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Conclusions
As you can see, a thread pool is a useful way to limit the amount of threads that a Java program will be using. This article presented a complete thread pool that can be used with Java. This thread pool can easily become the starting point for any application that you create, which requires a thread pool.
Listing 1—The ThreadPool Class
package com.heaton.threads;
import java.util.*;
/**
* Java Thread Pool
*
* This is a thread pool that for Java, it is
* simple to use and gets the job done. This program and
* all supporting files are distributed under the Limited
* GNU Public License (LGPL, http://www.gnu.org).
*
* This is the main class for the thread pool. You should
* create an instance of this class and assign tasks to it.
*
* For more information visit http://www.jeffheaton.com.
*
* @author Jeff Heaton (http://www.jeffheaton.com)
* @version 1.0
*/
public class ThreadPool {
/**
* The threads in the pool.
*/
protected Thread threads[] = null;
/**
* The backlog of assignments, which are waiting
* for the thread pool.
*/
Collection assignments = new ArrayList(3);
/**
* A Done object that is used to track when the
* thread pool is done, that is has no more work
* to perform.
*/
protected Done done = new Done();
/**
* The constructor.
*
* @param size How many threads in the thread pool.
*/
public ThreadPool(int size)
{
threads = new WorkerThread[size];
for (int i=0;i<threads.length;i++) {
threads[i] = new WorkerThread(this);
threads[i].start();
}
}
/**
* Add a task to the thread pool. Any class
* which implements the Runnable interface
* may be assienged. When this task runs, its
* run method will be called.
*
* @param r An object that implements the Runnable interface
*/
public synchronized void assign(Runnable r)
{
done.workerBegin();
assignments.add(r);
notify();
}
/**
* Get a new work assignment.
*
* @return A new assignment
*/
public synchronized Runnable getAssignment()
{
try {
while ( !assignments.iterator().hasNext() )
wait();
Runnable r = (Runnable)assignments.iterator().next();
assignments.remove(r);
return r;
} catch (InterruptedException e) {
done.workerEnd();
return null;
}
}
/**
* Called to block the current thread until
* the thread pool has no more work.
*/
public void complete()
{
done.waitBegin();
done.waitDone();
}
protected void finalize()
{
done.reset();
for (int i=0;i<threads.length;i++) {
threads[i].interrupt();
done.workerBegin();
threads[i].destroy();
}
done.waitDone();
}
}
/**
* The worker threads that make up the thread pool.
*
* @author Jeff Heaton
* @version 1.0
*/
class WorkerThread extends Thread {
/**
* True if this thread is currently processing.
*/
public boolean busy;
/**
* The thread pool that this object belongs to.
*/
public ThreadPool owner;
/**
* The constructor.
*
* @param o the thread pool
*/
WorkerThread(ThreadPool o)
{
owner = o;
}
/**
* Scan for and execute tasks.
*/
public void run()
{
Runnable target = null;
do {
target = owner.getAssignment();
if (target!=null) {
target.run();
owner.done.workerEnd();
}
} while (target!=null);
}
}
Listing 2—The Done Class
package com.heaton.threads;
/**
*
* This is a thread pool for Java, it is
* simple to use and gets the job done. This program and
* all supporting files are distributed under the Limited
* GNU Public License (LGPL, http://www.gnu.org).
*
* This is a very simple object that
* allows the TheadPool to determine when
* it is done. This object implements
* a simple lock that the ThreadPool class
* can wait on to determine completion.
* Done is defined as the ThreadPool having
* no more work to complete.
*
* Copyright 2001 by Jeff Heaton
*
* @author Jeff Heaton (http://www.jeffheaton.com)
* @version 1.0
*/
public class Done {
/**
* The number of Worker object
* threads that are currently working
* on something.
*/
private int _activeThreads = 0;
/**
* This boolean keeps track of if
* the very first thread has started
* or not. This prevents this object
* from falsely reporting that the ThreadPool
* is done, just because the first thread
* has not yet started.
*/
private boolean _started = false;
/**
* This method can be called to block
* the current thread until the ThreadPool
* is done.
*/
synchronized public void waitDone()
{
try {
while ( _activeThreads>0 ) {
wait();
}
} catch ( InterruptedException e ) {
}
}
/**
* Called to wait for the first thread to
* start. Once this method returns the
* process has begun.
*/
synchronized public void waitBegin()
{
try {
while ( !_started ) {
wait();
}
} catch ( InterruptedException e ) {
}
}
/**
* Called by a Worker object
* to indicate that it has begun
* working on a workload.
*/
synchronized public void workerBegin()
{
_activeThreads++;
_started = true;
notify();
}
/**
* Called by a Worker object to
* indicate that it has completed a
* workload.
*/
synchronized public void workerEnd()
{
_activeThreads--;
notify();
}
/**
* Called to reset this object to
* its initial state.
*/
synchronized public void reset()
{
_activeThreads = 0;
}
}
Listing 3—The Example Worker Thread
import com.heaton.threads.*;
/**
* This class shows an example worker thread that can
* be used with the thread pool. It demonstrates the main
* points that should be included in any worker thread. Use
* this as a starting point for your own threads.
*
* @author Jeff Heaton (http://www.jeffheaton.com)
* @version 1.0
*/
public class TestWorkerThread implements Runnable {
static private int count = 0;
private int taskNumber;
protected Done done;
/**
*
* @param done
*/
TestWorkerThread()
{
count++;
taskNumber = count;
}
public void run()
{
for (int i=0;i<100;i+=2) {
System.out.println("Task number: " + taskNumber +
",percent complete = " + i );
try {
Thread.sleep((int)(Math.random()*500));
} catch (InterruptedException e) {
}
}
}
}
Listing 4—The ThreadPool Class
import com.heaton.threads.*;
/**
* Main class used to test the thread pool.
*
* @author Jeff Heaton (http://www.jeffheaton.com)
* @version 1.0
*/
public class TestThreadPool {
/**
* Main entry point.
*
* @param args No arguments are used.
*/
public static void main(String args[])
{
ThreadPool pool = new ThreadPool(10);
for (int i=1;i<25;i++) {
pool.assign(new TestWorkerThread());
}
pool.complete();
System.out.println("All tasks are done.");
}
}
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