FULL PRODUCT VERSION :
1.6.0
build 1.6.0-b105

ADDITIONAL OS VERSION INFORMATION :
ALL Windows (XP, 2003)

A DESCRIPTION OF THE PROBLEM :
Very high CPU load when decreasing the CorePoolSize of ThreadPoolExecutor.
Decreasing it at runtime through setCorePoolSize() .

It results from a change in the implementation of ThreadPoolExecutor.getTask() .

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run this simple code using JDK6:

package com.clearforest.bugs.jdk6;

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

public class CPUBugDemo
{
static private AtomicInteger sm_counter = new AtomicInteger(0);
static private ScheduledFuture[] sm_workers = null;
static private ScheduledThreadPoolExecutor sm_thrdPool = new ScheduledThreadPoolExecutor(10);

/**
* @param args
*/
public static void main(String[] args)
{

Runnable rnbl = new Runnable()
{
public void run()
{
Execute1();
}
};

sm_workers = new ScheduledFuture[10];
long lDelay = 100L;
for(int i = 0 ; i < sm_workers.length ; i++)
{
sm_workers[i] = sm_thrdPool.scheduleAtFixedRate(rnbl, lDelay, 1000L, TimeUnit.MILLISECONDS);
lDelay += (100L);
}

{
try
{
Thread.sleep(3000);
System.out.println("************************************************************");
// The next line will cause very high CPU load!
sm_thrdPool.setCorePoolSize(sm_thrdPool.getCorePoolSize() - 3);
}
catch(Exception ex)
{

}
}

}

static private void Execute1()
{
int tmpInt = sm_counter.getAndIncrement();
if( ( tmpInt % 10) == 0)
System.out.println(Long.toString(Thread.currentThread().getId()) + " ==> " + Integer.toString(tmpInt));
}
}


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No CPU load
ACTUAL -
Very high CPU load

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
package com.clearforest.bugs.jdk6;

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

public class CPUBugDemo
{
static private AtomicInteger sm_counter = new AtomicInteger(0);
static private ScheduledFuture[] sm_workers = null;
static private ScheduledThreadPoolExecutor sm_thrdPool = new ScheduledThreadPoolExecutor(10);

/**
* @param args
*/
public static void main(String[] args)
{

Runnable rnbl = new Runnable()
{
public void run()
{
Execute1();
}
};

sm_workers = new ScheduledFuture[10];
long lDelay = 100L;
for(int i = 0 ; i < sm_workers.length ; i++)
{
sm_workers[i] = sm_thrdPool.scheduleAtFixedRate(rnbl, lDelay, 1000L, TimeUnit.MILLISECONDS);
lDelay += (100L);
}

{
try
{
Thread.sleep(3000);
System.out.println("************************************************************");
// The next line will cause very high CPU load!
sm_thrdPool.setCorePoolSize(sm_thrdPool.getCorePoolSize() - 3);
}
catch(Exception ex)
{

}
}

}

static private void Execute1()
{
int tmpInt = sm_counter.getAndIncrement();
if( ( tmpInt % 10) == 0)
System.out.println(Long.toString(Thread.currentThread().getId()) + " ==> " + Integer.toString(tmpInt));
}
}

---------- END SOURCE ----------
-