Accessing Weakly Referenced Objects

[Alec Lee]

I am not quite familiar with the proper way to use WeakReference. I got some objects referenced by java.lang.ref.WeakReference. Occasionally, I want to inspect those objects. But since weakly referenced objects would be gc'ed at any time, how can I make sure they are still there by the time I inspect them?

Say, I first check that the WeakReference still contains a non-null value returned by get(). Does that mean this value will still be non-null when I use it some time later? By calling get() would I now create another strong reference to the object? And, if yes, does it mean I must set null to any variable I used to store the reference obtained from the get()?

[Ireneusz Kordal]

If your program holds strong reference to the 'weak' object - this object wont be 'nulled' by GC.

Make a simple test:

  
public class Test {
   
    public static void main(String... a) {

        WeakReference<Integer> i1 = new WeakReference(new Integer(1));
        WeakReference<Integer> i2 = new WeakReference(new Integer(2));
        WeakReference<Integer> i3 = new WeakReference(new Integer(3));

        // get strong references
        Integer a1 = i2.get();
        Integer a2 = i2.get();
        Integer a3 = i2.get();

        // hold strong reference only to i2
        a1 = a3 = null;

        // Now fill the memory
        List list = new LinkedList();
        try{
            while(true){
                list.add(new Object[10000]);
            }
        }catch(OutOfMemoryError ex){
            list = null;
        }

        System.out.println("i1 = " + i1.get());
        System.out.println("i2 = " + i2.get());
        System.out.println("i2 = " + i3.get());
     }
}

result is:
i1 = null
i2 = 2
i2 = null

If you get it at one time and it is not null - if you do not hold strong reference to the object, this object can be nulled even 5 ms later.