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本篇內(nèi)容主要講解“Java的線程池的工作原理”,感興趣的朋友不妨來(lái)看看。本文介紹的方法操作簡(jiǎn)單快捷,實(shí)用性強(qiáng)。下面就讓小編來(lái)帶大家學(xué)習(xí)“Java的線程池的工作原理”吧!
相信大家都用過(guò)線程池,對(duì)該類ThreadPoolExecutor應(yīng)該一點(diǎn)都不陌生了;
我們之所以要用到線程池,線程池主要用來(lái)解決線程生命周期開(kāi)銷問(wèn)題和資源不足問(wèn)題;
我們通過(guò)對(duì)多個(gè)任務(wù)重用線程以及控制線程池的數(shù)目可以有效防止資源不足的情況;
本章節(jié)就著重和大家分享分析一下JDK8的ThreadPoolExecutor核心類,看看線程池是如何工作的;
線程池在內(nèi)部實(shí)際上構(gòu)建了一個(gè)生產(chǎn)者消費(fèi)者模型,將任務(wù)的提交和任務(wù)執(zhí)行兩者解耦,并不直接關(guān)聯(lián),從而良好的緩沖任務(wù),復(fù)用線程。
線程池的運(yùn)行主要分成兩部分:任務(wù)管理、線程管理。
任務(wù)管理充當(dāng)生產(chǎn)者的角色。
當(dāng)任務(wù)提交后,線程池會(huì)判斷該任務(wù)后續(xù)的流轉(zhuǎn):
直接申請(qǐng)線程執(zhí)行該任務(wù);
緩沖到隊(duì)列中等待線程執(zhí)行;
拒絕該任務(wù)。
線程管理是消費(fèi)者的角色
它們被統(tǒng)一維護(hù)在線程池內(nèi),根據(jù)任務(wù)請(qǐng)求進(jìn)行線程的分配
當(dāng)線程執(zhí)行完任務(wù)后則會(huì)繼續(xù)獲取新的任務(wù)去執(zhí)行,
最終當(dāng)線程獲取不到任務(wù)的時(shí)候,線程就會(huì)被回收。
接下來(lái),我們會(huì)按照以下三個(gè)部分去詳細(xì)講解線程池運(yùn)行機(jī)制:
線程池如何維護(hù)自身狀態(tài)。
線程池如何管理任務(wù)。
線程池如何管理線程。
// 構(gòu)造器一 public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), defaultHandler); } // 構(gòu)造器二 public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, defaultHandler); } // 構(gòu)造器三 public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), handler); } // 構(gòu)造器四 public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) { if (corePoolSize < 0 || maximumPoolSize <= 0 || maximumPoolSize < corePoolSize || keepAliveTime < 0) throw new IllegalArgumentException(); if (workQueue == null || threadFactory == null || handler == null) throw new NullPointerException(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler; }
通過(guò)仔細(xì)查看構(gòu)造器代碼,發(fā)現(xiàn)最終都是調(diào)用構(gòu)造器四,緊接著賦值了一堆的字段,接下來(lái)我們先看看這些字段是什么含義;
corePoolSize:核心運(yùn)行的線程池?cái)?shù)量大小,當(dāng)線程數(shù)量超過(guò)該值時(shí),就需要將超過(guò)該數(shù)量值的線程放到等待隊(duì)列中;
maximumPoolSize:線程池最大能容納的線程數(shù)(該數(shù)量已經(jīng)包含了corePoolSize數(shù)量),當(dāng)線程數(shù)量超過(guò)該值時(shí),則會(huì)拒絕執(zhí)行處理策略;
workQueue:等待隊(duì)列,當(dāng)達(dá)到corePoolSize的時(shí)候,就將新加入的線程追加到workQueue該等待隊(duì)列中;當(dāng)然BlockingQueue類也是一個(gè)抽象類,也有很多子類來(lái)實(shí)現(xiàn)不同的隊(duì)列等待, 一般來(lái)說(shuō),阻塞隊(duì)列有一下幾種:
ArrayBlockingQueue
LinkedBlockingQueue
SynchronousQueue
DelayedWorkQueue
ArrayBlockingQueue,PriorityBlockingQueue使用較少,一般使用LinkedBlockingQueue和Synchronous。
keepAliveTime:表示線程沒(méi)有任務(wù)執(zhí)行時(shí)最多保持多久存活時(shí)間。
默認(rèn)情況下當(dāng)線程數(shù)量大于corePoolSize后keepAliveTime才會(huì)起作用,并生效,一旦線程池的數(shù)量小于corePoolSize后keepAliveTime又不起作用了;
但是如果調(diào)用了 allowCoreThreadTimeOut(boolean) 方法,在線程池中的線程數(shù)不大于corePoolSize時(shí),keepAliveTime參數(shù)也會(huì)起作用,直到線程池中的線程數(shù)為0;
threadFactory:新創(chuàng)建線程出生的地方;
handler:拒絕執(zhí)行處理抽象類,就是說(shuō)當(dāng)線程池在一些場(chǎng)景中,不能處理新加入的線程任務(wù)時(shí),會(huì)通過(guò)該對(duì)象處理拒絕策略;
策略一( CallerRunsPolicy ):只要線程池沒(méi)關(guān)閉,就直接用調(diào)用者所在線程來(lái)運(yùn)行任務(wù);
