Re: How to search for leaks

Lew <>
Thu, 15 Jan 2009 21:40:16 -0500
Lew wrote:

Avoid 'finalize()' except for cleanup of native memory that is outside
the ken of GC. This basically only comes up in JNI code. Otherwise,
'finalize()' is evil.

Mark Space wrote:

Isn't that what dispose() does, clean up memory allocated by the
(native) windowing system? I thought that's the memory that Knute
was talking about when he said his app would leak memory and blow up
after two and one half weeks.

John B. Matthews wrote:

In both Graphics and JDialog, finalize() calls dispose(). If you don't
call dispose() explicitly, the finalize() method should do it for you.


Only if the Java object becomes eligible for garbage collection. That could
be a long time coming.

IIUC, you may run out of memory in the interim. I've seen it described
as more of a clog than a leak.

Arne Vajh?j wrote:

I would try to avoid relying on finalizer if possible.

That's part of my point. You never can tell when or if a finalizer will run.
  Since the JVM does not monitor native memory, it will not know that there is
memory pressure to call the finalizers that release native memory.

The other part is not to add finalizers to one's own classes unless such
classes *explicitly* interact with JNI. The point of finalizers that work
with JNI is to free the native memory that has the same lifetime as the Java
object, so that it goes away when the Java object does.

With graphics components, there might be a whole lot of native memory in use
but not necessarily a whole lot of Java heap, so it could take GC a long time
to get around to freeing the objects and thus the native memory. Calling
'dispose()' explicitly gives native memory allocations a shorter lifetime than
the managed objects'.


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