");}boolean loadCompleted = false;try {//重点在这行，以接口类的class对象为key，value为其对应的工厂对象，构造方法中指定了接口对象knownMappers.put(type, new MapperProxyFactory<>(type));// It's important that the type is added before the parser is run// otherwise the binding may automatically be attempted by the// mapper parser. If the type is already known, it won't try.MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);parser.parse();loadCompleted = true;} finally {if (!loadCompleted) {knownMappers.remove(type);}}}}
}

正文：
进入sqlSession.getMapper(UserMapper.class)中

//DefaultSqlSession中的getMapper
public <T> T getMapper(Class<T> type) {return configuration.<T>getMapper(type, this);
}//configuration中的给getMapper
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {return mapperRegistry.getMapper(type, sqlSession);
}//MapperRegistry中的getMapper
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {//从MapperRegistry中的HashMap中拿MapperProxyFactoryfinal MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);if (mapperProxyFactory == null) {throw new BindingException("Type " + type + " is not known to the MapperRegistry.");}try {// 通过动态代理工厂生成示例。return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}
}//MapperProxyFactory类中的newInstance方法public T newInstance(SqlSession sqlSession) {// 创建了JDK动态代理的Handler类final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);// 调用了重载方法return newInstance(mapperProxy);}//MapperProxy类，实现了InvocationHandler接口
public class MapperProxy<T> implements InvocationHandler, Serializable {//省略部分源码	private final SqlSession sqlSession;private final Class<T> mapperInterface;private final Map<Method, MapperMethod> methodCache;// 构造，传入了SqlSession，说明每个session中的代理对象的不同的！public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {this.sqlSession = sqlSession;this.mapperInterface = mapperInterface;this.methodCache = methodCache;}//省略部分源码
}//重载的方法，由动态代理创建新示例返回。
protected T newInstance(MapperProxy<T> mapperProxy) {return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);}

在动态代理返回了示例后，我们就可以直接调用mapper类中的方法了，说明在MapperProxy中的invoke方法中已经为我们实现了方法。

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {try {//判断调用是是不是Object中定义的方法，toString，hashCode这类非。是的话直接放行。if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, args);} else if (isDefaultMethod(method)) {return invokeDefaultMethod(proxy, method, args);}} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);} final MapperMethod mapperMethod = cachedMapperMethod(method);// 重点在这：MapperMethod最终调用了执行的方法return mapperMethod.execute(sqlSession, args);}public Object execute(SqlSession sqlSession, Object[] args) {Object result;//判断mapper中的方法类型，最终调用的还是SqlSession中的方法switch (command.getType()) {case INSERT: {Object param = method.convertArgsToSqlCommandParam(args);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()) {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 {Object param = method.convertArgsToSqlCommandParam(args);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;}