前言

上次讲了mybatis 中Mapper注入spring源码分析，实质就是如何将一个接口放入Spring工厂中，有放就有取，因此这次介绍的是mybatis是如何从Spring工厂中取出相应的Mapper。

源码分析

MapperRegistry：以Mapper.class为key，MapperProxyFactory对象为value，对外用于生产Mapper接口对应的代理对象。
MapperProxyFactory：作用生产对应的Mapper代理。
MapperProxy：实现了InvocationHandler 接口，代理类。
MapperFactoryBean：获得代理对象，将其提供给Spring。

image.png

现在我们以上图为标准，从MapperFactoryBean开始一一分析源码。

MapperFactoryBean

其实在上一节已经提示过，这个类很重要。

image.png

以下是MapperFactoryBean的源码，其实代码还是比较简单的，不过这个类是连接Spring和mybatis的桥梁，Spring通过调用这个类的getObject()方法实例化代理对象，然后将其注入到Mapper上。

// 这是一个FactoryBean类
public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {

  private Class<T> mapperInterface;

  public MapperFactoryBean() {
    
  }
  //Spring实例化这个类，就是通过这个构造函数，mapperInterface 就是Mapper接口的class对象
  // 参数就是上一张截图中的xxxxx.addGenericArgumentValue(xxxxx)
  public MapperFactoryBean(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  protected void checkDaoConfig() {
    super.checkDaoConfig();

    Configuration configuration = getSqlSession().getConfiguration();
    if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
      try {
        //configuration 底层调用MapperRegistry类将mapperInterface保存起来，以便调用
        //getObject() 方法的时候能够获取Mapper接口对应的代理类。
        configuration.addMapper(this.mapperInterface);
      } catch (Exception e) {
    }
  }
/**
* 获取Mapper接口对应的代理类对象，后面依赖Mapper接口注入的对象就是这个代理对象。
这个方法的调用是在AbstractBeanFactory对象里面。具体到方法是getObjectForBeanInstance
**/
  @Override
  public T getObject() throws Exception {
  // 生成代理对象的调用链路
  // SqlSession -->Configuration-->MapperRegistry-->MapperProxyFactory->MapperProxy
    return getSqlSession().getMapper(this.mapperInterface);
  }

  @Override
  public Class<T> getObjectType() {
    return this.mapperInterface;
  }
  @Override
  public boolean isSingleton() {
    return true;
  }
}

DefaultSqlSession

public class DefaultSqlSession implements SqlSession{
  private final Configuration configuration;
....
  @Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }
...
}

Configuration

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

MapperRegistry

 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);
    }
  }

MapperProxyFactory

两个newInstance方法都是生成代理对象。

public class MapperProxyFactory<T> {

  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

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

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

}

MapperProxy

了解下Java的动态代理就知道了。

public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;

  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      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);
    // 执行sql 操作。
    return mapperMethod.execute(sqlSession, args);
  }
}

总结

这节本质就是动态代理+FactoryBean实现接口代理能够依赖注入。理论将了两节，下一节将动手实现接口放入spring工厂，以及能够依赖注入。