1.3. Class Loading Algorithm
Overview
The OSGi bundle class loading algorithm plays a key role in the OSGi framework. Ultimately, it is this algorithm that defines the relationship between a bundle and its dependencies.
Requirements on a bundle class loader
Here are some of the requirements that a bundle class loader must satisfy in order to support a flexible and consistent class loading architecture:
- In order to avoid loading multiple copies of a class, the bundle class loader must first of all try to find the class in one of its dependent bundles.
- The bundle class loader must never load a duplicate copy of a class into its class space.
Bundle class loading algorithm
The following is a simplified description of the bundle class loading algorithm (for example, it does not enumerate all of the ways in which class loading can fail). For a full description of the algorithm, see the Runtime Class Loading section of the OSGi Core Specification.
- If the class belongs to one of the
java.*packages or any packages listed in theorg.osgi.framework.bootdelegationproperty, the bundle class loader delegates to the parent class loader. - If the class belongs to one of the packages listed in
Import-Package, the bundle class loader delegates loading to the corresponding exporter bundle. - If the class belongs to one of the packages imported by
Require-Bundle, the bundle class loader delegates loading to the corresponding exporter bundle.NoteIt is strongly recommended that you avoid using theRequire-Bundleheader. OSGi dependencies are meant to be defined at package granularity, not bundle granularity. - Next, the bundle class loader looks for the class amongst its internal classes (inside its own JAR file).
- Next, the bundle class loader searches the internal classes of any fragments attached to the bundle.
- Finally, if the class belongs to one of the packages imported using
DynamicImport-Package, the bundle class loader delegates loading to the corresponding exporter bundle (if there is one).
