【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();    }}