Java应用如何使用shutdownhook

在线上Java程序中经常遇到进程程挂掉，一些状态没有正确的保存下来，这时候就需要在JVM关掉的时候执行一些清理现场的代码。JDK在1.3之后提供了Java Runtime.addShutdownHook(Thread hook)方法，可以注册一个JVM关闭的钩子，这个钩子可以在以下几种场景被调用：

1. 程序正常退出
2. 使用System.exit()
3. 终端使用Ctrl+C触发的中断
4. 系统关闭
5. 使用Kill pid命令干掉进程

在JDK中方法的声明：

public void addShutdownHook(Thread hook)
参数
hook -- 一个初始化但尚未启动的线程对象，注册到JVM钩子的运行代码。
异常
IllegalArgumentException -- 如果指定的钩已被注册，或如果它可以判定钩已经运行或已被运行
IllegalStateException -- 如果虚拟机已经是在关闭的过程中
SecurityException -- 如果存在安全管理器并且它拒绝的RuntimePermission（“shutdownHooks”）

中断信号

在Linux信号机制中，存在多种进程中断信号（Linux信号列表 ）。其中比较典型的有 SIGNKILL（9） 和 SIGNTERM（15）.

SIGNKILL（9） 和 SIGNTERM（15） 的区别在于：
SIGNKILL（9） 的效果是立即杀死进程. 该信号不能被阻塞, 处理和忽略。
SIGNTERM（15） 的效果是正常退出进程，退出前可以被阻塞或回调处理。并且它是Linux缺省的程序中断信号。

注：在使用kill -9 pid 时JVM注册的钩子不会被调用。 kill 命令默认信号是 -15 ，-9 时会强制关闭。通常在使⽤ kill -9 前，应该先使⽤ kill -15，给⽬标进程⼀个清理善后⼯作的机会。如果没有，可能会留下⼀些不完整的⽂件或状态，从⽽影响服务的再次启动。

demo

public class TestShutdownHook {
    static Timer timer = new Timer("job-timer");
    static AtomicInteger count = new AtomicInteger(0);

    /**
     * hook线程
     */
    static class CleanWorkThread extends Thread{
        @Override
        public void run() {
            System.out.println("clean some work.");
            timer.cancel();
            try {
                System.out.println("try sleep 2s.");
                Thread.sleep(2 * 1000);
                System.out.println("sleep 2s.");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args) throws InterruptedException {
        //将hook线程添加到运行时环境中去
        Runtime.getRuntime().addShutdownHook(new CleanWorkThread());
        System.out.println("ShutdownHook added");
        //简单模拟
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                count.getAndIncrement();
                System.out.println("doing job " + count);
                if (count.get() == 3) {  //执行了3次后退出
                    System.exit(0);
                }
            }
        }, 0, 2 * 1000);
    }
}

output:

ShutdownHook added
doing job 1
doing job 2
doing job 3
clean some work.
try sleep 2s.
sleep 2s.

源码分析

Runtime类中对ShutdownHook的操作都是通过工具类ApplicationShutdownHooks来实现的。

class ApplicationShutdownHooks {
    // IdentityHashMap的特性是 比较键（和值）时使用引用相等性代替对象相等性
    // 也就是说使用 == 而不是使用 equals，即允许存在值相同，但是引用不同的情况
    /* The set of registered hooks */
    private static IdentityHashMap<Thread, Thread> hooks;
    static {
        try {
            // 注册根shutdown(shutdown 槽)
            Shutdown.add(1 /* shutdown hook 调用顺序 */,
                false /* not registered if shutdown in progress，默认为false */,
                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() {}

    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);
    }

    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;
    }

    /* 遍历所有 application hooks 并启动线程*/
    static void runHooks() {
        Collection<Thread> threads;
        synchronized(ApplicationShutdownHooks.class) {
            threads = hooks.keySet();
            hooks = null;
        }

        for (Thread hook : threads) {
            hook.start(); // 此处说明shutdown hook 是乱序执行的
        }
        for (Thread hook : threads) {
            while (true) {
                try {
                    hook.join(); // hook 线程阻塞当前线程
                    break;
                } catch (InterruptedException ignored) {
                }
            }
        }
    }
}