策略二( AbortPolicy ):默認(rèn)策略,直接拋出RejectedExecutionException異常;
策略三( DiscardPolicy ):執(zhí)行空操作,什么也不干,拒絕任務(wù)后也不做任何回應(yīng);
策略四( DiscardOldestPolicy ):將隊(duì)列中存活最久的那個(gè)未執(zhí)行的任務(wù)拋棄掉,然后將當(dāng)前新的線程放進(jìn)去(LRU);
該對(duì)象RejectedExecutionHandler有四個(gè)實(shí)現(xiàn)類,即四種策略,讓我們有選擇性的在什么場(chǎng)景下該怎么使用拒絕策略;
largestPoolSize:變量記錄了線程池在整個(gè)生命周期中曾經(jīng)出現(xiàn)的最大線程個(gè)數(shù);
allowCoreThreadTimeOut:當(dāng)為true時(shí),核心線程也有超時(shí)退出的概念一說(shuō);
線程池運(yùn)行的狀態(tài),并不是用戶顯式設(shè)置的,而是伴隨著線程池的運(yùn)行,由內(nèi)部來(lái)維護(hù)。
線程池內(nèi)部使用一個(gè)變量維護(hù)兩個(gè)值:運(yùn)行狀態(tài)(runState)和線程數(shù)量 (workerCount),兩個(gè)關(guān)鍵參數(shù)的維護(hù)放在了一起。
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
ctl 這個(gè)AtomicInteger類型,是對(duì)線程池的運(yùn)行狀態(tài)和線程池中有效線程的數(shù)量進(jìn)行控制的一個(gè)字段。
原子變量值,是一個(gè)復(fù)核類型的成員變量,是一個(gè)原子整數(shù),借助高低位包裝了兩個(gè)概念,它同時(shí)包含兩部分的信息:線程池的運(yùn)行狀態(tài) (runState) 和線程池內(nèi)有效線程的數(shù)量 (workerCount),高3位保存runState,低29位保存workerCount,兩個(gè)變量之間互不干擾。
用一個(gè)變量去存儲(chǔ)兩個(gè)值,可避免在做相關(guān)決策時(shí),出現(xiàn)不一致的情況,不必為了維護(hù)兩者的一致,而占用鎖資源。
線程池也提供了若干方法去供用戶獲得線程池當(dāng)前的運(yùn)行狀態(tài)、線程個(gè)數(shù)。這里都使用的是位運(yùn)算的方式,相比于基本運(yùn)算,速度也會(huì)快很多。
關(guān)于內(nèi)部封裝的獲取生命周期狀態(tài)、獲取線程池線程數(shù)量的計(jì)算方法如以下代碼所示:
// 偏移位數(shù),常量值29,之所以偏移29,目的是將32位的原子變量值ctl的高3位設(shè)置為 // 線程池的狀態(tài),低29位作為線程池大小數(shù)量值;32-3 private static final int COUNT_BITS = Integer.SIZE - 3; //低29位都為1,高位都為0 // 線程池的最大容量值 private static final int CAPACITY = (1 << COUNT_BITS) - 1; // runState is stored in the high-order bits // 接受新任務(wù),并處理隊(duì)列任務(wù) private static final int RUNNING = -1 << COUNT_BITS;//111 // 不接受新任務(wù),但會(huì)處理隊(duì)列任務(wù) private static final int SHUTDOWN = 0 << COUNT_BITS;//000 // 不接受新任務(wù),不會(huì)處理隊(duì)列任務(wù),而且會(huì)中斷正在處理過(guò)程中的任務(wù) private static final int STOP = 1 << COUNT_BITS;//001 // 所有的任務(wù)已結(jié)束,workerCount為0,線程過(guò)渡到TIDYING狀態(tài),將會(huì)執(zhí)行terminated()鉤子方法 private static final int TIDYING = 2 << COUNT_BITS;//010 // terminated()方法已經(jīng)完成 private static final int TERMINATED = 3 << COUNT_BITS;//011 // Packing and unpacking ctl線程池的狀態(tài),原子變量值ctl的高三位 //計(jì)算當(dāng)前運(yùn)行狀態(tài),取高三位 private static int runStateOf(int c) { return c & ~CAPACITY; } //計(jì)算當(dāng)前線程數(shù)量,取低29位 private static int workerCountOf(int c) { return c & CAPACITY; } //通過(guò)狀態(tài)和線程數(shù)生成ctl private static int ctlOf(int rs, int wc) { return rs | wc; } HashSet<Worker> workers = new HashSet<Worker>(); // 存放工作線程的線程池;
ThreadPoolExecutor的運(yùn)行狀態(tài)有5種,分別為:
成員方法
