Tuesday, March 17, 2009

RHQ tip of the day: Overseer API (updated)

Sometimes it is necessary in RHQ-plugins to load resources when the plugin is loaded and more important to unload those when the plugin is unloaded from the plugin container.

( As you can see from the comments, Mazz has renamed the Interface class, so I updated this article. )

Mazz has recently added the PluginOverseer PluginLifecycleListener interface for exactly this purpose (see e.g RHQ-1584).

To use the api, you need to do two things:

  • Implement the interface org.rhq.core.pluginapi.plugin.PluginLifecycleListener
  • Wire the implementation class in the plugin descriptor.

With the following entry in the plugin descriptor, rhq-plugin.xml

<plugin name="MyPlugin"


  pluginLifecycleListener="MyPluginOverseer" .../>

The code for this could look like this:

package org.rhq.plugins.myPlugin;

import org.rhq.core.pluginapi.plugin.PluginContext;

import org.rhq.core.pluginapi.plugin. PluginLifecycleListener;


public class MyPluginOverseer

  implements PluginLifecycleListener {

  Class clazz;

// called on plugin load

public void initialize( PluginContext context ) {


  clazz = Class.forName("org.acme.Driver");



// called on plugin unload

public void shutdown() {

  clazz = null;



There are some real examples of the usage of this API, like e.g. in the Postgres-plugin.

As this API is added to RHQ, it applies to Jopr (trunk) and the upcoming release of JBoss ON as well.

I have also updated the plugin generator to support this new API - its version is now 1.2.1.


Greg said...

PluginOverSeer? only mazz could name something that. PluginLifecycleListener wouldn't have been more descriptive?

Heiko W. Rupp said...

The funny part is that Mazz even wrote in the Jira, that he does not like this name :)

John Mazz said...

See this JIRA: RHQ-1884

Done. As Heiko mentioned, I hated the name myself and I did solicit suggestions at the time - but I got nothing from nobody and as such, silence == acceptance. :-p

But, even though this new suggestion is a month late, it is good so I used it.

Heiko, I updated your plugin gen code to use the new "pluginLifecycleListener".