Notify SpringBoot application that its web server is up

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I need that my Springboot application knows when its web server (Tomcat) is up

I tried something using @WebListener, ServletContextListener and @ServletComponentScan

  @WebListener
  public class MyServletContextListener implements ServletContextListener {
      private static final Logger LOG = getLogger(MyServletContextListener.class);

      @Override
      public void contextInitialized(ServletContextEvent servletContextEvent) {
          LOG.info("Starting up!");
      }

      @Override
      public void contextDestroyed(ServletContextEvent servletContextEvent) {
          LOG.info("Shutting down!");
      }
  }

It seems to work: my listener is correctly notified.

However, I realized that if I try to join my Web server from this listener, the call fails

@Override
public void contextInitialized(ServletContextEvent servletContextEvent) {
    URI uri = HttpHelper.newAnyUriBuilder("localhost", 8181, false).path("actuator/health").build();
    new RestAPI().GET(uri); /// Throws
    LOG.info("Starting up!");
}

meaning that the web server is not completely UP AND READY

If I postpone the call, it succeeds:

@Override
public void contextInitialized(ServletContextEvent servletContextEvent) {
    new Thread(new Runnable() {
        @Override
        public void run() {
            try { Thread.sleep(1000); } catch (InterruptedException e) {} /// dirty but this is just a sample
            URI uri = HttpHelper.newAnyUriBuilder("localhost", 8181, false).path("actuator/health").build();
            new RestAPI().GET(uri); /// does not throw anymore

            LOG.info("Starting up!");
        }
    }).start();
}

Any idea how can I be notified correctly?

Thanks

spring-boot spring-mvc tomcat spring java
1 Answers

If you are using Spring Boot's embedded Tomcat server, you can listen for the WebServerInitializedEvent:

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.context.WebServerInitializedEvent;
import org.springframework.context.ApplicationListener;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class WebServerReadyExampleApplication {

    @Bean
    ApplicationListener<WebServerInitializedEvent> webServerInitializedListener() {
        return (event) -> {
            System.out.println("Web server ready");
        };
    }
    
    public static void main(String[] args) {
        SpringApplication.run(WebServerReadyExampleApplication.class, args);
    }
    

}

The listener will be called after Tomcat has started accepting connections:


  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::                (v2.7.2)

2022-07-23 10:22:05.295  INFO 6769 --- [           main] c.e.d.WebServerReadyExampleApplication   : Starting WebServerReadyExampleApplication using Java 17.0.1 on example-host with PID 6769 (/Users/example/web-server-ready-example/bin/main started by example in /Users/example/web-server-ready-example)
2022-07-23 10:22:05.297  INFO 6769 --- [           main] c.e.d.WebServerReadyExampleApplication   : No active profile set, falling back to 1 default profile: "default"
2022-07-23 10:22:05.935  INFO 6769 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2022-07-23 10:22:05.943  INFO 6769 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2022-07-23 10:22:05.943  INFO 6769 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.65]
2022-07-23 10:22:06.009  INFO 6769 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2022-07-23 10:22:06.009  INFO 6769 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 671 ms
2022-07-23 10:22:06.259  INFO 6769 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
Web server ready
2022-07-23 10:22:06.266  INFO 6769 --- [           main] c.e.d.WebServerReadyExampleApplication   : Started WebServerReadyExampleApplication in 1.247 seconds (JVM running for 1.577)
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