// 提交任務(wù),添加Runnable對(duì)象到線程池,由線程池調(diào)度執(zhí)行 public void execute(Runnable command) // c & 高3位為0,低29位為1的CAPACITY,用于獲取低29位的線程數(shù)量 private static int workerCountOf(int c) { return c & CAPACITY; } // 添加worker工作線程,根據(jù)邊界值來(lái)決定是否創(chuàng)建新的線程 private boolean addWorker(Runnable firstTask, boolean core) // c通常一般為ctl,ctl值小于0,則處于可以接受新任務(wù)狀態(tài) private static boolean isRunning(int c) // 拒絕執(zhí)行任務(wù)方法,當(dāng)線程池在一些場(chǎng)景中,不能處理新加入的線程時(shí),會(huì)通過(guò)該對(duì)象處理拒絕策略; final void reject(Runnable command) // 該方法被Worker工作線程的run方法調(diào)用,真正核心處理Runnable任務(wù)的方法 final void runWorker(Worker w) // c & 高3位為1,低29位為0的~CAPACITY,用于獲取高3位保存的線程池狀態(tài) private static int runStateOf(int c) { return c & ~CAPACITY; } // 不會(huì)立即終止線程池,而是要等所有任務(wù)緩存隊(duì)列中的任務(wù)都執(zhí)行完后才終止,但再也不會(huì)接受新的任務(wù) void shutdown() public void shutdown() // 立即終止線程池,并嘗試打斷正在執(zhí)行的任務(wù),并且清空任務(wù)緩存隊(duì)列,返回尚未執(zhí)行的任務(wù) public List<Runnable> shutdownNow() // worker線程退出 private void processWorkerExit(Worker w, boolean completedAbruptly)
首先,所有任務(wù)的調(diào)度都是由execute方法完成的,這部分完成的工作是:檢查現(xiàn)在線程池的運(yùn)行狀態(tài)、運(yùn)行線程數(shù)、運(yùn)行策略,決定接下來(lái)執(zhí)行的流程,是直接申請(qǐng)線程執(zhí)行,或是緩沖到隊(duì)列中執(zhí)行,亦或是直接拒絕該任務(wù)。其執(zhí)行過(guò)程如下:
/** * Executes the given task sometime in the future. The task * may execute in a new thread or in an existing pooled thread. * If the task cannot be submitted for execution, either because this * executor has been shutdown or because its capacity has been reached, * the task is handled by the current {@code RejectedExecutionHandler}. * @param command the task to execute * @throws RejectedExecutionException at discretion of * {@code RejectedExecutionHandler}, if the task * cannot be accepted for execution * @throws NullPointerException if {@code command} is null */ public void execute(Runnable command) { if (command == null) throw new NullPointerException(); /* * Proceed in 3 steps: * * 1. If fewer than corePoolSize threads are running, try to * start a new thread with the given command as its first * task. The call to addWorker atomically checks runState and * workerCount, and so prevents false alarms that would add * threads when it shouldn't, by returning false. * * 2. If a task can be successfully queued, then we still need * to double-check whether we should have added a thread * (because existing ones died since last checking) or that * the pool shut down since entry into this method. So we * recheck state and if necessary roll back the enqueuing if * stopped, or start a new thread if there are none. * * 3. If we cannot queue task, then we try to add a new * thread. If it fails, we know we are shut down or saturated * and so reject the task. */ int c = ctl.get(); // 獲取原子計(jì)數(shù)值最新值 if (workerCountOf(c) < corePoolSize) { // 判斷當(dāng)前線程池?cái)?shù)量是否小于核心線程數(shù)量 if (addWorker(command, true)) // 嘗試添加command任務(wù)到核心線程 return; c = ctl.get(); // 重新獲取當(dāng)前線程池狀態(tài)值,為后面的檢查做準(zhǔn)備。 } // 執(zhí)行到此,說(shuō)明核心線程任務(wù)數(shù)量已滿,新添加的線程入等待隊(duì)列, 這個(gè)是大于corePoolSize且小于maximumPoolSize if (isRunning(c) && workQueue.offer(command)) { // 如果線程池處于可接受任務(wù)狀態(tài),嘗試添加到等待隊(duì)列 int recheck = ctl.get(); // 雙重校驗(yàn) if (! isRunning(recheck) && remove(command)) // 如果線程池突然不可接受任務(wù),則嘗試移除該command任務(wù) reject(command); // 不可接受任務(wù)且成功從等待隊(duì)列移除任務(wù),則執(zhí)行拒絕策略操作, // 通過(guò)策略告訴調(diào)用方任務(wù)入隊(duì)情況 else if (workerCountOf(recheck) == 0) // 如果此刻線程數(shù)量為0的話將沒(méi)有Worker執(zhí)行新的task,所以增加一個(gè)Worker addWorker(null, false); // 添加一個(gè)Worker } // 執(zhí)行到此,說(shuō)明添加任務(wù)等待隊(duì)列已滿,所以嘗試添加一個(gè)Worker else if (!addWorker(command, false)) // 如果添加失敗的話,那么拒絕此線程任務(wù)添加 // 拒絕此線程任務(wù)添加 reject(command); }
