為什么關閉線程池線程池關閉的意義不僅僅在于結束線程執(zhí)行避免內存溢出因為大多使用的場景并非上述示例那樣朝生夕死。線程池一般是持續(xù)工作的全局場景如數(shù)據(jù)庫連接池。線程池大家都知道用來實現(xiàn)資源復用的。避免了線程頻繁的創(chuàng)建和銷毀的開銷。待到使用完畢后都要關閉線程池以釋放資源避免線程全部泄漏白白的浪費了內存。容易導致內存吃緊的時候造成死機。線程池有運行任務自然也有關閉任務線程池手動關閉無非是兩個方法其實無非就是兩個方法 shutdown()/shutdownNow()。自動關閉則需要注意線程池配置技巧。線程池手動關閉關閉方法線程池有兩個方法 shutdown()/shutdownNow()用來關閉二者有著重要的區(qū)別shutdown() 執(zhí)行后停止接受新任務會把隊列的任務執(zhí)行完畢。shutdownNow() 也是停止接受新任務但會中斷所有的任務將線程池狀態(tài)變?yōu)?stop。下段源碼是shutdown() 和shutdownNow()從中可以看到兩個方法的不同/** * Initiates an orderly shutdown in which previously submitted * tasks are executed, but no new tasks will be accepted. * Invocation has no additional effect if already shut down. * * pThis method does not wait for previously submitted tasks to * complete execution. Use {link #awaitTermination awaitTermination} * to do that. * * throws SecurityException {inheritDoc} */ public void shutdown() { final ReentrantLock mainLock this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(SHUTDOWN); interruptIdleWorkers(); onShutdown(); // hook for ScheduledThreadPoolExecutor } finally { mainLock.unlock(); } tryTerminate(); } /** * Attempts to stop all actively executing tasks, halts the * processing of waiting tasks, and returns a list of the tasks * that were awaiting execution. These tasks are drained (removed) * from the task queue upon return from this method. * * pThis method does not wait for actively executing tasks to * terminate. Use {link #awaitTermination awaitTermination} to * do that. * * pThere are no guarantees beyond best-effort attempts to stop * processing actively executing tasks. This implementation * cancels tasks via {link Thread#interrupt}, so any task that * fails to respond to interrupts may never terminate. * * throws SecurityException {inheritDoc} */ public ListRunnable shutdownNow() { ListRunnable tasks; final ReentrantLock mainLock this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(STOP); interruptWorkers(); tasks drainQueue(); } finally { mainLock.unlock(); } tryTerminate(); return tasks; }shutdown()將線程池的狀態(tài)修改為SHUTDOWN狀態(tài)然后調用interruptIdleWorkers方法來中斷空閑的線程。方法是安全的關閉線程池調用shutdown方法后不是立即關閉線程池而是在線程池中執(zhí)行很多任務或者等待隊列中執(zhí)行任務等待所有任務完成后關閉線程池shutdownNow() 方法的執(zhí)行邏輯將線程池狀態(tài)修改為STOP然后調用線程池里的所有線程的interrupt方法。shutdownNow()立即關閉線程池首先向線程池中的線程發(fā)送中斷信號嘗試中斷線程然后將等待隊列的任務返回調用人員讓調用人員補救這些任務。通?？梢愿鶕?jù)自己的業(yè)務需求選擇合適的方法停止線程池。例如通常可以用shutdown()的方法關閉完成提交的任務但如果情況緊急可以用shutdownnow方法加快線程池的結束速度。