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今天就跟大家聊聊有关Java中线程池的执行原理是什么,可能很多人都不太了解,为了让大家更加了解,小编给大家总结了以下内容,希望大家根据这篇文章可以有所收获。
线程池状态
首先认识两个贯穿线程池代码的参数:
runState:线程池运行状态
workerCount:工作线程的数量
线程池用一个32位的int来同时保存runState和workerCount,其中高3位是runState,其余29位是workerCount。代码中会反复使用runStateOf和workerCountOf来获取runState和workerCount。
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); private static final int COUNT_BITS = Integer.SIZE - 3; private static final int CAPACITY = (1 << COUNT_BITS) - 1; // 线程池状态 private static final int RUNNING = -1 << COUNT_BITS; private static final int SHUTDOWN = 0 << COUNT_BITS; private static final int STOP = 1 << COUNT_BITS; private static final int TIDYING = 2 << COUNT_BITS; private static final int TERMINATED = 3 << COUNT_BITS; // ctl操作 private static int runStateOf(int c) { return c & ~CAPACITY; } private static int workerCountOf(int c) { return c & CAPACITY; } private static int ctlOf(int rs, int wc) { return rs | wc; }
RUNNING:可接收新任务,可执行等待队列里的任务
SHUTDOWN:不可接收新任务,可执行等待队列里的任务
STOP:不可接收新任务,不可执行等待队列里的任务,并且尝试终止所有在运行任务
TIDYING:所有任务已经终止,执行terminated()
TERMINATED:terminated()执行完成
线程池状态默认从RUNNING开始流转,到状态TERMINATED结束,中间不需要经过每一种状态,但不能让状态回退。下面是状态变化可能的路径和变化条件:
Worker的创建
线程池是由Worker类负责执行任务,Worker继承了AbstractQueuedSynchronizer,引出了Java并发框架的核心AQS。
AbstractQueuedSynchronizer,简称AQS,是Java并发包里一系列同步工具的基础实现,原理是根据状态位来控制线程的入队阻塞、出队唤醒来处理同步。
AQS不会在这里展开讨论,只需要知道Worker包装了Thread,由它去执行任务。
调用execute将会根据线程池的情况创建Worker,可以归纳出下图四种情况:
public void execute(Runnable command) { if (command == null) throw new NullPointerException(); int c = ctl.get(); //1 if (workerCountOf(c) < corePoolSize) { if (addWorker(command, true)) return; c = ctl.get(); } //2 if (isRunning(c) && workQueue.offer(command)) { int recheck = ctl.get(); if (! isRunning(recheck) && remove(command)) //3 reject(command); else if (workerCountOf(recheck) == 0) //4 addWorker(null, false); } //5 else if (!addWorker(command, false)) //6 reject(command); }
标记1对应第一种情况,要留意addWorker传入了core,core=true为corePoolSize,core=false为maximumPoolSize,
新增时需要检查workerCount是否超过允许的最大值。
标记2对应第二种情况,检查线程池是否在运行,并且将任务加入等待队列。标记3再检查一次线程池状态,如果线程池忽然处于非运行状态,那就将等待队列刚加的任务删掉,再交给RejectedExecutionHandler处理。标记4发现没有worker,就先补充一个空任务的worker。
标记5对应第三种情况,等待队列不能再添加任务了,调用addWorker添加一个去处理。
标记6对应第四种情况,addWorker的core传入false,返回调用失败,代表workerCount已经超出maximumPoolSize,那就交给RejectedExecutionHandler处理。
private boolean addWorker(Runnable firstTask, boolean core) { //1 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); 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 } } //2 boolean workerStarted = false; boolean workerAdded = false; Worker w = null; try { w = new Worker(firstTask); final Thread t = w.thread; if (t != null) { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { // Recheck while holding lock. // Back out on ThreadFactory failure or if // shut down before lock acquired. int rs = runStateOf(ctl.get()); if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) { if (t.isAlive()) // precheck that t is startable throw new IllegalThreadStateException(); workers.add(w); int s = workers.size(); if (s > largestPoolSize) largestPoolSize = s; workerAdded = true; } } finally { mainLock.unlock(); } if (workerAdded) { t.start(); workerStarted = true; } } } finally { if (! workerStarted) addWorkerFailed(w); } return workerStarted; }
