garbage collection - Does setting Java objects to null do anything anymore?

Question

I was browsing some old books and found a copy of "Practical Java" by Peter Hagger. In the performance section, there is a recommendation to set object references to null when no longer needed.

In Java, does setting object references to null improve performance or garbage collection efficiency? If so, in what cases is this an issue? Container classes? Object composition? Anonymous inner classes?

I see this in code pretty often. Is this now obsolete programming advice or is it still useful?

Answer 1

It depends a bit on when you were thinking of nulling the reference.

If you have an object chain A->B->C, then once A is not reachable, A, B and C will all be eligible for garbage collection (assuming nothing else is referring to either B or C). There's no need, and never has been any need, to explicitly set references A->B or B->C to null, for example.

Apart from that, most of the time the issue doesn't really arise, because in reality you're dealing with objects in collections. You should generally always be thinking of removing objects from lists, maps etc by calling the appropiate remove() method.

The case where there used to be some advice to set references to null was specifically in a long scope where a memory-intensive object ceased to be used partway through the scope. For example:

{
  BigObject obj = ...
  doSomethingWith(obj);
  obj = null;             <-- explicitly set to null
  doSomethingElse();
}

The rationale here was that because obj is still in scope, then without the explicit nulling of the reference, it does not become garbage collectable until after the doSomethingElse() method completes. And this is the advice that probably no longer holds on modern JVMs: it turns out that the JIT compiler can work out at what point a given local object reference is no longer used.