public void manipulateCredential(Item item, Role role, String operation)
throws ProcessingException {
try {
DefaultPolicy policy = policyManager.buildSubtreePolicy(accessController.getAccreditableManager(), url);
Accreditable accreditable = (Accreditable) item;
if (operation.equals(ADD)) {
policy.addRole(accreditable, role);
} else if (operation.equals(DELETE)) {
policy.removeRole(accreditable, role);