博弈Java讲义 - Java线程之uncontrolled exceptions

前两天翻阅《Effective Java》发现一条提示慎用线程组。ThreadGroup提供的很多功能的实现是有瑕疵的。例如，我们可以调用activeCount获得该组中活动线程的数量，一旦这个数组进行了分配，并用enumerate方法遍历，如果线程数增加了，就有可能忽略掉调用activeCount后新增的线程。关于处理线程组逻辑，可以用线程池的executor代替。 
  也许ThreadGroup提供的有用的功能之一就是uncaughtException方法了。Java提供了强大的异常处理机制，有些异常可以通过try/catch捕获或者re-throw，这些都是checked exception,比如IOException和ClassNotFoundException,还有一些是不必要捕获处理的，如NumberFormatException. 
  在Java多线程中提供了一个层次化的机制帮助我们有效的处理uncaught exception. 当一个线程抛出异常时，JVM首先会调用thread里面指定的uncaught exception handler处理异常，如果在thread级没有设置handler，JVM会到当前线程所在的组的exception handler处理异常，如果线程组没有定义uncaughtException方法，JVM会继续寻找上一级的exception handler处理。如果一个handler都没有找到，这默认打出错误栈并推出程序。 
  让们看一个具体的例子： 
  

  import java.lang.Thread.UncaughtExceptionHandler;

public class MyUncaughtExceptionHandler implements UncaughtExceptionHandler {

    @Override
    public void uncaughtException(Thread t, Throwable e) {
        System.out.printf("Uncaught exception raised and captured in thread  %s : \n", t.getName());
    }

}

import java.lang.Thread.UncaughtExceptionHandler;

public class MyUncaughtExceptionHandler implements UncaughtExceptionHandler {

    @Override
    public void uncaughtException(Thread t, Throwable e) {
        System.out.printf("Uncaught exception raised and captured in thread  %s : \n", t.getName());
    }

}

public class MyThreadGroupWithExceptionHandler extends ThreadGroup {

    public MyThreadGroupWithExceptionHandler(String name) {
        super(name);
    }

    @Override
    public void uncaughtException(Thread t, Throwable e) {
        System.out.printf("Uncaught exception caught in thread group's uncaughtException %s.\n",this.getName());
        interrupt();
    }

}
public class TestUncaughtException {

    public static void main(String[] args){
        MyTask task1 = new MyTask();
        Thread t1 = new Thread(task1,"UncaughtException Task");
        t1.setUncaughtExceptionHandler(new MyUncaughtExceptionHandler());
        t1.start();


        MyThreadGroupWithExceptionHandler group1 = new MyThreadGroupWithExceptionHandler("mythread");
        MyTask task2 = new MyTask();
        Thread t2 = new Thread(group1, task2,"Task with group exception handler");
        t2.start();

        ThreadGroup group2 = new ThreadGroup("mythread");
        MyTask task3 = new MyTask();
        Thread.setDefaultUncaughtExceptionHandler(new MyDefaultExceptionHandler());
        Thread t3 = new Thread(group2, task3, "Task with exception caught by Thread.default");
        t3.start();

    }

}

  输出可见异常分别在Thread实例,ThreadGroup,Thread级别被捕获。