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java的优雅停机及源码实现

作者:互联网

 

   优雅停机? 这个名词我是服的,如果抛开专业不谈,多好的名词啊!

  其实优雅停机,就是在要关闭服务之前,不是立马全部关停,而是做好一些善后操作,比如:关闭线程、释放连接资源等。 

  再比如,就是不会让调用方的请求处理了一增,一下就中断了。而处理完本次后,再停止服务。

  Java语言中,我们可以通过Runtime.getRuntime().addShutdownHook()方法来注册钩子,以保证程序平滑退出。(其他语言也类似)

来个栗子:

 

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public class ShutdownGracefulTest {

    /**
     * 使用线程池处理任务
     */
    public static ExecutorService executorService = Executors.newCachedThreadPool();

    public static void main(String[] args) {

        //假设有5个线程需要执行任务
        for(int i = 0; i < 5; i++){
            final int id = i;
            Thread taski = new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(System.currentTimeMillis() + " : thread_" + id + " start...");
                    try {
                        TimeUnit.SECONDS.sleep(id);
                    }
                    catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(System.currentTimeMillis() + " : thread_" + id + " finish!");
                }
            });
            taski.setDaemon(true);
            executorService.submit(taski);
        }

        Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
            @Override
            public void run() {

                System.out.println(System.currentTimeMillis() + " : " + Thread.currentThread().getName() + " No1 shutdown hooking...");
                boolean shutdown = true;
                try {
                    executorService.shutdown();
                    System.out.println(System.currentTimeMillis() + " : " + Thread.currentThread().getName() +  " shutdown signal got, wait threadPool finish.");
                    executorService.awaitTermination(1500, TimeUnit.SECONDS);
                    boolean done = false;
                    System.out.println(System.currentTimeMillis() + " : " + Thread.currentThread().getName() +  " all thread's done.");
                }
                catch (InterruptedException e) {
                    e.printStackTrace();
                    // 尝试再次关闭
                    if(!executorService.isTerminated()) {
                        executorService.shutdownNow();
                    }
                }
                System.out.println(System.currentTimeMillis() + " : " + Thread.currentThread().getName() + " No1 shutdown done...");
            }
        }));

        Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    System.out.println(System.currentTimeMillis() + " : " + Thread.currentThread().getName() + " No2 shutdown hooking...");
                    Thread.sleep(1000);
                }
                catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(System.currentTimeMillis() + " : " + Thread.currentThread().getName() + " No2 shutdown done...");
            }
        }));

        System.out.println("main method exit...");
        System.exit(0);
    }
}
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运行结果如下:

  很明显,确实是优雅了,虽然最后收到了一关闭信号,但是仍然保证了任务的处理完成。很棒吧!

  那么,在实际应用中是如何体现优雅停机呢?

kill -15 pid 


通过该命令发送一个关闭信号给到jvm, 然后就开始执行 Shutdown Hook 了,你可以做很多:
  1. 关闭 socket 链接
  2. 清理临时文件
  3. 发送消息通知给订阅方,告知自己下线
  4. 将自己将要被销毁的消息通知给子进程
  5. 各种资源的释放
  ...

  而在平时工作中,我们不乏看到很多运维同学,是这么干的:

kill -9 pid

  如果这么干的话,jvm也无法了,kill -9 相当于一次系统宕机,系统断电。这会给应用杀了个措手不及,没有留给应用任何反应的机会。
  所以,无论如何是优雅不起来了。

  要优雅,是代码和运维的结合!

其中,线程池的关闭方式为:

executorService.shutdown();
executorService.awaitTermination(1500, TimeUnit.SECONDS);

  ThreadPoolExecutor 在 shutdown 之后会变成 SHUTDOWN 状态,无法接受新的任务,随后等待正在执行的任务执行完成。意味着,shutdown 只是发出一个命令,至于有没有关闭还是得看线程自己。
  ThreadPoolExecutor 对于 shutdownNow 的处理则不太一样,方法执行之后变成 STOP 状态,并对执行中的线程调用 Thread.interrupt() 方法(但如果线程未处理中断,则不会有任何事发生),所以并不代表“立刻关闭”。
    shutdown() :启动顺序关闭,其中执行先前提交的任务,但不接受新任务。如果已经关闭,则调用没有附加效果。此方法不等待先前提交的任务完成执行。
    shutdownNow():尝试停止所有正在执行的任务,停止等待任务的处理,并返回正在等待执行的任务的列表。当从此方法返回时,这些任务将从任务队列中耗尽(删除)。此方法不等待主动执行的任务终止。

    executor.awaitTermination(this.awaitTerminationSeconds, TimeUnit.SECONDS)); 控制等待的时间,防止任务无限期的运行(前面已经强调过了,即使是 shutdownNow 也不能保证线程一定停止运行)。


