再谈为了提醒明知故犯（在一坑里迭倒两次不是不多见），由于业务系统中大量使用了spring Boot embedded tomcat的模式运行，在一些运维脚本中经常看到Linux 中kill指令，然而它的使用也有些讲究，要思考如何能做到优雅停机。

何为优雅关机

就是为确保应用关闭时，通知应用进程释放所占用的资源

线程池，shutdown（不接受新任务等待处理完）还是shutdownNow（调用Thread.interrupt进行中断）socket 链接，比如：netty、mq告知注册中心快速下线（靠心跳机制客服早都跳起来了），比如：eureka清理临时文件，比如：poi各种堆内堆外内存释放

总之，进程强行终止会带来数据丢失或者终端无法恢复到正常状态，在分布式环境下还可能导致数据不一致的情况。

kill指令

kill -9 pid可以模拟了一次系统宕机，系统断电等极端情况，而kill -15 pid则是等待应用关闭，执行阻塞操作，有时候也会出现无法关闭应用的情况（线上理想情况下，是bug就该寻根溯源）

#查看jvm进程pidjps#列出所有信号名称kill -l # Windows下信号常量值# 简称  全称    数值 # INT   SIGINT     2       Ctrl+C中断# ILL   SIGILL     4       非法指令# FPE   SIGFPE     8       floating point exception(浮点异常)# SEGV  SIGSEGV    11      segment violation(段错误)# TERM  SIGTERM    5       Software termination signal from kill(Kill发出的软件终止)# BREAK SIGBREAK   21      Ctrl-Break sequence(Ctrl+Break中断)# ABRT  SIGABRT    22      abnormal termination triggered by abort call(Abort) #linux信号常量值# 简称  全称  数值  # HUP   SIGHUP      1    终端断线  # INT   SIGINT      2    中断（同 Ctrl + C）        # QUIT  SIGQUIT     3    退出（同 Ctrl + \）         # KILL  SIGKILL     9    强制终止         # TERM  SIGTERM     15    终止         # CONT  SIGCONT     18    继续（与STOP相反， fg/bg命令）         # STOP  SIGSTOP     19    暂停（同 Ctrl + Z）        #.... #可以理解为操作系统从内核级别强行杀死某个进程kill -9 pid #理解为发送一个通知，等待应用主动关闭kill -15 pid#也支持信号常量值全称或简写（就是去掉SIG后）kill -l KILL

思考：jvm是如何接受处理linux信号量的？

当然是在jvm启动时就加载了自定义SignalHandler，关闭jvm时触发对应的handle。

public interface SignalHandler {    SignalHandler SIG_DFL = new NativeSignalHandler(0L);    SignalHandler SIG_IGN = new NativeSignalHandler(1L);     void handle(Signal var1);}class Terminator {    private static SignalHandler handler = null;     Terminator() {    }    //jvm设置SignalHandler，在System.initializeSystemClass中触发    static void setup() {        if (handler == null) {            SignalHandler var0 = new SignalHandler() {                public void handle(Signal var1) {                    Shutdown.exit(var1.getNumber() + 128);//调用Shutdown.exit                }            };            handler = var0;             try {                Signal.handle(new Signal("INT"), var0);//中断时            } catch (IllegalArgumentException var3) {                ;            }             try {                Signal.handle(new Signal("TERM"), var0);//终止时            } catch (IllegalArgumentException var2) {                ;            }         }    }}

在了解Shutdown.exit之前，先看Runtime.getRuntime().addShutdownHook(shutdownHook);则是为jvm中增加一个关闭的钩子，当jvm关闭的时候调用。

public class Runtime {    public void addShutdownHook(Thread hook) {        SecurityManager sm = System.getSecurityManager();        if (sm != null) {            sm.checkPermission(new RuntimePermission("shutdownHooks"));        }        ApplicationShutdownHooks.add(hook);    }}class ApplicationShutdownHooks {    /* The set of registered hooks */    private static IdentityHashMap<Thread, Thread> hooks;    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);    }}//它含数据结构和逻辑管理虚拟机关闭序列class Shutdown {    /* Shutdown 系列状态*/    private static final int RUNNING = 0;    private static final int HOOKS = 1;    private static final int FINALIZERS = 2;    private static int state = RUNNING;    /* 是否应该运行所以finalizers来exit? */    private static boolean runFinalizersOnExit = false;    // 系统关闭钩子注册一个预定义的插槽.    // 关闭钩子的列表如下:    // (0) Console restore hook    // (1) Application hooks    // (2) DeleteOnExit hook    private static final int MAX_SYSTEM_HOOKS = 10;    private static final Runnable[] hooks = new Runnable[MAX_SYSTEM_HOOKS];    // 当前运行关闭钩子的钩子的索引    private static int currentRunningHook = 0;    /* 前面的静态字段由这个锁保护 */    private static class Lock { };    private static Object lock = new Lock();     /* 为native halt方法提供锁对象 */    private static Object haltLock = new Lock();     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) {//执行shutdown过程中不添加hook                if (state > RUNNING)//如果已经在执行shutdown操作不能添加hook                    throw new IllegalStateException("Shutdown in progress");            } else {//如果hooks已经执行完毕不能再添加hook。如果正在执行hooks时，添加的槽点小于当前执行的槽点位置也不能添加                if (state > HOOKS || (state == HOOKS && slot <= currentRunningHook))                    throw new IllegalStateException("Shutdown in progress");            }             hooks[slot] = hook;        }    }    /* 执行所有注册的hooks     */    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];                }                if (hook != null) hook.run();            } catch(Throwable t) {                if (t instanceof ThreadDeath) {                    ThreadDeath td = (ThreadDeath)t;                    throw td;                }            }        }    }    /* 关闭JVM的操作     */    static void halt(int status) {        synchronized (haltLock) {            halt0(status);        }    }    //JNI方法    static native void halt0(int status);    // shutdown的执行顺序：runHooks > runFinalizersOnExit    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();    }    //Runtime.exit时执行，runHooks > runFinalizersOnExit > halt    static void exit(int status) {        boolean runMoreFinalizers = false;        synchronized (lock) {            if (status != 0) runFinalizersOnExit = false;            switch (state) {            case RUNNING:       /* Initiate shutdown */                state = HOOKS;                break;            case HOOKS:         /* Stall and halt */                break;            case FINALIZERS:                if (status != 0) {                    /* Halt immediately on nonzero status */                    halt(status);                } else {                    /* Compatibility with old behavior:                     * Run more finalizers and then halt                     */                    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);        }    }    //shutdown操作，与exit不同的是不做halt操作(关闭JVM)    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();        }    }}

