Why doesn't an executor reuse a thread after completing your task?

This could happen if your task throws a RuntimeException.. then the Java Executor will not reuse that thread and would create a new thread. Consider the following example, if you comment the line "throw new RuntimeException("Runtime");" you will get the following output:

This is some thread : TG-ID-1
This is some thread : TG-ID-2
...
This is some thread : TG-ID-20
This is some thread : TG-ID-20
This is some thread : TG-ID-20
This is some thread : TG-ID-20
...
...
This is some thread : TG-ID-20
..
This is some thread : TG-ID-2
This is some thread : TG-ID-1

But if you leave the RuntimeException, your outpur will be:
This is some thread : TG-ID-1
...
This is some thread : TG-ID-71
This is some thread : TG-ID-72
This is some thread : TG-ID-73
...
etc..

import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicInteger;

public class TestExecutor {

    public static void main(String[] args) {
        new TestExecutor().test();
    }

    private void test() {
        BlockingQueue blockingQueue = new LinkedBlockingQueue();
        Executor executor = new ThreadPoolExecutor(
            20, 100, 5,
            TimeUnit.SECONDS,
            blockingQueue,
            new NativeThreadFactory(new ThreadGroup("My-T-grp"), "TG-ID"));

        for (int i=0; i<100; i++) {
            executor.execute(new Runnable() {
                public void run() {
                    System.out.println("This is some thread : " + Thread.currentThread().getName());
                    throw new RuntimeException("Runtime");
                }
            });
        }
    }


    public class NativeThreadFactory implements
        ThreadFactory {

        final ThreadGroup group;
        final AtomicInteger count;
        final String namePrefix;

        public NativeThreadFactory(final ThreadGroup group, final String namePrefix) {
            super();
            this.count = new AtomicInteger(1);
            this.group = group;
            this.namePrefix = namePrefix;
        }

        public Thread newThread(final Runnable runnable) {
            StringBuffer buffer = new StringBuffer();
            buffer.append(this.namePrefix);
            buffer.append('-');
            buffer.append(this.count.getAndIncrement());
            Thread t = new Thread(group, runnable, buffer.toString(), 0);
            t.setDaemon(false);
            t.setPriority(Thread.NORM_PRIORITY);
            return t;
        }

    }
}