Java任务调度Timer定时器怎么实现

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Timer任务调用相关类的UML图如下(仅包含部分变量与方法)：

使用时，需自己写一个类，继承TimerTask，重写run()方法，大致步骤如下：

1.Timer的成员变量thread初始化时会聚合queue

public class Timer {    /**     * The timer task queue.  This data structure is shared with the timer     * thread.  The timer produces tasks, via its various schedule calls,     * and the timer thread consumes, executing timer tasks as appropriate,     * and removing them from the queue when they're obsolete.     */    private final TaskQueue queue = new TaskQueue();    /**     * The timer thread.     */    private final TimerThread thread = new TimerThread(queue);

2.Timer构造方法有几个重载，最终会调用thread的start()方法，启动线程，调用run()方法

public Timer(String name) {    thread.setName(name);    thread.start();}

3.thread的run()方法核心逻辑为mainLoop()方法：如果queue中TimerTask[]为空 && newTasksMayBeScheduled = true，则queue.wait()进入阻塞状态

    private void mainLoop() {        while (true) {            try {                TimerTask task;                boolean taskFired;                synchronized(queue) {                    // Wait for queue to become non-empty                    while (queue.isEmpty() && newTasksMayBeScheduled)                        queue.wait();                    if (queue.isEmpty())                        break; // Queue is empty and will forever remain; die                    // Queue nonempty; look at first evt and do the right thing                    long currentTime, executionTime;                    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;                        if (taskFired = (executionTime<=currentTime)) {                            if (task.period == 0) { // Non-repeating, remove                                queue.removeMin();                                task.state = TimerTask.EXECUTED;                            } else { // Repeating task, reschedule                                queue.rescheduleMin(                                  task.period<0 ? currentTime   - task.period                                                : executionTime + task.period);                            }                        }                    }                    if (!taskFired) // Task hasn't yet fired; wait                        queue.wait(executionTime - currentTime);                }                if (taskFired)  // Task fired; run it, holding no locks                    task.run();            } catch(InterruptedException e) {            }        }    }

4.阻塞状态下，调用Timer类的schedule方法开启定时任务，schedule有几个重载，最终都是调用sched方法

5.sched方法中，queue.add(task),然后queue.notify()唤醒thread的线程

private void sched(TimerTask task, long time, long period) {    if (time < 0)        throw new IllegalArgumentException("Illegal execution time.");    // Constrain value of period sufficiently to prevent numeric    // overflow while still being effectively infinitely large.    if (Math.abs(period) > (Long.MAX_VALUE >> 1))        period >>= 1;    synchronized(queue) {        if (!thread.newTasksMayBeScheduled)            throw new IllegalStateException("Timer already cancelled.");        synchronized(task.lock) {            if (task.state != TimerTask.VIRGIN)                throw new IllegalStateException(                    "Task already scheduled or cancelled");            task.nextExecutionTime = time;            task.period = period;            task.state = TimerTask.SCHEDULED;        }        queue.add(task);        if (queue.getMin() == task)            queue.notify();    }}

6.therad的run()继续执行，进行一系列判断，最后调用TimerTask的run()方法，执行定时任务(见mainLoop方法)

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