捕獲子線程中的異常

如下代碼，在 main 線程中，是無法捕獲子線程的異常的。
catch 子句中的代碼不會被執行。

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();
    }
}

輸出：

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 創建線程池時指定自定義的 ThreadFactory
• 自定義的 ThreadFactory 工廠在生產線程時，爲其設置 UncaughtExceptionHandler 異常處理

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;
    }

}

輸出：

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

那麼主線程爲什麼不能捕獲子線程的異常呢？知乎上有如下解釋：

• 因爲execute方法會立即返回，所以早就運行出try塊了，實際上主線程已經提前結束。
• 設計的主要初衷是線程運行是互相獨立的，可以理解主線程也是一種普通的線程即可。如果線程之間異常互相干擾，那麼1000個線程，一個線程掛了，其它線程跟着遭殃，這是不合理的。

ref:

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