JDK动态代理demo

直接上代码：

public interface ShoppingService {
    void shopping();
}

被代理类：

public class ShoppingServiceImpl implements ShoppingService {
    @Override
    public void shopping() {
        System.out.println("shopping……");
    }
}

public class MyHandler implements InvocationHandler {
    private Object target;

    MyHandler(Object target){
        this.target = target;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if(method.getName().equals("shopping")){
            System.out.println("before shoping……");
            Object result = method.invoke(target,args);		
            System.out.println("after shopping");
            return result;
        }
        return method.invoke(target,args);
    }
}

**Object result = method.invoke(target,args);**这个到后面应该就看得懂了。

public class Client {
    public static void main(String[] args) {
        ShoppingServiceImpl shoppingServiceImpl = new ShoppingServiceImpl();

        // 保存代理类的字节码 
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        
        ShoppingService proxy = (ShoppingService) Proxy.newProxyInstance(Client.class.getClassLoader(),shoppingServiceImpl.getClass().getInterfaces(),new MyHandler(shoppingServiceImpl));
        proxy.shopping();

    }
}

运行结果：

探究 Proxy.newProxyInstance方法：

public static Object newProxyInstance(ClassLoader loader,
                                          Class[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);
        final Class[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }
        
        Class cl = getProxyClass0(loader, intfs);
        
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
            final Constructor cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            return cons.newInstance(new Object[]{h});
        } catch (IllegalAccessException|InstantiationException e) {
            throw new InternalError(e.toString(), e);
        } catch (InvocationTargetException e) {
           
        } catch (NoSuchMethodException e) {
            throw new InternalError(e.toString(), e);
        }
    }

上面的代码被我删减了，主要看这几个：

在 checkProxyAccess方法中，进行参数验证在getProxyClass0方法中，生成代理类Class或者从proxyClassCache中找已生成过的代理类。通过getConstructor方法，获取生成的代理类的构造方法。通过newInstance方法，生成实例对象，也就是最终的代理对象==$Proxy0==。

代理类：

public final class $Proxy0 extends Proxy implements ShoppingService {
    private static Method m1;
    private static Method m2;
    private static Method m3;
    private static Method m0;

    public $Proxy0(InvocationHandler var1) throws  {
        super(var1);
    }
    public final void shopping() throws  {
        try {
            super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("com.blue.proxy.ShoppingService").getMethod("shopping");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        }
    }
}

  上面代码被我删了很多，主要看shopping方法，里面的super.h.invoke(this, m3, (Object[])null) 去到了父类InvocationHandler,父类的invoke方法早就被MyHandler重写了。

本人理解有限，只是介绍了一个简单的流程。