java多线程设计模式(7) Worker Pattern （２）

最后，Main负责完成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()一开始放入了20个Task，然后动态添加了一些服务线程，并定期打印线程状态，运行结果如下：

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.

仔细观察：一开始只有两个服务器线程，因此线程状态都是忙，后来线程数增多，7个线程中的两个状态变成idle，说明处于wait()状态。

思考：本例的线程调度算法其实根本没有，因为这个应用是围绕TaskQueue设计的，不是以Thread Pool为中心设计的。因此，Task调度取决于TaskQueue的getTask()方法，你可以改进这个方法，例如使用优先队列，使优先级高的任务先被执行。

如果所有的服务器线程都处于busy状态，则说明任务繁忙，TaskQueue的队列越来越长，最终会导致服务器内存耗尽。因此，可以限制TaskQueue的等待任务数，超过最大长度就拒绝处理。许多Web服务器在用户请求繁忙时就会拒绝用户：HTTP 503 SERVICE UNAVAILABLE