class ActivitiHelper {

    public String getRequiredExpressionVariableAsString(Expression expression, DelegateExecution execution, String variableName) throws IllegalArgumentException
    {
        String variableString = getExpressionVariableAsString(expression, execution);

        if (StringUtils.isBlank(variableString))
        {
            throw new IllegalArgumentException("\"" + variableName + "\" must be specified.");
        }

        return variableString;
    }

}