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然而，創(chuàng )建和銷(xiāo)毀線(xiàn)程本身就有一定的開(kāi)銷(xiāo)，如果頻繁創(chuàng )建和銷(xiāo)毀線(xiàn)程，CPU和內存開(kāi)銷(xiāo)就不可忽略，垃圾收集器還必須負擔更多的工作。因此，線(xiàn)程池就是為了避免頻繁創(chuàng )建和銷(xiāo)毀線(xiàn)程。

每當服務(wù)器接受了一個(gè)新的請求后，服務(wù)器就從線(xiàn)程池中挑選一個(gè)等待的線(xiàn)程并執行請求處理。處理完畢后，線(xiàn)程并不結束，而是轉為阻塞狀態(tài)再次被放入線(xiàn)程池中。這樣就避免了頻繁創(chuàng )建和銷(xiāo)毀線(xiàn)程。

Worker Pattern實(shí)現了類(lèi)似線(xiàn)程池的功能。首先定義Task接口：

package com.crackj2ee.thread;
public interface Task {
    void execute();
}

線(xiàn)程將負責執行execute()方法。注意到任務(wù)是由子類(lèi)通過(guò)實(shí)現execute()方法實(shí)現的，線(xiàn)程本身并不知道自己執行的任務(wù)。它只負責運行一個(gè)耗時(shí)的execute()方法。

具體任務(wù)由子類(lèi)實(shí)現，我們定義了一個(gè)CalculateTask和一個(gè)TimerTask：

// CalculateTask.java
package com.crackj2ee.thread;
public class CalculateTask implements Task {
    private static int count = 0;
    private int num = count;
    public CalculateTask() {
        count++;
    }
    public void execute() {
        System.out.println("[CalculateTask " + num + "] start...");
        try {
            Thread.sleep(3000);
        }
        catch(InterruptedException ie) {}
        System.out.println("[CalculateTask " + num + "] done.");
    }
}

// TimerTask.java
package com.crackj2ee.thread;
public class TimerTask implements Task {
    private static int count = 0;
    private int num = count;
    public TimerTask() {
        count++;
    }
    public void execute() {
        System.out.println("[TimerTask " + num + "] start...");
        try {
            Thread.sleep(2000);
        }
        catch(InterruptedException ie) {}
        System.out.println("[TimerTask " + num + "] done.");
    }
}

以上任務(wù)均簡(jiǎn)單的sleep若干秒。

TaskQueue實(shí)現了一個(gè)隊列，客戶(hù)端可以將請求放入隊列，服務(wù)器線(xiàn)程可以從隊列中取出任務(wù)：

package com.crackj2ee.thread;
import java.util.*;
public class TaskQueue {
    private List queue = new LinkedList();
    public synchronized Task getTask() {
        while(queue.size()==0) {
            try {
                this.wait();
            }
            catch(InterruptedException ie) {
                return null;
            }
        }
        return (Task)queue.remove(0);
    }
    public synchronized void putTask(Task task) {
        queue.add(task);
        this.notifyAll();
    }
}

終于到了真正的WorkerThread，這是真正執行任務(wù)的服務(wù)器線(xiàn)程：

package com.crackj2ee.thread;
public class WorkerThread extends Thread {
    private static int count = 0;
    private boolean busy = false;
    private boolean stop = false;
    private TaskQueue queue;
    public WorkerThread(ThreadGroup group, TaskQueue queue) {
        super(group, "worker-" + count);
        count++;
        this.queue = queue;
    }
    public void shutdown() {
        stop = true;
        this.interrupt();
        try {
            this.join();
        }
        catch(InterruptedException ie) {}
    }
    public boolean isIdle() {
        return !busy;
    }
    public void run() {
        System.out.println(getName() + " start.");       
        while(!stop) {
            Task task = queue.getTask();
            if(task!=null) {
                busy = true;
                task.execute();
                busy = false;
            }
        }
        System.out.println(getName() + " end.");
    }
}

前面已經(jīng)講過(guò)，queue.getTask()是一個(gè)阻塞方法，服務(wù)器線(xiàn)程可能在此wait()一段時(shí)間。此外，WorkerThread還有一個(gè)shutdown方法，用于安全結束線(xiàn)程。

