Plugin System

Plugin System

JNode uses an OSGi-like plugin/extension architecture where each functional unit is a plugin with an XML descriptor defining its dependencies, exports, and extension points.

Overview

The plugin system is JNode's primary modularity mechanism. Every subsystem — filesystem drivers, network protocols, video drivers, shell commands — is packaged as a plugin. Plugins have:

• Identity — unique ID, version, provider
• Dependencies — <requires> / <import> declarations
• Exports — packages visible to other plugins
• Extensions — implementations registered at named extension points
• Lifecycle — loaded, started, stopped by the PluginManager

Key Components

Class / File	Role
core/src/core/org/jnode/plugin/Plugin.java	Base class for all plugins
core/src/core/org/jnode/plugin/PluginDescriptor.java	Describes a plugin (parsed from XML)
core/src/core/org/jnode/plugin/PluginManager.java	Manages plugin lifecycle
core/src/core/org/jnode/plugin/PluginRegistry.java	Registry of all known plugins
core/src/core/org/jnode/plugin/Extension.java	A plugin's contribution to an extension point
core/src/core/org/jnode/plugin/ExtensionPoint.java	A named hook where extensions register
core/src/core/org/jnode/plugin/ConfigurationElement.java	XML config data within an extension
core/src/core/org/jnode/plugin/PluginClassLoader.java	Per-plugin classloader
core/src/core/org/jnode/plugin/manager/DefaultPluginManager.java	Default lifecycle implementation
core/src/core/org/jnode/plugin/model/PluginDescriptorModel.java	XML descriptor parser
core/src/core/org/jnode/plugin/model/PluginsClassLoader.java	Parent classloader for all plugin classloaders

Descriptor XML Format

Each plugin has an XML file in <subproject>/descriptors/:

<pluginid="org.jnode.driver.video.vga"name="JNode Video Standard VGA driver"version="@VERSION@"license-name="lgpl"provider-name="JNode.org">
<!-- Dependencies on other plugins -->
<requires>
<importplugin="org.jnode.awt"/>
<importplugin="org.jnode.driver.bus.pci"/>
<importplugin="org.jnode.driver.video.vgahw"/>
</requires>
<!-- Packages this plugin makes visible to dependents -->
<runtime>
<libraryname="jnode-gui.jar">
<exportname="org.jnode.driver.video.vga.*"/>
</library>
</runtime>
<!-- Extensions contributed to other plugins' extension points -->
<extensionpoint="org.jnode.driver.bus.pci.device-factories">
<device-factoryclass="org.jnode.driver.video.vga.VGADriver"/>
</extension>
</plugin>

Descriptor Elements

Element	Purpose
<plugin>	Root: id, name, version, provider
<requires> / <import>	Declares dependencies
<runtime> / <library>	JAR files this plugin provides
<export>	Packages visible to importing plugins
<extension-point>	Defines a hook for other plugins to extend
<extension>	Contributes to another plugin's extension point
<fragment>	Fragment that attaches to a host plugin

How It Works

Plugin Loading (at boot)

1. InitJarProcessor extracts plugin JARs and descriptors from the init-jar
2. PluginDescriptorModel parses each XML descriptor
3. PluginRegistry stores all descriptors
4. DefaultPluginManager.startSystemPlugins() sorts by dependency order and activates each plugin

Classloader Hierarchy

BootClassLoader (VM built-in classes)
└── PluginsClassLoader (shared parent for all plugins)
├── PluginClassLoader [org.jnode.driver]
├── PluginClassLoader [org.jnode.fs]
├── PluginClassLoader [org.jnode.driver.video.vga]
└── ...

Each PluginClassLoader:

• Can see classes from its own JAR
• Can see classes from plugins listed in <requires> / <import>
• Cannot see classes from other plugins (isolation)

Extension Points

Extension points are the primary mechanism for loose coupling:

1. Plugin A declares <extension-point id="org.jnode.driver.bus.pci.device-factories">
2. Plugin B contributes <extension point="org.jnode.driver.bus.pci.device-factories">
3. At runtime, Plugin A queries its extension point to discover all contributors
4. ExtensionPoint.getExtensions() returns all Extension objects
5. Each Extension has ConfigurationElements with class names and parameters

This is how drivers register themselves: the PCI bus plugin defines an extension point for device factories, and each driver plugin contributes a factory.

Plugin Lists

File	Purpose
all/conf/default-plugin-list.xml	Plugins included in a standard build
all/conf/full-plugin-list.xml	All available plugins

The plugin list also declares the main class to launch:

<manifest>
<attributekey="Main-Class"value="org.jnode.shell.CommandShell"/>
</manifest>

Gotchas & Non-Obvious Behavior

• Build-time pre-loading — BootImageBuilder pre-loads plugin descriptors and bakes the PluginRegistry into the boot image. The registry field in Main.java is set at build time.
• No dynamic loading at runtime — Unlike Eclipse OSGi, JNode's plugin system is primarily static. All plugins are loaded at boot from the init-jar.
• Fragment vs. Plugin — A FragmentDescriptor attaches to a host plugin and shares its classloader. Used for classpath extensions.
• Version string — @VERSION@ in descriptors is replaced at build time by Ant.
• AutoUnzipPlugin — A special plugin type that auto-extracts ZIP content (used for classpath resources).
• Circular dependencies — Not supported. The build will fail if plugins have circular imports.

Related Pages

• Architecture — Where plugins fit in the system layers
• Boot-Sequence — How plugins are loaded during boot
• Build-System — How plugin descriptors are processed
• Driver-Framework — Drivers are plugins with extension points
• Network Device Plugin — Example plugin for network device drivers