如何确保这些对象被垃圾收集？

Question

I am currently writing a small server for an online game. It creates one "Server" object which handels connections and data transfer, and one "Game" object per created game session between two players.

The "Game" class extends "Thread", having an infinite loop in it's run() method.

Each "Game" object holds references to several other objects (the pieces on the board, so to say), and these objects themselves hold a reference to the corresponding "Game" object, because they need to communicate with each other on a regular basis.

Now, when a game has ended, and the "Game" object is no longer needed - what steps do I have to take to make sure that the "Game" object gets garbage collected?

My idea would be:

In the "Server" object: remove reference to the Game object in question.

In every object used by the Game object in question: remove (nullify?) reference to the Game object in question.

Break the infinite loop that is running the Game object's run() method.

Would this suffice, or are there other steps neccesary, or are some of these steps not neccesary?

Answer 1

This will suffice:

In the "Server" object: remove reference to the Game object in question.

If the game is not referenced from the root of your application, then it can be garbage collected, and all objects that it referenced as well (doesn't matter that they referenced the game back, since this whole graph of objects still isn't reachable from anywhere).

Answer 2

All you need to do is to remove the reference to the Game object in the Server . Even though Game has references to other objects and those objects to Game (forming cycles/circular references), they are still not reachable from the GC root and so will end up being garbage collected anyway.

Answer 3

Now, when a game has ended, and the "Game" object is no longer needed - what steps do I have to take to make sure that the "Game" object gets garbage collected?

So if your application really does not have any references to the objects in question, the the garbage collector will free them. The Game thread should finish (ie the run() method should return) so the Thread will be reaped. Then any objects that are help by the Game object will then in turn have their reference counters decremented allowing them to be freed.

That said, once and a while if you have a large amount of object bandwidth, the GC can be helped by setting fields in your Game object to be null to specifically remove the references in ownership objects. This is especially true, for example, with Android applications which are notoriously sensitive to GC periods since many old mobile devices are single core.

However, the only time you should need to specifically set the fields to be null is if you suspect that the garbage collector is falling behind and unable to reap the objects in a timely manner. This is somewhat rare.