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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