Aktuelle Version vom 4. Januar 2009, 21:47 Uhr

written by Christoph P; created Tuesday, December 30, 2008

introduction

"JPF provides a runtime engine that dynamically discovers and loads "plug-ins". A plug-in is a structured component that describes itself to JPF using a "manifest". JPF maintains a registry of available plug-ins and the functions they provide (via extension points and extensions).

One major goal of JPF is that the application (and its end-user) should not pay any memory or performance penalty for plug-ins that are installed, but not used. Plug-ins are added to the registry at application start-up or while the application is running but they are not loaded until they are called." http://jpf.sourceforge.net/

some characteristics and features:

• open source (LGPL)
• low barrier of entrance (compared to OSGi)
• easy configuration (single xml file for each plugin)
• ant tasks
• hot deployment
• lazy loading
• integrity check
• embedded documentation

architecture overview

in the following example i will show you a simple demo application, divided into three parts: the boot module (got the main method and fires up jpf), the core module (alias application plugin, will display the main window and provide extension points for menubar entries) and the export module (imports core and uses the extension point to add a single entry to the menubar).

+--------------------------------------------+
|                                            |
|                  boot                      |
|                                            |
+--------------------------------------------+
|              |              |              |
|     core     |    export    |      ...     |   
|  (plugin_0)  |  (plugin_1)  |  (plugin_n)  |
|              |              |              |
++++++++++++++++++++ JPF +++++++++++++++++++++
************** JVM, Classloader **************

as you can see the core plugin (extends ApplicationPlugin) itself is a plugin.

manifest file

the three main parts are declared in the so-called manifest file (plugin.xml):

• plugin: with jpf the whole application is separated into independent plugins (kind of module). it holds the code, resources, libraries and defines imports (requirements), exports, extension-points and extensions. the class attribute is necessary if this plugin is an application plugin.

<!-
	PLUG-IN
	This is container for all other plug-in manifest elements.
	
	"docs-path": path to documentation folder, relative to plug-in context (home) folder
-->
<!ELEMENT plugin (doc?, attributes?, requires?, runtime?,
                  (extension-point|extension)*)>
<!ATTLIST plugin id         CDATA #REQUIRED>
<!ATTLIST plugin version    CDATA #REQUIRED>
<!ATTLIST plugin vendor     CDATA #IMPLIED>
<!ATTLIST plugin class      CDATA #IMPLIED>
<!ATTLIST plugin docs-path  CDATA #IMPLIED>

• extension-point: an interface which is provided by the plugin so other can provide implementations. an extension-point is identified by its plugin-id and extension-point-id, anddefines parameters for that interface. usually i only use an parameter called "class" for the fully qualified name of a class which implements a certain java interface and then define other attributes as getter on that java interface).

<!-
	EXTENSION POINT
	Tags extension-point describe the places where the functionality of
	this plug-in can be extended.
	
	Extension point multiplicity attribute description:
	any             any number of extensions can be available
	one             only one extension can be available
	one-per-plugin  only one extension can be defined in one plug-in
	none            no extension can be defined for this point
                        (used to declare "abstract" extension points,
                        that can be only "inherited" by other points using "parent" attributes)
-->
<!ELEMENT extension-point (doc?, parameter-def*)>
<!ATTLIST extension-point id                CDATA #REQUIRED>
<!ATTLIST extension-point parent-plugin-id  CDATA #IMPLIED>
<!ATTLIST extension-point parent-point-id   CDATA #IMPLIED>
<!ATTLIST extension-point extension-multiplicity  (any | one | one-per-plugin | none) "any">

<!-
	EXTENSION POINT PARAMETER DEFINITION
	... documentation shortened ...
-->
<!ELEMENT parameter-def (doc?, parameter-def*)>
<!ATTLIST parameter-def id             CDATA #REQUIRED>
<!ATTLIST parameter-def multiplicity   (one | any | none-or-one | one-or-more)
                                       "one">
<!ATTLIST parameter-def type           (string | boolean | number | date | time | date-time | null | any | plugin-id
                                       | extension-point-id | extension-id | fixed | resource) "string">
<!ATTLIST parameter-def custom-data    CDATA #IMPLIED>
<!ATTLIST parameter-def default-value  CDATA #IMPLIED>

• extension: could be thought as an implementation for a given interface. a plugin can extend the extension-point of another plugin and pass concrete values as arguments (most of the time only a "class" attribute which holds the full qualified name of a class).

