How does "this" escape the constructor in Java?

Question

I've heard about this happening in non thread-safe code due to improperly constructed objects but I really don't have the concept down, even after reading about in in Goetz's book. I'd like to solidify my understanding of this code smell as I maybe doing it and not even realize it. Please provide code in your explanation to make it stick, thanks.

Answer 1

Example : in a constructor, you create an event listener inner class (it has an implicit reference to the current object), and register it to a list of listener.
=> So your object can be used by another thread, even though it did not finish executing its constructor.

     public class A {        private boolean isIt;       private String yesItIs;        public A() {         EventListener el = new EventListener() { ....};         StaticListeners.register(el);         isIt = true;         yesItIs = "yesItIs";       }      } 

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An additional problem that could happen later : the object A could be fully created, made available to all threads, use by another thread ... except that that thread could see the A instance as created, yesItIs with it "yesItIs" value, but not isIt! Believe it or not, this could happen ! What happen is:

=> synchronization is only half about blocking thread, the other half is about inter-thread visibility.

The reason for that Java choice is performance : inter-thread visibility would kill performance if all data would be shared with all threads, so only synchronized data is guaranteed to be shared...