How to use `proxy` parameter of InvocationHandler#invoke(...) method?

Question

I have following code:

public class Test { public static void main(String[] args) { InvocationHandler ih = new MyHandler(); ClassLoader cl = Test.class.getClassLoader(); Class[] mapClass = {Map.class}; ((Map)Proxy.newProxyInstance(cl,mapClass,ih)).put("hello", 11); ((Map)Proxy.newProxyInstance(cl,mapClass,ih)).put("hi", 55); } } class MyHandler implements InvocationHandler { @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("\nInvoked method `" + method.getName() + "` args: " + Arrays.toString(args)); System.out.println(proxy.getClass()); //how to use proxy parameter? and what purposes it can be used? return 42; } }

Output:

Invoked method `put` args: [hello, 11] class com.sun.proxy.$Proxy0 Invoked method `put` args: [hi, 55] class com.sun.proxy.$Proxy0

Please tell me:
how can I use the proxy parameter? Non native methods calls yield stackoverflow error.
for what purposes it can be used?

The only use I can think of for them is implementing equals and hashCode — tgdavies
– tgdavies, Commented Aug 27, 2021 at 6:51
@tgdavies calling equals and hashcode methods on the proxy parameter gives java.lang.StackOverflowError. — the Hutt
– the Hutt, Commented Aug 27, 2021 at 7:02
I suppose the reason for the stackOverflow is : InvocationHandler: "When a method is invoked on a proxy instance, the method invocation is encoded and dispatched to the invoke method of its invocation handler.", so calling a method on the proxy from with the invoke-method invokes it again. — Hulk
– Hulk, Commented Aug 27, 2021 at 7:20

tgdavies · Accepted Answer · 2021-08-27 07:25:13Z

You can implement equals and hashCode (with the same semantics as the Object implementations) like this:

import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; import java.util.ArrayList; import java.util.List; public class ProxyEg { interface Foo { } public static void main(String[] args) { List<Foo> foos = new ArrayList<>(); InvocationHandler ih = new InvocationHandler() { @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { if (method.getName().equals("equals")) { return proxy == args[0]; } else if (method.getName().equals("hashCode")) { return System.identityHashCode(proxy); } return null; } }; for (int i = 0; i < 2; i++) { foos.add((Foo)Proxy.newProxyInstance(ClassLoader.getSystemClassLoader(),new Class[] {Foo.class}, ih)); } // with an 'empty' InvocationHandler all of these lines will throw UnsupportedOperationException System.out.println(foos.get(0).equals(foos.get(1))); System.out.println(foos.get(0).equals(foos.get(0))); System.out.println(foos.get(0).hashCode()); System.out.println(foos.get(1).hashCode()); } }

Thx! I think the instanceof operator can also be used, if we want to use same handler for proxies of various types.

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