我通常是按照以下方式關閉線程池的/** * author Evan Walker * version 1.0 * desc 線程池關閉demo */ public class ThreadPoolExecutorDemo { private final static Logger logger LoggerFactory.getLogger(ThreadPoolExecutorDemo.class); //起的線程數(shù) private static final Integer THREAD_NUM 5; //創(chuàng)建線程池 ThreadPoolExecutor pool new ThreadPoolExecutor(THREAD_NUM, THREAD_NUM * 2, 0, TimeUnit.SECONDS, new LinkedBlockingQueue(100)); public void ThreadPoolExecutorClose(Runnable command){ long start System.currentTimeMillis(); for (int i0; i 5; i) { pool.execute(command); } pool.shutdown(); try { while (!pool.awaitTermination(1, TimeUnit.SECONDS)) { logger.info(線程還在執(zhí)行。。。); } }catch (InterruptedException e){ logger.error(ThreadPoolExecutor InterruptedException ,e); } long end System.currentTimeMillis(); logger.info(一共處理了【{}】, (end - start)); } }也可以設置一個shutdownHook來關閉線程池Runtime.getRuntime().addShutdownHook(new Thread(() - { try { pool.shutdown(); while(!pool.awaitTermination(10,TimeUnit.SECONDS)) {} } catch(Exception e) { logger.error(pool shutdown has exception ,e); } }));pool.awaitTermination(1, TimeUnit.SECONDS) 會每隔一秒鐘檢查一次是否執(zhí)行完畢狀態(tài)為 TERMINATED當從 while 循環(huán)退出時就表明線程池已經完全終止了。關閉狀態(tài)判斷1 isShutdownboolean isShutdown()此方法在線程已經開始關閉的時候返回true其他時候返回false即是否調用了線程池的shutdown或shutdownnow方法返回true不代表線程池已經終止了還有可能是需要線程池需要等待正在執(zhí)行的任務或任務隊列中的任務執(zhí)行完。2 isTerminated此方法返回true表示線程池的所有任務都執(zhí)行完畢了線程池終結了如果線程池調用了shutdown但是任務還沒執(zhí)行完isShutdown返回true而isTerminated返回false。3.awaittermination方法是判斷線程池是否完全關閉與isterminated相似但接受等待時間。調用該方法可能發(fā)生以下情況1等待期間(包括進入等待狀態(tài))線程池關閉提交的任務(包括執(zhí)行中和隊列中等待的)全部完成相當于線程池結束方法返回true2等待超時后最初的線程池沒有發(fā)生結束法回到false3等待期間線程中斷方法會拋出互聯(lián)網(wǎng)異常。線程池自動關閉ThreadPoolExecutor類源碼中有這樣一句話A pool that is no longer referenced in a program and has no remaining threads will be shutdown automatically.沒有引用指向且沒有剩余線程的線程池將會自動關閉。先來看一下ThreadPoolExecutor參數(shù)最全的構造方法/** * param corePoolSize the number of threads to keep in the pool, even * if they are idle, unless {code allowCoreThreadTimeOut} is set * 核心線程數(shù)即使是空閑狀態(tài)也可以在線程池存活的線程數(shù)量除非 * allowCoreThreadTimeOut 設置為 true。 * param keepAliveTime when the number of threads is greater than * the core, this is the maximum time that excess idle threads * will wait for new tasks before terminating. * 存活時間對于超出核心線程數(shù)的線程空閑時間一旦達到存活時間就會被銷毀。 */ public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueueRunnable workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) { ... ... }方案一自動關閉線程池核心線程數(shù)為 0 并指定線程存活時間public class ThreadPoolTest { public static void main(String[] args) { // 重點關注 corePoolSize 和 keepAliveTime其他參數(shù)不重要 ThreadPoolExecutor executor new ThreadPoolExecutor(0, 5, 15L, TimeUnit.SECONDS, new LinkedBlockingQueue(15)); for (int i 0; i 20; i) { executor.execute(() - { // 簡單地打印當前線程名稱 System.out.println(Thread.currentThread().getName()); }); } } }通過執(zhí)行打印可以發(fā)現(xiàn)線程池在15秒之后自動關閉因為它核心線程核心線程數(shù)為 0 并指定線程存活時間且后續(xù)沒有繼續(xù)使用到該線程池在其最后一個線程執(zhí)行完畢后15秒之后會優(yōu)雅的關閉線程池。