Shutdown

下面来看下Shutdown的源码

class Shutdown {
    // Shutdown 状态 枚举
    private static final int RUNNING = 0;
    private static final int HOOKS = 1;
    private static final int FINALIZERS = 2;
    // 存储 Shutdown状态
    private static int state = RUNNING;

    // 是否退出时执行所有回收器
    private static boolean runFinalizersOnExit = false;

    // shutdown hook的 预注册 逻辑:
    // (0) Console restore hook ：处理命令行
    // (1) Application hooks
    // (2) DeleteOnExit hook
    private static final int MAX_SYSTEM_HOOKS = 10; // 最大hook slot 数是10
    private static final Runnable[] hooks = new Runnable[MAX_SYSTEM_HOOKS];

    // 当前运行的hook在数组中的位置
    private static int currentRunningHook = 0;

    // 保护 static 字段的锁类
    private static class Lock { };
    // 类锁，用于处理hooks 数组
    private static Object lock = new Lock();

    /* 类锁的key,用于保证halt方法的并发 */
    private static Object haltLock = new Lock();

    /* 该方法被 Runtime.runFinalizersOnExit() 调用 */
    static void setRunFinalizersOnExit(boolean run) {
        synchronized (lock) {
            runFinalizersOnExit = run;
        }
    }

    /**
     * 添加新的shutdown，检查状态和配置
     *
     * 除了注册DeleteOnExitHook的时侯，registerShutdownInProgress 参数默认设为false
     * application shutdown hooks 在运行中的时候，可能会向DeleteOnExitHook添加文件
     * File.deleteOnExit() 会调用 DeleteOnExitHook.add(path);
     *
     * @params slot  退出钩子的索引位置, 在shutdown期间按顺序执行
     * @params registerShutdownInProgress true to allow the hook
     *               to be registered even if the shutdown is in progress.
     * @params hook  the hook to be registered
     *
     * @throw IllegalStateException
     *        if registerShutdownInProgress is false and shutdown is in progress; or
     *        if registerShutdownInProgress is true and the shutdown process
     *           already passes the given slot
     */
    static void add(int slot, boolean registerShutdownInProgress, Runnable hook) {
        synchronized (lock) {
            if (hooks[slot] != null)
                throw new InternalError("Shutdown hook at slot " + slot + " already registered");

            if (!registerShutdownInProgress) {
                if (state > RUNNING)
                    throw new IllegalStateException("Shutdown in progress");
            } else {
                // 1. 已经执行完了hook 阶段
                // 2. 还在处理hook阶段，但是已经处理完了这个slot
                if (state > HOOKS || (state == HOOKS && slot <= currentRunningHook))
                    throw new IllegalStateException("Shutdown in progress");
            }

            hooks[slot] = hook;
        }
    }

    /* 运行所有的shutdownhook
     */
    private static void runHooks() {
        for (int i=0; i < MAX_SYSTEM_HOOKS; i++) {
            try {
                Runnable hook;
                synchronized (lock) {
                    // 确保数组的可见性
                    currentRunningHook = i;
                    hook = hooks[i];
                }
                if (hook != null) hook.run(); // 注意此处是顺序执行的
                // 使用的是run,而不是start()
            } catch(Throwable t) {
                if (t instanceof ThreadDeath) {
                    ThreadDeath td = (ThreadDeath)t;
                    throw td;
                }
            }
        }
    }
    // 强制结束，status是非零退出码
    static void halt(int status) {
        synchronized (haltLock) {
            // 此处加类锁,是为了防止处理 delete-on-shutdown 文件列表时的并发问题
            halt0(status);
        }
    }
    static native void halt0(int status);

