Spring Boot WebFlux Application Startup Sequence

September 3, 2019

Abstract

This article describes the activities that happen and the events that occur when a Spring Boot WebFlux application starts.

1. Application startup sequence

spring-boot-webflux-app-startup-sequence-flow

2. Event listeners

Applications can react to events published by the Spring framework by registering listeners.

MyEventListener.java
package rahulb.experiments.springappstartup.listener;

import org.springframework.context.ApplicationEvent;
import org.springframework.context.ApplicationListener;

public class MyEventListener implements ApplicationListener<ApplicationEvent> {

    @Override
    public void onApplicationEvent(ApplicationEvent event) {

        // Handle the event

        System.out.printf("Received %s%n", event.getClass().getSimpleName());
    }
}
META-INF/spring.factories
org.springframework.context.ApplicationListener=\
rahulb.experiments.springappstartup.listener.MyEventListener

3. Sample console logs

If the event listeners, beans and application runners print logs on the console, the console log would appear as follows:

The timestamp, severity, thread name, PID, logger name, etc. have been deleted from the log for brevity.
Console log
Received ApplicationStartingEvent (1)

Received ApplicationEnvironmentPreparedEvent

 :: Spring Boot ::        (v2.1.7.RELEASE)

Received ApplicationContextInitializedEvent

Starting Application on xxx with PID xxxxx ...

No active profile set, falling back to default profiles: default

Received ApplicationPreparedEvent

Controller.init() was called (2)

Received ContextRefreshedEvent (3)

o.s.b.web.embedded.netty.NettyWebServer  : Netty started on port(s): 8080 (4)

Received ReactiveWebServerInitializedEvent (5)

Started Application in 10.574 seconds (JVM running for 10.953)

Received ApplicationStartedEvent (6)

ApplicationRunner.run(...) was called

Received ApplicationReadyEvent (7)
1 ApplicationStartingEvent is published even before the loggers are ready.
2 The beans are instantiated and their init method is invoked after ApplicationPreparedEvent.
3 ContextRefreshedEvent is published after the beans are instantiated and initialized.
4 The web server is started after ContextRefreshedEvent.
5 ReactiveWebServerInitializedEvent is published after the web server is started.
6 ApplicationStartedEvent is published before the application runners are run.
7 ApplicationReadyEvent is published after the application runners are run.

References