JAVA EE: Custom Events in Spring | Spring Custom Events

Tuesday, 25 September 2018

Custom Events in Spring | Spring Custom Events | Spring event handling

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Overview: Custom Event Handling in Spring

The Spring Framework provides robust built-in support for event-driven architecture through its ApplicationContext event handling mechanism. While Spring comes with standard events (like ContextRefreshedEvent and ContextClosedEvent), you can also create and publish your own custom events to decouple components across your application.

In this guide, you will learn how to build, publish, and listen for custom events in Spring applications.

Core Components of Spring Event Handling

Custom event handling in Spring revolves around three essential elements:

Custom Event Class: Extends ApplicationEvent to encapsulate the event data or message payload.
Event Publisher: Uses ApplicationEventPublisher (or implements ApplicationEventPublisherAware) to fire the event to all registered listeners.
Event Listener: Implements ApplicationListener<CustomEvent> to handle and process the event when it is triggered.

Step-by-Step Implementation

Create the Custom Event: Define a class extending ApplicationEvent and pass the source object to the super constructor.
Publish the Event: Inject ApplicationEventPublisher into your publisher service and invoke publishEvent(new CustomEvent(this)).
Handle the Event: Implement the onApplicationEvent(CustomEvent event) method in your listener bean to execute the required action asynchronously or synchronously.

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Custom Events in Spring | Spring Custom Events | Spring event handling

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0%20http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.ram.core</groupId>

    <artifactId>SpringDemo</artifactId>

    <packaging>jar</packaging>

    <version>1.0-SNAPSHOT</version>

    <name>SpringDemo</name>

    <url>http://maven.apache.org</url>

    <properties>

        <spring.version>5.0.2.RELEASE</spring.version>

    </properties>

    <dependencies>

        <dependency>

            <groupId>junit</groupId>

            <artifactId>junit</artifactId>

            <version>3.8.1</version>

            <scope>test</scope>

        </dependency>

        <!-- Spring 5 dependencies -->

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-core</artifactId>

            <version>${spring.version}</version>

        </dependency>

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-context</artifactId>

            <version>${spring.version}</version>

        </dependency>

    </dependencies>

</project>

CustomEvent.java

package com.ram.events;

import org.springframework.context.ApplicationEvent;

public class CustomEvent extends ApplicationEvent

{

    public CustomEvent(Object source)

    {

        super(source);

    }

    public String toString()

    {

        return "My Custom Event";

    }

}

CustomEventPublisher.java

package com.ram.eventpublisher;

import org.springframework.context.ApplicationEventPublisher;

import org.springframework.context.ApplicationEventPublisherAware;

import com.ram.events.CustomEvent;

public class CustomEventPublisher implements ApplicationEventPublisherAware

{

    private ApplicationEventPublisher publisher;

    public void setApplicationEventPublisher(

            ApplicationEventPublisher publisher)

    {

        this.publisher = publisher;

    }

    public void publish()

    {

        CustomEvent customEvent = new CustomEvent(this);

        publisher.publishEvent(customEvent);

    }

}

CustomEventHandler.java

packagecom.ram.eventhand
ler;

importorg.springframework.context.ApplicationListener;

importcom.ram.events.CustomEvent;

publicclass CustomEventHandler implements ApplicationListener<CustomEvent>
{
publicvoid onApplicationEvent(CustomEvent event)
{
System.out.println("Received custom event = "+event.toString());
}
}

applicationContext.xml

<beans xmlns="http://www.springframework.org/schema/beans"

    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

    xsi:schemaLocation="http://www.springframework.org/schema/beans

http://www.springframework.org/schema/beans/spring-beans-4.3.xsd">

    <bean id="customEventHandler" class=" com.ram.eventhandler.CustomEventHandler" />

    <bean id="customEventPublisher" class="com.ram.eventpublisher.CustomEventPublisher" />

</beans>

App.java

package com.ram.core;

import org.springframework.context.ConfigurableApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.ram.eventpublisher.CustomEventPublisher;

public class App

{

    public static void main(String[] args)

    {

        ConfigurableApplicationContext context = new ClassPathXmlApplicationContext(

                "applicationContext.xml");

        CustomEventPublisher customEventPublisher = (CustomEventPublisher) context

                .getBean("customEventPublisher");

        customEventPublisher.publish();

        customEventPublisher.publish();

    }

}

Output

Aug 28, 2018 10:28:25 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@610694f1: startup date [Tue Aug 28 10:28:25 IST 2018]; root of context hierarchy

Aug 28, 2018 10:28:25 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

INFO: Loading XML bean definitions from class path resource [applicationContext.xml]

Received custom event = My Custom Event

Received custom event = My Custom Event