JDK 自帶定時器 Timer 核心原理代碼

 主要邏輯: new 一個 Timer 後, timer  內部維護一個隊列 queue,並開啓一個死循環線程 從
queue 中取任務執行( 觸發時間到了就執行,未到就 wait ),timer.schedule( TimerTask ) 其實就是 將 任務體放入隊列中  ( ps: timer 運行多個 TimerTask 時只要一個沒有捕獲拋出的異常異常,則此 timer  便停止了,所以建議使用 ScheduledExecutorService 代替 Timer )

private void mainLoop() {
        while (true) {
            try {
                TimerTask task;
                boolean taskFired;
                synchronized(queue) {
                    // Wait for queue to become non-empty
                    while (queue.isEmpty() && newTasksMayBeScheduled){
                        // 任務隊列中沒有待調度的任務,just  wait!
                        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
                        // 觸發時間還沒到, 延遲 wait
                        queue.wait(executionTime - currentTime);
                    }
                }
                if (taskFired){  // Task fired; run it, holding no locks
                    // 觸發時間到了,執行任務體( 這裏的 task  即是 實現了 Runnable 接口的 TimerTask 任務體 )
                    task.run();
                }
            } catch(InterruptedException e) {
            }
        }
    }