java动态代理怎么实现

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Java 动态代理可以通过以下几种方式实现：

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();}
}

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。

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();}
}