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

public class ShutdownHookDemo {
	// 线程池
    private static ExecutorService executorService = Executors.newFixedThreadPool(3);

    static {
        Runtime.getRuntime().addShutdownHook(new Thread(() -> {
            System.out.println("开始执行钩子方法...");
            // 关闭线程池
            executorService.shutdown();
            try {
            	// 等待60秒
                System.out.println(executorService.awaitTermination(60, TimeUnit.SECONDS));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("结束执行钩子方法...");
        }));
    }

    public static void main(String[] args) throws InterruptedException {
        System.out.println("程序开始启动...");
        // 向线程池添加10个任务
        for (int i = 0; i < 10; i++) {
            Thread.sleep(1000);
            final int finalI = i;
            executorService.execute(() -> {
                try {
                    Thread.sleep(4000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Task " + finalI + " execute...");
            });
            System.out.println("Task " + finalI + " is in thread pool...");
        }
    }
}

在命令行窗口中运行程序，在10个任务都提交到线程池之后，任务都还未处理完成之前，使用 Ctrl+C 中断程序，最终在虚拟机关闭之前，调用了关闭钩子，关闭线程池，并且等待60秒让所有任务执行完成。

Shutdown Hook 在 Spring 中的运用

Shutdown Hook 在 Spring 中是如何运用的呢。通过源码分析，Springboot 项目启动时会判断 registerShutdownHook 的值是否为 true，默认是 true，如果为真则向虚拟机注册关闭钩子。

private void refreshContext(ConfigurableApplicationContext context) {
	refresh(context);
	if (this.registerShutdownHook) {
		try {
			context.registerShutdownHook();
		}
		catch (AccessControlException ex) {
			// Not allowed in some environments.
		}
	}
}

@Override
public void registerShutdownHook() {
	if (this.shutdownHook == null) {
		// No shutdown hook registered yet.
		this.shutdownHook = new Thread() {
			@Override
			public void run() {
				synchronized (startupShutdownMonitor) {
				    // 钩子方法
					doClose();
				}
			}
		};
		// 底层还是使用此方法注册钩子
		Runtime.getRuntime().addShutdownHook(this.shutdownHook);
	}
}

protected void doClose() {
	// Check whether an actual close attempt is necessary...
	if (this.active.get() && this.closed.compareAndSet(false, true)) {
		if (logger.isDebugEnabled()) {
			logger.debug("Closing " + this);
		}

		LiveBeansView.unregisterApplicationContext(this);

		try {
			// 发布Spring 应用上下文的关闭事件，让监听器在应用关闭之前做出响应处理
			publishEvent(new ContextClosedEvent(this));
		}
		catch (Throwable ex) {
			logger.warn("Exception thrown from ApplicationListener handling ContextClosedEvent", ex);
		}

		// Stop all Lifecycle beans, to avoid delays during individual destruction.
		if (this.lifecycleProcessor != null) {
			try {
			    // 执行lifecycleProcessor的关闭方法
				this.lifecycleProcessor.onClose();
			}
			catch (Throwable ex) {
				logger.warn("Exception thrown from LifecycleProcessor on context close", ex);
			}
		}

		// 销毁容器里所有单例Bean
		destroyBeans();

		// 关闭BeanFactory
		closeBeanFactory();

		// Let subclasses do some final clean-up if they wish...
		onClose();

		// Reset local application listeners to pre-refresh state.
		if (this.earlyApplicationListeners != null) {
			this.applicationListeners.clear();
			this.applicationListeners.addAll(this.earlyApplicationListeners);
		}

		// Switch to inactive.
		this.active.set(false);
	}
}

@Override
public void destroy() {
	shutdown();
}

/**
 * Perform a shutdown on the underlying ExecutorService.
 * @see java.util.concurrent.ExecutorService#shutdown()
 * @see java.util.concurrent.ExecutorService#shutdownNow()
 */
public void shutdown() {
	if (logger.isInfoEnabled()) {
		logger.info("Shutting down ExecutorService" + (this.beanName != null ? " '" + this.beanName + "'" : ""));
	}
	if (this.executor != null) {
		if (this.waitForTasksToCompleteOnShutdown) {
			this.executor.shutdown();
		}
		else {
			for (Runnable remainingTask : this.executor.shutdownNow()) {
				cancelRemainingTask(remainingTask);
			}
		}
		awaitTerminationIfNecessary(this.executor);
	}
}