Spring优雅停止服务的实现及原理分析

背景

服务能被优雅停机，指的是：在接收到停机指令后，服务程序应该能拒绝新的请求, 但应该继续完成已经接收请求。

解决方案

Graceful Shutdown Spring Boot Applications 这篇文章提供了一个经典的实现优雅停机的实现。

import org.apache.catalina.connector.Connector;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.web.embedded.tomcat.TomcatConnectorCustomizer;
import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextClosedEvent;
import java.util.concurrent.Executor;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class GracefulShutdown implements TomcatConnectorCustomizer, ApplicationListener<ContextClosedEvent> {
    private static final Logger log = LoggerFactory.getLogger(GracefulShutdown.class);
    private volatile Connector connector;
    @Override
    public void customize(Connector connector) {
        this.connector = connector;
    }
    @Override
    public void onApplicationEvent(ContextClosedEvent event) {
        this.connector.pause();
        Executor executor = this.connector.getProtocolHandler().getExecutor();
        if (executor instanceof ThreadPoolExecutor) {
            try {
                ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) executor;
                threadPoolExecutor.shutdown();
                if (!threadPoolExecutor.awaitTermination(30, TimeUnit.SECONDS)) {
                    log.warn("Tomcat thread pool did not shut down gracefully within "
                            + "30 seconds. Proceeding with forceful shutdown");
                }
            } catch (InterruptedException ex) {
                Thread.currentThread().interrupt();
            }
        }
    }
}

以上代码实现了两个功能：

1. 实现TomcatConnectorCustomizer，作用是获取tomcat的connector，方便后续对tomcat的连接做操作。
2. 实现ApplicationListener，监听ApplicationContext发布的事件，该例中监听的是ContextClosedEvent事件，监听到此类事件后把tomcat中connector的线程池shutdown并等待30s，如果所有线程都结束则正常退出，否则发出报警。这里也体现了设计模式中的观察者模式，实现交互对象之间的松耦合。

设计好listener之后还需要将该listener注册到ApplicationContext中，方便其接受ApplicationContext发布的事件。同时还需将该listener注册到tomcat中，方便获取该Spring应用集成的tomcat的connector。注册的代码如下：

@Bean
public GracefulShutdown gracefulShutdown() {
    return new GracefulShutdown();
}
@Bean
public ConfigurableServletWebServerFactory webServerFactory(final GracefulShutdown gracefulShutdown) {
    TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
    factory.addConnectorCustomizers(gracefulShutdown);
    return factory;
}