Java 线程异常处理

如果我们调用一个线程处理任务，但是当线程发生异常的时候我们调用线程的地方无法捕捉到异常。
用例子验证：我在main中把调用线程的代码放到try-catch里面，但是结果却没有捕捉到任何异常。

package thread;
class MRunnable implements Runnable{

    @Override
    public void run() {
        throw new RuntimeException("This is an exception from Runnable");
        
    }
    
}

public class TestException {

    public static void main(String[] args) {
        try{
            new Thread(new MRunnable2()).start();
            
        }catch (Exception e) {
            System.out.println("catch: "+e.getMessage());
        }

    }

}

执行结果：

Exception in thread "Thread-0" java.lang.RuntimeException: This is an exception from Runnable
    at thread.MRunnable2.run(TestException.java:6)
    at java.lang.Thread.run(Unknown Source)

从结果看，console并没有打印出“catch:”，证明没有捕捉到。
怎么才能在线程外捕捉到异常呢？
在Thread类中有个UncaughtExceptionHandler接口，如果有异常，JVM会调用dispatchUncaughtException()方法。

  /**
     * Dispatch an uncaught exception to the handler. This method is
     * intended to be called only by the JVM.
     */
    private void dispatchUncaughtException(Throwable e) {
        getUncaughtExceptionHandler().uncaughtException(this, e);
    }

因此我们只要一个实现了UncaughtExceptionHandler接口的类就可以了。

package thread;

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

class MRunnable implements Runnable {

    @Override
    public void run() {
        throw new RuntimeException("This is an exception from Runnable");

    }

}

class MThreadFactory implements ThreadFactory {

    @Override
    public Thread newThread(Runnable r) {
        Thread thread = new Thread(r);
        thread.setUncaughtExceptionHandler(new MUncaughtExceptionHandler());
        return thread;
    }

}

class MUncaughtExceptionHandler implements Thread.UncaughtExceptionHandler {

    @Override
    public void uncaughtException(Thread t, Throwable e) {
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>");
        System.out.println("catch: "+e.getMessage());
        System.out.println("<<<<<<<<<<<<<<<<<<<<<<<<<<<");
    }

}

public class TestException {

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool(new MThreadFactory());

        executorService.execute(new MRunnable());

    }

}

执行结果：

>>>>>>>>>>>>>>>>>>>>>>>>>>>
catch: This is an exception from Runnable
<<<<<<<<<<<<<<<<<<<<<<<<<<<