The Eclipse platform consists of many plug-ins, which are bundles of code that provide some functionality to the entire system. Plug-ins contribute functionality to the system by implementing pre-defined extension points. You can provide extension points in your own plug-in to allow other plug-ins to extend your functionality.

A plug-in describes itself to the system using an OSGi manifest (MANIFEST.MF) file and a plug-in manifest (plugin.xml) file. The Eclipse platform maintains a registry of installed plug-ins and the function they provide. As Equinox, the OSGi runtime, is at the core of Eclipse, you can think of a plug-in as an OSGi bundle. The main difference between plug-ins and bundles is that plug-ins use extension points for interaction between bundles.

Plug-ins take a lazy-loading approach, where they can be installed and available on the registry but will not be activated until the user requests some functionality residing in the plug-in.

MANIFEST.MF, usually located in the META-INF directory, deals with the runtime details for your plug-in. Editing of the manifest can be done through the editor provided, or directly in the MANIFEST.MF tab. The following is an example of one such manifest for a simple plug-in:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Myplugin
Bundle-SymbolicName: com.dzone.tests.myplugin
Bundle-Version: 1.0.0.qualifier
Bundle-Activator: com.dzone.tests.myplugin.Activator
Require-Bundle: org.eclipse.ui, org.eclipse.core.runtime
Bundle-ActivationPolicy: lazy
Bundle-RequiredExecutionEnvironment: JavaSE-1.6

The Eclipse OSGi Framework implements the complete OSGi R4.1 Framework specification and all of the Core Framework services. Here we list the most common manifest headers and directives.

Plug-in Runtime

The Require-bundle manifest header has some extra functionality to help you manage your runtime dependencies. Bundles can be marked as optional dependencies by annotating the bundle with ;resolution:=optional.

You can also manage which version of the bundle your dependent on needs to be present using the ;bundleversion=' <values>' annotation. Here, the <values> that we refer to are a range of versions where you can specify minimum and maximum version ranges. The syntax of this range value is illustrated through these examples:

Additional Eclipse Bundle Headers

Eclipse provides a number of addition bundle headers and directives. These extra headers are not part of the OSGi R4.1 specification, but allow developers to use additional Eclipse OSGi Framework functionality.

Additional directives
are available to manage
the access restriction of
exported packages.x-internalThe default value for this
property is false. When
internal packages are
specified as true using this
option, the Eclipse PDE
discourages their use.x-friendsThis option is similar to
x-internal, but allows
certain bundles to use the
exported packages that
have this option. Other
bundles are discouraged.
The x-internal option
takes precedence over
x-friends.

Export-Package:
org.eclipse.
foo.internal;
x-internal:=trueExport-Package:
org.eclipse.foo.
formyfriends;
x-friends:='org.
eclipse.foo.
friend1'

This allows you to set
particular rules for your
bundle before it can start.osgi.nl for language
osgi.os for operating
system
osgi.arch for
architecture
osgi.ws for windowing
system

All entries in the manifest can be internationalized by moving them to a separate plugin.properties file.

With the Manifest.MF file looking after the runtime dependencies, plugin.xml deals with the plug-in extensions and extension points.

An extension allows you to extend the functionality of another plug-in in your system. An extension can be added through the plug-in editor's Extensions tab, or to your plugin.xml.

<extension point='org.eclipse.ui.preferencePages'>
   <page
     class='com.dzone.tests.myplugin.preferences.
       SamplePreferencePage'
           id='com.dzone.tests.myplugin.preferences.
       SamplePreferencePage'
     name='Sample Preferences'>
   </page>
</extension>

Each extension point has a XML schema which specifies the elements and attributes that make up the extension. As you can see in the listing above, each extension point has a unique identifier. The <page> element above is specified in the XML schema for the org.eclipse.ui.preferencesPages extension.

You can also define your own extension points, and we will detail that process in a later section.

The plug-in class is a representation of your plug-in running in the Eclipse platform. A plug-in class in Eclipse must extend org. eclipse.core.runtime.Plugin, which is an abstract class that provides generic facilities for managing plug-ins. When using the project wizard in the PDE, this class typically gets assigned Activator as its default name. Whatever name you assign to this plug-in class, it must be the same as that mentioned in the Bundle-Activator directive of your MANIFEST.MF.

