Trying to figure out when when weak reference starts to be removed

Question

I am reading about Weak References.
I am using the code to study from here. It is very simple.

private void doFunction() throws InterruptedException {  
        Map<Integer, String> map = new HashMap<Integer, String>();   
        myMap = new WeakReference<Map<Integer, String>>(map);  
        map = null;   
        int i = 0;    
        while (true) {    
            if (myMap != null && myMap.get() != null) {  
                myMap.get().put(i++, "test" + i);   
                System.out.println("im still working!!!!");    
            }
            else {   

                System.out.println("*******im free at:"+i+"*******");  
                Thread.sleep(5000);  
                if(myMap != null){  
                    System.out.println("*******myMap is not null*******");  
                }  
            }           

    }

I did not request a small heap size or any size via –Xms and –Xmx but was able to see the values removed from the cache when i == 15312.
So after 15312 objects in the Map the GC starts to remove entries.
My question: Isn't 15312 too low for a 32 bit machine with 4 GB of memory? I was expecting a much higher value before the references are starting to be removed.
Am I wrong on this? How can one evaluate at which points the objects will start to be removed?

You can try to guess...Simply put, it's good to know about weak and phantom references, but the idea of having GC is not to think too much about this stuff.. — hovanessyan, Jun 21 '12 at 08:20

score 2 · Answer 1 · answered Jun 21 '12 at 08:20

The point of the WeakReference is that you do not care when the VM will garbage collect the stuff. The VM will then do what it it considers is The Right Thing To Do in terms of the memory footprint & performance mix.

The fact the GC is starting to collect your objects after 15312 objects could merely be a coincidence, it'll also be dependent on how the GC is implemented and/or configured (a different version of the JVM might collect differently). Again, if you're using WeakReferences, you should expect your stuff will be collected at a random point in time, and efforts to try and control when this happens are futile.

So no, 15312 isn't "low for a 32 bit machine with 4GB of memory, since the VM could collect much earlier than "when the memory is full", or it could collect "never until the memory is full" - this is implementation & configuration dependent, and unless you're doing very specifically targeted performance optimization, in a very specific usage scenario, for a specific VM version on a specific architecture, time spent trying to change this is probably wasted.

`when the memory is full`.But a program has a virtual memory of 4GB.When is the memory full?I am trying to understand when the JVM thinks the memory is full.`15312` integers in the heap is not that many — Jim, Jun 21 '12 at 08:24
Make the sleep() shorter and you will see higher numbers. At least the Oracle JVM will collect weakly referenced objects more or less at the first opportunity it detects them. — Durandal, Jun 21 '12 at 13:10
@Jim: Your VM doesn't have 4GB of Heap if you don't force it. And the VM considers memory is full when you get an OutOfMemoryException. — Romain, Jun 22 '12 at 11:37

Trying to figure out when when weak reference starts to be removed

1 Answers1