java - 在 java shutdown hook 中添加守护线程与非守护线程的区别

Question

我在stackoverflow中看到过这个讨论。但我不清楚在 ShutdownHook 中将线程标记为守护进程是否与将其标记为非守护进程相同？

Thread t = new Thread(this::someMethod, "shutdown_hook");
t.setDaemon(true);
Runtime.getRuntime().addShutdownHook(t);

如果我不在上面的代码中执行 t.setDaemon(true); ，行为是否会相同。

我使用的是java 8。

Answer 1

无论关闭钩子(Hook)线程是否为守护进程，都没有区别。

作为 Runtime.addShutdownHook 的规范说，

When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking the exit method.

和

Once the shutdown sequence has begun it can be stopped only by invoking the halt method

JDK 实现遵循这些规则。正如我们在 source code 中看到的那样, runHooks 启动钩子(Hook)线程并等待所有线程完成:

    for (Thread hook : threads) {
        hook.start();
    }
    for (Thread hook : threads) {
        while (true) {
            try {
                hook.join();
                break;
            } catch (InterruptedException ignored) {
            }
        }
    }