面試官：執行緒池執行程序中遇到例外會發生什么，怎樣處理？-有解無憂

執行緒遇到未處理的例外就結束了

這個好理解，當執行緒出現未捕獲例外的時候就執行不下去了，留給它的就是垃圾回收了，

執行緒池中執行緒頻繁出現未捕獲例外

當執行緒池中執行緒頻繁出現未捕獲的例外，那執行緒的復用率就大大降低了，需要不斷地創建新執行緒，

做個實驗：

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;
  });});
 }
}

新建一個只有一個執行緒的執行緒池，每隔0.1s提交一個任務，任務中是一個1/0的計算，

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)

可見每次執行的執行緒都不一樣，之前的執行緒都沒有復用，原因是因為出現了未捕獲的例外，

我們把例外捕獲試試：

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

可見當例外捕獲了，執行緒就可以復用了，

問題來了，我們的代碼中例外不可能全部捕獲

如果要捕獲那些沒被業務代碼捕獲的例外，可以設定Thread類的uncaughtExceptionHandler屬性，這時使用ThreadFactoryBuilder會比較方便，ThreadFactoryBuilder是guava提供的ThreadFactory生成器，

new ThreadFactoryBuilder()
.setNameFormat("customThread %d")
.setUncaughtExceptionHandler((t, e) -> System.out.println(t.getName() + "發生例外" + 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() + "發生例外" + 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("執行緒" + Thread.currentThread().getName() + "執行");
    int j = 1 / 0;
   });
  });
 }
}

執行緒customThread 0執行
UncaughtExceptionHandler捕獲到：customThread 0發生例外/ by zero
執行緒customThread 1執行
UncaughtExceptionHandler捕獲到：customThread 1發生例外/ by zero
執行緒customThread 2執行
UncaughtExceptionHandler捕獲到：customThread 2發生例外/ by zero
執行緒customThread 3執行
UncaughtExceptionHandler捕獲到：customThread 3發生例外/ by zero
執行緒customThread 4執行
UncaughtExceptionHandler捕獲到：customThread 4發生例外/ by zero

可見，結果并不是我們想象的那樣，執行緒池中原有的執行緒沒有復用！所以通過UncaughtExceptionHandler想將例外吞掉使執行緒復用這招貌似行不通，它只是做了一層例外的保底處理，

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() + "發生例外" + 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("執行緒" + Thread.currentThread().getName() + "執行");
    int j = 1 / 0;
   });
   try {
    future.get();
   } catch (InterruptedException e) {
    e.printStackTrace();
   } catch (ExecutionException e) {
    e.printStackTrace();
   }
  });
 }
}

執行緒customThread 0執行
java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
執行緒customThread 0執行
java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
執行緒customThread 0執行
java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
執行緒customThread 0執行
java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
執行緒customThread 0執行
java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero

通過submit提交執行緒可以屏蔽執行緒中產生的例外，達到執行緒復用，當get()執行結果時例外才會拋出，

原因是通過submit提交的執行緒，當發生例外時，會將例外保存，待future.get();時才會拋出，

這是Futuretask的部分run()方法，看setException：

public void run() {
    try {
        Callable<V> 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();
    }
}

將例外存在outcome物件中，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outcome是例外時才拋出，

總結

1、執行緒池中執行緒中例外盡量手動捕獲

2、通過設定ThreadFactory的UncaughtExceptionHandler可以對未捕獲的例外做保底處理，通過execute提交任務，執行緒依然會中斷，而通過submit提交任務，可以獲取執行緒執行結果，執行緒例外會在get執行結果時拋出，

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

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