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線程遇到未處理的異常就結(jié)束了

這個(gè)好理解，當(dāng)線程出現(xiàn)未捕獲異常的時(shí)候就執(zhí)行不下去了，留給它的就是垃圾回收了。

線程池中線程頻繁出現(xiàn)未捕獲異常

當(dāng)線程池中線程頻繁出現(xiàn)未捕獲的異常，那線程的復(fù)用率就大大降低了，需要不斷地創(chuàng)建新線程。

public?class?ThreadExecutor?{

?private?ThreadPoolExecutor?threadPoolExecutor?=?new?ThreadPoolExecutor(1,?1,?60,?TimeUnit.SECONDS,
???new?ArrayBlockingQueue<>(200),?new?ThreadFactoryBuilder().setNameFormat("customThread?%d").build());

?@Test
?public?void?test()?{
??IntStream.rangeClosed(1,?5).forEach(i?->?{
???try?{
????Thread.sleep(100);
???}?catch?(InterruptedException?e)?{
????e.printStackTrace();
???}
???threadPoolExecutor.execute(()?->?{
?????int?j?=?1/0;
??});});
?}
}

新建一個(gè)只有一個(gè)線程的線程池，每隔0.1s提交一個(gè)任務(wù)，任務(wù)中是一個(gè)1/0的計(jì)算。

Exception?in?thread?"customThread?0"?java.lang.ArithmeticException:?/?by?zero
?at?thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
?at?java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
?at?java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
?at?java.lang.Thread.run(Thread.java:748)
Exception?in?thread?"customThread?1"?java.lang.ArithmeticException:?/?by?zero
?at?thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
?at?java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
?at?java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
?at?java.lang.Thread.run(Thread.java:748)
Exception?in?thread?"customThread?2"?java.lang.ArithmeticException:?/?by?zero
?at?thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
?at?java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
?at?java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
?at?java.lang.Thread.run(Thread.java:748)
Exception?in?thread?"customThread?3"?java.lang.ArithmeticException:?/?by?zero
?at?thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
?at?java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
?at?java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
?at?java.lang.Thread.run(Thread.java:748)
Exception?in?thread?"customThread?4"?java.lang.ArithmeticException:?/?by?zero
?at?thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
?at?java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
?at?java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
?at?java.lang.Thread.run(Thread.java:748)
Exception?in?thread?"customThread?5"?java.lang.ArithmeticException:?/?by?zero
?at?thread.ThreadExecutor.lambda$null$0(ThreadExecutor.java:25)
?at?java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
?at?java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
?at?java.lang.Thread.run(Thread.java:748)

可見每次執(zhí)行的線程都不一樣，之前的線程都沒有復(fù)用。原因是因?yàn)槌霈F(xiàn)了未捕獲的異常。

我們把異常捕獲試試：

public?class?ThreadExecutor?{

?private?ThreadPoolExecutor?threadPoolExecutor?=?new?ThreadPoolExecutor(1,?1,?60,?TimeUnit.SECONDS,
???new?ArrayBlockingQueue<>(200),?new?ThreadFactoryBuilder().setNameFormat("customThread?%d").build());

?@Test
?public?void?test()?{
??IntStream.rangeClosed(1,?5).forEach(i?->?{
???try?{
????Thread.sleep(100);
???}?catch?(InterruptedException?e)?{
????e.printStackTrace();
???}
???threadPoolExecutor.execute(()?->?{
????try?{
?????int?j?=?1?/?0;
????}?catch?(Exception?e)?{
?????System.out.println(Thread.currentThread().getName()?+"?"+?e.getMessage());
????}
???});
??});
?}
}

customThread?0?/?by?zero
customThread?0?/?by?zero
customThread?0?/?by?zero
customThread?0?/?by?zero
customThread?0?/?by?zero