而如果將corePoolSize設置為大于0的數(shù)字再運行以上代碼那么線程池將一直處于等待狀態(tài)而不能關閉因為核心線程不受keepAliveTime控制所以會一直存活程序也將一直不能結束。運行效果如下 (corePoolSize設置為5其他參數(shù)不變)線程池工作時當有新的任務到來時程序會先判斷線程池當前線程數(shù)是否達到corePoolSize(核心線程數(shù))沒達到則創(chuàng)建線程執(zhí)行任務達到則嘗試將任務放入任務隊列 (workQueue)。如果將corePoolSize設置為0的話新到來的任務會永遠優(yōu)先被放入任務隊列然后等待被處理這顯然會影響程序的執(zhí)行效率。所以就有了下面的解決方案。方案二自動關閉線程池通過 allowCoreThreadTimeOut 控制核心線程存活時間從JDK 1.6開始ThreadPoolExecutor類新增了一個allowCoreThreadTimeOut字段/** * If false (default), core threads stay alive even when idle. * If true, core threads use keepAliveTime to time out waiting * for work. * 默認為false核心線程處于空閑狀態(tài)也可一直存活 * 如果設置為true核心線程的存活狀態(tài)將受keepAliveTime控制超時將被銷毀 */ private volatile boolean allowCoreThreadTimeOut;這個字段值默認為false可使用allowCoreThreadTimeOut()方法對其進行設置如果設置為true那么核心線程數(shù)也將受keepAliveTime控制此方法源碼如下public void allowCoreThreadTimeOut(boolean value) { // 核心線程存活時間必須大于0一旦開啟keepAliveTime 也必須大于0 if (value keepAliveTime 0) throw new IllegalArgumentException(Core threads must have nonzero keep alive times); // 將 allowCoreThreadTimeOut 值設為傳入的參數(shù)值 if (value ! allowCoreThreadTimeOut) { allowCoreThreadTimeOut value; // 開啟后清理所有的超時空閑線程包括核心線程 if (value) interruptIdleWorkers(); } }public class ThreadPoolTest { public static void main(String[] args) { // 這里把corePoolSize設為5keepAliveTime保持不變 ThreadPoolExecutor executor new ThreadPoolExecutor(5, 5, 15L, TimeUnit.SECONDS, new LinkedBlockingQueue(15)); // 允許核心線程超時銷毀 executor.allowCoreThreadTimeOut(true); for (int i 0; i 20; i) { executor.execute(() - { System.out.println(Thread.currentThread().getName()); }); } } }運行結果可以看到程序在打印結束后等待了15s然后自行退出說明線程池已自動關閉也就是allowCoreThreadTimeOut()方法發(fā)揮了作用。這樣我們就實現(xiàn)了可自動關閉且核心線程數(shù)不為0的線程池。方案三自動關閉線程池線程池中的線程設置為守護線程public class ThreadPoolTest { public static void main(String[] args) { ThreadPoolExecutor executor new ThreadPoolExecutor(3, 5, 15L, TimeUnit.SECONDS, new LinkedBlockingQueue(15),new ThreadFactory() { public Thread newThread(Runnable r) { Thread thread new Thread(r, r.hashCode()); thread.setDaemon(true);//設置成守護線程 return thread; } }); for (int i 0; i 20; i) { executor.execute(() - { // 簡單地打印當前線程名稱 System.out.println(Thread.currentThread().getName()); }); } } }雖然corePoolSize0而且沒有設置allowCoreThreadTimeOut但是在創(chuàng)建線程池時通過ThreadFactory指定了線程為守護線程。當線程打印結束后無需等待程序正常退出說明線程池關閉了。PS守護進程Daemon是運行在后臺的一種特殊進程。它獨立于控制終端并且周期性地執(zhí)行某種任務或等待處理某些發(fā)生的事件。也就是說守護線程不依賴于終端但是依賴于系統(tǒng)與系統(tǒng)“同生共死”。文章參考https://segmentfault.com/a/1190000021225019java線程池的關閉_線程池 非核心線程關閉的配置-CSDN博客 如何關閉線程池
