PF4J Plugin Framework for Java

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Each plugin passes through a pre-defined set of states. PluginState defines all possible states.
The primary plugin states are:

• CREATED
• DISABLED
• STARTED
• STOPPED

The DefaultPluginManager contains the following logic:

• all plugins are resolved & loaded
• DISABLED plugins are NOT automatically STARTED by pf4j in startPlugins() BUT you may manually start (and therefore enable) a DISABLED plugin by calling startPlugin(pluginId) instead of enablePlugin(pluginId) + startPlugin(pluginId)
• only STARTED plugins may contribute extensions. Any other state should not be considered ready to contribute an extension to the running system.

The differences between a DISABLED plugin and a STARTED plugin are:

• a STARTED plugin has executed Plugin.start(), a DISABLED plugin has not
• a STARTED plugin may contribute extension instances, a DISABLED plugin may not

DISABLED plugins still have valid class loaders and their classes can be manually loaded and explored, but the resource loading - which is important for inspection - has been handicapped by the DISABLED check.

As integrators of pf4j evolve their extension APIs it will become a requirement to specify a minimum system version for loading plugins. Loading & starting a newer plugin on an older system could result in runtime failures due to method signature changes or other class differences.

For this reason was added a manifest attribute (in PluginDescriptor) to specify a ‘requires’ version which is a minimum system version on x.y.z format, or a SemVer Expression. Also DefaultPluginManager contains a method to specify the system version of the plugin manager and the logic to disable plugins on load if the system version is too old (if you want total control, please override isPluginValid()). This works for both loadPlugins() and loadPlugin().

PluginStateListener defines the interface for an object that listens to plugin state changes. You can use addPluginStateListener() and removePluginStateListener() from PluginManager if you want to add or remove a plugin state listener.

Your application, as a PF4J consumer, has full control over each plugin (state). So, you can load, unload, enable, disable, start, stop and delete a certain plugin using PluginManager (programmatically).