简单的线程池实现

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为了节省系统在并发时不断创建和销毁线程所带来的额外开销，就需要引入线程池。线程池的基本思想是，线程使用完后并不销毁，而是一直处于等待状态，如果下一个任务进入，可以再使用这个线程执行，这样就减少了线程的创建和销毁。
首先是线程池的实现：

package com.mr.smart.future.simplethreadpool;

import java.util.List;
import java.util.Vector;

public class ThreadPool {
    private static ThreadPool instance = null;
    //空闲的线程队列
    private List<PThread> 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 getThreadCounter() {
        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(PThread repoolingThread) {
        if (!isShutDown) {
            idleThreads.add(repoolingThread);
        } else {
            repoolingThread.shutDown();
        }
    }

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

    //执行任务
    public synchronized void start(Runnable target) {
        PThread thread = null;
        if (idleThreads.size() > 0) {
            int lastIndex = idleThreads.size() - 1;
            thread = (PThread) idleThreads.get(lastIndex);
            idleThreads.remove(lastIndex);
            //立即执行这个任务
            thread.setTarget(target);
        } else {
            threadCounter++;
            thread = new PThread(target, "PThread #" + threadCounter, this);
            // 启动这个线程
            thread.start();
        }
    }


}

配合线程：

package com.mr.smart.future.simplethreadpool;

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

    //构造函数
    public PThread(Runnable target, String name, ThreadPool pool) {
        super(name);
        this.pool = pool;
        this.target = target;
    }

    public Runnable getTarget() {
        return target;
    }

    public boolean isIdle() {
        return isIdle;
    }

    @Override
    public void run() {
        //只要没有关闭，则一直不结束该线程
        while (!isShutDown) {
            isIdle = false;
            if (target != null) {
                //运行任务
                target.run();
            }
            //任务结束了，到闲置状态
            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 e) {

            }
            isIdle = false;
        }
    }

    public synchronized void setTarget(Runnable newTarget) {
        target = newTarget;
        notifyAll();
    }

    //关闭线程
    public synchronized void shutDown() {
        isShutDown = true;
        notifyAll();
    }
}