<!-
	EXTENSION
	Tags extension describe the functionality the plug-in contribute to the system.
	
    "optional": if "true" than absense of required extension point will not cause runtime error
-->
<!ELEMENT extension (doc?, parameter*)>
<!ATTLIST extension plugin-id  CDATA #REQUIRED>
<!ATTLIST extension point-id   CDATA #REQUIRED>
<!ATTLIST extension id         CDATA #REQUIRED>
<!ATTLIST extension optional   (true | false) "false">

<!-
	EXTENSION PARAMETER
	
	If both attribute and tag "value" present, only attribute will be taken into account, the content of tag will be ignored.
	Parameter value will be validated according to parameter definition with corresponding ID.
-->
<!ELEMENT parameter (doc?, value?, parameter*)>
<!ATTLIST parameter id     CDATA #REQUIRED>
<!ATTLIST parameter value  CDATA #IMPLIED>

you also can have a look at the full DTD provided on the jpf website.

example

maven dependency

if you decide to use maven -and i really hope you did so already- just copy&paste these two dependencies into your pom: <source lang="xml"> <dependency> <groupId>net.sf.jpf</groupId> <artifactId>jpf</artifactId> <version>1.5</version> <scope>provided</scope> </dependency>

<dependency> <groupId>net.sf.jpf</groupId> <artifactId>jpf-boot</artifactId> <version>1.5</version> <scope>provided</scope> </dependency> </source>

this gives you access to the most recent jpf libraries (available december 2008) in provided scope, because the boot module got the two jars already in its classpath.

startuping up the framework

you can start the java plugin framework either by simply providing a boot.properties file in the project root and executing a utility jar (jpf-boot.jar), or by writing custom code which gives you full control of the boot procedure (preferred way). the last option is nevertheless required for unit tests.

let properties-file do the work (simple)

file boot.properties:

# application plugin id
org.java.plugin.boot.applicationPlugin = at.ac.tuwien.jsug.jpf.startup
# [ full | light | off ]
org.java.plugin.boot.integrityCheckMode = light
#org.java.plugin.boot.splashImage = ${applicationRoot}/splash.png

#-------------------------------------------------------------------------------
# JPF runtime configuration

org.java.plugin.PathResolver = org.java.plugin.standard.ShadingPathResolver
org.java.plugin.standard.ShadingPathResolver.shadowFolder = ${applicationRoot}/temp/.jpf-shadow
org.java.plugin.standard.ShadingPathResolver.unpackMode = smart
org.java.plugin.standard.ShadingPathResolver.excludes = CVS

#-------------------------------------------------------------------------------
# could be some more properties defined for own usage

executing following command should bring up your application: java -jar lib/jpf-boot.jar

here, the most important property is org.java.plugin.boot.applicationPlugin which looks for a plugin with the given id, looks up the manifest file and instantiates the type (ApplicationPlugin) defined by the class attribute.

take over full control (advanced)

file JpfBooter.java: <source lang="java"> package at.ac.tuwien.jsug.jpf.boot;

import javax.swing.SwingUtilities; import org.java.plugin.ObjectFactory; import org.java.plugin.PluginManager; import org.java.plugin.PluginManager.PluginLocation; import org.java.plugin.boot.DefaultPluginsCollector; import org.java.plugin.util.ExtendedProperties; import at.ac.tuwien.jsug.jpf.core.ICoreApplicationPlugin;

/**

* entry point for the application to bootstrap jpf and
* invoke CoreApplicationPlugin.startApplication().
*/

public class JpfBooter {

/** path to folder where plugins reside (either zipped, or unpacked as a simple folder) */ private static final String PLUGINS_REPOSITORY = "./plugins";