    /* （虫洞）实际上等同调用 java.lang.ref.Finalizer.runAllFinalizers */
    private static native void runAllFinalizers();


    /* 真正的shutdown逻辑. 
     * 如果不是为了支持 runFinalizersOnExit,这个方法会很简单，直接执行所有的hooks，然后 halt。
     * 现在我们需要关注当前是在执行 hooks 或 finalizers。
     * 在第二个用例，finalizer 可能直接调用exit(1) ,从而立刻终止。
     * 但是在第一个用例，任何对exit(n)的调用都会失效。
     * 注意，如果开启了on-exit finalizers，只有当  shutdown exit(0)  时才会执行。
     */
    private static void sequence() {
        synchronized (lock) {
            /* 防止守护线程在jvm 执行shutdown后调用exit()
             */
            if (state != HOOKS) return;
        }
        runHooks(); // 运行钩子方法
        boolean rfoe;
        synchronized (lock) {
            state = FINALIZERS; // 此处修改了state
            rfoe = runFinalizersOnExit;
        }
        if (rfoe) runAllFinalizers(); // 执行所有的 finalizer
    }


    /* 被Runtime.exit()调用, 或者被系统终止事件handler调用。
     */
    static void exit(int status) {
        boolean runMoreFinalizers = false;
        synchronized (lock) {
            if (status != 0) runFinalizersOnExit = false;  // 不正常退出
            switch (state) {
            case RUNNING:       /* Initiate shutdown */
                state = HOOKS;
                break;
            case HOOKS:         /* Stall and halt */
                break;
            case FINALIZERS:
                if (status != 0) {
                    // 非0状态立即停止
                    halt(status);
                } else {
                    // 兼容旧逻辑，执行finalizer后退出
                    runMoreFinalizers = runFinalizersOnExit;
                }
                break;
            }
        }
        if (runMoreFinalizers) {
            runAllFinalizers();
            halt(status);
        }
        synchronized (Shutdown.class) {
            /* Synchronize on the class object, causing any other thread
             * that attempts to initiate shutdown to stall indefinitely
             */
            sequence();
            halt(status);
        }
    }


    /* 通过JNI 调用
     * 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) {
            sequence();
        }
    }
}

shutdown的字段state的状态机如下图所示:

graph LR
id0((begin))-->id1(running)
id1-->|"Runtime.exit()"|id2(HOOKS)
id1-->|"shutdown()"|id2
id2-->|"sequence()"|id3(FINALIZERS)
id3-->id4((end))

delete-on-exit file list

java.io.File的deleteOnExit()方法可以在JVM退出时，删除文件。主要的实现就是DeleteOnExitHook.

class DeleteOnExitHook {
    private static LinkedHashSet<String> files = new LinkedHashSet<>();
    static {
        // DeleteOnExitHook 必须是最后一个被调用的shutdown hook。
        // Application shutdown hooks 可能会向 delete on exit file list中添加文件,
        // 此时shutdown的状态是 hooks， 所以将 registerShutdownInProgress 设置为 true.
        sun.misc.SharedSecrets.getJavaLangAccess()
            .registerShutdownHook(2 /* Shutdown hook invocation order */,
                true /* register even if shutdown in progress */,
                new Runnable() {
                    public void run() {
                       runHooks();
                    }
                }
        );
    }

    private DeleteOnExitHook() {}

    static synchronized void add(String file) {
        if(files == null) {
            // DeleteOnExitHook is running. Too late to add a file
            throw new IllegalStateException("Shutdown in progress");
        }

        files.add(file);
    }

    static void runHooks() {
        LinkedHashSet<String> theFiles;

        synchronized (DeleteOnExitHook.class) {
            theFiles = files;
            files = null;
        }

        ArrayList<String> toBeDeleted = new ArrayList<>(theFiles);

        // reverse the list to maintain previous jdk deletion order.
        // Last in first deleted.
        Collections.reverse(toBeDeleted);
        for (String filename : toBeDeleted) {
            (new File(filename)).delete();
        }
    }
}