Spring Boot 在关闭时，如果有请求没有响应完，在不同的容器会出现不同的结果，例如，在 Tomcat 和 Undertow 中会出现中断异常，那么就有可能对业务造成影响。所以，优雅停机非常有必要性，目前官方是没有提供很好的策略来实现。

Each SpringApplication registers a shutdown hook with the JVM to ensure that the ApplicationContext closes gracefully on exit. All the standard Spring lifecycle callbacks (such as the DisposableBean interface or the @PreDestroy annotation) can be used.

Spring Boot Application 在接收到停机信号后，可以通过 DisposableBean 接口 、 @PreDestroy 注解 或者 ContextClosedEvent 事件来处理优雅停机的相关逻辑。

版本信息

Spring Boot 版本： 2.0.0.RELEASE

Tomcat 优雅停机

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    /**
     * 用于接受 shutdown 事件
     */
    @Bean
    public GracefulShutdown gracefulShutdown() {
        return new GracefulShutdown();
    }

    @Bean
    public ServletWebServerFactory servletContainer() {
        TomcatServletWebServerFactory tomcat = new TomcatServletWebServerFactory();
        tomcat.addConnectorCustomizers(gracefulShutdown());
        return tomcat;
    }

    /**
     * 优雅关闭 Spring Boot
     */
    private class GracefulShutdown implements TomcatConnectorCustomizer, ApplicationListener<ContextClosedEvent> {
        private final Logger log = LoggerFactory.getLogger(GracefulShutdown.class);
        private volatile Connector connector;
        private final int waitTime = 30;
        @Override
        public void customize(Connector connector) {
            this.connector = connector;
        }
        @Override
        public void onApplicationEvent(ContextClosedEvent contextClosedEvent) {
            this.connector.pause();
            Executor executor = this.connector.getProtocolHandler().getExecutor();
            if (executor instanceof ThreadPoolExecutor) {
                try {
                    ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) executor;
                    threadPoolExecutor.shutdown();
                    if (!threadPoolExecutor.awaitTermination(waitTime, TimeUnit.SECONDS)) {
                        log.warn("Tomcat thread pool did not shut down gracefully within " + waitTime + " seconds. Proceeding with forceful shutdown");
                    }
                } catch (InterruptedException ex) {
                    Thread.currentThread().interrupt();
                }
            }
        }
    }

}

Undertow 优雅停机

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    /**
     * 优雅关闭 Spring Boot
     */
    @Component
    public class GracefulShutdown implements ApplicationListener<ContextClosedEvent> {

        @Autowired
        private GracefulShutdownWrapper gracefulShutdownWrapper;

        @Autowired
        private ServletWebServerApplicationContext context;

        @Override
        public void onApplicationEvent(ContextClosedEvent contextClosedEvent){
            gracefulShutdownWrapper.getGracefulShutdownHandler().shutdown();
            try {
                UndertowServletWebServer webServer = (UndertowServletWebServer)context.getWebServer();
                Field field = webServer.getClass().getDeclaredField("undertow");
                field.setAccessible(true);
                Undertow undertow = (Undertow) field.get(webServer);
                List<Undertow.ListenerInfo> listenerInfo = undertow.getListenerInfo();
                Undertow.ListenerInfo listener = listenerInfo.get(0);
                ConnectorStatistics connectorStatistics = listener.getConnectorStatistics();
                while (connectorStatistics.getActiveConnections() > 0){}
            }catch (Exception e){
                // Application Shutdown
            }
        }
    }
}

@Component
public class GracefulShutdownWrapper implements HandlerWrapper{

    private GracefulShutdownHandler gracefulShutdownHandler;

    @Override
    public HttpHandler wrap(HttpHandler handler) {
        if(gracefulShutdownHandler == null) {
            this.gracefulShutdownHandler = new GracefulShutdownHandler(handler);
        }
        return gracefulShutdownHandler;
    }

    public GracefulShutdownHandler getGracefulShutdownHandler() {
        return gracefulShutdownHandler;
    }

}

@Component
@AllArgsConstructor
public class UndertowExtraConfiguration {

    private final GracefulShutdownWrapper gracefulShutdownWrapper;

    @Bean
    public UndertowServletWebServerFactory servletWebServerFactory() {
        UndertowServletWebServerFactory factory = new UndertowServletWebServerFactory();
        factory.addDeploymentInfoCustomizers(deploymentInfo -> deploymentInfo.addOuterHandlerChainWrapper(gracefulShutdownWrapper));
        factory.addBuilderCustomizers(builder -> builder.setServerOption(UndertowOptions.ENABLE_STATISTICS, true));
        return factory;
    }

}

Jetty 优雅停机

默认支持所有请求完毕后再关闭，缺点：客户端接收不到响应，有待改进！

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