GH-3118: MessagingGW: Don't proxy default methods

[artembilan]

Fixes #3118

The GatewayProxyFactoryBean proxies all the methods in the provided interface,
including default and static.
This is not what is expected from end-users.

• Proxy only abstract methods from the provided interface
• Introduce a DefaultMethodInvokingMethodInterceptor to call default
  method on the interface using a MethodHandle approach for those methods
  calling

[garyrussell]

While this generally makes sense; it's a severely breaking change and, at least, it needs to be an option at the class or method level.

Consider

public interfact Foo {

    String bar(String baz);

    default String qux(String fiz) {
        throw new UnsupportedOperationException();
    }

}

A user might reasonably expect to be able to override qux with an SI flow.

Furthermore, someone might already be using it that way.

[artembilan]

it's a severely breaking change

Oh! I didn't think it that way. "How an unusual behavior can be OK?!" 😄

So, proxyDeafultMethods on the <gateway> and @MessagingGateway be it!
Or if you would like to proxy only some specific one, you can use a @Gateway explicitly on that method.

Is this OK?

[garyrussell]

Yes; that would be sufficient (and false by default is ok too).

Do we really need all that MethodHandle complexity; we can't use normal reflection?

[artembilan]

we can't use normal reflection?

No, because there is no target for method call in case of proxy against just an interface.
I saw somewhere only a solution with creating a class as an implementation on the fly and then use it as a target for that default method call.

The MethodHandle complexity here is with the same reason Spring Data has: proxy an interface without any class implementation.
More over: pay attention to the docs change - we still are not good with JDK interfaces anyway.
Here is some article on the matter: https://rmannibucau.wordpress.com/2014/03/27/java-8-default-interface-methods-and-jdk-dynamic-proxies/
And SO on the matter: https://stackoverflow.com/questions/26206614/java8-dynamic-proxy-and-default-methods

[artembilan]

it needs to be an option at the class or method level.

After more thinking I treat this issue as a 5.3 theme: no one complained so far about default methods, plus it really may feel as an expected behavior to have those methods proxied as well.
So, changing behavior (even if it is kinda bug fix) should go to the next version.

[artembilan]

OK. Introduced a proxyDefaultMethods options and incorporated the change against 5.3-WIP branch.

Thanks