Using Embedded Felix in Application Container or Application Container Bundle in Karaf and is OSGi a complete IoC container?

Question

Imagine you want to have a highly modular web application in Java. Components are already using Spring framework also. Some components cover only pure Logic and some others also come with some presentation GUIs in HTML. The application is not Portlet compatible but we need to have dynamic menu which promotes the list of installed modules and links to the pages or REST APIs. Since it’s Java, using OSGI is recommended but I got too much confused with the recent features in OSGI (introduced past few years) and I have few questions:

• Now OSGI is not only Module Management System but also IoC Container! It almost contains equivalent annotations for many of Spring IoC features. So How we can use Spring enabled modules inside OSGI? Can we eliminate Spring for our new designs and completely rely on OSGI? Or for dependency Injection within the modules we can use Spring and for Dependency injection over the Modules we can use OSGI?

• For real-life large scale applications we have two options: Using Embedded Felix within application Container (e.g. Tomcat) or Using OSGI Bundle of Application Container (Jetty or Tomcat) within KARAF. Which approach is better? Which one is more scalable?

Answer 1

Researching OSGi can get confusing because there are 15 years of articles and examples out there - a lot of which are valid but may be out of date.

You could consider removing Spring if it was just used as an IoC container. OSGi DS provides @Component and @Reference annotations (like spring Autowire) if DS is enabled in your OSGi container. The new OSGi enroute project has some modern examples of performing IoC and providing REST services.

Replacing more of the features might be more work. Running the latest Spring in OSGi may not work due to the classloader differences in OSGi and the spring projects move away from OSGi.

Answer 2

OSGi is no IoC container at all. There are some technologies for injection support that run on OSGi though. The most important are declarative services and blueprint. Spring is not really supported. There is some Spring DM support but this code is not maintained for ages.

So you should use one of the two supported IoC containers above. For aries blueprint I have written support for CDI annotations using the maven-blueprint-plugin. This may be your best bet to convert the application. I recommend to first convert your spring application to use only CDI annotations and only then start the OSGi migration. Such a migration is not easy. Make sure you get some good coaching and consulting.

Embedded Felix on a servlet container is good if most of you application is non OSGi and you just want to use bundles for a very small part. Karaf is better if you want to write your whole application for OSGi.