标记1的第一段代码,目的很简单,是为workerCount加一。至于为什么代码写了这么长,是因为线程池的状态在不断
变化,并发环境下需要保证变量的同步性。外循环判断线程池状态、任务非空和队列非空,内循环使用CAS机制保证workerCount正确地递增。不了解CAS可以看认识非阻塞的同步机制CAS,后续增减workerCount都会使用CAS。
标记2的第二段代码,就比较简单。创建一个新Worker对象,将Worker添加进workers里(Set集合)。成功添加后,启动worker里的线程。在finally里判断线程是否启动成功,不成功直接调用addWorkerFailed。
private void addWorkerFailed(Worker w) { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { if (w != null) workers.remove(w); decrementWorkerCount(); tryTerminate(); } finally { mainLock.unlock(); } }
addWorkerFailed将减少已经递增的workerCount,并且调用tryTerminate结束线程池。
Worker的执行
Worker(Runnable firstTask) { setState(-1); // inhibit interrupts until runWorker this.firstTask = firstTask; this.thread = getThreadFactory().newThread(this); } public void run() { runWorker(this); }
Worker在构造函数里采用ThreadFactory创建Thread,在run方法里调用了runWorker,看来是真正执行任务的地方。
final void runWorker(Worker w) { Thread wt = Thread.currentThread(); Runnable task = w.firstTask; w.firstTask = null; w.unlock(); // allow interrupts boolean completedAbruptly = true; try { //1 while (task != null || (task = getTask()) != null) { w.lock(); //2 if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) && !wt.isInterrupted()) wt.interrupt(); try { //3 beforeExecute(wt, task); Throwable thrown = null; try { 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; //4 w.completedTasks++; w.unlock(); } } completedAbruptly = false; //5 } finally { //6 processWorkerExit(w, completedAbruptly); } }
标记1进入循环,从getTask获取要执行的任务,直到返回null。这里达到了线程复用的效果,让线程处理多个任务。
标记2是一个比较复杂的判断,保证了线程池在STOP状态下线程是中断的,非STOP状态下线程没有被中断。如果你不了解Java的中断机制,看如何正确结束Java线程这篇。
标记3调用了run方法,真正执行了任务。执行前后提供了beforeExecute和afterExecute两个方法,由子类实现。
标记4里的completedTasks统计worker执行了多少任务,最后累加进completedTaskCount变量,可以调用相应方法返回一些统计信息。
标记5的变量completedAbruptly表示worker是否异常终止,执行到这里代表执行正常,后续的方法需要这个变量。
标记6调用processWorkerExit结束,后面会分析。
接着来看worker从等待队列获取任务的getTask方法:
private Runnable getTask() { boolean timedOut = false; // Did the last poll() time out? for (;;) { int c = ctl.get(); int rs = runStateOf(c); //1 // Check if queue empty only if necessary. if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; } int wc = workerCountOf(c); //2 // Are workers subject to culling? boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) { if (compareAndDecrementWorkerCount(c)) return null; continue; } //3 try { Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }
标记1检查线程池的状态,这里就体现出SHUTDOWN和STOP的区别。如果线程池是SHUTDOWN状态,还会先处理完等待队列的任务;如果是STOP状态,就不再处理等待队列里的任务了。
标记2先看allowCoreThreadTimeOut这个变量,false时worker空闲,也不会结束;true时,如果worker空闲超过keepAliveTime,就会结束。接着是一个很复杂的判断,好难转成文字描述,自己看吧。注意一下wc>maximumPoolSize,出现这种可能是在运行中调用setMaximumPoolSize,还有wc>1,在等待队列非空时,至少保留一个worker。
标记3是从等待队列取任务的逻辑,根据timed分为等待keepAliveTime或者阻塞直到有任务。
最后来看结束worker需要执行的操作:
