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About Michal Jastak

Michał is a Chief Technology Officer in Java Division of AIS.PL, company developing mostly Web Applications of different kind, usually e-Government related.

JBoss Modules Example – Modular Web Application

Recently I read Why there is no standard for developing real modular web applications? by Patroklos Papapetrou (also featured in Java Code Geeks). Inspired by this article I decided to check JBoss Modules in action. This post describes my experiment step by step.

I’ve started with the following goal in mind – to create a web application using some service defined by my own JBoss module. The service prepared by me was pretty simple. I named it Echo Service:

package warlock.echo;

public interface EchoService {

    String echo(String param);
    
}

and placed into a separate jar file, named echo-api. Then I implemented this service:

package warlock.echo.impl;

import warlock.echo.EchoService;

public class DefaultEchoService implements EchoService {
    
    public String echo(String param) {
        return param;
    }
        
}

and placed the implementation in a new jar file, named echo-module. Having in mind that my web application should have knowledge of the service API only, not the specific implementation, I decided to go with the way described in Creating Extensible Applications With the Java Platform – that choice required adding to echo-module jar special file under: META-INF/services/warlock.echo.EchoService, holding the “pointer” to the service implementation (fully qualified name of implementing class).

At this point, I retrieved and unpacked JBoss Application Server 7, stepped into the unpacked JBoss, and then into the modules directory. In this directory I’ve added the following structure:

Two jar files visible here are mentioned above, module.xml file is the definition of my JBoss Module – named ‘warlock.echo’, and has the following content:

<?xml version="1.0" encoding="UTF-8"?>

<module xmlns="urn:jboss:module:1.0" name="warlock.echo">

    <resources>
        <resource-root path="echo-module-1.0.0-SNAPSHOT.jar" />
        <resource-root path="echo-api-1.0.0-SNAPSHOT.jar" />
    </resources>

</module>

After the completion of the JBoss Module definition, I’ve prepared a simple Spring Framework based application (which uses echo-api jar only during the compilation of project and doesn’t use the echo-module jar at all) with only one Controller:

package warlock;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;

import warlock.echo.EchoService;

@Controller
@RequestMapping("/echo.html")
public class EchoController {

    @Autowired
    private EchoService service;
    
    @RequestMapping(method = RequestMethod.GET)
    @ResponseBody
    public String handleGet() {
        return service.echo("It workzzzzz!");
    }
}

As you can see, the Controller returns as response body the result of Echo Service call for some string. Now to the most important part – Echo Service definition in Web Application:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

    ...

    <bean class="org.springframework.beans.factory.serviceloader.ServiceFactoryBean">
        <property name="serviceType" value="warlock.echo.EchoService" />
    </bean>

    ...
    
</beans>

I know, one thing is bothering you :) – how the hell will Echo Service implementation be found if we don’t add either echo-api and echo-module jars to the Web Application?!

Well, this is the beauty itself ;) – We just need one more thing – WEB-INF/jboss-deployment-structure.xml file:

<jboss-deployment-structure>
    <deployment>
        <dependencies>
            <module name="warlock.echo" services="export" />
        </dependencies>
    </deployment>
</jboss-deployment-structure>

This way we tell JBoss that this application depends on the ‘warlock.echo’ module and the services defined in this module. The rest is pure JBoss Module magic ;)

Lectures for the dessert:

Reference: Modular Web Application based on JBoss Modules from our JCG partner Warlock at the “Warlock’s Thoughts” blog.

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One comment

  1. If I package the sample application in an ear file. The modules are not working. It is throwing ClassNotFound Exception. Any idea how to get modules working with ear?

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