How does `this` reference to an outer class escape through publishing inner class instance?

Question

This was asked slightly differently earlier but asking for a yes/no answer but I\'m looking for the explanation that\'s missing from the book (Java Concurrency in Practice),

Answer 1

The key point here is that it is often easy to forget that an in-lined anonymous object still has a reference to its parent object and that's how this code fragment is exposing a not-yet-completely-initialised instance of itself.

Imagine EventSource.registerListener immediately calls EventLister.doSomething()! That doSomething will be called on an object whose parent this is incomplete.

public class ThisEscape {

    public ThisEscape(EventSource source) {
        // Calling a method
        source.registerListener(
                // With a new object
                new EventListener() {
                    // That even does something
                    public void onEvent(Event e) {
                        doSomething(e);
                    }
                });
        // While construction is still in progress.
    }
}

Doing it this way would plug the hole.

public class TheresNoEscape {

    public TheresNoEscape(EventSource source) {
        // Calling a method
        source.registerListener(
                // With a new object - that is static there is no escape.
                new MyEventListener());
    }

    private static class MyEventListener {

        // That even does something
        public void onEvent(Event e) {
            doSomething(e);
        }
    }
}