线程池代码
import java.util.List;
import java.util.Vector;
public class ThreadPool
{
private static ThreadPool instance_ = null;
//定义优先级别常数，空闲的线程按照优先级不同分别存放在三个vector中
public static final int LOW_PRIORITY = 0;
public static final int NORMAL_PRIORITY = 1;
public static final int HIGH_PRIORITY = 2;
//保存空闲线程的List，或者说它是"池"
private List<PooledThread>[] idleThreads_;
private boolean shutDown_ = false;
private int threadCreationCounter_; //以创建的线程的个数
private boolean debug_ = false; //是否输出调试信息
//构造函数，因为这个类视作为singleton实现的，因此构造函数为私有
private ThreadPool()
{
// 产生空闲线程.三个vector分别存放分别处在三个优先级的线程的引用
List[] idleThreads = {new Vector(5), new Vector(5), new Vector(5)};
idleThreads_ = idleThreads;
threadCreationCounter_ = 0;
}
public int getCreatedThreadsCount() {
return threadCreationCounter_;
}
//通过这个函数得到线程池类的实例
public static ThreadPool instance() {
if (instance_ == null)
instance_ = new ThreadPool();
return instance_;
}
public boolean isDebug() {
return debug_;
}
//将线程repoolingThread从新放回到池中，这个方式是同步方法。
//这个方法会在多线程的环境中调用，设计这个方法的目的是让工作者线程
//在执行完target中的任务后，调用池类的repool()方法，
//将线程自身从新放回到池中。只所以这么做是因为线程池并不能预见到
//工作者线程何时会完成任务。参考PooledThread的相关代码。
protected synchronized void repool(PooledThread repoolingThread)
{
if (!shutDown_)
{
if (debug_)
{
System.out.println("ThreadPool.repool() : repooling ");
}
switch (repoolingThread.getPriority())
{
case Thread.MIN_PRIORITY :
{
idleThreads_[LOW_PRIORITY].add(repoolingThread);
break;
}
case Thread.NORM_PRIORITY :
{
idleThreads_[NORMAL_PRIORITY].add(repoolingThread);
break;
}
case Thread.MAX_PRIORITY :
{
idleThreads_[HIGH_PRIORITY].add(repoolingThread);
break;
}
default :
throw new IllegalStateException("Illegal priority found while repooling a Thread!");
}
notifyAll();//通知所有的线程
}
else
{
if (debug_)
{
System.out.println("ThreadPool.repool() : Destroying incoming thread.");
}
repoolingThread.shutDown();//关闭线程
}
if (debug_)
{
System.out.println("ThreadPool.recycle() : done.");
}
}
public void setDebug(boolean newDebug)
{
debug_ = newDebug;
}
//停止池中所有线程
public synchronized void shutdown()
{
shutDown_ = true;
if (debug_)
{
System.out.println("ThreadPool : shutting down ");
}
for (int prioIndex = 0; prioIndex <= HIGH_PRIORITY; prioIndex++)
{
List prioThreads = idleThreads_[prioIndex];
for (int threadIndex = 0; threadIndex < prioThreads.size(); threadIndex++)
{
PooledThread idleThread = (PooledThread) prioThreads.get(threadIndex);
idleThread.shutDown();
}
}
notifyAll();
if (debug_)
{
System.out.println("ThreadPool : shutdown done.");
}
}
//以Runnable为target，从池中选择一个优先级为priority的线程创建线程
//并让线程运行。
public synchronized void start(Runnable target, int priority)
{
PooledThread thread = null; //被选出来执行target的线程
List idleList = idleThreads_[priority];
if (idleList.size() > 0)
{
//如果池中相应优先级的线程有空闲的，那么从中取出一个
//设置它的target，并唤醒它
//从空闲的线程队列中获取
int lastIndex = idleList.size() - 1;
thread = (PooledThread) idleList.get(lastIndex);
idleList.remove(lastIndex);
thread.setTarget(target);
}
//池中没有相应优先级的线程
else
{
threadCreationCounter_++;
// 创建新线程，
thread = new PooledThread(target, "PooledThread #" + threadCreationCounter_, this);
// 新线程放入池中
switch (priority)
{
case LOW_PRIORITY :
{
