求求你們別再用 kill -9 了，這才是 Spring Boot 停機的正確方式！！！

再談為了提醒明知故犯（在一坑裡迭倒兩次不是不多見），由於業務系統中大量使用了spring Boot embedded tomcat的模式運行，在一些運維腳本中經常看到Linux 中 kill 指令，然而它的使用也有些講究，要思考如何能做到優雅停機。何為優雅關機就是為確保應用關閉時，通知應用進程釋 ...

再談為了提醒明知故犯（在一坑裡迭倒兩次不是不多見），由於業務系統中大量使用了spring Boot embedded tomcat的模式運行，在一些運維腳本中經常看到Linux 中 kill 指令，然而它的使用也有些講究，要思考如何能做到優雅停機。

何為優雅關機

就是為確保應用關閉時，通知應用進程釋放所占用的資源

線程池，shutdown（不接受新任務等待處理完）還是shutdownNow（調用 Thread.interrupt進行中斷）
socket 鏈接，比如：netty、mq
告知註冊中心快速下線（靠心跳機制客服早都跳起來了），比如：eureka
清理臨時文件，比如：poi
各種堆內堆外記憶體釋放

總之，進程強行終止會帶來數據丟失或者終端無法恢復到正常狀態，在分散式環境下還可能導致數據不一致的情況。

kill指令

kill -9 pid 可以模擬了一次系統宕機，系統斷電等極端情況，而kill -15 pid 則是等待應用關閉，執行阻塞操作，有時候也會出現無法關閉應用的情況（線上理想情況下，是bug就該尋根溯源）

#查看jvm進程pid
jps
#列出所有信號名稱
kill -l
 
# Windows下信號常量值
# 簡稱  全稱    數值 
# INT   SIGINT     2       Ctrl+C中斷
# ILL   SIGILL     4       非法指令
# FPE   SIGFPE     8       floating point exception(浮點異常)
# SEGV  SIGSEGV    11      segment violation(段錯誤)
# TERM  SIGTERM    5       Software termination signal from kill(Kill發出的軟體終止)
# BREAK SIGBREAK   21      Ctrl-Break sequence(Ctrl+Break中斷)
# ABRT  SIGABRT    22      abnormal termination triggered by abort call(Abort)
 
#linux信號常量值
# 簡稱  全稱  數值  
# HUP   SIGHUP      1    終端斷線  
# INT   SIGINT      2    中斷（同 Ctrl + C）        
# QUIT  SIGQUIT     3    退出（同 Ctrl + \）         
# KILL  SIGKILL     9    強制終止         
# TERM  SIGTERM     15    終止         
# CONT  SIGCONT     18    繼續（與STOP相反， fg/bg命令）         
# STOP  SIGSTOP     19    暫停（同 Ctrl + Z）        
#....
 
#可以理解為操作系統從內核級別強行殺死某個進程
kill -9 pid 
#理解為發送一個通知，等待應用主動關閉
kill -15 pid
#也支持信號常量值全稱或簡寫（就是去掉SIG後）
kill -l KILL

思考：jvm是如何接受處理linux信號量的？

當然是在jvm啟動時就載入了自定義SignalHandler，關閉jvm時觸發對應的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設置SignalHandler，在System.initializeSystemClass中觸發
    static void setup() {
        if (handler == null) {
            SignalHandler var0 = new SignalHandler() {
                public void handle(Signal var1) {
                    Shutdown.exit(var1.getNumber() + 128);//調用Shutdown.exit
                }
            };
            handler = var0;
 
            try {
                Signal.handle(new Signal("INT"), var0);//中斷時
            } catch (IllegalArgumentException var3) {
                ;
            }
 
            try {
                Signal.handle(new Signal("TERM"), var0);//終止時
            } catch (IllegalArgumentException var2) {
                ;
            }
 
        }
    }
}

Runtime.addShutdownHook

在瞭解Shutdown.exit之前，先看Runtime.getRuntime().addShutdownHook(shutdownHook);則是為jvm中增加一個關閉的鉤子，當jvm關閉的時候調用。