首先檢測(cè)線程池運(yùn)行狀態(tài),如果不是RUNNING,則直接拒絕,線程池要保證在RUNNING的狀態(tài)下執(zhí)行任務(wù)。
如果workerCount < corePoolSize,則創(chuàng)建并啟動(dòng)一個(gè)線程來(lái)執(zhí)行新提交的任務(wù)。
如果workerCount >= corePoolSize,且線程池內(nèi)的阻塞隊(duì)列未滿,則將任務(wù)添加到該阻塞隊(duì)列中。
如果workerCount >= corePoolSize && workerCount < maximumPoolSize,且線程池內(nèi)的阻塞隊(duì)列已滿,則創(chuàng)建并啟動(dòng)一個(gè)線程來(lái)執(zhí)行新提交的任務(wù)。
如果workerCount >= maximumPoolSize,并且線程池內(nèi)的阻塞隊(duì)列已滿, 則根據(jù)拒絕策略來(lái)處理該任務(wù), 默認(rèn)的處理方式是直接拋異常。就用RejectedExecutionHandler來(lái)執(zhí)行拒絕策略;
/** * Checks if a new worker can be added with respect to current * pool state and the given bound (either core or maximum). If so, * the worker count is adjusted accordingly, and, if possible, a * new worker is created and started, running firstTask as its * first task. This method returns false if the pool is stopped or * eligible to shut down. It also returns false if the thread * factory fails to create a thread when asked. If the thread * creation fails, either due to the thread factory returning * null, or due to an exception (typically OutOfMemoryError in * Thread.start()), we roll back cleanly. * * @param firstTask the task the new thread should run first (or * null if none). Workers are created with an initial first task * (in method execute()) to bypass queuing when there are fewer * than corePoolSize threads (in which case we always start one), * or when the queue is full (in which case we must bypass queue). * Initially idle threads are usually created via * prestartCoreThread or to replace other dying workers. * * @param core if true use corePoolSize as bound, else * maximumPoolSize. (A boolean indicator is used here rather than a * value to ensure reads of fresh values after checking other pool * state). * @return true if successful */ private boolean addWorker(Runnable firstTask, boolean core) { // 外層循環(huán),負(fù)責(zé)判斷線程池狀態(tài),處理線程池狀態(tài)變量加1操作 retry: for (;;) { // 狀態(tài)總體相關(guān)值:運(yùn)行狀態(tài) + 執(zhí)行線程任務(wù)數(shù)量 int c = ctl.get(); // 讀取狀態(tài)值 - 運(yùn)行狀態(tài) int rs = runStateOf(c); // Check if queue empty only if necessary. // 滿足下面兩大條件的,說(shuō)明線程池不能接受任務(wù)了,直接返回false處理 // 主要目的就是想說(shuō),只有線程池的狀態(tài)為 RUNNING 狀態(tài)時(shí),線程池才會(huì)接收 // 新的任務(wù),增加新的Worker工作線程 // 線程池的狀態(tài)已經(jīng)至少已經(jīng)處于不能接收任務(wù)的狀態(tài)了 if (rs >= SHUTDOWN && //目的是檢查線 程池是否處于關(guān)閉狀態(tài) ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty())) return false; // 