可見當(dāng)異常捕獲了，線程就可以復(fù)用了。另外，多線程系列面試題和答案全部整理好了，微信搜索Java技術(shù)棧，在后臺(tái)發(fā)送：面試，可以在線閱讀。

問題來了，我們的代碼中異常不可能全部捕獲

如果要捕獲那些沒被業(yè)務(wù)代碼捕獲的異常，可以設(shè)置Thread類的uncaughtExceptionHandler屬性。這時(shí)使用ThreadFactoryBuilder會(huì)比較方便，ThreadFactoryBuilder是guava提供的ThreadFactory生成器。

new?ThreadFactoryBuilder()
.setNameFormat("customThread?%d")
.setUncaughtExceptionHandler((t,?e)?->?System.out.println(t.getName()?+?"發(fā)生異常"?+?e.getCause()))
.build()

修改之后：

public?class?ThreadExecutor?{

?private?static?ThreadPoolExecutor?threadPoolExecutor?=?new?ThreadPoolExecutor(1,?1,?60,?TimeUnit.SECONDS,
???new?ArrayBlockingQueue<>(200),
???new?ThreadFactoryBuilder()
?????.setNameFormat("customThread?%d")
?????.setUncaughtExceptionHandler((t,?e)?->?System.out.println("UncaughtExceptionHandler捕獲到："?+?t.getName()?+?"發(fā)生異常"?+?e.getMessage()))
?????.build());

?@Test
?public?void?test()?{
??IntStream.rangeClosed(1,?5).forEach(i?->?{
???try?{
????Thread.sleep(100);
???}?catch?(InterruptedException?e)?{
????e.printStackTrace();
???}

???threadPoolExecutor.execute(()?->?{
????System.out.println("線程"?+?Thread.currentThread().getName()?+?"執(zhí)行");
????int?j?=?1?/?0;
???});
??});
?}
}

線程customThread?0執(zhí)行
UncaughtExceptionHandler捕獲到：customThread 0發(fā)生異常/ by zero
線程customThread?1執(zhí)行
UncaughtExceptionHandler捕獲到：customThread 1發(fā)生異常/ by zero
線程customThread?2執(zhí)行
UncaughtExceptionHandler捕獲到：customThread 2發(fā)生異常/ by zero
線程customThread?3執(zhí)行
UncaughtExceptionHandler捕獲到：customThread 3發(fā)生異常/ by zero
線程customThread?4執(zhí)行
UncaughtExceptionHandler捕獲到：customThread 4發(fā)生異常/ by zero

可見，結(jié)果并不是我們想象的那樣，線程池中原有的線程沒有復(fù)用！所以通過UncaughtExceptionHandler想將異常吞掉使線程復(fù)用這招貌似行不通。它只是做了一層異常的保底處理。

推薦一個(gè) Spring Boot 基礎(chǔ)教程及實(shí)戰(zhàn)示例：https://www.javastack.cn/categories/Spring-Boot/

將excute改成submit試試

public?class?ThreadExecutor?{

?private?static?ThreadPoolExecutor?threadPoolExecutor?=?new?ThreadPoolExecutor(1,?1,?60,?TimeUnit.SECONDS,
???new?ArrayBlockingQueue<>(200),
???new?ThreadFactoryBuilder()
?????.setNameFormat("customThread?%d")
?????.setUncaughtExceptionHandler((t,?e)?->?System.out.println("UncaughtExceptionHandler捕獲到："?+?t.getName()?+?"發(fā)生異常"?+?e.getMessage()))
?????.build());

?@Test
?public?void?test()?{
??IntStream.rangeClosed(1,?5).forEach(i?->?{
???try?{
????Thread.sleep(100);
???}?catch?(InterruptedException?e)?{
????e.printStackTrace();
???}

