Split Packages

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A split packages is caused where two or more bundles export the same package name and version, usually with different contents. For example, two JARs that each contain a subset of the classes comprising a whole package.

Split packages are a problem for the Java Virtual Machine because the "default" access level for a class, method or field depends on membership of the same runtime package (a runtime package is a Java package as loaded by a particular class loader). Therefore if we split a package across multiple JARs and load those JARs in different class loaders, the JVM can throw IllegalAccessErrors even when accessing resources that have the same package name. The javac compiler cannot catch such problems because it has no idea of the runtime class loading architecture.

In OSGi, a package is the primary unit of sharing between modules, and therefore a package must be coherent. When a package is imported by a bundle, exactly one exporter of that package is chosen, so if a package is split across multiple bundles then an importer will only ever see a subset of the package. As a result, it is a Best Practice to Avoid Split Packages.

Note that using Require-Bundle does allow a bundle to aggregate all the parts of a split package, by depending on bundles by their bundle symbolic name rather than by their exports. However, using Require-Bundle is also considered to be against best practice and it does not solve any of the other major issues with split packages.

However, if you are dealing with legacy code and cannot avoid split packages, you can use the following to "combine" the split package together for bundles which wish to import the combined package. Bundle b and c export portions of the split package.

Bundle-SymbolicName: b
Export-Package: com.company.util;b=split;mandatory:=b
Bundle-SymbolicName: c
Export-Package: com.company.util;c=split;mandatory:=c

Bundle a requires both bundles b and c which combines the package and then exports the combined package. Other bundles which import the package will wire to bundle a since they do not (and should not) have the mandatory attributes on their import.

Bundle-SymbolicName: a
Require-Bundle: b, c
Export-Package: com.company.util
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