《Dubbo 实现原理与源码解析 —— 精品合集》 《Netty 实现原理与源码解析 —— 精品合集》
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《Spring Boot 实现原理与源码解析 —— 精品合集》 《Java 面试题 + Java 学习指南》

摘要: 原创出处 blog.csdn.net/alex_xfboy/article/details/90404691/ 「布道」欢迎转载,保留摘要,谢谢!


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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pid
jps
#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) {
;
}

}
}
}

Runtime.addShutdownHook

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();
}
}
}

spring 3.2.12

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();
}

spring boot

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 http://127.0.0.1:8088/shutdown ,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shutdown
endpoints.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IP
management.port=8088
management.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;
}
}
}

文章目录
  1. 1. 何为优雅关机
  2. 2. kill指令
  3. 3. Runtime.addShutdownHook
  4. 4. spring 3.2.12
  5. 5. spring boot