The class has start and stop methods that refer to the BundleContext and are provided by the BundleActivator interface. These methods allow you to deal with the plug-ins lifecycle, so that you can do both initialization and cleanup activities at the appropriate times. When overriding these methods be sure to always call the superclass Implementations.

Bundle Context

A BundleContext is associated with your plug-in when it is started. As well as providing information about the plug-in, the BundleContext can provide information about other plug-ins in the system. By providing a listener to BundleEvent, you can monitor the lifecycle of any other plug-in.

Bundle

The terms Bundle and Plug-in may be used interchangeably when discussing Eclipse. The Bundle class provides us with the OSGi unit of modularity. There are six states associated with bundles:

Plug-ins are normally set to load lazily, so that the code isn't loaded into memory until it is required. This is normally a good thing as you don't want to affect the startup time of Eclipse. If you do require your plug-in to start up and load when Eclipse launches, you can use the org.eclipse.ui.startup extension point.

<extension point='org.eclipse.ui.startup'>
   <startup class='com.myplugin.StartupClass'>
</extension>

The startup class listed above must implement the org.eclipse.ui.IStartup interface which provides an earlyStartup() method. The method is called in a separate thread after the workbench initializes.

The Eclipse platform provides a number of extension points that you can hook into, to provide additional functionality. The concept behind an extension point is that a class provides some extendable behavior, and publishes this behavior as an extension point. In order to run this code, the plug-in requires a host '" in this case your own plug-in.

In your plugin.xml you take this extension point and provide extra information to help it run. You will usually need to provide some class that implements a particular interface in order to do this.

Here we will run through some useful extension points in the Eclipse platform. Note, that to make some of these available for your plug-in, you will usually need to add dependencies.

As well as being a user of extension points, a plug-in can provide its own extensions for other plug-ins. Extension points allow loose coupling of functionality '" your plug-in exposes a set of interfaces and an extension point definition for others to use.

Extension Point Definition

You can create your extension point through the plugin.xml file, or through the Add button in the Extension Points tab of the plug-in editor.

For identifying your extension point you need to provide a unique identifier and a human readable name. At this point you can also point to a schema file and edit it afterwards. An extension point schema must have .exsd as its suffix.

Figure 1: The New Extension Point Wizard

Defining an Extension Point Schema

The PDE provides an editor for defining your .exsd file, consisting of three tabs. First, the Overview tab allows you to provide documentation and examples for your extension point. This is an essential step if you want your extension point to be adopted. Next, the Definition tab presents a graphical way to define your schema, while the Source tab allows editing of the .exsd XML definition.

Figure 2: The Extension Point editor

When creating your extension point, you will first want to create one or more elements with attributes that will be used. Each extension point attribute has a number of associated properties:

Once you have created your elements and attributes for the extension point, the element can be added to a sequence for this extension. You can control the multiplicity of your extension here.

The mapping of XML to extension point declaration is simple; for users of your point, an xml element in the extension point will always appear on the left hand side tree, as part of the extension point declaration, while the xml attributes will appear as extension point attributes.

The Code Behind an Extension Point

With the extension point defined, the producer of this needs to provide some implementation that makes use of any extension point contributions.

To get a list of all the implementers of your extension point you can query the extension registry as follows, providing your extension point identifier as the parameter.

IConfigurationElement[] config = Platform.getExtensionRegistry()
   .getConfigurationElementsFor('myextid');

To use the implementing extension point, you can get the object from the IConfigurationElement.

final Object o = config[i].createExecutableExtension('class');

The PDE plug-in editor provides a number of useful utilities for working with your plug-ins. The Dependencies tab in particular is essential for organizing your runtime.

From here you can investigate the plug-in dependency hierarchy, starting with your plug-in as the root. You can also see which plug-ins are dependent on your own plug-in, as well as find any unused dependencies. This can be useful if you previously added a dependency to use an extension point, but have found that it is since no longer required. Finally, and most importantly, the tab provides a utility for investigating for cyclic dependencies.

Another useful tool for plug-in development is the Plug-in Registry view. This can be accessed from the Window>Show View>Other..>Plug-in Development category. This view will display all the plug-ins that are currently available in your Eclipse installation.