thread.setPriority(Thread.MIN_PRIORITY);
break;
}
case NORMAL_PRIORITY :
{
thread.setPriority(Thread.NORM_PRIORITY);
break;
}
case HIGH_PRIORITY :
{
thread.setPriority(Thread.MAX_PRIORITY);
break;
}
default :
{
thread.setPriority(Thread.NORM_PRIORITY);
break;
}
}
//启动这个线程
thread.start();
}
}
}
工作者线程代码：
public class PooledThread extends Thread
{
private ThreadPool pool_; // 池中线程需要知道自己所在的池
private Runnable target_; // 线程的任务
private boolean shutDown_ = false;
private boolean idle_ = false;//设置是否让线程处于等待状态
private PooledThread() {
super();
}
private PooledThread(Runnable target)
{
super(target); //初始化父类
}
private PooledThread(Runnable target, String name)
{
super(target, name);
}
public PooledThread(Runnable target, String name, ThreadPool pool)
{
super(name);
pool_ = pool;
target_ = target;
}
private PooledThread(String name)
{
super(name);//初始化父类
}
private PooledThread(ThreadGroup group, Runnable target)
{
super(group, target);
}
private PooledThread(ThreadGroup group, Runnable target, String name)
{
super(group, target, name);
}
private PooledThread(ThreadGroup group, String name)
{
super(group, name);
}
public java.lang.Runnable getTarget()
{
return target_;
}
public boolean isIdle()
{
return idle_;//返回当前的状态
}
//工作者线程与通常线程不同之处在于run()方法的不同。通常的线程，
//完成线程应该执行的代码后，自然退出，线程结束。
//虚拟机在线程结束后收回分配给线程的资源，线程对象被垃圾回收。]
//而这在池化的工作者线程中是应该避免的，否则线程池就失去了意义。
//作为可以被放入池中并重新利用的工作者线程，它的run()方法不应该结束，
//随意，在随后可以看到的实现中，run()方法执行完target对象的代码后，
//就将自身repool()，然后调用wait()方法，使自己睡眠而不是退出循环和run()。
//这就使线程池实现的要点。
public void run()
{
// 这个循环不能结束，除非池类要求线程结束
// 每一次循环都会执行一次池类分配给的任务target
while (!shutDown_)
{
idle_ = false;
if (target_ != null)
{
target_.run(); // 运行target中的代码
}
idle_ = true;
try
{
//线程通知池重新将自己放回到池中
pool_.repool(this); //
//进入池中后睡眠，等待被唤醒执行新的任务，
//这里是线程池中线程于普通线程的run()不同的地方。
synchronized (this)
{
wait();
}
}
catch (InterruptedException ie)
{
}
idle_ = false;
}
//循环这里不能结束，否则线程结束，资源被VM收回，
//就无法起到线程池的作用了
}
public synchronized void setTarget(java.lang.Runnable newTarget)
{//设置新的target，并唤醒睡眠中的线程
target_ = newTarget; // 新任务
notifyAll(); // 唤醒睡眠的线程
}
public synchronized void shutDown()
{
shutDown_ = true;
notifyAll();
}
}
测试代码：
public static void main(String[] args)
{
System.out.println("Testing ThreadPool ");
System.out.println("Creating ThreadPool ");
ThreadPool pool = ThreadPool.instance();
pool.setDebug(true);
class TestRunner implements Runnable
{
public int count = 0;
public void run()
{
System.out.println("Testrunner sleeping 5 seconds ");
//此方法使本线程睡眠5秒
synchronized (this)
{
try
{
wait(5000);//等待5秒时间
}
catch (InterruptedException ioe)
{
}
}
System.out.println("Testrunner leaving ");
count++;
}
}
System.out.println("Starting a new thread ");
TestRunner runner = new TestRunner();
pool.start(runner, pool.HIGH_PRIORITY);
System.out.println("count : " + runner.count);
System.out.println("Thread count : " + pool.getCreatedThreadsCount());
pool.shutdown();
}
}
结果
Testing ThreadPool 
Creating ThreadPool 
Starting a new thread Testrunner sleeping 5 seconds count : 0Thread count : 1ThreadPool : shutting down ThreadPool : shutdown done.Testrunner leaving ThreadPool.repool() : Destroying incoming thread.ThreadPool.recycle() : done.
