Java: Is it possible to add / attach a proxy to a live object instance during runtime?

Question

I know Java supports proxies. I've been checking Javassist but not sure whether it can support the following trick:

public class Hello {

    public void hi() {
        System.out.println("hi");
    }
}

Hello hello = new Hello();

Hello proxyHello = createProxy(hello);
proxyHello.hi(); // method will be intercepted by a predefined proxy

Is it possible to do something like that?

Answer 1

What is available in the JDK only allows to create proxies that implement a set of interfaces, if you want to create a proxy of a Class, you will need to use Javassist but to be able to do it you need a constructor that is accessible from the class where you create your proxy.

Here is a way to implement what you need:

final Hello hello = new Hello() {
    public void hi() {
        System.out.println("Hello World");
    }
};
ProxyFactory factory = new ProxyFactory();
factory.setSuperclass(Hello.class);
MethodHandler handler = (self, m, proceed, args) -> {
    // This allows to proxy even non accessible methods, it could not be
    // needed depending on your context
    if (!m.isAccessible())
        m.setAccessible(true);
    return m.invoke(hello, args);
};
Hello proxy = (Hello)factory.create(new Class<?>[0], new Object[0], handler);
proxy.hi();

Output:

Hello World

In this example:

1. First I create my factory
2. Then I provide the super class
3. Then I define how to handle all method calls on the proxy (here I delegate everything to my live object)
4. Finally I create my proxy (here I use the default constructor to create the proxy).

More details here

Assuming that you have a constructor with one String argument you will need to modify the last line as next:

Hello proxy = (Hello)factory.create(
    new Class<?>[]{String.class}, new Object[]{"bar"}, handler
);

Here I call a constructor with one argument of type String and I provided bar as value.