FAQ What is a plug-in fragment?
Sometimes it is useful to make part of a plug-in optional, allowing it to be installed, uninstalled, or updated independently from the rest of the plug-in. For example, a plug-in may have a library that is specific to a particular operating system or windowing system or a language pack that adds translations for the plug-ins messages. In these situations, you can create a fragment that is associated with a particular host plug-in. On disk, a fragment looks almost exactly the same as a plug-in, except for a few cosmetic differences.
- The manifest is stored in a file called fragment.xml instead of plugin.xml.
- The top-level element in the manifest is called fragment and has two extra attributes plugin-id and plugin-version for specifying the ID and version number of the host plug-in.
- The fragment manifest does not need its own requires element. The fragment will automatically inherit the requires element of its host plug-in. It can add requires elements if it needs access to plug-ins that are not required by the host plug-in.
Apart from these differences, a fragment appears much the same as a normal plug-in. A fragment can specify libraries, extensions, and other files. When it is loaded by the platform loader, a fragment is logically, but not physically, merged into the host plug-in. The end result is exactly the same as if the fragments manifest were copied into the plug-in manifest, and all the files in the fragment directory appear as if they were located in the plug-ins install directory. Thus, a runtime library supplied by a fragment appears on the classpath of its host plug-in. In fact, a Java class in a fragment can be in the same package as a class in the host and will even have access to package-visible methods on the hosts classes. The methods find and openStream on Plugin, which take as a parameter a path relative to the plug-ins install directory, can be used to locate and read resources stored in the fragment install directory.
This FAQ was originally published in Official Eclipse 3.0 FAQs. Copyright 2004, Pearson Education, Inc. All rights reserved. This text is made available here under the terms of the Eclipse Public License v1.0.