java遊戲編程(1-2)線程池

線程池顧名思義，它是執行所有任務的一組線程，也許你要限制同時網路連接或I/o串連所用的線程數，或要對處理器工作量很大的任務控制最大線程數。例子：

清單1.1ThreadPool。java

import java.util.LinkedList;

/**
    A thread pool is a group of a limited number of threads that
    are used to execute tasks.（線程池是一組線程，限制執行任務的線程數）
*/
public class ThreadPool extends ThreadGroup {

    private boolean isAlive;
    private LinkedList taskQueue;
    private int threadID;
    private static int threadPoolID;

    /**
        Creates a new ThreadPool.（建立新的線程)
        @param numThreads The number of threads in the pool.
    */
    public ThreadPool(int numThreads) {
        super("ThreadPool-" + (threadPoolID++));
        setDaemon(true);

        isAlive = true;

        taskQueue = new LinkedList();
        for (int i=0; i<numThreads; i++) {
            new PooledThread().start();
        }
    }

    /**
        Requests a new task to run. This method returns
        immediately, and the task executes on the next available
        idle thread in this ThreadPool.(請求新的任務。這個方法立即返回，任務在池中下一閑機線程中執行）
        <p>Tasks start execution in the order they are received.
        @param task The task to run. If null, no action is taken.
        @throws IllegalStateException if this ThreadPool is
        already closed.
    */
    public synchronized void runTask(Runnable task) {
        if (!isAlive) {
            throw new IllegalStateException();
        }
        if (task != null) {
            taskQueue.add(task);
            notify();
        }

    }

    protected synchronized Runnable getTask()
        throws InterruptedException
    {
        while (taskQueue.size() == 0) {
            if (!isAlive) {
                return null;
            }
            wait();
        }
        return (Runnable)taskQueue.removeFirst();
    }

    /**
        Closes this ThreadPool and returns immediately. All
        threads are stopped, and any waiting tasks are not
        executed. Once a ThreadPool is closed, no more tasks can
        be run on this ThreadPool.（關閉這個線程池並立即返回。所有線程停止，不執行任何等待的任務。關閉一個線程池之後，這個線程池不能再運行任何任務）
    */
    public synchronized void close() {
        if (isAlive) {
            isAlive = false;
            taskQueue.clear();
            interrupt();
        }
    }

    /**
        Closes this ThreadPool and waits for all running threads
        to finish. Any waiting tasks are executed.
    */
    public void join() {
        // notify all waiting threads that this ThreadPool is no
        // longer alive
        synchronized (this) {
            isAlive = false;
            notifyAll();
        }

        // wait for all threads to finish（這個線程並等待所有運行線程完成，執行任何等待的任務)
        Thread[] threads = new Thread[activeCount()];
        int count = enumerate(threads);
        for (int i=0; i<count; i++) {
            try {
                threads[i].join();
            }
            catch (InterruptedException ex) { }
        }
    }

    /**
        A PooledThread is a Thread in a ThreadPool group, designed
        to run tasks (Runnables).
    */
    private class PooledThread extends Thread {

        public PooledThread() {
            super(ThreadPool.this,
                "PooledThread-" + (threadID++));
        }

        public void run() {
            while (!isInterrupted()) {

                // get a task to run
                Runnable task = null;
                try {
                    task = getTask();
                }
                catch (InterruptedException ex) { }

                // if getTask() returned null or was interrupted,
                // close this thread by returning.
                if (task == null) {
                    return;
                }

                // run the task, and eat any exceptions it throws
                try {
                    task.run();
                }
                catch (Throwable t) {
                    uncaughtException(this, t);
                }
            }
        }
    }
}

現在介紹ThreadGroup.ThreadGroup包括一組線程和一些修改線程的方法，如下代碼中，使用ThreadGroup類的interrupt()方法用於在關閉線程池是停止等待所有等待的線程。下面用清單1.2 所示的ThreadPoolTest類測試ThreadPool對象。測試中，ThreadPoolTest會線上程池中啟動幾個線程，並指定幾個任務讓線程池去運行。因為程式要取得任務和線程數的變元，所以在測試時，要在命令列中輸入如下代碼：java ThreadPoolTest 8,4

清單1.2ThreadPoolTest.java

public class ThreadPoolTest {

    public static void main(String[] args) {
        if (args.length != 2) {
            System.out.println("Tests the ThreadPool task.");
            System.out.println(
                "Usage: java ThreadPoolTest numTasks numThreads");
            System.out.println(
                "  numTasks - integer: number of task to run.");
            System.out.println(
                "  numThreads - integer: number of threads " +
                "in the thread pool.");
            return;
        }
        int numTasks = Integer.parseInt(args[0]);
        int numThreads = Integer.parseInt(args[1]);

        // create the thread pool
        ThreadPool threadPool = new ThreadPool(numThreads);

        // run example tasks
        for (int i=0; i<numTasks; i++) {
            threadPool.runTask(createTask(i));
        }

        // close the pool and wait for all tasks to finish.
        threadPool.join();
    }

    /**
        Creates a simple Runnable that prints an ID, waits 500
        milliseconds, then prints the ID again.
    */
    private static Runnable createTask(final int taskID) {
        return new Runnable() {
            public void run() {
                System.out.println("Task " + taskID + ": start");

                // simulate a long-running task
                try {
                    Thread.sleep(500);
                }
                catch (InterruptedException ex) { }

                System.out.println("Task " + taskID + ": end");
            }
        };
    }

}
由於向控制台列印訊息幾乎不需要時間，所以ThreadPoolTest類通過休眠500毫秒來類比時間任務。