mybatis - mapper.java 的创建

一. Mapper.java 创建过程

在前面注册 bean 的时候, 对beanClass 进行了替换, 为 MapperFactoryBean. 那么创建实例的时候, 会调用 MapperFactoryBean 的 getObject() 方法得到实例. 

  @Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }

  public SqlSession getSqlSession() {
    return this.sqlSession;
  }

这里的的 sqlSession 就是 前面在配置类中创建的 SqlSessionTemplate 实例.

所以 getMapper 调用的就是 SqlSessionTemplate 的 getMapper 方法. 具体调用过程有点曲折, 最后会调用  MapperRegistry 的 getMapper 方法

  // SqlSessionTemplate.java
  @Override
  public <T> T getMapper(Class<T> type) {
    return getConfiguration().getMapper(type, this);
  }

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  // Configuration.java
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

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  // MapperRegistry.java
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      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);
    }
  }

newInstance 调用的是  MapperProxyFactory 的方法:

  //MapperProxyFactory.java
  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }


  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

从这里可以看到, 对于 UserMapper.java 接口, 采用了 jdk代理 的方式来创建一个实例. 且代理处理类为:  MapperProxy