Tuesday, April 29, 2008

GWT and JPA with Servlets

I am doing some investigation of GWT and in writing an application, I wanted to integrate the servlet end of my application with a JPA services oriented architecture (either through the servlet itself, or a standalone JPA service). I have written a few JPA applications and there are several things I like about JPA:

  • There is a nice model(bean)/service view of the world.
  • Defining your persistence behavior on your JavaBeans "feels" right.
  • Takes care of all the ugly connection/pooling stuff for you.
  • You do not need to be a service "guru" (personally I prefer the business layer and presentation layer) to get a working application with persistent storage.

While there are certainly disadvantages with JPA, I wanted to leverage my JPA knowledge, tools and services with a GWT application using RPC-Servlets. The nice thing about using RPC-Servlets is you can code your servlet in a very DSL like manner. (I was going to look into REST and Restlets but decided GWT was enough to learn for now). Getting JPA and RPC-Servlets proved rather difficult at first, and I almost abandoned the approach in favor of a pure servlet "service" approach delivering JSON or XML objects and then using HTTPRequests to get the responses from GWT.

My first attempt was to try and stick my servlet(s) that referenced my JPA services outside of the GWT client packages and then try and run these within the hosted tomcat within GWT. This proved problematic since GWT/tomcat bundles an earlier version of the xerces library which is not compatible with the persistence.xml xsi schema. (I also wasn't very convinced this would work anways since I am not sure if the imbedded tomcat runs with a 1.4-compliant JDK or not (thus preventing JPA annotations)). So I needed another approach. I had read that GWT hosted browser could run without the Tomcat imbedded, hence allowing you to work and debug within the hosted browser, but use a standard OOTB tomcat service. This appealed to me alot, since I can easily setup a dynamic web project in eclipse to host my servlet/service and I can work in the natural hosted browser of GWT. After a little tweaking I got this to work in eclipse and so far, while not as clean as a pure GWT hosted-mode environment I am able to do everything I need to do.

Application Description

I am going to write an application which allows me to store user profiles (website profiles, not Windows profiles) in a database with encrypted passwords. If you are like me, after signing up for a few sites, you can never remember you user ids and passwords. The application will be called ProfileManager and it will be written in GWT as a web-application, using JPA to access the database through a ProfileService.

GWT Application Setup

I created an eclipse project for my GWT application using the projectCreator and applicationCreator scripts provided by GWT with the -eclipse flag. This provides you with the basic scaffolding necessary for the creation of a GWT application. For the purposes of this blog, the package path of my module is org.javad.profile.gwt.ProfileManager. Executing the ProfileManager.launch script launches the GWT toolkit development shell and my application. I am not going to go into more details here as these procedures are well documented in text and websites.

Step 1: Create the Service Interface

My service interface under the package org.javad.profile.gwt.client.rpc. The interface has to extend the google interface RemoteService. For simplicity this interface defines only a single method getAll( ) that looks like the following:

package org.javad.profile.gwt.client.rpc;

import java.util.List;
import com.google.gwt.user.client.rpc.RemoteService;

public interface RPCProfileService extends RemoteService {
   public List getAll( );
As more functionality is added, I will add the signatures to the service interface. Along with the interface, I need an Asynchronous interface which is defined in the same package:

package org.javad.profile.gwt.client.rpc;
import com.google.gwt.user.client.rpc.AsyncCallback;

public interface RPCProfileServiceAsync {
   public void getAll( AsyncCallback callback );

Step 2: Create your Serializable Bean

Since JPA uses annotations, we unfortunately have to translate the JPA POJO to a GWT DAO. One product you might want to look at is HiberObjects. For this project, I created a simplified model of my JPA POJO called "ProfileBean" under org.javad.profile.gwt.client.model. Since this overview does not use the ProfileBean directly, I am not going to say any more on it, other than you'd need it for a full GWT Application.

Step 3: Create an Externalized JAR

In order to implement the service interface (and retrieve ProfileBean objects) you need to bundle these into an external JAR to associate to your web project. Within eclipse you can do this with the File->Export ...->JAR File functionality. This JAR forms the externalized view of our service which we'll need to implement within the web project.

Step 4: Modify the .launch Script

When you create your project, the GWT toolkit created a ProfileManager.launch script which you can use to launch your application. The problem is, this launch script will launch the Toolkit Development shell with an imbedded Tomcat, will attempt to connect on port 8888 (default for GWT toolkit) and will not include a web-application root name, which if you are deploying with web project will be needed.

