Does assigning objects to null in Java impact garb

Question

Does assigning an unused object reference to null in Java improve the garbage collection process in any measurable way?

My experience with Java (and C#) has taught me that is often counter intuitive to try and outsmart the virtual machine or JIT compiler, but I've seen co-workers use this method and I am curious if this is a good practice to pick up or one of those voodoo programming superstitions?

Answer 1

At least in java, it's not voodoo programming at all. When you create an object in java using something like

Foo bar = new Foo();

you do two things: first, you create a reference to an object, and second, you create the Foo object itself. So long as that reference or another exists, the specific object can't be gc'd. however, when you assign null to that reference...

bar = null ;

and assuming nothing else has a reference to the object, it's freed and available for gc the next time the garbage collector passes by.

Answer 2

public class JavaMemory {
    private final int dataSize = (int) (Runtime.getRuntime().maxMemory() * 0.6);

    public void f() {
        {
            byte[] data = new byte[dataSize];
            //data = null;
        }

        byte[] data2 = new byte[dataSize];
    }

    public static void main(String[] args) {

        JavaMemory jmp = new JavaMemory();
        jmp.f();

    }

}

Above program throws OutOfMemoryError. If you uncomment data = null;, the OutOfMemoryError is solved. It is always good practice to set the unused variable to null

Answer 3

I was working on a video conferencing application one time and noticed a huge huge huge difference in performance when I took the time to null references as soon as I didn't need the object anymore. This was in 2003-2004 and I can only imagine the GC has gotten even smarter since. In my case I had hundreds of objects coming and going out of scope every second, so I noticed the GC when it kicked in periodically. However after I made it a point to null objects the GC stopped pausing my application.

So it depends on what your doing...

Answer 4

I assume the OP is referring to things like this:

private void Blah()
{
    MyObj a;
    MyObj b;

    try {
        a = new MyObj();
        b = new MyObj;

        // do real work
    } finally {
        a = null;
        b = null;
    }
}

In this case, wouldn't the VM mark them for GC as soon as they leave scope anyway?

Or, from another perspective, would explicitly setting the items to null cause them to get GC'd before they would if they just went out of scope? If so, the VM may spend time GC'ing the object when the memory isn't needed anyway, which would actually cause worse performance CPU usage wise because it would be GC'ing more earlier.

Answer 5

Yes.

From "The Pragmatic Programmer" p.292:

By setting a reference to NULL you reduce the number of pointers to the object by one ... (which will allow the garbage collector to remove it)

Answer 6

Good article is today's coding horror.

The way GC's work is by looking for objects that do not have any pointers to them, the area of their search is heap/stack and any other spaces they have. So if you set a variable to null, the actual object is now not pointed by anyone, and hence could be GC'd.

But since the GC might not run at that exact instant, you might not actually be buying yourself anything. But if your method is fairly long (in terms of execution time) it might be worth it since you will be increasing your chances of GC collecting that object.

The problem can also be complicated with code optimizations, if you never use the variable after you set it to null, it would be a safe optimization to remove the line that sets the value to null (one less instruction to execute). So you might not actually be getting any improvement.

So in summary, yes it can help, but it will not be deterministic.