Using GWT to create an OSGi-aware web application

Angelo van der Sijpt — Thu, 09 Jul 2009 20:14:29 +0000

Update 2010-02-20 Both Pax Runner 1.3.0 and GWT 2.0 have caused quite some changes to this post. I have tried to stay up to date as well as I could (the zipped project now uses GWT 2.0), but you might find some inconsistencies when following the tutorial.

Google Web Toolkit is cool, and so is OSGi. However, when building a web UI for Apache ACE, I found out that creating a web application that can use OSGi services is not that easy. By the end of this tutorial, you will have created a GWT project that delivers a usable jar. If you’re impatient, skip to the end for the downloadable Eclipse project.

Step 1: Create a GWT project

Create a regular GWT project using the regular webAppCreator; this will give you a project that includes an Ant buildfile, we will need that later on.

angelos:workspace angelos$ ./gwt-mac-1.6.4/webAppCreator -out GwtDemo net.luminis.gwt.gwtdemo
Created directory GwtDemo/src
Created directory GwtDemo/war
Created directory GwtDemo/war/WEB-INF
Created directory GwtDemo/war/WEB-INF/lib
Created directory GwtDemo/src/net/luminis/gwt
Created directory GwtDemo/src/net/luminis/gwt/client
Created directory GwtDemo/src/net/luminis/gwt/server
Created file GwtDemo/src/net/luminis/gwt/gwtdemo.gwt.xml
Created file GwtDemo/war/gwtdemo.html
Created file GwtDemo/war/gwtdemo.css
Created file GwtDemo/war/WEB-INF/web.xml
Created file GwtDemo/src/net/luminis/gwt/client/gwtdemo.java
Created file GwtDemo/src/net/luminis/gwt/client/GreetingService.java
Created file GwtDemo/src/net/luminis/gwt/client/GreetingServiceAsync.java
Created file GwtDemo/src/net/luminis/gwt/server/GreetingServiceImpl.java
Created file GwtDemo/build.xml
Created file GwtDemo/README.txt
Created file GwtDemo/.project
Created file GwtDemo/.classpath
Created file GwtDemo/gwtdemo.launch
Created file GwtDemo/war/WEB-INF/lib/gwt-servlet.jar

If you want to, you can import this project directly into your Eclipse. If you check the mark “use Google Web Toolkit” in the project properties, you can use all the same goodies that creating the project in Eclipse would have given you. Remember to replace the buildpath entries for gwt-user.jar and gwt-dev-*.jar by a Library import for GWT.

Step 2: Include the necessary OSGi references

Create an ‘ext’ directory, and add org.osgi.core.jar to that. In Eclipse, add this jar to your build path.

Step 3: Use OSGi services from your web applicaiton

We will first add a simple Activator on the server side.

package net.luminis.gwt.server;
 
import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
 
public class Activator implements BundleActivator {
    private static BundleContext m_context;
 
    public static BundleContext getContext() {
        return m_context;
    }
 
    public void start(BundleContext context) throws Exception {
        m_context = context;
    }
 
    public void stop(BundleContext context) throws Exception {
    }
}

Then, we up the GreetingServiceImpl to actually use this BundleContext (note that we use it directly here, but you could use it to get other services, create a ServiceTracker, etc.)

public String greetServer(String input) {
  String serverInfo = getServletContext().getServerInfo();
  String userAgent = getThreadLocalRequest().getHeader("User-Agent");
  return "Hello, " + input + "!
 
I am running " + serverInfo
    + ".
 
It looks like you are using:" + userAgent +
    "The framework we run from has " + Activator.getContext().getBundles().length + " bundles in it.";
}

Step 4: Add OSGi dependencies for the compiler

Add our OSGi dependencies to the classpath, so the compiler can find all of it.

Right, let’s give it a try!

angelos:GwtDemo angelos$ ant war
Buildfile: build.xml
 
...some output removed...
 
war:
[zip] Building zip: /Users/angelos/workspace/workspace/GwtDemo/gwtdemo.war
 
BUILD SUCCESSFUL
Total time: 36 seconds

You will find a war in your project directory now. There still is one ingredient we need. We need to make this into a proper bundle. We can use bnd to help us with that.

Step 5: use bnd to create a proper war

Download bnd, and put into a lib directory, and add it to your buildfile.

We create a new target that transforms our war into a jar.


    
    Import-Package: junit.framework;resolution:=optional, com.google.gwt.*;resolution:=optional, org.w3c.*;resolution:=optional, sun.misc;resolution:=optional, javax.imageio;resolution:=optional, javax.servlet.*;resolution:=optional, *
Bundle-Name: GWT Demo
Bundle-ClassPath: WEB-INF/classes, WEB-INF/lib/gwt-servlet.jar
Bundle-SymbolicName: net.luminis.gwt.gwtdemo
Webapp-Context: gwtdemo
Bundle-Activator: net.luminis.gwt.server.Activator

What’s happening here?

we copy our war to the same file, but with a jar extension,
we create a file for bnd to use, stating that we
- want optional imports for junit and the gwt benchmarks, and non-optional imports for everything else (that what the * is for),
- have some classes that we want bnd to scan for finding dependencies,
- want to use a given Webapp-Context (this is a Pax war extender specific entry),
let bnd do its magic,
update our manifest: we put the . back on the classpath. This is important for the web application to find all resources, but if we would tell bnd to do it like this, it would treat . as the root of the classpath.
Finally, we delete that temporary bnd file.

What does that give us?

angelos:GwtDemo angelos$ ant jar
Buildfile: build.xml
 
...some output removed...
 
jar:
[copy] Copying 1 file to /Users/angelos/workspace/workspace/GwtDemo
[bndwrap] gwtdemo 41 910305
[bndwrap] Warnings
[bndwrap] Superfluous export-package instructions: [WEB-INF.classes.net, gwtdemo.gwt.standard.images, WEB-INF, gwtdemo, WEB-INF.classes.net.luminis.gwt, gwtdemo.gwt.standard, WEB-INF.classes.net.luminis, WEB-INF.lib, WEB-INF.classes, gwtdemo.gwt.standard.images.ie6, WEB-INF.classes.net.luminis.gwt.client, WEB-INF.classes.net.luminis.gwt.server, gwtdemo.gwt]
[jar] Updating jar: /Users/angelos/workspace/workspace/GwtDemo/gwtdemo.jar
[delete] Deleting: /Users/angelos/workspace/workspace/GwtDemo/gwtdemo.bnd
 
BUILD SUCCESSFUL
Total time: 23 seconds

That’s it! You can now deploy this jar into a framework that uses the pax web tools. Right, let’s give that a try.

Download pax-runner, and unzip that somewhere. Copy in your new jar, an try the following command

angelos:pax-runner angelos$ sh bin/pax-run.sh --profiles=war,compendium gwtdemo.jar

Now visit http://localhost:8080/gwtdemo:

Summary

So, what did we need?

A fairly regular GWT project, create with an Ant file,
some code that tries to use OSGi services,
some bnd magic to make the war into a jar,
Pax tools to get it all running quickly.

If you don’t want to use pax runner, you can need to deploy pax-web-extender-war(jar, snapshot 23 June) and an http server, preferably pax-web-service(jar), into your framework.

You can download the gwtdemo Eclipse project to play around with it. I have not provided the GWT runtime in this download; you should edit line 4 of the build.xml to point to your installation of GWT.

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