【多线程】一个简单的线程池Demo

首先介绍一下为什么要使用线程池？使用线程池的原因是因为线程的创建和销毁的代价是比较高的，而且是与业务无关的。一般情况下，我们会想着把CPU用在处理具体的业务上面，而不是用在辅助业务的创建和销毁上面，所以就有了线程池。

线程池类：

public class ThreadPool {
    private static ThreadPool instance = null;
    //空闲的线程队列
    private List<Worker> idleThreads;
    //已有的线程总数
    private int threadCounter;
    private boolean isShutDown = false;

    private ThreadPool(){
        this.idleThreads = new Vector(5);
        threadCounter = 0;
    }

    public int getCreatedThreadsCount(){
        return threadCounter;
    }

    //取得线程池的实例
    public synchronized static ThreadPool getInstance(){
        if(instance == null){
            instance = new ThreadPool();
        }
        return instance;
    }

    //将线程放入池中
    protected synchronized void repool(Worker repoolingThread){
        if(!isShutDown){
            idleThreads.add(repoolingThread);
        }else{
            //关闭线程
            repoolingThread.shutDown();
        }
    }

    //停止池中的所有线程
    public synchronized void shutdown(){
        isShutDown = true;
        for(int threadIndex = 0; threadIndex < idleThreads.size(); threadIndex++){
            Worker idleThread = (Worker)idleThreads.get(threadIndex);
            idleThread.shutDown();
        }
    }

    //执行任务
    public synchronized void start(Runnable target){
        Worker thread = null;
        //如果有空闲线程，则直接使用
        if(idleThreads.size() > 0){
            int lastIndex = idleThreads.size() - 1;
            thread = (Worker)idleThreads.get(lastIndex);
            idleThreads.remove(lastIndex);
            //立即执行这个任务
            thread.setTarget(target);
        }else{
            //没有空闲线程，则创建新线程
            threadCounter++;
            //创建新线程
            thread = new Worker(target, "PThread #"+threadCounter, this);
            //启动这个线程
            thread.start();
        }
    }
}

Worker类：

public class Worker extends Thread{
    //线程池
    private ThreadPool pool;
    //任务
    private Runnable target;
    private boolean isShutDown = false;
    private boolean isIdle = false;

    public Worker(Runnable target, String name, ThreadPool pool){
        super(name);
        this.pool = pool;
        this.target = target;
    }

    public Runnable getTarget(){
        return target;
    }

    public boolean isIdle(){
        return isIdle;
    }

    public void run(){
        while(!isShutDown){
            isIdle = false;
            if(target != null){
                //运行任务
                target.run();
            }
            //任务结束了
            isIdle = true;
            try{
                //该任务结束后，不关闭线程，而是放入线程池空闲队列
                pool.repool(this);
                synchronized(this){
                    //线程空闲，等待新的任务到来
                    wait();
                }
            }catch(InterruptedException ie){

            }
            isIdle = false;
        }
    }

    public synchronized void setTarget(java.lang.Runnable newTarget){
        target = newTarget;
        //设置了任务之后，通知run方法开始执行这个任务
        notifyAll();
    }

    public synchronized void shutDown(){
        isShutDown = true;
        notifyAll();
    }
}