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线程池在提交的任务在处理过程中发生了异常，却没有捕获到，导致异常只是输出在控制台，这通常需要把异常记录下来1、通过观察ThreadGroup的构造方法知道，当调用线程组的构造方法时，会获取当前线程所属的线程组，作为当前正在创建的线程组的parent线程组。2、通过在main方法中获取线程组信息可知，当前main线程所属的线程组是main线程组，而main线程组的parent线程组是system线程组。3、ThreadGroup的uncaughtException实现如下
public void uncaughtException(Thread t, Throwable e) {if (parent != null) {parent.uncaughtException(t, e);} else {Thread.UncaughtExceptionHandler ueh =Thread.getDefaultUncaughtExceptionHandler();if (ueh != null) {ueh.uncaughtException(t, e);} else if (!(e instanceof ThreadDeath)) {/* 【异常输出信息的红色是这里输出来的】 */System.err.print("Exception in thread \""+ t.getName() + "\" ");e.printStackTrace(System.err);}}}4、Thread类有个dispatchUncaughtException方法，当Thread线程运行run方法时，异常抛出来而没有处理时将会把异常抛给虚拟机，虚拟机将会交给该Thread对象的dispatchUncaughtException方法处理，可以从如下代码看到，如果没有给Thread设置具体的UncaughtExceptionHandler，将会给线程组处理，而线程组一直委托给parent处理，所以最后到了system线程组处理，由于它的parent是null，所以就使用System.err输出了异常信息
private void dispatchUncaughtException(Throwable e) {getUncaughtExceptionHandler().uncaughtException(this, e);
}
public UncaughtExceptionHandler getUncaughtExceptionHandler() {return uncaughtExceptionHandler != null ?uncaughtExceptionHandler : group;
}5、线程池中默认的DefaultThreadFactory的实现如下
static class DefaultThreadFactory implements ThreadFactory {private static final AtomicInteger poolNumber = new AtomicInteger(1);private final ThreadGroup group;private final AtomicInteger threadNumber = new AtomicInteger(1);private final String namePrefix;DefaultThreadFactory() {SecurityManager s = System.getSecurityManager();/* 交给的是当前线程所属的线程组 */group = (s != null) ? s.getThreadGroup() :Thread.currentThread().getThreadGroup();namePrefix = "pool-" +poolNumber.getAndIncrement() +"-thread-";}public Thread newThread(Runnable r) {/* 创建的线程 所指定的线程组是在构造方法中设置的 */Thread t = new Thread(group, r,namePrefix + threadNumber.getAndIncrement(),0);if (t.isDaemon())t.setDaemon(false);if (t.getPriority() != Thread.NORM_PRIORITY)t.setPriority(Thread.NORM_PRIORITY);return t;}
}

测试

public class ThrTask implements Runnable {private int a, b;private ThrTask(int a, int b) {this.a = a;this.b = b;}@Overridepublic void run() {double re = a / b;System.out.println(re);}public static void main(String[] args) {ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 1, TimeUnit.SECONDS, new SynchronousQueue<>(), new ThreadFactory() {@Overridepublic Thread newThread(Runnable r) {Thread t = new Thread(Thread.currentThread().getThreadGroup(), r, "myThread");if (t.isDaemon()) t.setDaemon(false);t.setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {@Overridepublic void uncaughtException(Thread t, Throwable e) {System.out.println("线程" + t.getName() + "出现异常" + e);// record this self..}});return t;}});for (int i = 0; i < 5; i++) {//将不会抛出异常// threadPoolExecutor.submit(new ThrTask(5, i));//将会抛出异常threadPoolExecutor.execute(new ThrTask(5, i));}}
}