java多线程设计模式Worker Thread（线程池）

想象一个场景，一个工厂在生产玩具，在一个车间里，有几个工人，每次生产部件准备好车间外的人就将部件放到车间的一个桌子上，工人每次做完一个玩具就从桌子上取部件。在这里，注意到，部件并不是直接交给工人的，另外一点，工人并不是做完一个部件就回家换个新人，后者在现实有点滑稽，但是在程序中却对应一个典型的线程使用方法：线程池。

所谓线程池，就是对线程的复用，当线程执行完任务之后就继续取其他任务执行，而不是销毁启动新线程执行其他任务。因为线程的启动对于系统性能开销比较大，所以这样对于系统性能的提高很有好处。

来个典型程序范例：

   首先是请求，即玩具的部件

public class Request {    private final String name;    private final int number;    private static final Random random = new Random();    public Request(String name, int number) {        this.name = name;        this.number = number;    }    public void execute(){        System.out.println(Thread.currentThread().getName()+"execute"+this);        try {            Thread.sleep(1000);        } catch (InterruptedException e) {            e.printStackTrace();        }    }    @Override    public String toString() {        return "Request from" + name + "No."+ number;    }

也就是拥有name和number并且execute的时候打印出字段的一个简单类。

然后是ClientThread，负责将请求放入RequestQueue中，即将部件放到桌子上。

public class ClientThread extends  Thread{    private final Channel channel;    private static final Random random = new Random();    public ClientThread(Channel channel,String name) {        super(name);        this.channel = channel;    }    @Override    public void run() {        for (int i = 0; true; i++) {            Request request = new Request(getName(),i);            channel.putRequest(request);            try {                Thread.sleep(random.nextInt(1000));            } catch (InterruptedException e) {                e.printStackTrace();            }        }    }}

Channel类，可以当做车间

public class Channel {    private static final int MAX_REQUEST = 100;    private final Request[] requestQueue;    private int tail;    private int head;    private int count;    private final WorkerThread[] thraedPool;    public Channel(int thraedSize) {        this.requestQueue = new Request[MAX_REQUEST];        head = 0;        tail = 0;        count = 0;        thraedPool = new WorkerThread[thraedSize];        for (int i = 0; i < thraedPool.length; i++) {            thraedPool[i] = new WorkerThread("Worker-"+ i,this);        }    }    public void startWorker(){        for (int i = 0; i < thraedPool.length; i++) {            thraedPool[i].start();        }    }    public synchronized void putRequest(Request request){        while (count >= requestQueue.length){                try {                    wait();                } catch (InterruptedException e) {                    e.printStackTrace();                }        }        requestQueue[tail] = request;        tail = (tail+1)%requestQueue.length;        count++;        notifyAll();    }    public synchronized Request takeRequest(){        while (count <= 0){            try {                wait();            } catch (InterruptedException e) {                e.printStackTrace();            }        }        Request request = requestQueue[head];        head = (head + 1)%requestQueue.length;        count--;        notifyAll();        return  request;    }}

Requestqueue可以当做桌子，是一个数量有限的请求队列。threadPool是一个工人线程的数组，这就是一个线程池。在这里提供了putRequest和takeRequest方法，分别是往请求队列放入请求和取出请，这里使用了上一篇博文讲到的生产者消费者模式 java多线程设计模式之消费者生产者模式。确保了WorkerThread和ClientThread之间可以友好合作。

工人线程：

public class WorkerThread extends Thread{    private final Channel channel;    public WorkerThread(String name,Channel channel) {        super(name);        this.channel = channel;    }    @Override    public void run() {        while (true){            Request request = channel.takeRequest();            request.execute();        }    }}

这里就是一个不断从请求队列中取出请求然后执行请求的过程，保证了工人线程的复用，并不会执行完一个请求任务就销毁。

最后是Main：

public class Main {    public static void main(String[] args){        Channel channel = new Channel(5);        channel.startWorker();        new ClientThread(channel,"A").start();        new ClientThread(channel,"B").start();        new ClientThread(channel,"C").start();    }}

结果：

Worker-4executeRequest fromBNo.4Worker-3executeRequest fromCNo.10Worker-2executeRequest fromANo.6Worker-1executeRequest fromBNo.5Worker-0executeRequest fromCNo.11Worker-4executeRequest fromCNo.12Worker-2executeRequest fromANo.7Worker-3executeRequest fromCNo.13Worker-1executeRequest fromBNo.6Worker-0executeRequest fromBNo.7Worker-2executeRequest fromANo.8Worker-3executeRequest fromCNo.15Worker-4executeRequest fromCNo.14

可以看出线程执行任务的线程就是WorkerThread1,2,3,4,5五个，它们不断执行来自ClientThread A,B,C的请求任务。