深入解析Java Timer源码机制

Java定时器Timer源码详解

Java中的Timer类用于在后台线程中安排任务执行，支持一次性或重复执行。以下从源码角度分析其实现原理。

Timer类核心结构

Timer类主要依赖两个核心组件：TaskQueue和TimerThread。TaskQueue是一个优先级队列，用于存储待执行的任务；TimerThread是执行任务的线程。

private final TaskQueue queue = new TaskQueue();
private final TimerThread thread = new TimerThread(queue);

任务调度流程

当调用schedule()或scheduleAtFixedRate()方法时，任务会被添加到TaskQueue中。队列根据任务的触发时间（nextExecutionTime）进行排序，确保最早执行的任务在队列头部。

private void sched(TimerTask task, long time, long period) {
    synchronized(queue) {
        if (!thread.newTasksMayBeScheduled)
            throw new IllegalStateException("Timer already cancelled");
        synchronized(task.lock) {
            task.nextExecutionTime = time;
            task.period = period;
            task.state = TimerTask.SCHEDULED;
        }
        queue.add(task);
        if (queue.getMin() == task)
            queue.notify();
    }
}

任务执行线程

TimerThread是Timer的核心执行线程，不断从队列中获取任务并执行。如果队列为空，线程会等待；如果获取到任务但未到执行时间，线程会休眠至执行时间。

private void mainLoop() {
    while (true) {
        try {
            TimerTask task;
            boolean taskFired;
            synchronized(queue) {
                while (queue.isEmpty() && newTasksMayBeScheduled)
                    queue.wait();
                if (queue.isEmpty())
                    break;
                long currentTime, executionTime;
                task = queue.getMin();
                synchronized(task.lock) {
                    if (task.state == TimerTask.CANCELLED) {
                        queue.removeMin();
                        continue;
                    }
                    currentTime = System.currentTimeMillis();
                    executionTime = task.nextExecutionTime;
                    if (taskFired = (executionTime<=currentTime)) {
                        if (task.period == 0) {
                            queue.removeMin();
                            task.state = TimerTask.EXECUTED;
                        } else {
                            queue.rescheduleMin(
                              task.period<0 ? currentTime   - task.period
                                            : executionTime + task.period);
                        }
                    }
                }
                if (!taskFired)
                    queue.wait(executionTime - currentTime);
            }
            if (taskFired)
                task.run();
        } catch(InterruptedException e) {}
    }
}

关键设计要点

线程安全：通过synchronized关键字确保队列操作和任务状态修改的原子性。
优先级队列：使用二叉堆实现的优先级队列，保证任务按时间顺序执行。
延迟控制：通过wait(timeout)实现精确的延迟执行，避免忙等待。

使用注意事项

单线程执行：所有任务在同一个线程中顺序执行，长时间任务会阻塞后续任务。
异常处理：任务抛出未捕获异常会导致线程终止，后续任务无法执行。
替代方案：Java 5+推荐使用ScheduledThreadPoolExecutor，提供更好的灵活性和错误处理。

通过分析源码可见，Timer的实现简洁但存在局限性，适合简单的定时任务场景。复杂场景建议使用更现代的调度框架。

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