/** plugin id of the core module, defined in it's plugin.xml class attribute */ private static final String CORE_PLUGIN_ID = "at.ac.tuwien.jsug.jpf.core";

public static void main(String[] args) { SwingUtilities.invokeLater(new Runnable() { public void run() { new App().start(); }}); }

private void start() { // instantiate necessary objects final PluginManager manager = ObjectFactory.newInstance().createManager(); final DefaultPluginsCollector collector = new DefaultPluginsCollector(); final ExtendedProperties props = new ExtendedProperties();

// prepare configuration props.setProperty("org.java.plugin.boot.pluginsRepositories", PLUGINS_REPOSITORY);

try { collector.configure(props); // examine plugins repository for plugins manager.publishPlugins(collector.collectPluginLocations().toArray(new PluginLocation[] {}));

// finally retrieve the core plugin and start it up final ICoreApplicationPlugin corePlugin = (ICoreApplicationPlugin) manager.getPlugin(CORE_PLUGIN_ID); corePlugin.startApplication(); } catch (Exception e) { e.printStackTrace(); } } } </source>

create the main boot part

besides the plugins/ folder and a log4j configuration file, there is only the JpfBooter class (a listing was shown in the preceding section).

the project structure should look like this:

* src/main/java/
  - at.ac.tuwien.jsug.jpf.boot.JpfBooter.java
* src/main/resources/
  - log4j.properties
* plugins/
  - at.ac.tuwien.jsug.jpf.core/ ... target output for core plugin
  - at.ac.tuwien.jsug.jpf.export/ ... target output for export plugin

create the core application plugin

the application plugin has its manifest file (plugin.xml) and three types: the core application interface and its implementation and an interface for the menubar extension point.

after creating the files, the project structure should look like this:

* src/main/java/
  - at.ac.tuwien.jsug.jpf.core.CoreApplicationPlugin.java
  - at.ac.tuwien.jsug.jpf.core.ICoreApplicationPlugin.java
  - at.ac.tuwien.jsug.jpf.core.IMenuBarPlugin.java
* src/main/resources/
  - plugin.xml

file plugin.xml: <source lang="xml"> <?xml version="1.0" ?> <!DOCTYPE plugin PUBLIC "-//JPF//Java Plug-in Manifest 1.0" "http://jpf.sourceforge.net/plugin_1_0.dtd"> <plugin id="at.ac.tuwien.jsug.jpf.core" version="0.0.1" class="at.ac.tuwien.jsug.jpf.core.CoreApplicationPlugin">

<runtime> <library id="core" path="/" type="code"> <export prefix="*" /> </library> </runtime>

<extension-point id="MenuBar"> <parameter-def id="class" /> </extension-point> </plugin> </source>

file ICoreApplicationPlugin.java: <source lang="java"> package at.ac.tuwien.jsug.jpf.core;

public interface ICoreApplicationPlugin { void startApplication(); } </source>

file CoreApplicationPlugin.java: <source lang="java"> package at.ac.tuwien.jsug.jpf.core;

public class CoreApplicationPlugin implements ICoreApplicationPlugin { public void startApplication() { final JFrame frame = new JFrame(); frame.getContentPane().add(new JLabel("JPF Core Module"));

// ... lookup all extensions and load them into the menubar ... see code down below in "use the extension" section

frame.pack(); frame.setVisible(true); } } </source>

file IMenuBarPlugin.java: <source lang="java"> package at.ac.tuwien.jsug.jpf.core;

public interface IMenuBarPlugin { String getTitle(); void execute(); } </source>

create an extension

a plugin now simply declares the core plugin as a requirement in its manifest file (and therefore has access to the IMenuBarPlugin interface). then a proper implementation can be written for the menubar extension.

after creating the files, the project structure should look like this:

* src/main/java/
  - at.ac.tuwien.jsug.jpf.export.ExportMenuBarPlugin.java
* src/main/resources/
  - plugin.xml

file plugin.xml: <source lang="xml"> <?xml version="1.0" ?> <!DOCTYPE plugin PUBLIC "-//JPF//Java Plug-in Manifest 1.0" "http://jpf.sourceforge.net/plugin_1_0.dtd"> <plugin id="at.ac.tuwien.jsug.jpf.export" version="0.0.1">

