Java 动态代理 - class 实现多个接口

Question

I'm looking at some Java dynamic proxies examples. A basic one which simply intercepts a method call and add some print-out:

public interface IVehicle {
    public void forward();
}

public class Car implements IVehicle {
    private String name;

    public Car(String name) {this.name = name;}

    public void forward() {
        System.out.println("Car " + name + " forward");
    }
}

public class VehicleHandler implements InvocationHandler {
    private Car v;
    public VehicleHandler(Car v) {this.v = v;}
    public Object invoke(Object proxy, Method m, Object[] args) throws Throwable {
        System.out.println("Vehicle Handler: Invoking " + m.getName());
        return m.invoke(v, args);
    }
}

public class Client {
    public static void main(String[] args) {
        Car c = new Car("audi");
        ClassLoader cl = Car.class.getClassLoader();
        IVehicle v = (IVehicle) Proxy.newProxyInstance(cl,
            new Class[] {IVehicle.class}, new VehicleHandler(c));

        v.forward();
    }
}

This example works well and prints out handler and object messages. But then I'm thinking what if the Car class in above example implements multiple interfaces and I call all methods in all implemented interfaces?

I edited above example to (Changes are surrounded by **):

public interface IVehicle {
    public void forward();
}

**public interface IVehicle2 {
    public void forward2();
}**

public class Car implements IVehicle, **IVehicle2** {
    private String name;

    public Car(String name) {this.name = name;}

    public void forward() {
        System.out.println("Car " + name + " forward");
    }

    **public void forward2() {
        System.out.println("Car " + name + " forward2");
    }**
}

public class VehicleHandler implements InvocationHandler {
    private Car v;
    public VehicleHandler(Car v) {this.v = v;}
    public Object invoke(Object proxy, Method m, Object[] args) throws Throwable {
        System.out.println("Vehicle Handler: Invoking " + m.getName());
        return m.invoke(v, args);
    }
}

public class Client {
    public static void main(String[] args) {
        Car c = new Car("audi");
        ClassLoader cl = Car.class.getClassLoader();
        **Car** v = **(Car)** Proxy.newProxyInstance(cl,
            new Class[] {IVehicle.class, **IVehicle2.class**}, new VehicleHandler(c));

        v.forward();
        **v.forward2();**
    }
}

I got below error:

Exception in thread "main" java.lang.ClassCastException: class com.sun.proxy.$Proxy0 cannot be cast to class Car

My questions are: 1. why do I get above error? 2. How do I correctly do proxy if I want to call both forward() and forward2() in above example?

Thanks a lot!

Answer 1

You're implementing the Proxy class, ie Car at compile time. The idea of dynamic proxy is to have the 'proxy class' generated at runtime, ie the proxy class implementation at compile time is not needed.

In this code snippet

**Car** v = **(Car)** Proxy.newProxyInstance(cl,
        new Class[] {IVehicle.class, **IVehicle2.class**}, new VehicleHandler(c));

Have the proxy instance assigned to one of interface that it implements. Based on the interface it's assigned to, the corresponding method can be invoked on the proxy.

If you want a single proxy that can invoke either of the interface's method, then have a common interface that extends both IVehicle and IVehicle2 ; and then use the common interface as the proxy class, ie (assign proxy rcvd. through Proxy.newProxyInstance to the common interface.

Note: the Car class can be used for other purposes it may have, including it being the loader class. When using it as the proxy class, ie**Car** v = **(Car)** Proxy.newProxyInstance it's not a dynamic proxy class.