Groovy classpath not set up properly by <groovy> Ant task - ant

Question

My Groovy script depends on some libraries. This is what I have at the top of my script.
#Grapes([
#Grab(group = 'net.sf.json-lib', module = 'json-lib', version = '2.3',
classifier = 'jdk15'),
#Grab(group = 'org.codehaus.groovy.modules.http-builder',
module = 'http-builder', version = '0.7.1'),
#Grab(group = 'commons-cli', module = 'commons-cli', version = '1.2')])
When I run the script from command line using groovy executable, everything works properly. The artefacts get downloaded and the script runs.
However, if I try to execute the same script from Apache Ant using <groovy src="myscript.groovy" fork="true" /> (simplified), the artefacts also get resolved and downloaded but then I get [groovy] Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/commons/cli/ParseException.
The Apache Ant installation uses groovy-all-2.3.6.jar that I have copied from the Groovy installation that I have installed, so they should be pretty much identical.
What am I missing? How can I make the <groovy> task work and use the downloaded jars?
UPDATE I
The issue I believe is that Grape and Ant use different classloaders so the artifacts aren't visible to Ant. Can you try adding this: #GrabConfig(systemClassLoader = true) to your groovy script after the #Grape annotations?
If I do that I get General error during conversion: No suitable ClassLoader found for grab.
UPDATE II
I have also tried this now:
import groovy.grape.Grape;
Grape.grab(group:"commons-cli", module:"commons-cli", version:"1.2", classLoader:this.class.classLoader.rootLoader)
//...
It does not help. I get compile time error then:
[groovy] Exception in thread "main" Script Failed: org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
[groovy] C:\Users\xxx\AppData\Local\Temp\embedded_script_in_2825216891785993632groovy_Ant_task: 18: unable to resolve class groovyx.net.http.HTTPBuilder
[groovy] # line 18, column 1.
[groovy] import groovyx.net.http.HTTPBuilder
[groovy] ^

Answer 1

Get rid of the #Grab and use the static .grab() method Grape offers. It allows you to specify a the rootLoader classloader which Ant can see:
import groovy.grape.Grape;
Grape.grab(group:"commons-cli", module:"commons-cli", version:"1.2", classLoader:this.class.classLoader.rootLoader)
<repeat for rest of #Grab>