Classloading is a serious issue when creating OSGi based applications. As each bundle is bound to it’s own classpath, sometimes the reflection mechanism will not work as expected. So, there are basicly two ways of solving the problem of extending a bundle’s classpath (when using Equinox…), either create a feature or use Buddy Classloading. But which solutions fits best for what problem?
A fragment is an OSGi mechanism to allow a dynamic extension of a bundle at runtime. The fragment itself is not a true component as other bundles may not define a dependency on the fragment. It is used for different purposes such as testing, internationalisation or logging configuration. You can add system dependent code or configuration files to a generic implementation. The good thing is, that the extended code has no dependency on the fragment i.e. does not necessarily require the fragment for running (for example testing fragments).
A fragment is structured as a common bundle. It may be created using a wizard (in Eclipse) or by adding a line to the manifest.mf of a common bundle defining the so called fragment host:
This advises the runtime to merge the fragment’s content with it’s host at runtime.
Fragments are pure OSGi and provide the flexibility of extending a component dynamicly at runtime without breaking the component architecture. Some third party classpath requirements can be solved this way, for example providing a logging configuration.
Some third party classpath requirements cannot be matched using fragments.
Buddy Cassloading is somewhat a hack (in my opinion), as it adds a bundle’s classpath to another’s – you could call it ‘reverse-dependency’. In OSGi terms, this is explicitly not wanted as a bundle should be treated as a component. But since many Java libraries got used to a single classloader environment, they still expect an exclusive access to any class of the application using reflection. So, in some situations, you can’t really avoid using Buddy Classloading in an Equinox environment.
The manifest.mf of the bundle to be extended receives an additional line to define a Buddy Policy:
This defines the bundle to be aware of registered bundles which entend the classpath.
The manifest.mf of the bundle to extend receives two additional lines:
This defines the bundle to extend the classpath of the listed bundles. Additionally, the registered Buddy Policy requires the extending bundle to have a dependency on the extended bundle.
Buddy Classloading easily enables using common reliable libraries without any problems. Additionally, the extending bundle can still be treated as a ‘normal’ bundle, other bundles may have a dependency on the bundle and use the exported packages as provided.
Buddy Classloading is not ‘the OSGi way’ as it corrupts the idea of having a component based application with clearly separated bundles defining an interface and dependencies. Buddy Classloading is an answer to a technical problem and should not be used for application design.