（源码解析）mybatis调用链之获取对应mapper动态代理对象

调用SqlSession的getMapper方法，获取对应mapper。

这里以SqlSession实现类DefaultSqlSession为例，

DefaultSqlSession

......

@Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

......

调用Configuration的getMapper方法，

Configuration

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

从Configuration的mapper接口的动态代理注册中心MapperRegistry中获取，调用MapperRegistry的getMapper方法。

MapperRegistry

......

//记录了mapper接口与对应MapperProxyFactory之间的关系
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();
  
......

@SuppressWarnings("unchecked")
  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);
    }
  }
 ......

可以看出，getMapper方法从knownMappers中获取到MapperProxyFactory对象，调用MapperProxyFactory的newInstance方法。

MapperProxyFactory

//mapper接口的class对象
  private final Class<T> mapperInterface;
//key是mapper接口中的某个方法的method对象，value是对应的MapperMethod，MapperMethod对象不记录任何状态信息，所以它可以在多个代理对象之间共享
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();

......

public T newInstance(SqlSession sqlSession) {
     //每次调用都会创建新的MapperProxy对象
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

初始化MapperProxy实例对象，MapperProxy类实现了InvocationHandler接口，它是增强mapper接口的实现

MapperProxy

  private final SqlSession sqlSession;//记录关联的sqlsession对象
  private final Class<T> mapperInterface;//mapper接口对应的class对象；
//key是mapper接口中的某个方法的method对象，value是对应的MapperMethod，MapperMethod对象不记录任何状态信息，所以它可以在多个代理对象之间共享
  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())) {//如果是Object本身的方法不增强
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //从缓存中获取mapperMethod对象，如果缓存中没有，则创建一个，并添加到缓存中
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //调用execute方法执行sql
    return mapperMethod.execute(sqlSession, args);
  }
  
  ......

最终调用MapperProxyFactory 的 protected T newInstance(MapperProxy mapperProxy)方法，创建mapper接口的动态代理对象

MapperProxyFactory

@SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    //创建实现了mapper接口的动态代理对象
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }