Java GC - is there a way to determine which objects collected

Question

I'm trying to monitor the gc activity in my app, using -verbosegc flag. I can see there are full and minor collections, but is there a way to determine (subscrbing to events/vm flags/whatever) which objects actually collected?

Thanks!

Answer 1

For general information about objects in memory I would suggest you look into jvisualvm (it is in the bin folder of your JDK). It has alot of useful information about what the VM is doing as your program runs, including information about the various objects, and memory state.

If you want something more specific you can use WeakReferences and ReferenceQueues. This option might be viable if you are only interested in objects of a few type. You can create a WeakReference to the objects as they are created with a common ReferenceQueue, and then have another thread check the Queue periodically (note that the queue only says the objects are reachable, not that they are actually collected):

static ReferenceQueue<MyObject> MY_QUEUE = new ReferenceQueue<MyObject>();
static class MyReference extends WeakReference<MyObject>{
  public final String name;
  public MyReference(MyObject o, ReferenceQueue<MyObject> q){
    super(o, q);
    name = o.toString();
  }
}

static{
  Thread t = new Thread(){
    public void run(){
      while(true){
        MyReference r = (MyReference)MY_QUEUE.remove();
        System.out.println(r.name+" eligible for collection");
      }
    }
  }
  t.setDaemon(true);
  t.start();
}

public MyObject(){
  //normal init...
  new MyReference(this, MY_QUEUE);
}