When I actually got around to trying this, I could not make it work.

While the code worked fine when run as a Groovy script on its own, once it was inside the Jenkins JVM, for some reason "rootLoader" was null. I'm not enough of a Groovy Guru to understand the difference between the two loaders, but I am still stumped and have no workaround.

Cool . . . just made it to my SQL server from inside the plugin with the following change instead of "this":

manager.hudson.class.classLoader.addURL(new URL("file:///" + filePath))

Frank, how did that work? For me that says:

groovy.lang.MissingPropertyException: No such property: manager for class: Script1

fmerrow, bviktor would be higly interested in that, too

I've been having the same problem. I'm using v2.2 of the plugin. I've tried using the "Additional Classpath" fields, gone into the Jenkins file in /etc/sysconfig and added a "-cp" options to the JENKINS_JAVA_OPTIONS setting, and tried using the classload in the form: manager.hudson.class.classLoader.addURL(new URL("file:///" + filePath))

None of it seemed to work. In the end I had to create a "Trigger parameterized build on other project" then specify my script in a Build step. The classpath variables appeared to work there.

I was able to get the following to work

in my postbuild task

this.class.classLoader.parseClass("/home/jenkins/GitlabPostbuildReporter.groovy")
GitlabPostbuildReporter.newInstance(manager).report()

in my file on disk at /home/jenkins/GitlabPostbuildReporter.groovy

class GitlabPostbuildReporter {
  def manager
  public GitlabPostbuildReporter(manager){
    if(manager == null) {
      throw new RuntimeException("Manager object musn't be null")
    }
    this.manager = manager
  }
  public def report() {
    // do work with manager object
  }
}

Hi!
I am doing the same like Zach in the last comment but getting error message
groovy.lang.MissingPropertyException: No such property: XXX for class: Script1

does anyone have any idea why it can happen?

thanks

Also spent hours trying to make it working but got lots various errors.
Finally could make it working the following way:

Groovy file:

class GitlabPostbuildReporter {
  def manager
  
  public GitlabPostbuildReporter(Object manager){
    if(manager == null) {
      throw new RuntimeException("Manager object can't be null")
    }
    this.manager = manager
  }
  
  public def report() {
    // do work with manager object
    manager.addBadge("db_in.gif", "Stored to DB")
  }
}

On Jenkins side create a groovy script post build action:

// Ayrat Natfullin: this code passes manager object to external groovy script 
// so you may operate with current job
import hudson.model.*
  
import groovy.lang.GroovyClassLoader;
import groovy.lang.GroovyObject;
 
import java.io.File;

// short ref for printing out in Jenkins console
out = manager.listener.logger.&println

def file = new File("D:/tools/Jenkins/scripts/GitlabPostbuildReporter.groovy")
// this helps you to make sure script exists and Jenkins has access to it
out(file.exists())

// Loading script using String parameter doesn't work so switch to using File instead
Class groovy = this.class.classLoader.parseClass(new File("D:/tools/Jenkins/scripts/GitlabPostbuildReporter.groovy"));
GroovyObject groovyObj = (GroovyObject) groovy.newInstance(manager);
// example of how to print out all existing methods
out(groovyObj.metaClass.methods*.name.sort().unique());
// call report that will add Badge icon to current build (see script above)
groovyObj.report();

Also the original question was about "Additional groovy classpath": in Jenkins ver. 1.619 it works.
I've moved all logic to java uber jar, added jar to classpath in Groovy Script Post Build action and is able to make http requests from Java.

Ayrat, thanks a lot! your solution works for me!