捕获子线程中的异常

如下代码，在 main 线程中，是无法捕获子线程的异常的。
catch 子句中的代码不会被执行。

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class NaiveExceptionHandling {
    public static void main(String[] args) {
        ExecutorService exec = Executors.newCachedThreadPool();
        try {
            exec.execute(() -> {
                throw new RuntimeException();
            });
        } catch (RuntimeException e) {
            System.out.println("this msg will not de printed");
        }
        exec.shutdown();
    }
}

输出：

Exception in thread "pool-1-thread-1" java.lang.RuntimeException
	at NaiveExceptionHandling.lambda$main$0(NaiveExceptionHandling.java:9)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

Process finished with exit code 0

改造：

• newCachedThreadPool 创建线程池时指定自定义的 ThreadFactory
• 自定义的 ThreadFactory 工厂在生产线程时，为其设置 UncaughtExceptionHandler 异常处理

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;

public class NaiveExceptionHandling {

    public static void main(String[] args) {
        ExecutorService exec2 = Executors.newCachedThreadPool(new HandleThreadFactory());
        exec2.execute(() -> {
            throw new RuntimeException();
        });
        exec2.shutdown();
    }
}

class HandleThreadFactory implements ThreadFactory {
    @Override
    public Thread newThread(Runnable r) {
        System.out.println("create thread t");
        Thread t = new Thread(r);
        System.out.println("set uncaughtException for t");
        t.setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
            @Override
            public void uncaughtException(Thread t, Throwable e) {
                System.out.println("caught " + e);
            }
        });
        return t;
    }

}

输出：

create thread t
set uncaughtException for t
caught java.lang.RuntimeException

那么主线程为什么不能捕获子线程的异常呢？知乎上有如下解释：

• 因为execute方法会立即返回，所以早就运行出try块了，实际上主线程已经提前结束。
• 设计的主要初衷是线程运行是互相独立的，可以理解主线程也是一种普通的线程即可。如果线程之间异常互相干扰，那么1000个线程，一个线程挂了，其它线程跟着遭殃，这是不合理的。

ref:

1. https://blog.csdn.net/wild46cat/article/details/80808555
2. 《Java 编程思想（第4版）》 P672
3. https://www.zhihu.com/question/67790293