<requires> <import plugin-id="at.ac.tuwien.jsug.jpf.core" /> </requires>

<runtime> <library id="src" path="/" type="code" /> </runtime>

<extension plugin-id="at.ac.tuwien.jsug.jpf.core" point-id="MenuBar" id="ExportMenuBar"> <parameter id="class" value="at.ac.tuwien.jsug.jpf.export.ExportMenuBarPlugin" /> </extension> </plugin> </source>

file ExportMenuBarPlugin.java: <source lang="java"> package phudy.jpf.pluginexport;

import at.ac.tuwien.jsug.jpf.core.IMenuBarPlugin;

public class ExportMenuBarPlugin implements IMenuBarPlugin { public String getTitle() { return "Export"; }

public void execute() { System.out.println("exec export"); } } </source>

use the extension

you might want to use the following handy methods for retrieving plugins: <source lang="java"> /**

* overloaded method setting default "class" attribute name
* @see #fetchPlugins(Plugin,String,String,String)
*/

public static <T> List<T> fetchPlugins( final Plugin plugin, final String extPointPluginId, final String extPointId) throws Exception { return fetchPlugins(plugin, extPointPluginId, extPointId, "class"); }

/**

* fetches all extensions for the given extension point qualifiers
*/

@SuppressWarnings("unchecked") public static <T> List<T> fetchPlugins( final Plugin plugin, final String extPointPluginId, final String extPointId, final String attributeName) throws Exception { final List<T> plugins = new LinkedList<T>();

final PluginManager manager = plugin.getManager();

final ExtensionPoint extPoint = manager.getRegistry().getExtensionPoint(extPointPluginId, extPointId); for (final Extension extension : extPoint.getConnectedExtensions()) { // LOG.info("Processing extension point: " + extension);

final PluginDescriptor extensionDescriptor = extension.getDeclaringPluginDescriptor(); manager.activatePlugin(extensionDescriptor.getId()); final ClassLoader classLoader = manager.getPluginClassLoader(extensionDescriptor); final String pluginClassName = extension.getParameter(attributeName).valueAsString(); final Class<T> pluginClass = (Class<T>) classLoader.loadClass(pluginClassName); final T pluginInstance = pluginClass.newInstance(); plugins.add(pluginInstance); }

return Collections.unmodifiableList(plugins); } </source>

file CoreApplicationPlugin: <source lang="java"> package at.ac.tuwien.jsug.jpf.core;

public class CoreApplicationPlugin implements ICoreApplicationPlugin {

private static final String EXTPOINT_ID_MENUBAR = "MenuBar";

public void startApplication() { final JFrame frame = new JFrame(); frame.getContentPane().add(new JLabel("JPF Core Module"));

// setup menubar final List<IMenuBarPlugin> menuBarPlugins = JpfUtil.fetchPlugins(this, this.getDescriptor().getId(), EXTPOINT_ID_MENUBAR); setJMenuBar(newMenuBar(menuBarPlugins));

frame.pack(); frame.setVisible(true); }

private JMenuBar newMenuBar(final List<IMenuBarPlugin> menuBarPlugins) { final JMenuBar bar = new JMenuBar(); final JMenu menu = new JMenu("Application");

for (int i = 0; i < menuBarPlugins.size(); i++) { final IMenuBarPlugin plugin = menuBarPlugins.get(i); final JMenuItem item = new JMenuItem(plugin.getTitle());

item.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent event) { plugin.execute(); }});

menu.add(item); }

bar.add(menu); return bar; } } </source>

appendix

links

• http://jpf.sourceforge.net ... official website of the JPF project
• http://page.mi.fu-berlin.de/oezbek/jpf/ ... JPF code generator
• http://maven.koecke.net/maven-jpf-plugin/ ... mojo to do the cumbersomely work of packaging (unfortunately, somehow unfinished).

notes

• although it is more common to write "plug-in" instead of "plugin" i have choosen not to write the additional "-" character :)