最后是ThreadPool，負責管理所有的服務(wù)器線(xiàn)程，還可以動(dòng)態(tài)增加和減少線(xiàn)程數：

package com.crackj2ee.thread;
import java.util.*;
public class ThreadPool extends ThreadGroup {
    private List threads = new LinkedList();
    private TaskQueue queue;
    public ThreadPool(TaskQueue queue) {
        super("Thread-Pool");
        this.queue = queue;
    }
    public synchronized void addWorkerThread() {
        Thread t = new WorkerThread(this, queue);
        threads.add(t);
        t.start();
    }
    public synchronized void removeWorkerThread() {
        if(threads.size()>0) {
            WorkerThread t = (WorkerThread)threads.remove(0);
            t.shutdown();
        }
    }
    public synchronized void currentStatus() {
        System.out.println("-----------------------------------------------");
        System.out.println("Thread count = " + threads.size());
        Iterator it = threads.iterator();
        while(it.hasNext()) {
            WorkerThread t = (WorkerThread)it.next();
            System.out.println(t.getName() + ": " + (t.isIdle() ? "idle" : "busy"));
        }
        System.out.println("-----------------------------------------------");
    }
}

currentStatus()方法是為了方便調試，打印出所有線(xiàn)程的當前狀態(tài)。

最后，Main負責完成main()方法：

package com.crackj2ee.thread;
public class Main {
    public static void main(String[] args) {
        TaskQueue queue = new TaskQueue();
        ThreadPool pool = new ThreadPool(queue);
        for(int i=0; i<10; i++) {
            queue.putTask(new CalculateTask());
            queue.putTask(new TimerTask());
        }
        pool.addWorkerThread();
        pool.addWorkerThread();
        doSleep(8000);
        pool.currentStatus();
        pool.addWorkerThread();
        pool.addWorkerThread();
        pool.addWorkerThread();
        pool.addWorkerThread();
        pool.addWorkerThread();
        doSleep(5000);
        pool.currentStatus();
    }
    private static void doSleep(long ms) {
        try {
            Thread.sleep(ms);
        }
        catch(InterruptedException ie) {}
    }
}

main()一開(kāi)始放入了20個(gè)Task，然后動(dòng)態(tài)添加了一些服務(wù)線(xiàn)程，并定期打印線(xiàn)程狀態(tài)，運行結果如下：

worker-0 start.
[CalculateTask 0] start...
worker-1 start.
[TimerTask 0] start...
[TimerTask 0] done.
[CalculateTask 1] start...
[CalculateTask 0] done.
[TimerTask 1] start...
[CalculateTask 1] done.
[CalculateTask 2] start...
[TimerTask 1] done.
[TimerTask 2] start...
[TimerTask 2] done.
[CalculateTask 3] start...
-----------------------------------------------
Thread count = 2
worker-0: busy
worker-1: busy
-----------------------------------------------
[CalculateTask 2] done.
[TimerTask 3] start...
worker-2 start.
[CalculateTask 4] start...
worker-3 start.
[TimerTask 4] start...
worker-4 start.
[CalculateTask 5] start...
worker-5 start.
[TimerTask 5] start...
worker-6 start.
[CalculateTask 6] start...
[CalculateTask 3] done.
[TimerTask 6] start...
[TimerTask 3] done.
[CalculateTask 7] start...
[TimerTask 4] done.
[TimerTask 7] start...
[TimerTask 5] done.
[CalculateTask 8] start...
[CalculateTask 4] done.
[TimerTask 8] start...
[CalculateTask 5] done.
[CalculateTask 9] start...
[CalculateTask 6] done.
[TimerTask 9] start...
[TimerTask 6] done.
[TimerTask 7] done.
-----------------------------------------------
Thread count = 7
worker-0: idle
worker-1: busy
worker-2: busy
worker-3: idle
worker-4: busy
worker-5: busy
worker-6: busy
-----------------------------------------------
[CalculateTask 7] done.
[CalculateTask 8] done.
[TimerTask 8] done.
[TimerTask 9] done.
[CalculateTask 9] done.

仔細觀(guān)察：一開(kāi)始只有兩個(gè)服務(wù)器線(xiàn)程，因此線(xiàn)程狀態(tài)都是忙，后來(lái)線(xiàn)程數增多，7個(gè)線(xiàn)程中的兩個(gè)狀態(tài)變成idle，說(shuō)明處于wait()狀態(tài)。

思考：本例的線(xiàn)程調度算法其實(shí)根本沒(méi)有，因為這個(gè)應用是圍繞TaskQueue設計的，不是以Thread Pool為中心設計的。因此，Task調度取決于TaskQueue的getTask()方法，你可以改進(jìn)這個(gè)方法，例如使用優(yōu)先隊列，使優(yōu)先級高的任務(wù)先被執行。

如果所有的服務(wù)器線(xiàn)程都處于busy狀態(tài)，則說(shuō)明任務(wù)繁忙，TaskQueue的隊列越來(lái)越長(cháng)，最終會(huì )導致服務(wù)器內存耗盡。因此，可以限制 TaskQueue的等待任務(wù)數，超過(guò)最大長(cháng)度就拒絕處理。許多Web服務(wù)器在用戶(hù)請求繁忙時(shí)就會(huì )拒絕用戶(hù)：HTTP 503 SERVICE UNAVAILABLE