面試官：線程池執行過程中遇到異常會發生什么，怎樣處理？

線程遇到未處理的異常就結束了

這個好理解，當線程出現未捕獲異常的時候就執行不下去了，留給它的就是垃圾回收了。

線程池中線程頻繁出現未捕獲異常

當線程池中線程頻繁出現未捕獲的異常，那線程的復用率就大大降低了，需要不斷地創建新線程。

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;
  });});
 }
}

新建一個只有一個線程的線程池，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)

可見每次執行的線程都不一樣，之前的線程都沒有復用。原因是因為出現了未捕獲的異常。

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

可見當異常捕獲了，線程就可以復用了。

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務代碼捕獲的異常，可以設置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() + "執行");
    int j = 1 / 0;
   });
  });
 }
}

線程customThread 0執行
UncaughtExceptionHandler捕獲到：customThread 0發生異常/ by zero
線程customThread 1執行
UncaughtExceptionHandler捕獲到：customThread 1發生異常/ by zero
線程customThread 2執行
UncaughtExceptionHandler捕獲到：customThread 2發生異常/ by zero
線程customThread 3執行
UncaughtExceptionHandler捕獲到：customThread 3發生異常/ by zero
線程customThread 4執行
UncaughtExceptionHandler捕獲到：customThread 4發生異常/ by zero

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

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() + "執行");
    int j = 1 / 0;
   });
   try {
    future.get();
   } catch (InterruptedException e) {
    e.printStackTrace();
   } catch (ExecutionException e) {
    e.printStackTrace();
   }
  });
 }
}

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

通過submit提交線程可以屏蔽線程中產生的異常，達到線程復用。當get()執行結果時異常才會拋出。

原因是通過submit提交的線程，當發生異常時，會將異常保存，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();
    }
}

將異常存在outcome對象中，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outcome是異常時才拋出。

總結

1、線程池中線程中異常盡量手動捕獲

2、通過設置ThreadFactory的UncaughtExceptionHandler可以對未捕獲的異常做保底處理，通過execute提交任務，線程依然會中斷，而通過submit提交任務，可以獲取線程執行結果，線程異常會在get執行結果時拋出。

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

版權聲明：本文為博主原創文章，遵循 CC 4.0 BY-SA 版權協議，轉載請附上原文出處鏈接和本聲明。

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