为什么对uncaughtexception的处理似乎发生在发生异常行为的死线程中？

Question

@Slf4j
public class Main {

    public static void main(String[] args) throws InterruptedException {
        Thread t = new Thread() {

            public void run() {
                try {
                    //do some stuff
                    LOG.debug("about to throw a RE from a worker thead");
                } catch (Exception e) {
                    e.printStackTrace();
                }

                throw new RuntimeException("purposeful!");
            }

        };
        LOG.debug("in main thead");
        t.setUncaughtExceptionHandler(
                (thread, throwable) -> LOG.debug("in main thread; catching exception from worker. uncaught exception from worker thread: {} from {}",
                        throwable.getMessage(), thread.getName()));
        t.start();
        TimeUnit.SECONDS.sleep(10);
        LOG.debug("done-main thead");
    }
}

如果运行，将产生以下输出：

10:19:50.249 [main] DEBUG com.foo.services.search.Main - in main thead
10:19:50.258 [Thread-0] DEBUG com.foo.services.search.Main - about to throw a RE from a worker thead
10:19:50.258 [Thread-0] DEBUG com.foo.services.search.Main - in main thread; catching exception from worker. uncaught exception from worker thread: purposeful! from Thread-0
10:20:00.258 [main] DEBUG com.foo.services.search.Main - done-main thead

为什么当thread-0完成时，throwable-catching-activity似乎发生在这个完成的线程中？

Answer 1

表单doc中，声明此方法仅由JVM调用：

    /**
     * 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);
    }

出于好奇，我找到了HotSpot的相关源代码:https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/thread.cpp

正如您在thread.cpp中看到的，当thread退出时会触发此逻辑：

// For any new cleanup additions, please check to see if they need to be applied to
// cleanup_failed_attach_current_thread as well.
void JavaThread::exit(bool destroy_vm, ExitType exit_type) {
  // ........
  if (!destroy_vm) {
    if (uncaught_exception.not_null()) {
      EXCEPTION_MARK;
      // Call method Thread.dispatchUncaughtException().
      Klass* thread_klass = vmClasses::Thread_klass();
  //...
}