Application eventing using Spring Framework
An eventing mechanism is a powerful tool for decoupling different components of application modules. Application level events are also useful for maintaining state changes in an application. In this article, we will create a simple application to Spring framework based eventing mechanism to publish events and listening to the events. This article presumes the working maven installation.
Let’s start by creating a simple maven project “spring-app-event-demo” by executing the following command on your terminal application:
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mvn archetype:generate -DgroupId=com.wordpress.omanandj -DartifactId=spring-app-event-demo -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false |
Spring eventing framework works on the Base Object ApplicationEvent. Now, create an interface “IAppEventPublisher” to publish your event.
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package com.wordpress.omanandj.event;import org.springframework.context.ApplicationEvent;public interface IAppEventPublisher { public void publish(ApplicationEvent event);} |
Now we will implement IAppEventPublisher to publish our demo events. Spring has interface ApplicationEventPublisher to publish the events and ApplicationEventPublisherAware to make the spring bean aware of eventing framework. Create a class AppEventPublisher which implements interfaces ApplicationEventPublisherAware and IAppEventPublisher as:
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package com.wordpress.omanandj.event;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.context.ApplicationEvent;import org.springframework.context.ApplicationEventPublisher;import org.springframework.context.ApplicationEventPublisherAware;import org.springframework.stereotype.Component;@Componentpublic class AppEventPublisher implements ApplicationEventPublisherAware, IAppEventPublisher { private ApplicationEventPublisher publisher; private static final Logger LOG = LoggerFactory.getLogger(AppEventPublisher.class); @Override @Autowired public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher) { this.publisher =applicationEventPublisher; } public void publish(ApplicationEvent event) { LOG.info("publishing event {}", event); publisher.publishEvent(event); }} |
You may have noticed above, I have added annotation @Component to create class AppEventPublisher as a spring bean.
Before creating our event listener, first, create a model class NewAppEvent for our application event.
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package com.wordpress.omanandj.event;import org.springframework.context.ApplicationEvent;public class NewAppEvent extends ApplicationEvent{ /** * Create a new ApplicationEvent. * * @param source the component that published the event (never {@code null}) */ public NewAppEvent(Object source) { super(source); }} |
For creating the Application event listener, you need to implement ApplicationListener<E extends ApplicationEvent>. Create a listener AppEventListener, which will log any event it receives.
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package com.wordpress.omanandj.event;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import org.slf4j.impl.Log4jLoggerFactory;import org.springframework.context.ApplicationEvent;import org.springframework.context.ApplicationListener;import org.springframework.stereotype.Component;@Componentpublic class AppEventListener implements ApplicationListener<NewAppEvent> { private static final Logger LOG = LoggerFactory.getLogger(AppEventListener.class); @Override public void onApplicationEvent(NewAppEvent event) { LOG.info("Event Occured {} " , event); }} |
You are almost done with your basic spring application eventing framework. Now you need to configure all the spring beans. Let us create a java class AppConfiguration to specify spring configuration:
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package com.wordpress.omanandj;import com.wordpress.omanandj.config.AppConfiguration;import com.wordpress.omanandj.event.IAppEventPublisher;import com.wordpress.omanandj.event.NewAppEvent;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import org.springframework.context.ApplicationContext;import org.springframework.context.annotation.AnnotationConfigApplicationContext;/** * App Event Demo * */public class App{ private static final Logger LOG = LoggerFactory.getLogger(App.class); public static void main( String[] args ) { ApplicationContext context = new AnnotationConfigApplicationContext(AppConfiguration.class); IAppEventPublisher eventPublisher = context.getBean(IAppEventPublisher.class); LOG.info("Hello World for App Event using Spring"); for(int i = 0; i < 100; ++i) { eventPublisher.publish(new NewAppEvent("Publishing a new App" + i)); } }} |
You can download the complete source code from my git repository.