[JAVA] Event processing is performed in Spring.

What is Spring's "event"?

Spring Framework can generate "events" in the middle of the service. By using this, the processing can be made "loosely coupled", so the processing of the service can be divided into pieces.

Since it works just by sandwiching it between services, it seems to be effective for log output and cache trends. Since it has an independent configuration, it is effective when you want to prevent the processing of the service from becoming long, or when you want someone else to create some of the functions.

This time, instead of implementing the annotations implemented from Spring 4.3, we will make it by implementing the interface and gorigori.

Flow until the event is issued.

1. Create your own event
2. Create an event listener
3. Implement the service
4. Register event listeners and services in DI
5. Start the service with Servlet!

1. Create an event.

First, create an event class that inherits ** ApplicationEvent **. ** MyBeanEvent () ** in this is executed.

MyBeanEvent.java

public class MyBeanEvent extends ApplicationEvent {

	public MyBeanEvent(Object source) {
		super(source);
		//TODO auto-generated constructor stub
		System.out.println("Create MyBeanEvent");
	}

}

2. Create an event listener

Then create an event listener Just implement an interface called ** ApplicationListner **.

MyBeanEventListener.java

public class MyBeanEventListener implements ApplicationListener<MyBeanEvent> {

	@Override
	public void onApplicationEvent(MyBeanEvent event) {
		//TODO auto-generated method stub
		System.out.println("Event Has Finished!!");
	}

}

This event listener is always called after the event ends. It might be good to write an end log

3. Create a service.

Then create a ** service ** that runs the event. (I will omit the place to register the bean in DI!)

MyBeanEventService.java

public class MyBeanEventService implements ApplicationEventPublisherAware{

	@Autowired
	private MyBeanEve beanEve;
	private ApplicationEventPublisher publisher;
	
	@Override
	public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher) {
		//TODO auto-generated method stub
		this.publisher = applicationEventPublisher;
		System.out.println("publisher set!!");
		
	}
	
	public void doService(String message) {
		System.out.println("Service Started!");
		beanEve.addMessage(message);
		publisher.publishEvent(new MyBeanEvent(this));
	}

}

Here, the processing that actually works is the doService part. By ** setApplicationEventListenerPublisher ** Will publish an event. publisher is a convenience class that publishes events. You are now executing event processing.

4. Register event listeners and services in DI

Finally, register the event listener and service in the Java config. The Spring Framework is now ready to publish the event. (* ^-^ *)

MyBeanEveConfig.java

@Configuration
@ComponentScan(basePackages = "com.TsugaruInfo.bean")
public class MyBeanEveConfig {
	
	@Bean
	MyBeanEve myBean2() {
		return new MyBeanEve();
	}
	
	@Bean
	public MySampleApplicationListener addListener() {
		return new MySampleApplicationListener();
	}
	
	@Bean
	public MyBeanEventListener mybeanListener() {
		return new MyBeanEventListener();
	}
	
	@Bean
	public MyBeanEventService mybeanService() {
		return new MyBeanEventService();
	}
}

5. Start the service with Servlet!

Finally, run the service on the Servlet or controller. This time it is executed by Servlet. It may not work because I changed it a little (/ _;)

MySampleServlet.java

@WebServlet("/sample")
public class MySampleServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
	
	ApplicationContext app;
		
	@Autowired
	private MyBeanEventService myService;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public void init() throws ServletException{
        super.init();
        SpringBeanAutowiringSupport
        	.processInjectionBasedOnCurrentContext(this);
    }
    
	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		String message = request.getParameter("message");
		myService.doService(message);
		response.sendRedirect("sample");
	}

Run

Let's embed this Servlet and start the server.

You should see the characters ** publisher set !! ** printed on the console. (Leave other characters It's proof that the service is properly registered in DI.

Let's access it for a moment and move the method.

From the characters on the console ** Service → Event → Event Listener ** You can see that it works in the order of.

Summary

If you use it well, you can do the processing you want to use before the service, which is a convenient function. However, for that purpose, the design of what process to divide is very important. ~~ It is better to let the team know the relationship between the service and the event (ry ~~

Think about your design and do smart Spring programming!

Reference book Spring Framework 5 Introduction to programming https://www.amazon.co.jp/Spring-Framework-%E3%83%97%E3%83%AD%E3%82%B0%E3%83%A9%E3%83%9F%E3%83%B3%E3%82%B0%E5%85%A5%E9%96%80-%E6%8E%8C%E7%94%B0-%E6%B4%A5%E8%80%B6%E4%B9%83/dp/4798053740