This document, together with the hello-world plugin, shows you how to get started with the plugin development.

What Can Plugins Do?

Hudson defines extensibility points, which are interfaces or abstract classes that model an aspect of a build system. Those interfaces define contracts of what need to be implemented, and Hudson allows plugins to contribute those implementations. See this document for more about extension points.

In this document, we'll be implementing a Builder that says hello. (built-in builders include Ant, Maven, and shell script. Builders build a project.)

Other resources

Besides this tutorial, there are other tutorials and examples available on line:

Stephen Connolly's 7 part tutorial (Writing a Hudson plugin)
Quick start guide in Japanese

Setting Up Environment

To develop a plugin, you need Maven 2 (why?) and JDK 6.0 or later. If this is the first time you use Maven, make sure Maven can download stuff over the internet.

With a fairly recent version of Maven (ie. 2.0.9 or newer) you should only need to add the following to your ~/.m2/settings.xml (Windows users will find them in %USERPROFILE%\.m2\settings.xml):

<settings>

<pluginGroups>
  <pluginGroup>org.jvnet.hudson.tools</pluginGroup>
</pluginGroups>

</settings>

This will let you use short names of Hudson Maven plugins (ie. hpi:create instead of org.jvnet.hudson.tools:maven-hpi-plugin:1.23:create).

If you are using Maven version earlier than 2.0.9, you'll need to follow additional steps.

Creating a New Plugin

To start a new plugin, run the following maven command:

$ mvn -cpu hpi:create

This will ask you a few question, like the groupId (the maven jargon for the package name) and the artifactId (the maven jargon for your project name), then create a skeleton plugin from which you can start with. Make sure you can build this:

$ cd newly-created-directory
$ mvn package

Setting up a productive environment with your IDE

NetBeans

NetBeans users can use the IDE's Maven support to open the project directly. (Bundled in 6.7 and up; available from Plugin Manager for 6.5.) The archetype may also be available right from the New Project dialog.

IntelliJ IDEA

IntelliJ 7.0 (or later) users can load pom.xml directly from IDE, and you should see all the source code of libraries and Hudson core all the way to the bottom.

Eclipse

Use Eclipse 3.3 or later to avoid a bug in Eclipse 3.2 (more details.)
Eclipse users can run the following Maven command to generate Eclipse project files:

$ mvn -DdownloadSources=true -DdownloadJavadocs=true eclipse:eclipse

Alternatively, Eclipse users can install the maven2 eclipse plug-in, to open a Maven project directly in IDE.

If you get the error message Unable to find a plugin class. Did you put @plugin in javadoc? this maybe caused by eclipse and maven both use the target folder for build output.
Run mvn clean before building with maven or change the output path.

Plugin Workspace Layout

The plugin workspace consists of the following major pieces:

pom.xml

Maven uses it for building your plugin

src/main/java

Java source files of the plugin

src/main/resources

Jelly/Groovy views of the plugin. See this document for more about it.

src/main/webapp

Static resources of the plugin, such as images and HTML files.

Source Code

Let's take a look at the source code. A plugin's main entry point is a PluginImpl class that extends from Plugin. Once Hudson detects your plugin class (via its inheritance relationship from Plugin), it will create an instance, and invokes methods.

Most of the time, a plugin class just registers extension points, and your main work involves in implementing those extension points. See the source code for more about how a Builder is implemented and what it does.

Debugging a Plugin

NetBeans 6.7+ users can just hit Debug. For all others, run the following command to launch Hudson with your plugin:
Unix:

$ export MAVEN_OPTS="-Xdebug -Xrunjdwp:transport=dt_socket,server=y,address=8000,suspend=n"
$ mvn hpi:run

Windows:

> set MAVEN_OPTS=-Xdebug -Xrunjdwp:transport=dt_socket,server=y,address=8000,suspend=n
> mvn hpi:run

If you open http://localhost:8080/ in your browser, you should see the Hudson page running in Jetty. The MAVEN_OPTS portion launches this whole thing with the debugger port 8000, so you should be able to start a debug session to this port from your IDE.

Once this starts running, keep it running. Jetty will pick up all the changes automatically.

When you make changes to view files in src/main/resources or resource files in src/main/webapp, just hit F5 in your browser to see the changes.
When you change Java source files. Compile them in your IDE (NetBeans 6.7+: Debug > Apply Code Changes) and Jetty should automatically redeploy Hudson to pick up those changes. There is no need to run mvn at all.
MAVEN_OPTS can be used to specify all sorts of other JVM parameters, like -Xmx

Changing port

If you need to launch the Hudson on a different port than 8080, set the port through the system property port

$ mvn hpi:run -Dport=8090

Distributing a Plugin

To create a distribution image of your plugin, run the following Maven goal:

$ mvn package

This should create target/*.hpi file. Other users can use Hudson's web UI to upload this plugin to Hudson (or place it in $HUDSON_HOME/plugins.)

Hosting a Plugin on java.net

If you got to this point, you should definitely consider hosting your plugin on java.net. Move on to this document for how to do that.

Other tips

Consider running Maven like mvn -o ... to avoid hitting repositories every time. This will make various operations considerably faster.
Subscribe to users@hudson.dev.java.net from here so that we can get in touch with you.
When you bump up the version of Hudson you depend on, make sure to run mvn clean once, in particular to delete target/work that Jetty uses. Newer versions may just use work, not target/work. Otherwise your Jetty may continue to pick up old left-over jar files.