Figure 4: Plug-in Registry

Logging

It is recommended to log to a file, rather than using System. out. The Activator or plug-in class provides a facility to access the plug-in logging mechanism through the getLog() method, returning the org.eclipse.core.runtime.ILog interface.

Each log entry using this framework is of type IStatus. Any CoreExceptions thrown in Eclipse have an associated IStatus object. An implementation of this interface, Status, is available for use. There is also a MultiStatus class which allows multiple statuses to be logged at once.

Since Eclipse 3.4, p2 has been used as the method to provision your application with new or updated plug-ins. For build managers who have used the Update Site mechanism before, there doesn't need to be any change.

To create an update site you can use the wizard provided to create a new site.xml file. Using the Software Updates menu, users can point to your update site on the web and download the plug-in.

By adding some extra functionality over this simple implementation, you can leverage p2 to add extra meta data to your update site, which will make the installation experience faster for end users.

p2 Update Site Publisher

The UpdateSite Publisher application is provided by p2 to generate an artifact.xml and content.xml files for your standard update site. You can run this application in headless mode using org.eclipse.equinox.p2.publisher.UpdateSitePublisher. The following shows an example of how to run this application, taken from the p2 wiki.

java -jar <targetProductFolder>/plugins/org.eclipse.equinox.
launcher_*.jar
 -application org.eclipse.equinox.p2.publisher.UpdateSitePublisher
 -metadataRepository file:/<some location>/repository
 -artifactRepository file:/<some location>/repository
 -source /< location with a site.xml>
 -configs gtk.linux.x86
 -compress
 -publishArtifacts

Read more about p2 at http://wiki.eclipse.org/Equinox/p2

When developing your plug-in, you should be aware of the wide variety of projects available in the Eclipse eco-system that help make your development easier and faster. This section gives an overview of just a few of the useful projects that exist, and explains how they can be used in your project.

Eclipse Modeling Project
http://eclipse.org/modeling/

The Eclipse Modelling Project provides a large set of tools for model driven development. The most popular part of this project is the Eclipse Modelling Framework (EMF). Using this technology, you can define a model in the score format, generate Java code to represent, serialize and de-serialize the model. Other tools within the modeling project utilize EMF to provide more specialized frameworks for developers.

The Connected Data Objects (CDO) project provides a three tier architecture for distributed and shared models.

The Graphical Modelling Framework (GMF) allows you to generate graphical editors for your model based on EMF and the Graphical Editing Framework (GEF). For developers who want to provide textual editor for their own language or DSL, XText provides a EBNF grammar language and generates a parser, meta-model and Eclipse text editor from this input.

Eclipse Communication Framework
http://eclipse.org/ecf

If your plugin requires any communication functionality, the ECF project is the first place to look. ECF consists of a number of bundles that expose various communication APIs. These APIs range from instant messaging, dynamic service discovery, file transfer to remote and distributed OSGi. Real-time shared editing functionality is also available in the framework, allowing you to collaborate remotely on anything that you are editing within your plug-in's environment.

Business Intelligence and Reporting Tools
http://eclipse.org/birt

BIRT is an open source reporting system based on Eclipse. BIRT provides both programmatic access to report creation, as well as functionality to create your own report template within the Eclipse IDE. While BIRT allows you to generate reports in file formats such as PDF, it is also possible to use BIRT on an application server to serve reports through a web browser.

Equinox
http://eclipse.org/equinox

As we have described in this card, Equinox is the Eclipse implementation of the OSGi R4 core framework specification, and provides the real runtime for all your plug-ins. However, as well as running your plug-ins on the desktop on an instance of Eclipse, you can take Equinox and run it on a server, allowing your plug-in to run on browsers as well as the desktop.

Rich Ajax Platform
http://eclipse.org/rap

With the emergence of the web as a real platform for rich applications, the Rich Ajax Platform allows you to take a standard RCP project, and with some minor modifications, make it deployable to the web. This idea of single-sourcing is key to the RAP project, and reduces the burden for developers to make an application ready for either the desktop or the web.

The same programming model is used, while qooxdoo is used for the client side presentation of your SWT and JFace widgets.