為什么關閉線程池線程池關閉的意義不僅僅在于結束線程執(zhí)行避免內存溢出因為大多使用的場景并非上述示例那樣朝生夕死。線程池一般是持續(xù)工作的全局場景如數(shù)據(jù)庫連接池。線程池大家都知道用來實現(xiàn)資源復用的。避免了線程頻繁的創(chuàng)建和銷毀的開銷。待到使用完畢后都要關閉線程池以釋放資源避免線程全部泄漏白白的浪費了內存。容易導致內存吃緊的時候造成死機。線程池有運行任務自然也有關閉任務線程池手動關閉無非是兩個方法其實無非就是兩個方法 shutdown()/shutdownNow()。自動關閉則需要注意線程池配置技巧。線程池手動關閉關閉方法線程池有兩個方法 shutdown()/shutdownNow()用來關閉二者有著重要的區(qū)別shutdown() 執(zhí)行后停止接受新任務會把隊列的任務執(zhí)行完畢。shutdownNow() 也是停止接受新任務但會中斷所有的任務將線程池狀態(tài)變?yōu)?stop。下段源碼是shutdown() 和shutdownNow()從中可以看到兩個方法的不同/** * Initiates an orderly shutdown in which previously submitted * tasks are executed, but no new tasks will be accepted. * Invocation has no additional effect if already shut down. * * pThis method does not wait for previously submitted tasks to * complete execution. Use {link #awaitTermination awaitTermination} * to do that. * * throws SecurityException {inheritDoc} */ public void shutdown() { final ReentrantLock mainLock this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(SHUTDOWN); interruptIdleWorkers(); onShutdown(); // hook for ScheduledThreadPoolExecutor } finally { mainLock.unlock(); } tryTerminate(); } /** * Attempts to stop all actively executing tasks, halts the * processing of waiting tasks, and returns a list of the tasks * that were awaiting execution. These tasks are drained (removed) * from the task queue upon return from this method. * * pThis method does not wait for actively executing tasks to * terminate. Use {link #awaitTermination awaitTermination} to * do that. * * pThere are no guarantees beyond best-effort attempts to stop * processing actively executing tasks. This implementation * cancels tasks via {link Thread#interrupt}, so any task that * fails to respond to interrupts may never terminate. * * throws SecurityException {inheritDoc} */ public ListRunnable shutdownNow() { ListRunnable tasks; final ReentrantLock mainLock this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(STOP); interruptWorkers(); tasks drainQueue(); } finally { mainLock.unlock(); } tryTerminate(); return tasks; }shutdown()將線程池的狀態(tài)修改為SHUTDOWN狀態(tài)然后調用interruptIdleWorkers方法來中斷空閑的線程。方法是安全的關閉線程池調用shutdown方法后不是立即關閉線程池而是在線程池中執(zhí)行很多任務或者等待隊列中執(zhí)行任務等待所有任務完成后關閉線程池shutdownNow() 方法的執(zhí)行邏輯將線程池狀態(tài)修改為STOP然后調用線程池里的所有線程的interrupt方法。shutdownNow()立即關閉線程池首先向線程池中的線程發(fā)送中斷信號嘗試中斷線程然后將等待隊列的任務返回調用人員讓調用人員補救這些任務。通?？梢愿鶕?jù)自己的業(yè)務需求選擇合適的方法停止線程池。例如通常可以用shutdown()的方法關閉完成提交的任務但如果情況緊急可以用shutdownnow方法加快線程池的結束速度。