一、什么是ShutdownHook？
Runtime.java中相关方法源码
public void addShutdownHook(Thread hook) {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
sm.checkPermission(new RuntimePermission("shutdownHooks"));
}
ApplicationShutdownHooks.add(hook);
}
public boolean removeShutdownHook(Thread hook) {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
sm.checkPermission(new RuntimePermission("shutdownHooks"));
}
return ApplicationShutdownHooks.remove(hook);
}ApplicationShutdownHooks.java
class ApplicationShutdownHooks {
/* The set of registered hooks */
private static IdentityHashMap<Thread, Thread> hooks;
static {
try {
Shutdown.add(1 /* shutdown hook invocation order */,
false /* not registered if shutdown in progress */,
new Runnable() {
public void run() {
runHooks();
}
}
);
hooks = new IdentityHashMap<>();
} catch (IllegalStateException e) {
// application shutdown hooks cannot be added if
// shutdown is in progress.
hooks = null;
}
}
private ApplicationShutdownHooks() {}
/* Add a new shutdown hook. Checks the shutdown state and the hook itself,
* but does not do any security checks.
*/
static synchronized void add(Thread hook) {
if(hooks == null)
throw new IllegalStateException("Shutdown in progress");
if (hook.isAlive())
throw new IllegalArgumentException("Hook already running");
if (hooks.containsKey(hook))
throw new IllegalArgumentException("Hook previously registered");
hooks.put(hook, hook);
}
/* Remove a previously-registered hook. Like the add method, this method
* does not do any security checks.
*/
static synchronized boolean remove(Thread hook) {
if(hooks == null)
throw new IllegalStateException("Shutdown in progress");
if (hook == null)
throw new NullPointerException();
return hooks.remove(hook) != null;
}
/* Iterates over all application hooks creating a new thread for each
* to run in. Hooks are run concurrently and this method waits for
* them to finish.
*/
static void runHooks() {
Collection<Thread> threads;
synchronized(ApplicationShutdownHooks.class) {
threads = hooks.keySet();
hooks = null;
}
for (Thread hook : threads) {
hook.start();
}
for (Thread hook : threads) {
try {
hook.join();
} catch (InterruptedException x) { }
}
}
}二、java进程平滑退出的意义
很多时候，我们会有这样的一些场景，比如说nginx反向代理若干个负载均衡的web容器，又或者微服务架构中存在的若干个服务节点，需要进行无间断的升级发布。
在重启服务的时候，除非我们去变更nginx的配置，否则重启很可能会导致正在执行的线程突然中断，本来应该要完成的事情只完成了一半，并且调用方出现错误警告。
如果能有一种简单的方式，能够让进程在退出时能执行完当前正在执行的任务，并且让服务的调用方将新的请求定向到其他负载节点，这将会很有意义。
自己注册ShutdownHook可以帮助我们实现java进程的平滑退出。
三、java进程平滑退出的思路
- 在服务启动时注册自己的ShutdownHook
- ShutdownHook在被运行时，首先不接收新的请求，或者告诉调用方重定向到其他节点
- 等待当前的执行线程运行完毕，如果五秒后仍在运行，则强制退出