The Java platform has traditionally been distributed in 3 editions, each providing a different type of runtime environment:

Each environment aims to provide a base level of functionality on top of which developers can add their own code to create applications. For example Java SE provides networking and security libraries together with graphical user interface toolkits to facilitate the development of desktop and simple client-server applications. Java EE takes this a stage further by adding a number of 'enterprise services' such as transactions, messaging, and persistence that allow much more robust and scalable 'enterprise applications' to be developed. These services are typically combined together inside a JEE application server in order to provide a standard runtime environment for enterprise applications but it is often the case that some are never used.

Services that are never used are undesirable as they can take up valuable resources such as CPU and memory resulting in lower performance. They may also clutter up the environment with redundant configuration files, complicating maintenance and adding unnecessary complexity. Given these drawbacks it would be better if there was a way to create a custom runtime environment containing only those services that were needed. You would then be able to create your own custom 'Java Edition' tailored to your requirements.

JBoss Microcontainer aims to provide these capabilities by allowing services, created using Plain Old Java Objects (POJOs), to be deployed into a standard Java SE runtime environment in a controlled manner to create a customized environment for your applications. Dependencies are fully managed to ensure that new services cannot be deployed until services they depend on have first been deployed. Where it makes sense to do so you can even redeploy services at runtime providing that you access them via the microcontainer bus. Undeploying a service causes all dependent services to be undeployed first in order to maintain the integrity of the system.

JBoss Application Server 5.0 uses the microcontainer to integrate enterprise services together with a Servlet/JSP container, EJB container, deployers and management utilities in order to provide a standard Java EE environment. If you need additional services then you can simply deploy these on top of Java EE to provide the functionality you need. Likewise you are free to remove any services that you don't need simply by changing the configuration. You can even use the microcontainer to do this in other environments such as Tomcat and GlassFish since you can plug in different classloading models during the service deployment phase.

Since JBoss Microcontainer is very lightweight and deals with POJOs it can also be used to deploy services into a Java ME runtime environment. This opens up new possibilities for mobile applications that can now take advantage of enterprise services without requiring a full JEE application server.

In common with other lightweight containers JBoss Microcontainer uses dependency injection to wire individual POJOs together to create services. Configuration is performed using either annotations or XML depending on where the information is best located. Finally unit testing is made extremely simple thanks to a helper class that extends JUnit to setup the test environment, allowing you to access POJOs and services from your test methods using just a few lines of code.