java我在使用jdk动态代理时遇到问题

Question

在使用SpringAOP和cglib之前，现在我替换了一个简单的示例。我发现执行方法sayHello1（）和sayHello2（）都会输出“之前”和“之后”哦，天哪，这很难，你明白我在说什么吗？我现在快发疯了。T.T

public interface HelloWorld {
    void sayHello1(String say);
    void sayHello2(String say);
}

public static class HelloWorldImpl implements HelloWorld {
    @Override
    public void sayHello1(String say) { System.out.println(say); }
    @Override
    public void sayHello2(String say) { System.out.println(say); }
}

public static class Invocation implements InvocationHandler {
    private final Object target;

    public Invocation(Object target) { this.target = target; }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("before..."); // TODO method before
        Object object = method.invoke(target, args);
        System.out.println("after..."); // TODO method after
        return object;
    }
}

public static void main(String[] args) {
    HelloWorld helloWorld = (HelloWorld) Proxy.newProxyInstance(
        ClassLoader.getSystemClassLoader(),
        new Class[] { HelloWorld.class },
        new Invocation(new HelloWorldImpl())
    );
    helloWorld.sayHello1("Hello World1 ...");
    helloWorld.sayHello2("Hello World2 ...");
}

Answer 1

你是说你想要这样的东西

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  System.out.println("Instrumenting " + method);
  Object result;
  switch (method.getName()) {
    case "sayHello1":
      System.out.println("before A");
      result = method.invoke(target, args);
      System.out.println("after A");
      break;
    case "sayHello2":
      System.out.println("before B");
      // Let's change the argument just for fun
      args[0] = "changed argument";
      result = method.invoke(target, args);
      System.out.println("after B");
      break;
    default:
      result = method.invoke(target, args);
  }
  return result;
}

这将产生以下控制台日志：

Instrumenting public abstract void de.scrum_master.spring.q62001911.HelloWorld.sayHello1(java.lang.String)
before A
Hello World1 ...
after A
Instrumenting public abstract void de.scrum_master.spring.q62001911.HelloWorld.sayHello2(java.lang.String)
before B
changed argument
after B

当然，您可以打印进一步的信息，或者通过参数类型区分同名的重载方法。试试这样

method.getParameterTypes();
method.getParameterCount();
method.getReturnType();

这无聊吗？没错，但还是很简单。这就是为什么AspectJ或Spring AOP及其优雅的切入点+建议模型更易于使用的原因，因为它们已经完成了工作，并且向您隐藏了内部的复杂性