JAVA创建代理时的lang.ClassCastException

Question

我的目标是从class创建一个实例，它实现了一个interface并扩展了另一个class

。。。实体注释：

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.FIELD})
public @interface Entity {

    String visibileName();

}

。。。实现IEventDesignDialog

      public class EventDesignDialog implements IEventDesignDialog{

        private String show;
        private String dateAndTimeDisplayFormat;
        private String eventType;


        @Entity(visibileName = "Show")
        public String getShow() {
            return this.show;
        }

        @Entity(visibileName = "Date And Time display format")
        public String getDateAndTimeDisplayFormat() {
            return this.dateAndTimeDisplayFormat;
        }

        @Entity(visibileName = "Event Type")
        public String getEventType() {
            System.out.println("get event type method invokde successfully");
            return this.eventType;
        }
}

IEventDesignDialog接口：

public interface IEventDesignDialog extends IPage{

    public String getShow();

    public String getDateAndTimeDisplayFormat();

    public String getEventType();


}

IPage接口：

public interface IPage {

}

动态代理实现：

public class IPageProxy implements InvocationHandler {
    private List<Method> entityMethods;



    private Class <? extends IPage> screenClazz;

    public IPageProxy(final Class <? extends IPage> screenClazz) {
        entityMethods = new ArrayList<>();
        getEntityAnnotatedMethods(screenClazz);
        // Accept the real implementation to be proxied
        this.screenClazz = screenClazz;
    }


    /**
     * create an page instance
     * @param type
     * @param
     * @return
     * @throws InstantiationException
     * @throws IllegalAccessException
     */
    public static IPage getInstance(final Class<? extends IPage> type)
            throws InstantiationException, IllegalAccessException {

        List<Class<?>> interfaces = new ArrayList<>();
        interfaces.addAll(Arrays.asList(type.getInterfaces()));

        return (IPage) Proxy.newProxyInstance(
                type.getClassLoader(),
                findInterfaces(type),
                new IPageProxy(type)
             );

        /*return (IPage) Proxy.newProxyInstance(type.getClassLoader(),
               interfaces.toArray(new Class<?>[interfaces.size()])
                , new IPageProxy(type));*/
    }


    /**
     * get all methods that annotated with @Entity annotation
     * and add it for entityMethods array List
     * @param screenClazz
     */
    private void getEntityAnnotatedMethods(final Class <? extends IPage>  screenClazz) {
        // Scan each interface method for the specific annotation
        // and save each compatible method
        for (final Method m : screenClazz.getDeclaredMethods()) {
            if (m.isAnnotationPresent(Entity.class)) {
                entityMethods.add(m);
            }
        }
    }


    static Class<?>[] findInterfaces(final Class<? extends IPage> type) {
        Class<?> current = type;

        do {
            final Class<?>[] interfaces = current.getInterfaces();

            if (interfaces.length != 0) {
                return interfaces;
            }
        } while ((current = current.getSuperclass()) != Object.class);

        throw new UnsupportedOperationException("The type does not implement any interface");
    }



    @Override
    public Object invoke(
            final Object proxy,
            final Method method,
            final Object[] args) throws InvocationTargetException, IllegalAccessException {
        // A method on MyInterface has been called!
        // Check if we need to go call it directly or if we need to
        // execute something else before!


        if (entityMethods.contains(method)) {
            // The method exist in our to-be-proxied list
            // Execute something and the call it
            // ... some other things
            System.out.println("Something else");
        }

        // Invoke original method
        return method.invoke(screenClazz, args);
    }

}

主要类别：

public class Main {

    public static void main(String[] args) {
        try {

            ((EventDesignDialog)getInstance(EventDesignDialog.class)).getEventType();
        } catch (InstantiationException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }


    @SuppressWarnings("unchecked")
    public static <T extends IPage> T getInstance(final Class<? extends IPage> type) throws InstantiationException, IllegalAccessException {
        return (T) IPageProxy.getInstance(type);
    }

}

将引发以下异常：

  Exception in thread "main" java.lang.ClassCastException: com.sun.proxy.$Proxy2 cannot be cast to abc.EventDesignDialog
    at abc.Main.main(Main.java:8)

Answer 1

您正在扩展Screen，这意味着它不是interface
只有在层次结构中存在基interface时，动态代理才能工作

interfaces.size() == 0

因此，代理不能实现任何interface，显然它不是Screen层次结构的一部分

---

如果Screen是interface，那么您的方法仍然太复杂。这个

public static Screen getInstance(Class<? extends Screen> type)

这就足够了

---

您仍然会收到一个异常，因为

Class#getInterfaces

返回由这个类实现的interface

这意味着如果在EventDesignDialog.class上调用它，它将返回一个空数组
这意味着如果在EntityDesignDialog.class上调用它，它仍然会返回一个空数组
在Screen.class上调用它时，它将返回

[IPage.class]

你需要在层次结构中循环使用

Class#getSuperclass

直到你找到合适的interface

一个可能的实现可能看起来像

static Class<?>[] findInterfaces(final Class<?> type) {
    Class<?> current = type;

    do {
        final Class<?>[] interfaces = current.getInterfaces();

        if (interfaces.length != 0) {
            return interfaces;
        }
    } while ((current = current.getSuperclass()) != Object.class);

    throw new UnsupportedOperationException("The type does not implement any interface");
}

这意味着您需要将代码更改为

return (IPage) Proxy.newProxyInstance(
              type.getClassLoader(),
              findInterfaces(type),
              new IPageProxy(type)
           );

但是，由于您已经知道结果将是一个IPage代理，您可以

return (IPage) Proxy.newProxyInstance(
              type.getClassLoader(),
              new Class[] { IPage.class },
              new IPageProxy(type)
           );

---

这里

public static IPage getInstance(final Class<? extends IPage> type)

你正在返回一个IPage，但是在这里

((EventDesignDialog)getInstance(EventDesignDialog.class))

你试图把它下放到一个更具体的类型。这是不可能的，因为代理不是EventDesignDialog类型，但它只实现了IPage接口

由于动态代理是基于接口的，您将被迫处理接口
尝试强制转换为具体类总是会引发异常

如果你需要一个IEventDesignDialog，你需要一个新的代理