ThreadPoolExecutor中是如何做到线程复用的?
咱们晓得,一个线程在创立的时候会指定一个线程工作,当执行完这个线程工作之后,线程主动销毁。然而线程池却能够复用线程,一个线程执行完线程工作后不销毁,继续执行另外一个线程工作。那么它是如何做到的?这得从addWorker()说起
addWorker()
先看上半局部addWorker()
private boolean addWorker(Runnable firstTask, boolean core) { retry: for (;;) { int c = ctl.get(); int rs = runStateOf(c); // 对边界设定的查看 if (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty())) return false; for (;;) { int wc = workerCountOf(c); if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize)) return false; if (compareAndIncrementWorkerCount(c)) break retry; c = ctl.get(); // Re-read ctl if (runStateOf(c) != rs) continue retry; // else CAS failed due to workerCount change; retry inner loop }}
retry:可能有些同学没用过,它只是一个标记,它的下一个标记就是for循环,在for循环外面调用continue/break再紧接着retry标记时,就示意从这个中央开始执行continue/break操作,但这不是咱们关注的重点。
从下面的代码,咱们能够看出,ThreadPoolExecutor在创立线程时,会将线程封装成工作线程worker,并放入工作线程组中,而后这个worker重复从阻塞队列中拿工作去执行。这个addWorker是excute办法中调用的
咱们接着看下半局部
private boolean addWorker(Runnable firstTask, boolean core) { // 上半局部 retry: for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. if (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty())) return false; for (;;) { int wc = workerCountOf(c); // core是ture,须要创立的线程为外围线程,则先判断以后线程是否大于外围线程 // 如果core是false,证实须要创立的是非核心线程,则先判断以后线程数是否大于总线程数 // 如果不小于,则返回false if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize)) return false; if (compareAndIncrementWorkerCount(c)) break retry; c = ctl.get(); // Re-read ctl if (runStateOf(c) != rs) continue retry; // else CAS failed due to workerCount change; retry inner loop } } // 下半局部 boolean workerStarted = false; boolean workerAdded = false; Worker w = null; try { // 创立worker对象 w = new Worker(firstTask); final Thread t = w.thread; if (t != null) { // 获取线程全局锁 final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { int rs = runStateOf(ctl.get()); // 判断线程池状态 if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) { if (t.isAlive()) // precheck that t is startable throw new IllegalThreadStateException(); // 将以后线程增加到线程组 workers.add(w); int s = workers.size(); // 如果线程组中的线程数大于最大线程池数 largestPoolSize赋值s if (s > largestPoolSize) largestPoolSize = s; // 增加胜利 workerAdded = true; } } finally { mainLock.unlock(); } // 增加胜利后执行线程 if (workerAdded) { t.start(); workerStarted = true; } } } finally { // 增加失败后执行 addWorkerFailed if (! workerStarted) addWorkerFailed(w); } return workerStarted; }
再看 addWorkerFailed(),与上边相同,相当于一个回滚操作,会移除失败的工作线程
private void addWorkerFailed(Worker w) { // 同样须要全局锁 final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { if (w != null) workers.remove(w); decrementWorkerCount(); tryTerminate(); } finally { mainLock.unlock(); } }
Worker
咱们接着看Worker对象
private final class Worker extends AbstractQueuedSynchronizer implements Runnable { /** * This class will never be serialized, but we provide a * serialVersionUID to suppress a javac warning. */ private static final long serialVersionUID = 6138294804551838833L; /** Thread this worker is running in. Null if factory fails. */ final Thread thread; /** Initial task to run. Possibly null. */ Runnable firstTask; /** Per-thread task counter */ volatile long completedTasks; Worker(Runnable firstTask) { setState(-1); // inhibit interrupts until runWorker this.firstTask = firstTask; this.thread = getThreadFactory().newThread(this); } /** Delegates main run loop to outer runWorker */ public void run() { runWorker(this); } //..... // 省略下边代码 }
Worker类实现了Runnable接口,所以Worker也是一个线程工作。在构造方法中,创立了一个线程,回过头想想addWorker()里为啥能够t.start()应该很分明了吧, 并且在构造方法中调用了线程工厂创立了一个线程实例,咱们上节讲过线程工厂。其实这也不是关注的重点,重点是这个runWorker()
final void runWorker(Worker w) { // 获取以后的线程实例 Thread wt = Thread.currentThread(); // 间接从第一个工作开始执行 Runnable task = w.firstTask; // 获取完之后把worker的firstTask置为null 避免下次获取到 w.firstTask = null; // 线程启动之后,通过unlock办法开释锁 w.unlock(); // allow interrupts // 线程异样退出时 为 true boolean completedAbruptly = true; try { // Worker执行firstTask或从workQueue中获取工作,直到工作为空 while (task != null || (task = getTask()) != null) { // 获取锁以避免在工作执行过程中产生中断 w.lock(); // 判断边界值 如果线程池中断 则中断线程 if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) && !wt.isInterrupted()) wt.interrupt(); try { // 相当于钩子办法 beforeExecute(wt, task); Throwable thrown = null; try { // 执行工作 task.run(); } catch (RuntimeException x) { thrown = x; throw x; } catch (Error x) { thrown = x; throw x; } catch (Throwable x) { thrown = x; throw new Error(x); } finally { afterExecute(task, thrown); } } finally { task = null; w.completedTasks++; w.unlock(); } } completedAbruptly = false; } finally { processWorkerExit(w, completedAbruptly); } }
首先去执行创立这个worker时就有的工作,当执行完这个工作后,worker的生命周期并没有完结,在while循环中,worker会一直地调用getTask办法从阻塞队列中获取工作而后调用task.run()执行工作,从而达到复用线程的目标。只有getTask办法不返回null,此线程就不会退出。
咱们接着看getTask()
private Runnable getTask() { boolean timedOut = false; // Did the last poll() time out? for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; } int wc = workerCountOf(c); boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; // 如果运行线程数超过了最大线程数,然而缓存队列曾经空了,这时递加worker数量。 // 如果有设置容许线程超时或者线程数量超过了外围线程数量,并且线程在规定工夫内均未poll到工作且队列为空则递加worker数量 if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) { if (compareAndDecrementWorkerCount(c)) return null; continue; } try { // 如果timed为true,则会调用workQueue的poll办法获取工作. // 超时工夫是keepAliveTime。如果超过keepAliveTime时长, // 如果timed为false, 则会调用workQueue的take办法阻塞在以后。 // 队列中有工作退出时,线程被唤醒,take办法返回工作,并执行。 Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }
大家有没有想过这里为啥要用take和poll,它们都是出队的操作,这么做有什么益处?
take & poll
咱们说take()办法会将外围线程阻塞挂起,这样一来它就不会占用太多的cpu资源,直到拿到Runnable 而后返回。
如果allowCoreThreadTimeOut设置为true,那么外围线程就会去调用poll办法,因为poll可能会返回null,所以这时候外围线程满足超时条件也会被销毁
非核心线程会workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS),如果超时还没有拿到,下一次循环判断compareAndDecrementWorkerCount就会返回null,Worker对象的run()办法循环体的判断为null,工作完结,而后线程被零碎回收 。
再回头看一下runWorker()是不是设计的很奇妙~
结束语
本节内容不是很好了解,想持续探讨的同学能够持续浏览它的源码,这部分内容理解一下就好,其实咱们从源码中能够看到大量的线程状态查看,代码写的很强壮,能够从中学习一下