我通常是按照以下方式關閉線程池的/** * author Evan Walker * version 1.0 * desc 線程池關閉demo */ public class ThreadPoolExecutorDemo { private final static Logger logger LoggerFactory.getLogger(ThreadPoolExecutorDemo.class); //起的線程數(shù) private static final Integer THREAD_NUM 5; //創(chuàng)建線程池 ThreadPoolExecutor pool new ThreadPoolExecutor(THREAD_NUM, THREAD_NUM * 2, 0, TimeUnit.SECONDS, new LinkedBlockingQueue(100)); public void ThreadPoolExecutorClose(Runnable command){ long start System.currentTimeMillis(); for (int i0; i 5; i) { pool.execute(command); } pool.shutdown(); try { while (!pool.awaitTermination(1, TimeUnit.SECONDS)) { logger.info(線程還在執(zhí)行。。。); } }catch (InterruptedException e){ logger.error(ThreadPoolExecutor InterruptedException ,e); } long end System.currentTimeMillis(); logger.info(一共處理了【{}】, (end - start)); } }也可以設置一個shutdownHook來關閉線程池Runtime.getRuntime().addShutdownHook(new Thread(() - { try { pool.shutdown(); while(!pool.awaitTermination(10,TimeUnit.SECONDS)) {} } catch(Exception e) { logger.error(pool shutdown has exception ,e); } }));pool.awaitTermination(1, TimeUnit.SECONDS) 會每隔一秒鐘檢查一次是否執(zhí)行完畢狀態(tài)為 TERMINATED當從 while 循環(huán)退出時就表明線程池已經完全終止了。關閉狀態(tài)判斷1 isShutdownboolean isShutdown()此方法在線程已經開始關閉的時候返回true其他時候返回false即是否調用了線程池的shutdown或shutdownnow方法返回true不代表線程池已經終止了還有可能是需要線程池需要等待正在執(zhí)行的任務或任務隊列中的任務執(zhí)行完。2 isTerminated此方法返回true表示線程池的所有任務都執(zhí)行完畢了線程池終結了如果線程池調用了shutdown但是任務還沒執(zhí)行完isShutdown返回true而isTerminated返回false。3.awaittermination方法是判斷線程池是否完全關閉與isterminated相似但接受等待時間。調用該方法可能發(fā)生以下情況1等待期間(包括進入等待狀態(tài))線程池關閉提交的任務(包括執(zhí)行中和隊列中等待的)全部完成相當于線程池結束方法返回true2等待超時后最初的線程池沒有發(fā)生結束法回到false3等待期間線程中斷方法會拋出互聯(lián)網(wǎng)異常。線程池自動關閉ThreadPoolExecutor類源碼中有這樣一句話A pool that is no longer referenced in a program and has no remaining threads will be shutdown automatically.沒有引用指向且沒有剩余線程的線程池將會自動關閉。先來看一下ThreadPoolExecutor參數(shù)最全的構造方法/** * param corePoolSize the number of threads to keep in the pool, even * if they are idle, unless {code allowCoreThreadTimeOut} is set * 核心線程數(shù)即使是空閑狀態(tài)也可以在線程池存活的線程數(shù)量除非 * allowCoreThreadTimeOut 設置為 true。 * param keepAliveTime when the number of threads is greater than * the core, this is the maximum time that excess idle threads * will wait for new tasks before terminating. * 存活時間對于超出核心線程數(shù)的線程空閑時間一旦達到存活時間就會被銷毀。 */ public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueueRunnable workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) { ... ... }方案一自動關閉線程池核心線程數(shù)為 0 并指定線程存活時間public class ThreadPoolTest { public static void main(String[] args) { // 重點關注 corePoolSize 和 keepAliveTime其他參數(shù)不重要 ThreadPoolExecutor executor new ThreadPoolExecutor(0, 5, 15L, TimeUnit.SECONDS, new LinkedBlockingQueue(15)); for (int i 0; i 20; i) { executor.execute(() - { // 簡單地打印當前線程名稱 System.out.println(Thread.currentThread().getName()); }); } } }通過執(zhí)行打印可以發(fā)現(xiàn)線程池在15秒之后自動關閉因為它核心線程核心線程數(shù)為 0 并指定線程存活時間且后續(xù)沒有繼續(xù)使用到該線程池在其最后一個線程執(zhí)行完畢后15秒之后會優(yōu)雅的關閉線程池。