在spring中通过ContextClosedEvent事件来触发一些动作（可以拓展），主要通过LifecycleProcessor.onClose来做stopBeans。由此可见spring也基于jvm做了拓展。

public abstract class AbstractApplicationContext extends DefaultResourceLoader { public void registerShutdownHook() {  if (this.shutdownHook == null) {   // No shutdown hook registered yet.   this.shutdownHook = new Thread() {    @Override    public void run() {     doClose();    }   };   Runtime.getRuntime().addShutdownHook(this.shutdownHook);  } } protected void doClose() {  boolean actuallyClose;  synchronized (this.activeMonitor) {   actuallyClose = this.active && !this.closed;   this.closed = true;  }   if (actuallyClose) {   if (logger.isInfoEnabled()) {    logger.info("Closing " + this);   }    LiveBeansView.unregisterApplicationContext(this);    try {    //发布应用内的关闭事件    publishEvent(new ContextClosedEvent(this));   }   catch (Throwable ex) {    logger.warn("Exception thrown from ApplicationListener handling ContextClosedEvent", ex);   }    // 停止所有的Lifecycle beans.   try {    getLifecycleProcessor().onClose();   }   catch (Throwable ex) {    logger.warn("Exception thrown from LifecycleProcessor on context close", ex);   }    // 销毁spring 的 BeanFactory可能会缓存单例的 Bean.   destroyBeans();    // 关闭当前应用上下文（BeanFactory）   closeBeanFactory();    // 执行子类的关闭逻辑   onClose();    synchronized (this.activeMonitor) {    this.active = false;   }  } } }public interface LifecycleProcessor extends Lifecycle { /**  * Notification of context refresh, e.g. for auto-starting components.  */ void onRefresh();  /**  * Notification of context close phase, e.g. for auto-stopping components.  */ void onClose();}

到这里就进入重点了，spring boot中有spring-boot-starter-actuator模块提供了一个 restful 接口，用于优雅停机。

执行请求curl -X POST ，待关闭成功则返回提示。

注：线上环境该url需要设置权限，可配合 spring-security使用或在nginx中限制内网访问

#启用shutdownendpoints.shutdown.enabled=true#禁用密码验证endpoints.shutdown.sensitive=false#可统一指定所有endpoints的路径management.context-path=/manage#指定管理端口和IPmanagement.port=8088management.address=127.0.0.1#开启shutdown的安全验证（spring-security）endpoints.shutdown.sensitive=true#验证用户名security.user.name=admin#验证密码security.user.password=secret#角色management.security.role=SUPERUSER

spring boot的shutdown原理也不复杂，其实还是通过调用AbstractApplicationContext.close实现的。

@ConfigurationProperties(    prefix = "endpoints.shutdown")public class ShutdownMvcEndpoint extends EndpointMvcAdapter {    public ShutdownMvcEndpoint(ShutdownEndpoint delegate) {        super(delegate);    }    //post请求    @PostMapping(        produces = {"application/vnd.spring-boot.actuator.v1+json", "application/json"}    )    @ResponseBody    public Object invoke() {        return !this.getDelegate().isEnabled() ? new ResponseEntity(Collections.singletonMap("message", "This endpoint is disabled"), HttpStatus.NOT_FOUND) : super.invoke();    }}@ConfigurationProperties(    prefix = "endpoints.shutdown")public class ShutdownEndpoint extends AbstractEndpoint<Map<String, Object>> implements ApplicationContextAware {    private static final Map<String, Object> NO_CONTEXT_MESSAGE = Collections.unmodifiableMap(Collections.singletonMap("message", "No context to shutdown."));    private static final Map<String, Object> SHUTDOWN_MESSAGE = Collections.unmodifiableMap(Collections.singletonMap("message", "Shutting down, bye..."));    private ConfigurableApplicationContext context;     public ShutdownEndpoint() {        super("shutdown", true, false);    }    //执行关闭    public Map<String, Object> invoke() {        if (this.context == null) {            return NO_CONTEXT_MESSAGE;        } else {            boolean var6 = false;             Map var1;             class NamelessClass_1 implements Runnable {                NamelessClass_1() {                }                 public void run() {                    try {                        Thread.sleep(500L);                    } catch (InterruptedException var2) {                        Thread.currentThread().interrupt();                    }                    //这个调用的就是AbstractApplicationContext.close                    ShutdownEndpoint.this.context.close();                }            }             try {                var6 = true;                var1 = SHUTDOWN_MESSAGE;                var6 = false;            } finally {                if (var6) {                    Thread thread = new Thread(new NamelessClass_1());                    thread.setContextClassLoader(this.getClass().getClassLoader());                    thread.start();                }            }             Thread thread = new Thread(new NamelessClass_1());            thread.setContextClassLoader(this.getClass().getClassLoader());            thread.start();            return var1;        }    }}

原文：blog.csdn.net/alex_xfboy/article/details/90404691