Re: A bit confused with SoftReference - when exactly is it collected?

From:

"Oliver Wong" <owong@castortech.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 13 Nov 2006 16:25:38 -0500

Message-ID:

<lp56h.81244$jG4.250577@wagner.videotron.net>

"Thomas Kellerer" <TAAXADSCBIXW@spammotel.com> wrote in message
news:4rs5jmFqhfslU1@mid.individual.net...

Hello,

I'm using a SoftReference to a StringBuffer to build up log messages that
are later displayed in a Swing GUI. As this might potentially grow but I
don't want the messages to be causing an OOME (as most of them will be
written into the log file anyway) I am storing the actual StringBuffer in
a SoftReference.

My understand was, that a SoftReference will not be collected until the
JVM runs out of memory. But it seems that when I run a lengthy task that
does create a lot of temporary objects, the soft reference is cleared even
though the available memory (max heap) is big enough.

The only explanation I have: the JVM will already collect SoftReferences
before even expanding the current heap (even if way below the -Xmx value),
or - even worse - just at will ;)

Can anybody shed some light on this?

The javadocs explain exactly what guarantees are and are not made with
regards to SoftReference. As usual for things intimately tied with the
garbage collector, a lot of its behaviour is implementation defined.

Currently this does not really help me with my problem. Is there anyway to
implement an object that *only* gets collected if the JVM cannot expand
the heap any more (-Xmx reached)?

Not to my knowledge. A simple fix for your design might be to strongly
store the last 10 log entries, and then softly store any entries beyond
that, which may or may not get reclaimed by the GC, depending on its mood.

- Oliver