private void processWorkerExit(Worker w, boolean completedAbruptly) { //1 if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted decrementWorkerCount(); //2 final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { completedTaskCount += w.completedTasks; workers.remove(w); } finally { mainLock.unlock(); } //3 tryTerminate(); int c = ctl.get(); //4 if (runStateLessThan(c, STOP)) { if (!completedAbruptly) { int min = allowCoreThreadTimeOut ? 0 : corePoolSize; if (min == 0 && ! workQueue.isEmpty()) min = 1; if (workerCountOf(c) >= min) return; // replacement not needed } addWorker(null, false); } }
正常情况下,在getTask里就会将workerCount减一。标记1处用变量completedAbruptly判断worker是否异常退出,如果是,需要补充对workerCount的减一。
标记2将worker处理任务的数量累加到总数,并且在集合workers中去除。
标记3尝试终止线程池,后续会研究。
标记4处理线程池还是RUNNING或SHUTDOWN状态时,如果worker是异常结束,那么会直接addWorker。如果allowCoreThreadTimeOut=true,并且等待队列有任务,至少保留一个worker;如果allowCoreThreadTimeOut=false,workerCount不少于corePoolSize。
总结一下worker:线程池启动后,worker在池内创建,包装了提交的Runnable任务并执行,执行完就等待下一个任务,不再需要时就结束。
线程池的关闭
线程池的关闭不是一关了事,worker在池里处于不同状态,必须安排好worker的”后事”,才能真正释放线程池。ThreadPoolExecutor提供两种方法关闭线程池:
shutdown:不能再提交任务,已经提交的任务可继续运行;
shutdownNow:不能再提交任务,已经提交但未执行的任务不能运行,在运行的任务可继续运行,但会被中断,返回已经提交但未执行的任务。
public void shutdown() { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { checkShutdownAccess(); //1 安全策略机制 advanceRunState(SHUTDOWN); //2 interruptIdleWorkers(); //3 onShutdown(); //4 空方法,子类实现 } finally { mainLock.unlock(); } tryTerminate(); //5 }
shutdown将线程池切换到SHUTDOWN状态,并调用interruptIdleWorkers请求中断所有空闲的worker,最后调用tryTerminate尝试结束线程池。
public List<Runnable> shutdownNow() { List<Runnable> tasks; final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(STOP); interruptWorkers(); tasks = drainQueue(); //1 } finally { mainLock.unlock(); } tryTerminate(); return tasks; }
shutdownNow和shutdown类似,将线程池切换为STOP状态,中断目标是所有worker。drainQueue会将等待队列里未执行的任务返回。
interruptIdleWorkers和interruptWorkers实现原理都是遍历workers集合,中断条件符合的worker。
上面的代码多次出现调用tryTerminate,这是一个尝试将线程池切换到TERMINATED状态的方法。
final void tryTerminate() { for (;;) { int c = ctl.get(); //1 if (isRunning(c) || runStateAtLeast(c, TIDYING) || (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty())) return; //2 if (workerCountOf(c) != 0) { // Eligible to terminate interruptIdleWorkers(ONLY_ONE); return; } //3 final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) { try { terminated(); } finally { ctl.set(ctlOf(TERMINATED, 0)); termination.signalAll(); } return; } } finally { mainLock.unlock(); } // else retry on failed CAS } }
标记1检查线程池状态,下面几种情况,后续操作都没有必要,直接return。
RUNNING(还在运行,不能停)
TIDYING或TERMINATED(已经没有在运行的worker)
SHUTDOWN并且等待队列非空(执行完才能停)
标记2在worker非空的情况下又调用了interruptIdleWorkers,你可能疑惑在shutdown时已经调用过了,为什么又调用,而且每次只中断一个空闲worker?
你需要知道,shutdown时worker可能在执行中,执行完阻塞在队列的take,不知道要结束,所有要补充调用interruptIdleWorkers。每次只中断一个是因为processWorkerExit时,还会执行tryTerminate,自动中断下一个空闲的worker。
标记3是最终的状态切换。线程池会先进入TIDYING状态,再进入TERMINATED状态,中间提供了terminated这个空方法供子类实现。
调用关闭线程池方法后,需要等待线程池切换到TERMINATED状态。awaitTermination检查限定时间内线程池是否进入TERMINATED状态,代码如下:
public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException { long nanos = unit.toNanos(timeout); final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { for (;;) { if (runStateAtLeast(ctl.get(), TERMINATED)) return true; if (nanos <= 0) return false; nanos = termination.awaitNanos(nanos); } } finally { mainLock.unlock(); } }
看完上述内容,你们对Java中线程池的执行原理是什么有进一步的了解吗?如果还想了解更多知识或者相关内容,请关注亿速云行业资讯频道,感谢大家的支持。
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