Every OSGI bundle is a jar file,and before you compress the bundle to a jar file, a configuration file is necessary,the following is a configuration file example.it is very simple.it belongs to .mf format configuration file.
Bundle-Description: Demo Bundle
the most important properties in preceding file is Bundle-Activator and Import-Package,they are emphasized by an underline.
Bundle-Activator tells the framework which is the Activtor class,this is a kind of “main” class of a bundle,after we deploy this bundle to the OSGI framework ,the bundle will start from this class.so we can consider that the Activator class is an entrance of an bundle.
Import-Package tells the framework that out bundle needs to access the classes in specified package,in the example it is org.osgi.framework.Any bundle needs an special package which is org.osgi.framework,only by this it is a bundle.By the way, a bundle may also need other packages when it is necessary.