why osgi is used?

For me the best part of OSGi is that it promotes a service-oriented view of your system, which helps decoupling, testing and teamwork.

I don't think it has to be complex, as I tried to demonstrate with my "OSGi for mere mortals" example application [1] and slides [2] (both shameless plugs ;-)

[1] https://github.com/bdelacretaz/OSGi-for-mere-mortals

[2] http://www.slideshare.net/bdelacretaz/osgi-for-mere-mortals

IMHO OSGI on its own requires a a lot of boiler plate code for what it does. However if you use a library like Karaf & iPOJO most of the overhead is hidden and you get most of the benefits.

The main benefit is being able to control which versions are used and proper module isolation. For example you can

• have the container download the right versions from a maven repository like nexus.
• use multiple version of the same library and have the modules which need those use the right ones.
• stop, upgrade and start modules while the application is running.
• a web console to see the state of all your modules and manage them collectively or individually.

OSGi provides a modular system with versioning of the modules, and further a bit of life cycle management. It manages dependencies between modules, dynamic/lazy startup/uninstall of modules, lookup/discovery and such.

Part of this will become part of the java language. It enables a lesser complexity than a spaghetti wiring of classes.