Theseus Generic Plugin Engine

Plugin Archive File Format

Every plugin can be made available to the plugin engine in two formats. While developing plugins, it is often easiest to create a plugin project in an exploded directory structure, which is basically a representation of the plugin the way it needs to be formatted with any added directories and files that are specific for development purposes. In this format, a directory has four primary components.

The descriptor that every plugin must have to be a valid plugin is named plugin.xml. This is always found in the root directory of a plugin. The root directory of a plugin project will also contain a build.xml file (usually), possibly any helper files such as resource bundles for different languages, and so forth, although these may be found under other directories.

If a plugin contains a lifecycle class and any other compiled java code, these must be located in the /classes directory off of the root plugin directory. A plugin does not require a lifecycle class or any compiled classes. Plugins may contribute to extension points in other ways besides code, such as adding html files to an html help system plugin.

If a plugin depends on any third party libraries, these must be placed in the /lib directory off of the root directory. The plugin engine automatically adds any .jar and .zip files in the /lib directory as part of the plugins classpath. A plugin may contribute libraries without providing any other plugin code with the exception that all plugins that provide any libraries must provide a lifecycle class in the /classes directory. This is to make sure that all plugins are given a chance to do any initialization and cleanup when a plugin is first activated and later when it is destroyed or deactivated. At a bare minimum, a lifecycle class can simply implement the PluginListener interface and provide the handleEvent() method with no further code. A common use of plugins providing only libraries are wrappers around often used libraries, such as xml parsers, tools like ant, logging libraries and so forth. Learn more about how libraries can be used by dependent plugins here

The last useful plugin piece is the /resource directory. While you can choose to put any files like images, html pages and so forth anywhere, the /resource directory, if present, gets special attention with the engine. It is used by the getClass().getResource() and getClass().getClassLoader().getResource() methods to find any resources a plugin may wish to find within the confines of this directory.