JAX-RS the missing link between UI5 and HCP Java apps - adding EJB

This is part 2 of this blog and it assumes you have read Part 1.

In part 1 of this blog we have achieved a basic "ping" service that responds to a get request with just some static answer - "pong".

Normally you want to access a database and do some nice stuff. The best way to achieve a nice service layer is (in my opinion) to use EJB. You can control access rights, transactions etc. right by annotating your EJB in a nice and simple way. And you get your EntityManager or Datasource for JPA or JDBC directly injected without so much "boiler plate" code. So I want to show how to incopreate EJB in our setup. So the task I want to solve is adding an EJB with an entity manager injected and returning the result as a JSON object.

So first turn your project to a JPA project by adding this project facet in Eclipse.

Then we need a simple EJB:

/**
* EJB
*/
package com.sap.myapp;
 
import javax.ejb.LocalBean;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
 
/**
* Simple EJB
*/
@LocalBean
@Stateless
public class MyEJB {
 
  @PersistenceContext EntityManager em;
 
  public MyJSONResult doSomething() {
    MyJSONResult result = new MyJSONResult();
    result.setState(em != null ? "em is injected :-)" : " em not inject :-(");
    return result;
  }
}

It does nothing else as to give me the state of the injected entity manager. For the sake of completeness I have to add the MyJSONResult class, which is just a simple POJO:

/**
* JSON bean
*/
package com.sap.myapp;
 
/**
* Simple Bean as result of EJB call
*/
public class MyJSONResult {
  private String state;
 
  /**
   * @return the state
   */
  public String getState() {
    return state;
  }
 
  /**
   * @param state the state to set
   */
  public void setState(String state) {
    this.state = state;
  }
}

If we would have the Java Web Profile level 7 not 6 on HCP this would be very easy (and I could not write this blog) all you would have to do is add your EJB to your new service. So my service would look like:

/**
* REST
*/
package com.sap.myapp;
 
import javax.ejb.EJB;
import javax.servlet.ServletException;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
 
/**
* Simple REST Service
*/
@Path("/service")
public class MyService {
 
  @EJB MyEJB myEjb;
 
  @GET
  @Path("/")
  @Produces(MediaType.APPLICATION_JSON)
  public Response myService() throws ServletException {
    return Response.ok().entity(myEjb.doSomething()).build();
  }
}

This would result in a Nullpointer Exception as the EJB is not injected. So there is some more magic needed as long as we stay with Jave Web Profile 6. We have to do a JNDI lookup of the bean before we can use it. By doing JNDI we get a fully working container managed EJB and we have to do that in every service method that wants to use EJB.

So the extended service will look like the following:

/**
* REST
*/
package com.sap.myapp;
 
import javax.ejb.EJB;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.servlet.ServletException;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
 
/**
* Simple REST Service
*/
@Path("/service")
public class MyService {
 
  @EJB MyEJB myEjb;
 
  @GET
  @Path("/")
  @Produces(MediaType.APPLICATION_JSON)
  public Response ping() throws ServletException {
    lookupEJBs();
 
    return Response.ok().entity(myEjb.doSomething()).build();
  }
 
  private void lookupEJBs() throws ServletException {
    if (myEjb == null) {
      try {
        InitialContext ic = new InitialContext();
        myEjb = (MyEJB) ic.lookup("java:comp/env/ejb/MyEJB");
      }
      catch (NamingException e) {
        throw new ServletException(e);
      }
      catch (Exception e) {
        throw new ServletException(e);
      }
    }
  }
}

For the JNDI lookup to work you have to add your EJB to the web.xml:

  <ejb-local-ref>
    <ejb-ref-name>ejb/MyEJB</ejb-ref-name>
    <local>com.sap.myapp.MyEJB</local>
  </ejb-local-ref>

So you tried it all out and your ping-pong service works but your new service gives a 404? Then you forgot to add your new service class to "MyApplication" as we did with the PingService in Part 1

Thats it, if you call now your application with the relative path "/rest/service" you get a JSON response.

{"state":"em is injected :-)"}

Explore the power of Jersey, even stuff as file upload can be done with a few simple annotations. If someone is interested in that I may add a third part to this small blog series