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Why is WeakReference to a WeakRef object not recovered?

 1 year ago
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Why is WeakReference to a WeakRef object not recovered?

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While trying out some examples with WeakReferences,I just came across the below scenario. I am creating a hashmap and filled it with a weak reference on Employee object.Now I have two strong references, employee & weakReference . I have made both to null in the try block.Invoking the gc explicity, I get finally block run.

So after the gc run ,it is supposed to collect the weakReference object in the heap ,as there is no strong reference to it,But when I print the map it still has the key pointing to the old weakReference object.How is this possible?

            Employee employee = new Employee(11);
            WeakReference<Employee>weakReference=new WeakReference<Employee>(employee);
            Map map = new HashMap<WeakReference<Employee>, String>();
            map.put(weakReference, "Test");
            System.out.println(map);

        try {
            employee = null;
            weakReference=null;
            System.gc();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            System.out.println("Inside finally");
        }

        System.out.println(map);

And the ouput is

{[email protected]=Test}
Inside finally
{[email protected]=Test}


GC will not delete WeakReference itself, it will delete only the object it references, so map will still contain the WeakReference. What you should check is weakReference.get(). This should return null after gc(). Just in case, make a Thread.sleep(1000) before check, so that GC has some time to do its job


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