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);
    }
}
//它含數據結構和邏輯管理虛擬機關閉序列
class Shutdown {
    /* Shutdown 系列狀態*/
    private static final int RUNNING = 0;
    private static final int HOOKS = 1;
    private static final int FINALIZERS = 2;
    private static int state = RUNNING;
    /* 是否應該運行所以finalizers來exit? */
    private static boolean runFinalizersOnExit = false;
    // 系統關閉鉤子註冊一個預定義的插槽.
    // 關閉鉤子的列表如下:
    // (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];
    // 當前運行關閉鉤子的鉤子的索引
    private static int currentRunningHook = 0;
    /* 前面的靜態欄位由這個鎖保護 */
    private static class Lock { };
    private static Object lock = new Lock();
 
    /* 為native halt方法提供鎖對象 */
    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) {//執行shutdown過程中不添加hook
                if (state > RUNNING)//如果已經在執行shutdown操作不能添加hook
                    throw new IllegalStateException("Shutdown in progress");
            } else {//如果hooks已經執行完畢不能再添加hook。如果正在執行hooks時，添加的槽點小於當前執行的槽點位置也不能添加
                if (state > HOOKS || (state == HOOKS && slot <= currentRunningHook))
                    throw new IllegalStateException("Shutdown in progress");
            }
 
            hooks[slot] = hook;
        }
    }
    /* 執行所有註冊的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;
                }
            }
        }
    }
    /* 關閉JVM的操作
     */
    static void halt(int status) {
        synchronized (haltLock) {
            halt0(status);
        }
    }
    //JNI方法
    static native void halt0(int status);
    // shutdown的執行順序：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時執行，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，與exit不同的是不做halt操作(關閉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();
        }
    }
}

spring 3.2.12

在spring中通過ContextClosedEvent事件來觸發一些動作（可以拓展），主要通過LifecycleProcessor.onClose來做stopBeans。由此可見spring也基於jvm做了拓展。

推薦一個開源免費的 Spring Boot 最全教程：

https://github.com/javastacks/spring-boot-best-practice

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 {
    //發佈應用內的關閉事件
    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);
   }
 
   // 銷毀spring 的 BeanFactory可能會緩存單例的 Bean.
   destroyBeans();
 
   // 關閉當前應用上下文（BeanFactory）
   closeBeanFactory();
 
   // 執行子類的關閉邏輯
   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();
}

spring boot

到這裡就進入重點了，spring boot中有spring-boot-starter-actuator 模塊提供了一個 restful 介面，用於優雅停機。執行請求 curl -X POST http://127.0.0.1:8088/shutdown ，待關閉成功則返回提示。

Spring Boot 基礎就不介紹了，推薦看這個免費教程：

https://github.com/javastacks/spring-boot-best-practice

註：線上環境該url需要設置許可權，可配合 spring-security使用或在nginx中限制內網訪問

#啟用shutdown
endpoints.shutdown.enabled=true
#禁用密碼驗證
endpoints.shutdown.sensitive=false
#可統一指定所有endpoints的路徑
management.context-path=/manage
#指定管理埠和IP
management.port=8088
management.address=127.0.0.1

#開啟shutdown的安全驗證（spring-security）
endpoints.shutdown.sensitive=true
#驗證用戶名
security.user.name=admin
#驗證密碼
security.user.password=secret
#角色
management.security.role=SUPERUSER

spring boot的shutdown原理也不複雜，其實還是通過調用AbstractApplicationContext.close實現的。

@ConfigurationProperties(
    prefix = "endpoints.shutdown"
)
public class ShutdownMvcEndpoint extends EndpointMvcAdapter {
    public ShutdownMvcEndpoint(ShutdownEndpoint delegate) {
        super(delegate);
    }
    //post請求
    @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);
    }
    //執行關閉
    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();
                    }
                    //這個調用的就是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;
        }
    }
}

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