java.util.Timer 中schedule和scheduleAtFixedRate的区别

最近项目中大量用到Timer 总结一下.

schedule安排指定的任务在指定的时间开始进行重复的固定延迟执行

scheduleAtFixedRate安排指定的任务从指定的延迟后开始进行重复的固定延迟执行

参考http://blog.csdn.net/gtuu0123/archive/2010/11/27/6040159.aspx 对schedule和scheduleAtFixedRate的分析.

但它仅仅分析了部分,查看JDK的源代码java.util.Timer分析以下几段:

    public void schedule(TimerTask task, Date firstTime, long period) {
        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, firstTime.getTime(), -period);
    }

 对比

    public void scheduleAtFixedRate(TimerTask task, long delay, long period) {
        if (delay < 0)
            throw new IllegalArgumentException("Negative delay.");
        if (period <= 0)
            throw new IllegalArgumentException("Non-positive period.");
        sched(task, System.currentTimeMillis()+delay, period);
    }

 其调用void java.util.Timer .sched(TimerTask task, long time, long period)  方法,区别仅仅在于period传入方式.

再分析以下代码:

 /**
     * The main timer loop.  (See class comment.)
     */
    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) {
            }
        }
    }

 以while(true)方式来调用task.run()方法.由此分析TimerTask中的run方法不存在同步的问题 ,如果执行时间超出设置时间,会直接再次执行run()而没有时间间隔!!!!