The spec for ThreadPoolExecutor requires poolSize to not drop below
corePoolSize due to timeouts.

But:

import java.util.*;
import java.util.concurrent.*;

public class TimeOutShrink {
    static void print(ThreadPoolExecutor pool) {
System.out.printf("poolSize=%d core=%d max=%d%n",
pool.getPoolSize(),
pool.getCorePoolSize(),
pool.getMaximumPoolSize());
    }

    public static void main(String[] args) throws Throwable {
final int n = 4;
final CyclicBarrier barrier = new CyclicBarrier(2*n+1);
final ThreadPoolExecutor pool
= new ThreadPoolExecutor(n, 2*n, 5L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
final Runnable r = new Runnable() { public void run() {
try {
barrier.await();
barrier.await();
} catch (Throwable t) { t.printStackTrace(); }}};

for (int i = 0; i < 2*n; i++)
pool.execute(r);
barrier.await();
print(pool);
barrier.await();
Thread.sleep(10*1000);
print(pool);
pool.shutdown();
pool.awaitTermination(Long.MAX_VALUE, TimeUnit.SECONDS);
    }
}

prints:

poolSize=8 core=4 max=8
poolSize=0 core=4 max=8

but should print

poolSize=8 core=4 max=8
poolSize=4 core=4 max=8