Not EGC..... why?

[Vicken Karaoghlanian]

why gd is NOT eligible for GC?

class GarbageDump extends Thread {

public static void main(String [] args) {
GarbageDump gd = new GarbageDump();
gd.start();
gd = null;
// Eligible for collection?   //NO
}

public void run() {
int i = 0;
while(true) ++i;
}
}

[Jose Botella]

There are at least two reasons the make the thread not eligible for gc :
a) it is running
b) the thread object is referred from the ThreadGroup main
This code shows that even if a thread has finished a reference to it is still in the related thread group.

import java.util.*;
class GarbageDump extends Thread {
static Thread[] threads = new Thread[3];
public static void main(String [] args) throws InterruptedException {
GarbageDump gd = new GarbageDump();
gd.start();
gd = null;
        System.gc();
// Eligible for collection?   //NO
        Thread.currentThread().sleep(2000);
        printThreads(Thread.currentThread());
}
public void finalize() {
        System.out.println("finalizing");   
    }
public void run() {
System.out.println("running");
        printThreads(Thread.currentThread());
}
    private static void printThreads(Thread t) {
        t.getThreadGroup().enumerate(threads);
        System.out.println(Arrays.asList(threads));
    }
}
////////////////////////////////////////////////////////////////////
C:\java\test>java GarbageDump
running
[Thread[main,5,main], Thread[Thread-0,5,main], null]
[Thread[main,5,main], Thread[Thread-0,5,], null]

And this one shows that even nullifying the thread group, gd is not eligible for garbage collection. Presumably because is still referenced from somewhere else.

import java.util.*;
class GarbageDump extends Thread {
static Thread[] threads = new Thread[3];
public static void main(String [] args) throws InterruptedException {
ThreadGroup tg = new ThreadGroup("tg");
        new GarbageDump(tg, "gd").start();
        tg = null;
        System.gc();
// Eligible for collection?   //STILL NOT
        Thread.currentThread().sleep(2000);
        printThreads(Thread.currentThread());
}
public void finalize() {
        System.out.println("finalizing");   
    }
public void run() {
System.out.println("running");
        printThreads(Thread.currentThread());
}
    private static void printThreads(Thread t) {
        t.getThreadGroup().enumerate(threads);
        System.out.println(Arrays.asList(threads));
    }
    GarbageDump(ThreadGroup tg, String name) {
        super(tg, name);   
    }
}
//////////////////////////////////////////////////////////////////////////
C:\java\test>java GarbageDump
running
[Thread[gd,5,tg], null, null]
[Thread[main,5,main], null, null]

[ October 10, 2003: Message edited by: Jose Botella ]