The GWTShell command can take some arguments which we'll use to adjust this. Editing the ProfileManager.launch shell, you need to change the value of the stringAttribute " org.eclipse.jdt.launching.PROGRAM_ARGUMENTS"

  1. First, we need to tell the shell not to launch an imbedded Tomcat. This is done by specifiying the -noserver option.
  2. The port needs to be specified. In my case, my Tomcat is running on port 8080 which can be defined by specifying the -port 8080 option.
  3. Finally, we need to change the application which is launched by the application script by prepending the web-application name in front of the module's HTML file.

An example of this line from my ProfileManager.launch script looks like the following:

<stringAttribute key="org.eclipse.jdt.launching.PROGRAM_ARGUMENTS" value="-noserver -port 8080 -out www Profiles/org.javad.profile.gwt.ProfileManager/ProfileManager.html"/>

Now executing the script will open the Toolkit Shell an attempt to execute the GWT application with the correct port number and web-application name. A tomcat instance will not be started by the Toolkit Shell. The navigator window of course will not be able to connect to your application (since we have not hooked it up yet) so your window should look something like this:

Step 5: Make a call to the Service

Before we go on to the servlet project, it'll be helpful to know that we have the connection to the service working within GWT when the servlet is ready. To test this, I modified the the onModuleLoad( ) method of the application to simply dispatch a call to the RPC service. Obviously if you are going to use a DAO/Controller pattern this would be abstracted, but this is simply a test to know you are on the right path.

   RPCProfileServiceAsync service = (RPCProfileServiceAsync)GWT.create(RPCProfileService.class);
   ServiceDefTarget endpoint = (ServiceDefTarget) service;
   String moduleRelativeURL = GWT.getModuleBaseURL() + "servlet/ProfileServlet";
   AsyncCallback callback = new AsyncCallback() {
      public void onSuccess(Object result) {
         System.out.println("in callback");
      public void onFailure(Throwable caught) {

If we were to launch the application now, it would fail since there would be no response from the service. However when we complete the next section, we should see to "in callback" message shown in the console of the GWTShell.

Servlet Project For the servlet project, I am using Eclipse Europa with the WTP 2.0. This includes Dali, which allows you to easy define you JPA POJOs using the built in editor. Step 1: Create the Project Create the project within Eclipse using the "File->New->Project..." and selecting the dynamic web project under the Web project types. Enter a project name (this will be the default web-application name) so for mine I chose "Profiles". For the Project Facets step, make sure you choose the "Java Persistence" facet. This will allow you to manage your JPA objects.

Step 2: Add the GWT RPC library When we exported the RPC library from our GWT project (see Step 3 above) we created the contract that the servlet needs obey. We now need to import this JAR into the web project as a references library. After adding it as a referenced library, we also need to ensure that it is copied to the server deployment location. This is done by selecting the Properties of our project and selecting the necessary JARs under the J2EE Module Dependency option. I also included the TopLink JPA, MySQL (connector library) and gwt-servlet.jar as can be seen from the image below:

Step 3: Create your RPC Servlet

To implement our servlet, I created a class GWTProfileServlet which extends the RemoteServiceServlet and implements our RPCProfileService. This is located in the org.javad.profile.servlet package under the src variant of my web project.

package org.javad.profile.servlet;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import org.javad.profile.gwt.client.rpc.RPCProfileService;
import org.javad.profile.model.Profile;
import org.javad.profile.service.ProfileService;
import com.google.gwt.user.server.rpc.RemoteServiceServlet;

public class GWTProfileServlet extends RemoteServiceServlet implements RPCProfileService {
   public List getAll() {
      ProfileService service = ProfileService.getInstance();
      Collection profiles = service.getAll();
      System.out.println("debug: got all profiles" + profiles);
      return new ArrayList();

In the example above, I make a call to the ProfileService which is a JPA Enabled service for managing profile objects from the datastore. You could also add your JPA persistence code directly to the servlet here. I am also printing out a debug message just to show that I am getting the output in the servlet.

Step 4: Create the servlet mapping

Since we want the GWTProfileServlet to be mapped under the GWT Application, in the web.xml for the web project, you need to define the servlet-mapping that includes the GWT Module name. So for our application, the servlet path was appended to the module path, making a url-pattern name as follows:


Step 5: Copy over the base GWT Application

In order for the web application to properly server the GWT Application to the imbedded browser we need to provide some files to the Tomcat.

First compile the GWT Application that you wrote in the first section above using the ProfileManager-compile.cmd through the External Tools in Eclipse.

Create a folder under the www-root named "org.javad.profile.gwt.ProfileManager". In this folder copy the following files from the www-root folder from your GWT Project:

Now you should be able to start your application server, and launch the GWT Toolkit browser and debug the application (both the servlet and the GWT application) using the eclipse debugger.

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