MyBatis原理分析（通俗易懂）

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); } 

1. 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
2. 由SqlSessionFactory创建SqlSession 对象，没有手动设置的话事务默认开启
3. 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 } 

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

//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， 
    
      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 first propertiesElement(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 #631 environmentsElement(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：

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SimpleExecutor return 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Executor final Executor executor = configuration.newExecutor(tx, execType); //返回的是DefaultSqlSession 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Key CacheKey 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 #601 deferredLoads.clear(); if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) { 
       // issue #482 clearLocalCache(); } } 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。

 
        <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MapperProxyFactory private 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); } } } } } 

//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MapperProxyFactory final 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; }