No suitable classloader found for grab

Question

I have this at the beginning of a class:

@Grab(group = 'org.ccil.cowan.tagsoup', module = 'tagsoup', version = '1.2')
class MyClass{...

I'm trying to unit test this class, but whenever I try to run JUnit 4 tests, I get this error:

Caused by: java.lang.RuntimeException: No suitable ClassLoader found for grab
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at org.codehaus.groovy.reflection.CachedConstructor.invoke(CachedConstructor.java:77)
    at org.codehaus.groovy.runtime.callsite.ConstructorSite$ConstructorSiteNoUnwrapNoCoerce.callConstructor(ConstructorSite.java:102)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallConstructor(CallSiteArray.java:52)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callConstructor(AbstractCallSite.java:190)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callConstructor(AbstractCallSite.java:198)
    at groovy.grape.GrapeIvy.chooseClassLoader(GrapeIvy.groovy:163)
    at groovy.grape.GrapeIvy$chooseClassLoader.callCurrent(Unknown Source)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallCurrent(CallSiteArray.java:44)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:141)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:149)
    at groovy.grape.GrapeIvy.grab(GrapeIvy.groovy:227)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at org.codehaus.groovy.runtime.callsite.PogoMetaMethodSite$PogoCachedMethodSite.invoke(PogoMetaMethodSite.java:225)
    at org.codehaus.groovy.runtime.callsite.PogoMetaMethodSite.callCurrent(PogoMetaMethodSite.java:51)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallCurrent(CallSiteArray.java:44)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:141)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:153)
    at groovy.grape.GrapeIvy.grab(GrapeIvy.groovy:216)
    at groovy.grape.Grape.grab(Grape.java:131)
    at groovy.grape.Grape$grab.callStatic(Unknown Source)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallStatic(CallSiteArray.java:48)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callStatic(AbstractCallSite.java:165)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callStatic(AbstractCallSite.java:173)
    at ammoscanner.AmmoScanner.<clinit>(AmmoScanner.groovy)
    ... 30 more

Any ideas? I'm using groovy 1.7.5

Answer 1

The Problem

Looking at the source code, this exception is thrown whenever the supplied ClassLoader's name (or it's superclasses) is not groovy.lang.GroovyClassLoader or org.codehaus.groovy.tools.RootLoader. i.e. The target classloader must be an instance of the aforementioned classes (a bit restrictive IMHO).

A Solution

Currently I don't know how to configure a specific classloader using @Grape/@Grab/@GrabConfig annotations. The closest would be to use @GrabConfig(systemClassLoader=true), and ensure the System classloader is an instance of one of the above ClassLoader classes.

If anyone does know, please let me know (and I'll update this answer).

A Workaround

The following code will programmatically download your Grapes, and load them into the supplied GroovyClassLoader (admittedly, not quite what you want).

def loadGrapes(){
    ClassLoader classLoader = new groovy.lang.GroovyClassLoader()
    Map[] grapez = [[group : 'org.ccil.cowan.tagsoup', module : 'tagsoup', version : '1.2']]
    Grape.grab(classLoader: classLoader, grapez)
    println "Class: " + classLoader.loadClass('org.ccil.cowan.tagsoup.jaxp.SAXParserImpl')
}