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The jar libraries a plugin uses must be defined in the plugin manifest file ("plugin.xml"). It is possible to define a jar library as public, shared, or hidden to other plugins.
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The plugin class-loader is a url class-loader so theoretically it is possible to load jar libraries from a central server. This may interesting for example in case you run a server farm and wish to have a centralised library management for maintenance issues.<<<PluginCentral
Pitfalls
The Nutch plugin system can avoid dependency version conflicts between plugins and to some extend between a plugin and Nutch core dependencies. There are some situations where things go wrong, e.g., if an object is passed from core to a plugin but the plugin has a different class instance (and implementation) in its class loader.