Description of problem:

When calling RuntimeManagerFactory.newSingletonRuntimeManager() with the default MVELUserGroupCallback in OSGi, it fails with no specific exception. Using debugger, you can see that an InvocationTargetException was thrown with target CompileException which cannot be evaluated to string due to:

  Method threw 'java.lang.StringIndexOutOfBoundsException' exception. Cannot evaluate org.mvel2.CompileException.toString()

The cause of the CompileException is the following:

java.lang.ClassNotFoundException: org.jbpm.services.task.impl.model.UserImpl

The debugger reveals that the class context is:

class org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer
class org.mvel2.ast.NewObjectNode
class org.mvel2.compiler.ExecutableAccessor
...
class org.mvel2.MVELRuntime
...
class org.drools.core.util.MVELSafeHelper$RawMVELEvaluator
class org.jbpm.services.task.utils.MVELUtils
class org.jbpm.services.task.identity.MvelUserGroupCallbackImpl
class org.jbpm.services.task.HumanTaskConfigurator
class org.jbpm.runtime.manager.impl.factory.LocalTaskServiceFactory
class org.jbpm.runtime.manager.impl.SingletonRuntimeManager
class org.jbpm.runtime.manager.impl.RuntimeManagerFactoryImpl
...
class org.drools.karaf.itest.beans.ProcessWithPersistenceBean

I have tried to add the not found class' package to Import-Package header of the bundle, but it did not help.

I understand that MvelUserGroupCallback might not be used in production, so I am setting severity to medium - if that assumption is not true, feel free to raise severity to high.

In case this was not fixable with reasonable effort and MvelUserGroupCallback was really not intended for production use, we could document this as a known issue. 

Version-Release number of selected component (if applicable):
BRMS 6.2.2
Fuse 6.2.1

How reproducible:
always

Steps to Reproduce:
1. look at [1] in KieBlueprintjBPMPersistenceKarafIntegrationTest from PR https://github.com/droolsjbpm/droolsjbpm-integration/pull/315
2. comment out the line [1]
3. put a breakpoint at [2] (creating the RuntimeManager)
4. uncomment [3] (enable Karaf debugging)
5. run 'mvn clean test -Dtest=KieBlueprintjBPMPersistenceKarafIntegrationTest'
6. attach debugger and setp over to see the error

[1] https://github.com/droolsjbpm/droolsjbpm-integration/pull/315/files#diff-55f7600103b4324558f5c4e620ca577cR76
[2] https://github.com/droolsjbpm/droolsjbpm-integration/pull/315/files#diff-55f7600103b4324558f5c4e620ca577cR51
[3] https://github.com/droolsjbpm/droolsjbpm-integration/pull/315/files#diff-c4e2df544edb849ffb43b2b5bfdf87b0R86

Actual results:
MvelUserGroupCallback fails to initialize in OSGi environment.

Expected results:
MvelUserGroupCallback initializes successfully.