Mybatis中sqlSession.getMapper背后的原理

在通过MyBatis操作数据库之前我们一定先通过Session对象获取指定Mappper接口的代理对象。如下代码所示：

public class UserMapper{@Select(value="SELECT * FROM user")public List<User> findAll();
}

public static void main(String [] args){Configuration configuration = new Configuration();configuration.addMapper(UserMapper.class);SqlSessionFactory sqlSessionFactory = new DefaultSqlSessionFactory(configuration);// 获取SqlSession对象try(SqlSession sqlSession = sqlSessionFactory.openSession();){UserMapper userMapper = sqlSession.getMapper(UserMapper.class);List<User> userList = userMapper.findAll();}catch(Exception e){//TODO 异常信息}
}

调用链

1.DefaultSqlSession.调用getMapper方法 -> 2.Configuration.调用getMapper方法 -> 3.MapperRegistry调用getMapper方法|||-------------------------------------------------------------------------------------||||-->4.判断knownMappers缓存是否已经解析过指定的Mapper接口,没解析过抛错(在调用addMapper方法时就将接口添加到缓存中并对接口中的方法进行解析).解析过创建代理对象public <T> T getMapper(Class<T> type, SqlSession sqlSession) {final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);if (Objects.isNull(mapperProxyFactory)){throw new BindingException("Type " + type + " is not known to the MapperRegistry.");}try {return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}}|||-->  5.创建代理对象,MapperProxy实现了InvocationHandler接口，使用JDK动态代理创建public T newInstance(SqlSession sqlSession) {final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);return newInstance(mapperProxy);}protected T newInstance(MapperProxy<T> mapperProxy) {// 用JDK自带的动态代理生成映射器return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy);}

注意第5代逻辑：

   // 已解析过的Mapper接口及对应的代理工厂缓存private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();public <T> T getMapper(Class<T> type, SqlSession sqlSession) {final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);if (Objects.isNull(mapperProxyFactory)) {throw new BindingException("Type " + type + " is not known to the MapperRegistry.");}try {return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}}

先从缓存中获取接口对应的代理工厂，并判断是否存储代理工厂。不存在时抛错，表示此接口没有注册过无法创建代理对象.
何时向缓存中添加接口及对应的代理工厂呢？如下代码：

  public <T> void addMapper(Class<T> type) {// mapper必须是接口！才会添加if (type.isInterface()) {if ( knownMappers.containsKey((type)) {// 如果重复添加了，报错throw new BindingException("Type " + type + " is already known to the MapperRegistry.");}boolean loadCompleted = false;try {// 将mapper接口放入缓存并为其创建独立的mapper接口代理对象工厂// 每个Mapper接口有一个独立的代理工厂knownMappers.put(type, new MapperProxyFactory<T>(type));// 在运行分析器之前添加类型非常重要// 否则，绑定可能会由// 映射器解析器。如果类型已知，则不会尝试。MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);// 解析mapper接口及相应的mapper.xml文件parser.parse();loadCompleted = true;} finally {// 如果加载过程中出现异常需要再将这个mapper从mybatis中删除,这种方式比较丑陋吧，难道是不得已而为之？if (!loadCompleted) {knownMappers.remove(type);}}}
}

在注册Mapper 接口时，会向缓存中添加接口对应的代理工厂，以便后面通过接口获取代理对象调用方法。