java动态代理怎么实现

Java动态代理可以通过以下几种方式实现：

1. 使用java.lang.reflect.Proxy类：Proxy类提供了一个静态方法newProxyInstance()，可以通过传入目标对象的类加载器、目标对象实现的接口和InvocationHandler接口的实现类来创建代理对象。InvocationHandler接口的实现类负责处理代理对象的方法调用。代理对象在调用方法时，会把方法调用转发给InvocationHandler的invoke()方法，从而实现代理功能。

public interface Subject {
    void doSomething();
}

public class RealSubject implements Subject {
    public void doSomething() {
        System.out.println("RealSubject doSomething");
    }
}

public class MyInvocationHandler implements InvocationHandler {
    private Subject target;

    public MyInvocationHandler(Subject target) {
        this.target = target;
    }

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("Before method invoke");
        Object result = method.invoke(target, args);
        System.out.println("After method invoke");
        return result;
    }
}

public class Main {
    public static void main(String[] args) {
        Subject realSubject = new RealSubject();
        Subject proxySubject = (Subject) Proxy.newProxyInstance(realSubject.getClass().getClassLoader(),
                realSubject.getClass().getInterfaces(),
                new MyInvocationHandler(realSubject));
        proxySubject.doSomething();
    }
}

2. 使用CGLib库：CGLib是一个基于ASM框架的字节码生成库，它可以在运行时动态生成目标对象的子类，并覆盖其中的方法以实现代理功能。

public interface Subject {
    void doSomething();
}

public class RealSubject implements Subject {
    public void doSomething() {
        System.out.println("RealSubject doSomething");
    }
}

public class MyMethodInterceptor implements MethodInterceptor {
    public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
        System.out.println("Before method invoke");
        Object result = proxy.invokeSuper(obj, args);
        System.out.println("After method invoke");
        return result;
    }
}

public class Main {
    public static void main(String[] args) {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(RealSubject.class);
        enhancer.setCallback(new MyMethodInterceptor());
        Subject proxySubject = (Subject) enhancer.create();
        proxySubject.doSomething();
    }
}

需要注意的是，使用CGLib实现动态代理时，目标对象不能是final类或者包含final方法，因为CGLib是通过生成目标对象的子类来实现代理的。

3. 使用ByteBuddy库：ByteBuddy是一个轻量级的字节码操作库，它可以在运行时动态生成目标对象的子类，并覆盖其中的方法以实现代理功能。

public interface Subject {
    void doSomething();
}

public class RealSubject implements Subject {
    public void doSomething() {
        System.out.println("RealSubject doSomething");
    }
}

public class MyInterceptor implements MethodInterceptor {
    public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
        System.out.println("Before method invoke");
        Object result = proxy.invokeSuper(obj, args);
        System.out.println("After method invoke");
        return result;
    }
}

public class Main {
    public static void main(String[] args) throws Exception {
        Class<? extends Subject> proxyClass = new ByteBuddy()
                .subclass(Subject.class)
                .method(any()).intercept(MethodDelegation.to(MyInterceptor.class))
                .make()
                .load(RealSubject.class.getClassLoader())
                .getLoaded();
        Subject proxySubject = proxyClass.getDeclaredConstructor().newInstance();
        proxySubject.doSomething();
    }
}

需要注意的是，使用ByteBuddy实现动态代理时，需要引入net.bytebuddy:byte-buddy和net.bytebuddy:byte-buddy-agent两个依赖。

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