一. Mapper.java 創建過程

在前面註冊 bean 的時候, 對beanClass 進行了替換, 為 MapperFactoryBean. 那麼創建實例的時候, 會調用 MapperFactoryBean 的 getObject() 方法得到實例. 

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

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

這裡的的 sqlSession 就是 前面在配置類中創建的 SqlSessionTemplate 實例.

所以 getMapper 調用的就是 SqlSessionTemplate 的 getMapper 方法. 具體調用過程有點曲折, 最後會調用  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 調用的是  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);
  }

從這裡可以看到, 對於 UserMapper.java 介面, 採用了 jdk代理 的方式來創建一個實例. 且代理處理類為:  MapperProxy