而如果將corePoolSize設置為大于0的數(shù)字再運行以上代碼那么線程池將一直處于等待狀態(tài)而不能關閉因為核心線程不受keepAliveTime控制所以會一直存活程序也將一直不能結束。運行效果如下 (corePoolSize設置為5其他參數(shù)不變)線程池工作時當有新的任務到來時程序會先判斷線程池當前線程數(shù)是否達到corePoolSize(核心線程數(shù))沒達到則創(chuàng)建線程執(zhí)行任務達到則嘗試將任務放入任務隊列 (workQueue)。如果將corePoolSize設置為0的話新到來的任務會永遠優(yōu)先被放入任務隊列然后等待被處理這顯然會影響程序的執(zhí)行效率。所以就有了下面的解決方案。方案二自動關閉線程池通過 allowCoreThreadTimeOut 控制核心線程存活時間從JDK 1.6開始ThreadPoolExecutor類新增了一個allowCoreThreadTimeOut字段/** * If false (default), core threads stay alive even when idle. * If true, core threads use keepAliveTime to time out waiting * for work. * 默認為false核心線程處于空閑狀態(tài)也可一直存活 * 如果設置為true核心線程的存活狀態(tài)將受keepAliveTime控制超時將被銷毀 */ private volatile boolean allowCoreThreadTimeOut;這個字段值默認為false可使用allowCoreThreadTimeOut()方法對其進行設置如果設置為true那么核心線程數(shù)也將受keepAliveTime控制此方法源碼如下public void allowCoreThreadTimeOut(boolean value) { // 核心線程存活時間必須大于0一旦開啟keepAliveTime 也必須大于0 if (value keepAliveTime 0) throw new IllegalArgumentException(Core threads must have nonzero keep alive times); // 將 allowCoreThreadTimeOut 值設為傳入的參數(shù)值 if (value ! allowCoreThreadTimeOut) { allowCoreThreadTimeOut value; // 開啟后清理所有的超時空閑線程包括核心線程 if (value) interruptIdleWorkers(); } }public class ThreadPoolTest { public static void main(String[] args) { // 這里把corePoolSize設為5keepAliveTime保持不變 ThreadPoolExecutor executor new ThreadPoolExecutor(5, 5, 15L, TimeUnit.SECONDS, new LinkedBlockingQueue(15)); // 允許核心線程超時銷毀 executor.allowCoreThreadTimeOut(true); for (int i 0; i 20; i) { executor.execute(() - { System.out.println(Thread.currentThread().getName()); }); } } }運行結果可以看到程序在打印結束后等待了15s然后自行退出說明線程池已自動關閉也就是allowCoreThreadTimeOut()方法發(fā)揮了作用。這樣我們就實現(xiàn)了可自動關閉且核心線程數(shù)不為0的線程池。方案三自動關閉線程池線程池中的線程設置為守護線程public class ThreadPoolTest { public static void main(String[] args) { ThreadPoolExecutor executor new ThreadPoolExecutor(3, 5, 15L, TimeUnit.SECONDS, new LinkedBlockingQueue(15),new ThreadFactory() { public Thread newThread(Runnable r) { Thread thread new Thread(r, r.hashCode()); thread.setDaemon(true);//設置成守護線程 return thread; } }); for (int i 0; i 20; i) { executor.execute(() - { // 簡單地打印當前線程名稱 System.out.println(Thread.currentThread().getName()); }); } } }雖然corePoolSize0而且沒有設置allowCoreThreadTimeOut但是在創(chuàng)建線程池時通過ThreadFactory指定了線程為守護線程。當線程打印結束后無需等待程序正常退出說明線程池關閉了。PS守護進程Daemon是運行在后臺的一種特殊進程。它獨立于控制終端并且周期性地執(zhí)行某種任務或等待處理某些發(fā)生的事件。也就是說守護線程不依賴于終端但是依賴于系統(tǒng)與系統(tǒng)“同生共死”。文章參考https://segmentfault.com/a/1190000021225019java線程池的關閉_線程池 非核心線程關閉的配置-CSDN博客