Source Taste: Activiti: org.activiti.engine.impl.form.JuelFormEngine

//WHY Juel HERE? Take a look at the footnote
public class JuelFormEngine implements FormEngine {

  public String getName() {
    return "juel";
  }

  //At last, have an idea of formkey, and how to bind template and process inline script as well.
  public Object renderStartForm(StartFormData startForm) {
    if (startForm.getFormKey()==null) {
      return null;
    }
    //Probably we might get FreeMarker template by the following line via Activiti infrastructure.
    String formTemplateString = getFormTemplateString(startForm, startForm.getFormKey());
    
    ScriptingEngines scriptingEngines = Context.getProcessEngineConfiguration().getScriptingEngines();
    
    //Result Including business data? or just a template?
    return scriptingEngines.evaluate(formTemplateString, ScriptingEngines.DEFAULT_SCRIPTING_LANGUAGE, null);
  }

  public Object renderTaskForm(TaskFormData taskForm) {
    if (taskForm.getFormKey()==null) {
      return null;
    }
    String formTemplateString = getFormTemplateString(taskForm, taskForm.getFormKey());
    ScriptingEngines scriptingEngines = Context.getProcessEngineConfiguration().getScriptingEngines();
    TaskEntity task = (TaskEntity) taskForm.getTask();
    return scriptingEngines.evaluate(formTemplateString, ScriptingEngines.DEFAULT_SCRIPTING_LANGUAGE, task.getExecution());
  }

  protected String getFormTemplateString(FormData formInstance, String formKey) {
    String deploymentId = formInstance.getDeploymentId();
    
    ResourceEntity resourceStream = Context
      .getCommandContext()
      .getResourceManager()
      .findResourceByDeploymentIdAndResourceName(deploymentId, formKey);
    
    if (resourceStream == null) {
      throw new ActivitiException("Form with formKey '"+formKey+"' does not exist");
    }
    
    byte[] resourceBytes = resourceStream.getBytes();
    String formTemplateString = new String(resourceBytes);
    return formTemplateString;
  }
}

Footnote: JUEL is an implementation of the Unified Expression Language (EL), specified as part of the JSP 2.1 standard (JSR-245), which has been introduced in JEE5. Additionally, JUEL 2.2 implements the JSP 2.2 maintenance release specification for full JEE6 compliance.
Reference: http://juel.sourceforge.net/