內(nèi)層循環(huán),負(fù)責(zé)worker數(shù)量加1操作 for (;;) { // 獲取當(dāng)前worker線程數(shù)量 int wc = workerCountOf(c); if (wc >= CAPACITY || // 如果線程池?cái)?shù)量達(dá)到最大上限值CAPACITY // core為true時(shí)判斷是否大于corePoolSize核心線程數(shù)量 // core為false時(shí)判斷是否大于maximumPoolSize最大設(shè)置的線程數(shù)量 wc >= (core ? corePoolSize : maximumPoolSize)) return false; // 調(diào)用CAS原子操作,目的是worker線程數(shù)量加1 if (compareAndIncrementWorkerCount(c)) // break retry; c = ctl.get(); // Re-read ctl // CAS原子操作失敗的話,則再次讀取ctl值 if (runStateOf(c) != rs) // 如果剛剛讀取的c狀態(tài)不等于先前讀取的rs狀態(tài),則繼續(xù)外層循環(huán)判斷 continue retry; // else CAS failed due to workerCount change; retry inner loop // 之所以會(huì)CAS操作失敗,主要是由于多線程并發(fā)操作,導(dǎo)致workerCount // 工作線程數(shù)量改變而導(dǎo)致的,因此繼續(xù)內(nèi)層循環(huán)嘗試操作 } } boolean workerStarted = false; boolean workerAdded = false; Worker w = null; try { // 創(chuàng)建一個(gè)Worker工作線程對(duì)象,將任務(wù)firstTask, // 新創(chuàng)建的線程thread都封裝到了Worker對(duì)象里面 w = new Worker(firstTask); final Thread t = w.thread; if (t != null) { // 由于對(duì)工作線程集合workers的添加或者刪除, // 涉及到線程安全問(wèn)題,所以才加上鎖且該鎖為非公平鎖 final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { // Recheck while holding lock. // Back out on ThreadFactory failure or if // shut down before lock acquired. // 獲取鎖成功后,執(zhí)行臨界區(qū)代碼,首先檢查獲取當(dāng)前線程池的狀態(tài)rs int rs = runStateOf(ctl.get()); // 當(dāng)線程池處于可接收任務(wù)狀態(tài) // 或者是不可接收任務(wù)狀態(tài),但是有可能該任務(wù)等待隊(duì)列中的任務(wù) // 滿足這兩種條件時(shí),都可以添加新的工作線程 if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) { if (t.isAlive()) // precheck that t is startable throw new IllegalThreadStateException(); workers.add(w); // 添加新的工作線程到工作線程集合workers,workers是set集合 int s = workers.size(); if (s > largestPoolSize) // 變量記錄了線程池在整個(gè)生命周期中曾經(jīng)出現(xiàn)的最大線程個(gè)數(shù) largestPoolSize = s; workerAdded = true; } } finally { mainLock.unlock(); } if (workerAdded) { // 往workers工作線程集合中添加成功后,則立馬調(diào)用線程start方法啟動(dòng)起來(lái) t.start(); workerStarted = true; } } } finally { if (! workerStarted) // 如果啟動(dòng)線程失敗的話,還得將剛剛添加成功的線程共集合中移除并且做線 // 程數(shù)量做減1操作 addWorkerFailed(w); } return workerStarted; }
該方法是任務(wù)提交的一個(gè)核心方法,主要完成狀態(tài)的檢查,工作線程的創(chuàng)建并添加到線程集合切最后順利的話將創(chuàng)建的線程啟動(dòng);
addWorker(command, true):當(dāng)線程數(shù)小于corePoolSize時(shí),添加一個(gè)需要處理的任務(wù)command進(jìn)線程集合,如果workers數(shù)量超過(guò)corePoolSize時(shí),則返回false不需要添加工作線程;
addWorker(command, false):當(dāng)?shù)却?duì)列已滿時(shí),將新來(lái)的任務(wù)command添加到workers線程集合中去,若線程集合大小超過(guò)maximumPoolSize時(shí),則返回false不需要添加工作線程;
addWorker(null, false):放一個(gè)空的任務(wù)進(jìn)線程集合,當(dāng)這個(gè)空任務(wù)的線程執(zhí)行時(shí),會(huì)從等待任務(wù)隊(duì)列中通過(guò)getTask獲取任務(wù)再執(zhí)行,創(chuàng)建新線程且沒(méi)有任務(wù)分配,當(dāng)執(zhí)行時(shí)才去取任務(wù);
addWorker(null, true):創(chuàng)建空任務(wù)的工作線程到workers集合中去,在setCorePoolSize方法調(diào)用時(shí)目的是初始化核心工作線程實(shí)例;
任務(wù)調(diào)度是線程池的主要入口,當(dāng)用戶提交了一個(gè)任務(wù),接下來(lái)這個(gè)任務(wù)將如何執(zhí)行都是由這個(gè)階段決定的。了解這部分就相當(dāng)于了解了線程池的核心運(yùn)行機(jī)制。
