timer是基于PriorityQueue实现的(和PriorityBlockingQueue区别, 在于没有阻塞机制, 不是线程安全的), 优先级队列是堆数据结构的典型应用 默认情况下, 按照自然顺序(其实就是默认comparator的定义), 最小的元素排在堆头 当然也可以自己重新实现comparator接口, 比如timer就用reify重新实现了comparator接口
整个过程其实比较简单, 开个timer-thread, 不断check PriorityQueue里面时间最小的timer是否已经可以触发 如果可以, 就poll出来, 调用callback, 并sleep, 都很好理解
唯一需要说的是, 这里使用Semaphore, 信号量和lock相似, 都是用于互斥 不同在于, 信号量模拟资源管理, 所以不同于lock的排他, 信号量可以接收多个aquire(取决于配置) 另外一个比较大的区别, lock是解铃还须系铃人, 谁锁谁解, 而信号量无所谓, 任何线程都可以调用release, 或acquire 这里使用信号量, 是用于在cancel-timer时, 等待timer-thread结束
(defn cancel-timer [timer] (check-active! timer) (locking (:lock timer) (reset! (:active timer) false) (.interrupt (:timer-thread timer))) (.acquire (:cancel-notifier timer))) 因为cancel的过程就是将active置false, 然后就是调用acquire等待信号量cancel-notifier被释放 而timer-thread在线程结束前, 会release这个信号量(defnk mk-timer [:kill-fn (fn [& _] )] (let [queue (PriorityQueue. 10 (reify Comparator (compare [this o1 o2] (- (first o1) (first o2)) ) (equals [this obj] true ))) active (atom true) ;;标志位 lock (Object.) ;;创建lock对象, 由于PriorityQueue非线程安全, 所以使用locking来保证同时只有一个线程访问queue notifier (Semaphore. 0) ;;创建信号量, 初始为0 timer-thread (Thread. (fn [] (while @active (try ;;peek读但不从queue中取出, 先读出time看看, 符合条件再取出 (let [[time-secs _ _ :as elem] (locking lock (.peek queue))] (if (and elem (>= (current-time-secs) time-secs)) ;;无法保证恰好, 只要当前时间>=time-secs, 就可以执行, 可想而知对于afn必须不能耗时, 否则会影响其他timer ;; imperative to not run the function inside the timer lock ;; otherwise, it's possible to deadlock if function deals with other locks ;; (like the submit lock) (let [afn (locking lock (second (.poll queue)))] ;;poll从queue中取出 (afn)) ;;真正执行timer中的callback (Time/sleep 1000) )) (catch Throwable t ;; because the interrupted exception can be wrapped in a runtimeexception (when-not (exception-cause? InterruptedException t) (kill-fn t) (reset! active false) (throw t)) ))) (.release notifier)))] (.setDaemon timer-thread true) (.setPriority timer-thread Thread/MAX_PRIORITY) (.start timer-thread) {:timer-thread timer-thread :queue queue :active active :lock lock :cancel-notifier notifier}))
schedule其实就是往PriorityQueue里面插入timer
对于循环schdule, 就是在timer的callback里面, 再次schedule
(defnk schedule [timer delay-secs afn :check-active true] (when check-active (check-active! timer)) (let [id (uuid) ^PriorityQueue queue (:queue timer)] (locking (:lock timer) (.add queue [(+ (current-time-secs) delay-secs) afn id]) ))) (defn schedule-recurring [timer delay-secs recur-secs afn] (schedule timer delay-secs (fn this [] (afn) (schedule timer recur-secs this :check-active false)) ; this avoids a race condition with cancel-timer ))
Supervisor中的使用例子, 定期的调用hb函数更新supervisor的hb 在mk-timer时, 传入的kill-fn callback, 会在timer-thread发生exception的时候被调用
:timer (mk-timer :kill-fn (fn [t] (log-error t "Error when processing event") (halt-process! 20 "Error when processing an event") )) (schedule-recurring (:timer supervisor) 0 (conf SUPERVISOR-HEARTBEAT-FREQUENCY-SECS) heartbeat-fn) 本文章摘自博客园，原文发布日期：2013-07-02 相关资源：敏捷开发V1.0.pptx