注意:
  虚拟机会对多个shutdownhook以未知的顺序调用,都执行完后再退出。
  如果接收到 kill -15 pid 命令时,执行阻塞操作,可以做到等待任务执行完成之后再关闭 JVM。同时,也解释了一些应用执行 kill -15 pid 无法退出的问题,如:中断被阻塞了,或者hook运行了死循环代码。

 

实现原理:

Runtime.getRuntime().addShutdownHook(hook);        // 添加钩子,开启优雅之路

// 具体流程如下:

复制代码
    /**
     * Registers a new virtual-machine shutdown hook.
     *
     * @param   hook
     *          An initialized but unstarted <tt>{@link Thread}</tt> object
     *
     * @throws  IllegalArgumentException
     *          If the specified hook has already been registered,
     *          or if it can be determined that the hook is already running or
     *          has already been run
     *
     * @throws  IllegalStateException
     *          If the virtual machine is already in the process
     *          of shutting down
     *
     * @throws  SecurityException
     *          If a security manager is present and it denies
     *          <tt>{@link RuntimePermission}("shutdownHooks")</tt>
     *
     * @see #removeShutdownHook
     * @see #halt(int)
     * @see #exit(int)
     * @since 1.3
     */
    public void addShutdownHook(Thread hook) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("shutdownHooks"));
        }
        // 添加到 application 中
        ApplicationShutdownHooks.add(hook);
    }
    
    // java.lang.ApplicationShutdownHooks.add(hook);
    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 以map类型保存, k->k 形式存储,保证每一个钩子都是独立的
        hooks.put(hook, hook);
    }
    
    // java.lang.ApplicationShutdownHooks 会先注册一个静态块,添加一个任务到 Shutdown 中
    /* 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() {
                        // 即当该任务被调用时,调用自身的运行方法,使所有注册的 hook 运行起来
                        runHooks();
                    }
                }
            );
            hooks = new IdentityHashMap<>();
        } catch (IllegalStateException e) {
            // application shutdown hooks cannot be added if
            // shutdown is in progress.
            hooks = null;
        }
    }
    
    // runHooks 执行所有钩子线程,进行异步调用
    /* 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 {
                // 阻塞等待所有完成
                hook.join();
            } catch (InterruptedException x) { }
        }
    }
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到现在为止,我们已经知道关闭钩子是如何执行的,但是,还不是知道,该钩子是何时触发?

复制代码
    // java.lang.Shutdown.add() 该方法会jvm主动调用,从而触发 后续钩子执行
    /* Invoked by the JNI DestroyJavaVM procedure when the last non-daemon
     * thread has finished.  Unlike the exit method, this method does not
     * actually halt the VM.
     */
    static void shutdown() {
        synchronized (lock) {
            switch (state) {
            case RUNNING:       /* Initiate shutdown */
                state = HOOKS;
                break;
            case HOOKS:         /* Stall and then return */
            case FINALIZERS:
                break;
            }
        }
        synchronized (Shutdown.class) {
            // 执行序列
            sequence();
        }
    }
    // 而 sequence() 则会调用 runHooks(), 调用自定义的钩子任务
    private static void sequence() {
        synchronized (lock) {
            /* Guard against the possibility of a daemon thread invoking exit
             * after DestroyJavaVM initiates the shutdown sequence
             */
            if (state != HOOKS) return;
        }
        runHooks();
        boolean rfoe;
        synchronized (lock) {
            state = FINALIZERS;
            rfoe = runFinalizersOnExit;
        }
        if (rfoe) runAllFinalizers();
    }
    
    // 执行钩子,此处最多允许注册 10 个钩子,且进行同步调用,当然这是最顶级的钩子,钩子下还可以添加钩子,可以任意添加n个
    private static void runHooks() {
        for (int i=0; i < MAX_SYSTEM_HOOKS; i++) {
            try {
                Runnable hook;
                synchronized (lock) {
                    // acquire the lock to make sure the hook registered during
                    // shutdown is visible here.
                    currentRunningHook = i;
                    hook = hooks[i];
                }
                // 同步调用注册的hook, 即 前面看到 ApplicationShutdownHooks.runHooks()
                if (hook != null) hook.run();
            } catch(Throwable t) {
                if (t instanceof ThreadDeath) {
                    ThreadDeath td = (ThreadDeath)t;
                    throw td;
                }
            }
        }
    }
    
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如此,整个关闭流程完美了。

简化为: 

  1. 注册流程(应用主动调用)
    Runtime.addShutdownHook -> ApplicationShutdownHooks.add()/static -> java.lang.Shutdown.add()/shutdown()
  2. 执行流程(jvm自动调用)
    java.lang.Shutdown.shutdown()->sequence()->runHooks() -> ApplicationShutdownHooks.runHooks() -> hooks 最终

 

不要害怕今日的苦,你要相信明天,更苦!

标签:java,Thread,hooks,停机,System,hook,源码,shutdown,new
来源: https://www.cnblogs.com/shoshana-kong/p/14715280.html