/** * Main worker run loop. Repeatedly gets tasks from queue and * executes them, while coping with a number of issues: * * 1. We may start out with an initial task, in which case we * don't need to get the first one. Otherwise, as long as pool is * running, we get tasks from getTask. If it returns null then the * worker exits due to changed pool state or configuration * parameters. Other exits result from exception throws in * external code, in which case completedAbruptly holds, which * usually leads processWorkerExit to replace this thread. * * 2. Before running any task, the lock is acquired to prevent * other pool interrupts while the task is executing, and then we * ensure that unless pool is stopping, this thread does not have * its interrupt set. * * 3. Each task run is preceded by a call to beforeExecute, which * might throw an exception, in which case we cause thread to die * (breaking loop with completedAbruptly true) without processing * the task. * * 4. Assuming beforeExecute completes normally, we run the task, * gathering any of its thrown exceptions to send to afterExecute. * We separately handle RuntimeException, Error (both of which the * specs guarantee that we trap) and arbitrary Throwables. * Because we cannot rethrow Throwables within Runnable.run, we * wrap them within Errors on the way out (to the thread's * UncaughtExceptionHandler). Any thrown exception also * conservatively causes thread to die. * * 5. After task.run completes, we call afterExecute, which may * also throw an exception, which will also cause thread to * die. According to JLS Sec 14.20, this exception is the one that * will be in effect even if task.run throws. * * The net effect of the exception mechanics is that afterExecute * and the thread's UncaughtExceptionHandler have as accurate * information as we can provide about any problems encountered by * user code. * * @param w the worker */ final void runWorker(Worker w) { Thread wt = Thread.currentThread(); Runnable task = w.firstTask; w.firstTask = null; // allow interrupts 允許中斷 w.unlock(); boolean completedAbruptly = true; try { // 不斷從等待隊(duì)列blockingQueue中獲取任務(wù) // 之前addWorker(null, false)這樣的線程執(zhí)行時(shí), // 會(huì)通過(guò)getTask中再次獲取任務(wù)并執(zhí)行 while (task != null || (task = getTask()) != null) { w.lock(); // 上鎖,并不是防止并發(fā)執(zhí)行任務(wù), // 而是為了防止shutdown()被調(diào)用時(shí)不終止正在運(yùn)行的worker線程 // If pool is stopping, ensure thread is interrupted; // if not, ensure thread is not interrupted. This // requires a recheck in second case to deal with // shutdownNow race while clearing interrupt if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) && !wt.isInterrupted()) wt.interrupt(); try { // task.run()執(zhí)行前,由子類實(shí)現(xiàn) beforeExecute(wt, task); Throwable thrown = null; try { task.run(); // 執(zhí)行線程Runable的run方法 } catch (RuntimeException x) { thrown = x; throw x; } catch (Error x) { thrown = x; throw x; } catch (Throwable x) { thrown = x; throw new Error(x); } finally { // task.run()執(zhí)行后,由子類實(shí)現(xiàn) afterExecute(task, thrown); } } finally { task = null; w.completedTasks++; w.unlock(); } } completedAbruptly = false; } finally { processWorkerExit(w, completedAbruptly); } }
