java - Check if object can be fetched by garbage collector

Question

is there a way to check if an object can be fetched by the garbage collector?

Somewhere in my code I've got a reference to an object:

MyObject mo = myObject;

Then, via Eclipse Debugger, I get the objects memory location. Afterwards, I set the reference null:

mo = null;

Is there any way to check if the previously referenced object is now suitable for garbage collection or if there's somewhere another reference to it?

Thanks a lot,

Stefan

Answer 1

You cannot do this at runtime with an arbitrary object, and in fact it's not fully possible to do this deterministically. However, there are two options that may be suitable depending on your needs:

1. Take a heap dump after you set the reference to null, and then load it up in a heap analyzer tool such as jhat or a profiler that supports this. These tools should let you traverse the path from the GC roots and thus check if your object is still reachable or not.
2. Wrap the object in a PhantomReference with a given ReferenceQueue. When the reference is enqueued, you know that the object has been garbage collected. (Unfortunately, if the reference is unqueued it could be because the object is still reachable, or it could be because the GC just hasn't inspected the object yet. As with all GC-related questions, garbage collection is not a deterministic process!)

On the whole though, I agree that the best option is to be aware of memory leak issues and design your application to avoid them. If you do have a memory leak it should be obvious enough, and you can then focus your energies on finding the problem (again by dumping and analysing the heap for objects that are incorrectly reachable).

The steps above are relatively time-consuming, and shouldn't be something that you do after every change just to reassure yourself, but rather are tools you'd use to investigate a specific problem.