Create a main method in a Jersey web application

Question

I have created a web application using Jersey and Maven, and I want to execute some code when the server starts, not when a function is called.

@Path("/Users") 
public class User {

    public static void main(String[] args) throws InterruptedException {    
            System.out.println("Main executed");
           //I want to execute some code here
    }

    @GET
    @Path("/prof") 
    @Produces(MediaType.APPLICATION_JSON)
    public List<String> getFile(@QueryParam("userid") String userid, {

But the main method is never executed, I never see the output Main executed. I have also tried to create a new class with a normal main method, but it also never executes.

Answer 1

You need to create a ServletContextListener implementation, and register it in your web.xml file:

ApplicationServletContextListener.java:

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class ApplicationServletContextListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent arg0) {
        System.out.println("Application started");  
    }

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
    }
}

web.xml:

<web-app>
  <listener>
    <listener-class>
      com.yourapp.ApplicationServletContextListener
    </listener-class>
  </listener>
</web-app>

If using servlet 3.0, you can also just annotate the class with the @WebListener annotation instead of registering it in your web.xml file.

---

Alternatively, as another user points out, Jersey includes an ApplicationEventListener that you can use to perform an action once the application is initialized:

MyApplicationEventListener.java:

public class MyApplicationEventListener implements ApplicationEventListener {

    private volatile int requestCount = 0;

    @Override
    public void onEvent(ApplicationEvent event) {
        switch (event.getType()) {
            case INITIALIZATION_FINISHED:
                System.out.println("Application was initialized.");
                break;
            case DESTROY_FINISHED:
                System.out.println("Application was destroyed.");
                break;
        }
    }

    @Override
    public RequestEventListener onRequest(RequestEvent requestEvent) {
        requestCount++;
        System.out.println("Request " + requestCount + " started.");

        // return the listener instance that will handle this request.
        return new MyRequestEventListener(requestCnt);
    }
}

Note that this is not part of the JAX-RS standard, and will tie your application to using Jersey.