addWorker通過(guò)調(diào)用t.start()啟動(dòng)了線程,線程池的真正核心執(zhí)行任務(wù)的地方就在此runWorker中;
不斷的執(zhí)行我們提交任務(wù)的run方法,可能是剛剛提交的任務(wù),可能是隊(duì)列中等待的隊(duì)列,原因在于Worker工作線程類繼承了AQS類;
Worker重寫(xiě)了AQS的tryAcquire方法,不管先來(lái)后到,一種非公平的競(jìng)爭(zhēng)機(jī)制,通過(guò)CAS獲取鎖,獲取到了就執(zhí)行代碼塊,沒(méi)獲取到的話則添加到CLH隊(duì)列中通過(guò)利用LockSuporrt的park/unpark阻塞任務(wù)等待;
addWorker通過(guò)調(diào)用t.start()啟動(dòng)了線程,線程池的真正核心執(zhí)行任務(wù)的地方就在此runWorker中;
/** * Performs cleanup and bookkeeping for a dying worker. Called * only from worker threads. Unless completedAbruptly is set, * assumes that workerCount has already been adjusted to account * for exit. This method removes thread from worker set, and * possibly terminates the pool or replaces the worker if either * it exited due to user task exception or if fewer than * corePoolSize workers are running or queue is non-empty but * there are no workers. * * @param w the worker * @param completedAbruptly if the worker died due to user exception */ private void processWorkerExit(Worker w, boolean completedAbruptly) { // 如果突然中止,說(shuō)明runWorker中遇到什么異常了, // 那么正在工作的線程自然就需要減1操作了 if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted decrementWorkerCount(); final ReentrantLock mainLock = this.mainLock; // 執(zhí)行到此,說(shuō)明runWorker正常執(zhí)行完了, // 需要正常退出工作線程,上鎖正常操作移除線程 mainLock.lock(); try { completedTaskCount += w.completedTasks; // 增加線程池完成任務(wù)數(shù) workers.remove(w); // 從workers線程集合中移除已經(jīng)工作完的線程 } finally { mainLock.unlock(); } // 在對(duì)線程池有負(fù)效益的操作時(shí),都需要“嘗試終止”線程池,主要是判斷線程池是否滿足終止的狀態(tài); // 如果狀態(tài)滿足,但還有線程池還有線程,嘗試對(duì)其發(fā)出中斷響應(yīng),使其能進(jìn)入退出流程; // 沒(méi)有線程了,更新?tīng)顟B(tài)為tidying->terminated; tryTerminate(); int c = ctl.get(); // 如果狀態(tài)是running、shutdown,即tryTerminate()沒(méi)有成功終止線程池,嘗試再添加一個(gè)worker if (runStateLessThan(c, STOP)) { // 不是突然完成的,即沒(méi)有task任務(wù)可以獲取而完成的,計(jì)算min,并根據(jù)當(dāng)前worker數(shù)量判斷是否需要addWorker() if (!completedAbruptly) { int min = allowCoreThreadTimeOut ? 0 : corePoolSize; // 如果min為0,且workQueue不為空,至少保持一個(gè)線程 if (min == 0 && ! workQueue.isEmpty()) min = 1; // 如果線程數(shù)量大于最少數(shù)量,直接返回,否則下面至少要addWorker一個(gè) if (workerCountOf(c) >= min) return; // replacement not needed } // 只要worker是completedAbruptly突然終止的,或者線程數(shù)量小于要維護(hù)的數(shù)量,就新添一個(gè)worker線程,即使是shutdown狀態(tài) addWorker(null, false); } }
異常中止情況worker數(shù)量減1,正常情況就上鎖從workers中移除;
tryTerminate():在對(duì)線程池有負(fù)效益的操作時(shí),都需要“嘗試終止”線程池;
是否需要增加worker線程,如果線程池還沒(méi)有完全終止,仍需要保持一定數(shù)量的線程;
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