//根據(jù)時間進行優(yōu)先排序的隊列    
private final TaskQueue queue = new TaskQueue();

//消費線程，對queue中的定時任務進行編排和執(zhí)行
private final TimerThread thread = new TimerThread(queue);

//構造函數(shù)
public Timer(String name) {
        thread.setName(name);
        thread.start();
}

//在當前時間往后delay個毫秒開始執(zhí)行
public void schedule(TimerTask task, long delay) {...}
//在指定的time時間點執(zhí)行
public void schedule(TimerTask task, Date time) {...}

//從當前時間開始delay個毫秒數(shù)開始定期執(zhí)行，周期是period個毫秒數(shù)
public void schedule(TimerTask task, long delay, long period) {...}
////從指定的firstTime開始定期執(zhí)行，往后每次執(zhí)行的周期是period個毫秒數(shù)
public void schedule(TimerTask task, Date firstTime, long period){...}

public static void main(String[] args) {
        TimerTask task1 = new DemoTimerTask("Task1");
        TimerTask task2 = new DemoTimerTask("Task2");
        Timer timer = new Timer();
        timer.schedule(task1, 1000, 5000);
        timer.schedule(task2, 1000, 5000);
}
    
static class DemoTimerTask extends TimerTask {
        private String taskName;
        private DateFormat df = new SimpleDateFormat("HH:mm:ss---");
        
        public DemoTimerTask(String taskName) {
            this.taskName = taskName;
        }
        
        @Override
        public void run() {
            System.out.println(df.format(new Date()) + taskName + " is working.");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            System.out.println(df.format(new Date()) + taskName + " finished work.");
        }
}

public static void main(String[] args) {
        TimerTask task1 = new DemoTimerTask("Task1");
        TimerTask task2 = new DemoTimerTask("Task2");
        
        Timer timer = new Timer();
        timer.scheduleAtFixedRate(task1, 1000, 5000);
        timer.scheduleAtFixedRate(task2, 1000, 5000);
}
    
static class DemoTimerTask extends TimerTask {
        private String taskName;
        private DateFormat df = new SimpleDateFormat("HH:mm:ss---");
        
        public DemoTimerTask(String taskName) {
            this.taskName = taskName;
        }
        
        @Override
        public void run() {
            System.out.println(df.format(new Date()) + taskName + " is working.");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            System.out.println(df.format(new Date()) + taskName + " finished work.");
        }
}

private void mainLoop() {
        while (true) {
            try {
                TimerTask task;
                boolean taskFired;
                synchronized(queue) {
                    // 如果queue里面沒有要執(zhí)行的任務，則掛起TimerThread線程
                    while (queue.isEmpty() && newTasksMayBeScheduled)
                        queue.wait();
                    // 如果TimerThread被激活，queue里面還是沒有任務，則介紹該線程的無限循環(huán)，不再接受新任務
                    if (queue.isEmpty())
                        break; 

                    long currentTime, executionTime;
                    // 獲取queue隊列里面下一個要執(zhí)行的任務（根據(jù)時間排序，也就是接下來最近要執(zhí)行的任務）
                    task = queue.getMin();
                    synchronized(task.lock) {
                        if (task.state == TimerTask.CANCELLED) {
                            queue.removeMin();
                            continue;  // No action required, poll queue again
                        }
                        currentTime = System.currentTimeMillis();
                        executionTime = task.nextExecutionTime;
                        // taskFired表示是否需要立刻執(zhí)行線程，當task的下次執(zhí)行時間到達當前時間點時為true
                        if (taskFired = (executionTime<=currentTime)) {
                            //task.period==0表示這個任務只需要執(zhí)行一次，這里就從queue里面刪掉了
                            if (task.period == 0) { 
                                queue.removeMin();
                                task.state = TimerTask.EXECUTED;
                            } else { // Repeating task, reschedule
                                //針對task.period不等于0的任務，則計算它的下次執(zhí)行時間點
                                //task.period<0表示是fixed delay模式的任務
                                //task.period>0表示是fixed rate模式的任務
                                queue.rescheduleMin(
                                  task.period<0 ? currentTime   - task.period
                                                : executionTime + task.period);
                            }
                        }
                    }
                    // 如果任務的下次執(zhí)行時間還沒有到達，則掛起TimerThread線程executionTime - currentTime毫秒數(shù)，到達執(zhí)行時間點再自動激活
                    if (!taskFired) 
                        queue.wait(executionTime - currentTime);
                }
                // 如果任務的下次執(zhí)行時間到了，則執(zhí)行任務
                // 注意：這里任務執(zhí)行沒有另起線程，還是在TimerThread線程執(zhí)行的，所以當有任務在同時執(zhí)行時會出現(xiàn)阻塞
                if (taskFired)  
                    // 這里沒有try catch異常，當TimerTask拋出異常會導致整個TimerThread跳出循環(huán)，從而導致Timer失效
                    task.run();
            } catch(InterruptedException e) {
            }
        }
}