???Future?future?=?threadPoolExecutor.submit(()?->?{
????System.out.println("線程"?+?Thread.currentThread().getName()?+?"執(zhí)行");
????int?j?=?1?/?0;
???});
???try?{
????future.get();
???}?catch?(InterruptedException?e)?{
????e.printStackTrace();
???}?catch?(ExecutionException?e)?{
????e.printStackTrace();
???}
??});
?}
}

線程customThread?0執(zhí)行
java.util.concurrent.ExecutionException:?java.lang.ArithmeticException:?/?by?zero
線程customThread?0執(zhí)行
java.util.concurrent.ExecutionException:?java.lang.ArithmeticException:?/?by?zero
線程customThread?0執(zhí)行
java.util.concurrent.ExecutionException:?java.lang.ArithmeticException:?/?by?zero
線程customThread?0執(zhí)行
java.util.concurrent.ExecutionException:?java.lang.ArithmeticException:?/?by?zero
線程customThread?0執(zhí)行
java.util.concurrent.ExecutionException:?java.lang.ArithmeticException:?/?by?zero

通過submit提交線程可以屏蔽線程中產(chǎn)生的異常，達(dá)到線程復(fù)用。當(dāng)get()執(zhí)行結(jié)果時(shí)異常才會(huì)拋出。

原因是通過submit提交的線程，當(dāng)發(fā)生異常時(shí)，會(huì)將異常保存，待future.get();時(shí)才會(huì)拋出。最新面試題整理好了，點(diǎn)擊Java面試庫小程序在線刷題。

這是Futuretask的部分run()方法，看setException：

public?void?run()?{
????try?{
????????Callable?c?=?callable;
????????if?(c?!=?null?&&?state?==?NEW)?{
????????????V?result;
????????????boolean?ran;
????????????try?{
????????????????result?=?c.call();
????????????????ran?=?true;
????????????}?catch?(Throwable?ex)?{
????????????????result?=?null;
????????????????ran?=?false;
????????????????setException(ex);
????????????}
????????????if?(ran)
????????????????set(result);
????????}
????}?
}

protected?void?setException(Throwable?t)?{
????if?(UNSAFE.compareAndSwapInt(this,?stateOffset,?NEW,?COMPLETING))?{
????????outcome?=?t;
????????UNSAFE.putOrderedInt(this,?stateOffset,?EXCEPTIONAL);?//?final?state
????????finishCompletion();
????}
}

將異常存在outcome對(duì)象中，沒有拋出，再看get方法：

public?V?get()?throws?InterruptedException,?ExecutionException?{
????int?s?=?state;
????if?(s?<=?COMPLETING)
????????s?=?awaitDone(false,?0L);
????return?report(s);
}
private?V?report(int?s)?throws?ExecutionException?{
????Object?x?=?outcome;
????if?(s?==?NORMAL)
????????return?(V)x;
????if?(s?>=?CANCELLED)
????????throw?new?CancellationException();
????throw?new?ExecutionException((Throwable)x);
}

當(dāng)outcome是異常時(shí)才拋出。

總結(jié)

1、線程池中線程中異常盡量手動(dòng)捕獲

2、通過設(shè)置ThreadFactory的UncaughtExceptionHandler可以對(duì)未捕獲的異常做保底處理，通過execute提交任務(wù)，線程依然會(huì)中斷，而通過submit提交任務(wù)，可以獲取線程執(zhí)行結(jié)果，線程異常會(huì)在get執(zhí)行結(jié)果時(shí)拋出。

本文鏈接：https://blog.csdn.net/weixin_37968613/article/details/108407774

版權(quán)聲明：本文為博主原創(chuàng)文章，遵循 CC 4.0 BY-SA 版權(quán)協(xié)議，轉(zhuǎn)載請(qǐng)附上原文出處鏈接和本聲明。

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面試官：線程池執(zhí)行過程中遇到異常會(huì)發(fā)生什么，怎樣處理？

線程池中線程頻繁出現(xiàn)未捕獲異常

問題來了，我們的代碼中異常不可能全部捕獲

總結(jié)