how can I know whether the plugin is used by any jobs in jenkins

Jenkins had 600+ plugins, in the real system, we are used to install lots of plugins.

And sometimes, we want to remove some plugins to make system more clean or replace with another mature plugin (different name).

This needs to make sure no one/no job use those plugins or I need to notify them.

Are there any ways in configuration or somewhere in Jenkins system to know whether the plugin is used by any jobs ?

UPDATE 2013 Based on the answer below, I maintain the simple "plugin:keyword" mapping, like

plugin_keys = {
    "git":'scm class="hudson.plugins.git.GitSCM"',
    "copyartifact":"hudson.plugins.copyartifact.CopyArtifact",
        # and more      
}

And search the plugin keyword from the config.xml, all the information (plugins,jobs,config) can be fetched via jenkins remote API

it works for me.

UPDATE 2014.04.26 Later jenkins version, it seems the config.xml is changed to have plugin name there directly

Like

<com.coravy.hudson.plugins.github.GithubProjectProperty plugin="[email protected]">
<hudson.plugins.throttleconcurrents.ThrottleJobProperty plugin="[email protected]">
<hudson.plugins.disk__usage.DiskUsageProperty plugin="[email protected]"/>
<scm class="hudson.plugins.git.GitSCM" plugin="[email protected]">

Therefore I just check this plugin="<plugin name>" in config.xml, it works again

UPDATE 2014.05.05

See complete script in gist jenkins-stats.py

UPDATE 2018.6.7

There is plugin usage plugin support this (no REST API yet)

Here are 2 ways to find that information.

The easiest is probably to to grep the job config files:

E.g. when you know the class name (or package name) of your plugin (e.g. org.jenkinsci.plugins.unity3d.Unity3dBuilder):

find $JENKINS_HOME/jobs/ -name config.xml -maxdepth 2 | xargs grep Unity3dBuilder

Another is to use something like the scriptler plugin, but then you need more information about where the plugin is used in the build.

import hudson.model.*
import hudson.maven.*
import hudson.tasks.*

for(item in Hudson.instance.items) {
    //println("JOB : "+item.name);
    for (builder in item.builders){
      if (builder instanceof org.jenkinsci.plugins.unity3d.Unity3dBuilder) {
        println(">>" + item.name.padRight(50, " ") + "\t UNITY3D BUILDER with " + builder.unity3dName);
      }
    }
  }
}

Update: here's a small scriplet script that might ease you finding the relevant class names. It can certainly be improved:

import jenkins.model.*;
import hudson.ExtensionFinder;

List<ExtensionFinder> finders = Jenkins.instance.getExtensionList(ExtensionFinder.class);

for (finder in finders) {
  println(">>> " + finder);
  if (finder instanceof hudson.ExtensionFinder.GuiceFinder) {
    println(finder.annotations.size());
    for (key in finder.annotations.keySet()) {
       println(key);
    }
  } else if (finder instanceof ruby.RubyExtensionFinder) {
    println(finder.parsedPlugins.size());
    for (plugin in finder.parsedPlugins) {
      for (extension in plugin.extensions) {
        println("ruby wrapper for " + extension.instance.clazz);
      }
    }
  } else if (finder instanceof hudson.cli.declarative.CLIRegisterer) {
    println(finder.discover(Jenkins.instance));
    for (extension in finder.discover(Jenkins.instance)) {
      println("CLI wrapper for " + extension.instance.class);
      // not sure what to do with those      
    }
  } else {
    println("UNKNOWN FINDER TYPE"); 
  }
}

(inlined scriplet from my original listJenkinsExtensions submission to http://scriptlerweb.appspot.com which seems down)

Don't forget to backup!

As of early 2018 there is a "Plugins Usage Plugin" that gives you a nice list of the plugins and where they are used. We've noticed that depending on the system sometimes it doesn't seems to catch all the plugins, but it gives a really lovely list of the plugins and all jobs related to a specific plugin in an expandable list.

https://plugins.jenkins.io/plugin-usage-plugin

> Plugins used in pipeline scripts would not be listed normally as used by jobs, because they are used dynamically in Jenkinsfiles.

I can't comment because I don't have enough reputation, but if I could, I would point out that the broken link provided by coffeebreaks for the small scriplet script mentioned in the accepted answer can be found on the Internet Archive, at this link:

https://web.archive.org/web/20131103111754/http://scriptlerweb.appspot.com/script/show/97001

In case that link breaks, here is the content of the script:

import jenkins.model.*;
import hudson.ExtensionFinder;

List<ExtensionFinder> finders = Jenkins.instance.getExtensionList(ExtensionFinder.class);

for (finder in finders) {
  println(">>> " + finder);
  if (finder instanceof hudson.ExtensionFinder.GuiceFinder) {
    println(finder.annotations.size());
    for (key in finder.annotations.keySet()) {
       println(key);
    }
  } else if (finder instanceof ruby.RubyExtensionFinder) {
    println(finder.parsedPlugins.size());
    for (plugin in finder.parsedPlugins) {
      for (extension in plugin.extensions) {
        println("ruby wrapper for " + extension.instance.clazz);
      }
    }
  } else if (finder instanceof hudson.cli.declarative.CLIRegisterer) {
    println(finder.discover(Jenkins.instance));
    for (extension in finder.discover(Jenkins.instance)) {
      println("CLI wrapper for " + extension.instance.class);
      // not sure what to do with those      
    }
  } else {
    println("UNKNOWN FINDER TYPE"); 
  }
}

I wrote this parameterized Jenkins job that searches config files. You only need to know what tags the plugin generates in the config file and to use that tag's name as parameter needle:

cd  $JENKINS_HOME
cd jobs
echo searching for $needle
find . -name config.xml -type f -exec grep $needle /dev/null {} \;