理解线程池，自己实现一个线程池

线程池本质是一个生产者-消费者模式，一边维护一些线程执行任务，一边由主线程添加一些任务。现在我们抛弃源码中一些繁杂的状态判断，自己写一个线程池。

public class poolT {　　 //可能频繁增删任务，链表队列效率较高
    private final BlockingQueue<Runnable> workQueue = new LinkedBlockingQueue<Runnable>();
    private final HashSet<Work> workers = new HashSet<Work>();
    private static int num = 3;

    public poolT(int num) {
        this.num = num;
        for (int i = 0; i < num; i++) {
            Work w = new Work();
            w.start();
            workers.add(w);
        }
    }

    public void addWork(Runnable r) {
        workQueue.add(r);
    }

    public void close() throws Exception {

        while (!workQueue.isEmpty()) {
            Thread.sleep(500);
        }
        for (Work work : workers) {
            // 通知正在运行的结束
            work.setDrop();
            // 强制结束还在等待的
            if (work.getState() == Thread.State.WAITING) {
                work.interrupt();
            }
        }
        Thread.sleep(2000);
        for (Work work : workers) {
            System.out.println(work.getName() + "状态:" + work.getState());
        }
    }

    // 内部线程封装
    private class Work extends Thread {
        Runnable r = null;
        // 结束线程标志位
        private boolean hasRunning = true;

        public void setDrop() {
            this.hasRunning = false;
        }

        public void run() {
            try {
                while (hasRunning || !workQueue.isEmpty()) {
                    // 阻塞线程执行
                    r = workQueue.take();
                    if (r != null) {
                        r.run();
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) throws Exception {
        poolT p = new poolT(4);
        for (int i = 0; i < 2; i++) {
            Runnable newRun = new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(1000);
                        System.out.println(Thread.currentThread().getName() + "运行任务;");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            };
            p.addWork(newRun);
        }
        p.close();
        System.out.println("主程序完毕");
    }
}

这里面我使用了一个阻塞队列，当任务添加时，由队列随机选取一个空闲线程进行处理，没有任务时，进行阻塞。

当然也可以不用阻塞队列，不过需要自己进行同步

public class MyThreadPool {

    List<Runnable> taskList = new LinkedList<Runnable>();

    private List<MyThread> threadList = new LinkedList<MyThread>();

    private static MyThreadPool threadPool;

    public MyThreadPool(int num) {
        for (int i = 0; i < num; i++) {
            threadList.add(new MyThread());
        }
        for (MyThread thread : threadList) {
            thread.start();
        }
    }

      public void destroy() {
            while (!taskList.isEmpty()) {// 如果还有任务没执行完成，就先睡会吧
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            // 工作线程停止工作，且置为null
            for (MyThread thread : threadList) {
                thread.setDistroy();
            }
        }  

    public void execute(Runnable run) {

        synchronized (taskList) {
            taskList.add(run);
            taskList.notify();
        }
    }

    private class MyThread extends Thread {
        public boolean hasRun = true;

        private void setDistroy() {
            this.hasRun = false;
        }

        @Override
        public void run() {
            while (hasRun) {
                Runnable r = null;
                System.out.println(Thread.currentThread().getName() + "is running");
                synchronized (taskList) {
                    if (taskList.isEmpty() && hasRun) {
                        try {
                            taskList.wait(20);
                        } catch (InterruptedException e) {

                            e.printStackTrace();
                        }
                    } else {
                        r = taskList.remove(0);
                    }
                }
                if (r != null) {
                    r.run();
                }
            }
        }
    }

    public static void main(String[] args) throws Exception {
//         ExecutorService excutor=Executors.newFixedThreadPool(3);
        MyThreadPool pool =new MyThreadPool(4);
        pool.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(500);
                    System.out.println("任务一");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

            }
        });
        pool.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(500);
                    System.out.println("任务贰");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

            }
        });

        System.out.println("End");
        pool.destroy();
    }
}

参考：http://blog.csdn.net/hsuxu/article/details/8985931