Java一个简单的线程池实现

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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 : 
0
Thread count : 
1
ThreadPool : shutting down 

ThreadPool : shutdown done
.
Testrunner leaving  

ThreadPool
.repool() : Destroying incoming thread.
ThreadPool
.recycle() : done.