private <R> R modifyResource(
      DBBroker broker, Resource resource, DatabaseItemModifier<DocumentImpl, R> modifier)
      throws XMLDBException, LockException, PermissionDeniedException, EXistException,
          SyntaxException {
    final TransactionManager transact = broker.getBrokerPool().getTransactionManager();
    final Txn transaction = transact.beginTransaction();

    DocumentImpl document = null;
    try {
      document = ((AbstractEXistResource) resource).openDocument(broker, Lock.WRITE_LOCK);
      final SecurityManager sm = broker.getBrokerPool().getSecurityManager();
      if (!document.getPermissions().validate(user, Permission.WRITE)
          && !sm.hasAdminPrivileges(user)) {
        throw new XMLDBException(
            ErrorCodes.PERMISSION_DENIED,
            "you are not the owner of this resource; owner = "
                + document.getPermissions().getOwner());
      }

      final R result = modifier.modify(document);

      broker.storeXMLResource(transaction, document);
      transact.commit(transaction);

      return result;

    } catch (final EXistException ee) {
      transact.abort(transaction);
      throw ee;
    } catch (final XMLDBException xmldbe) {
      transact.abort(transaction);
      throw xmldbe;
    } catch (final LockException le) {
      transact.abort(transaction);
      throw le;
    } catch (final PermissionDeniedException pde) {
      transact.abort(transaction);
      throw pde;
    } catch (final SyntaxException se) {
      transact.abort(transaction);
      throw se;
    } finally {
      transact.close(transaction);
      if (document != null) {
        ((AbstractEXistResource) resource).closeDocument(document, Lock.WRITE_LOCK);
      }
    }
  }
  private <R> R readResource(
      DBBroker broker, Resource resource, DatabaseItemReader<DocumentImpl, R> reader)
      throws XMLDBException, PermissionDeniedException {

    DocumentImpl document = null;
    try {
      document = ((AbstractEXistResource) resource).openDocument(broker, Lock.READ_LOCK);

      return reader.read(document);

    } finally {
      if (document != null) {
        ((AbstractEXistResource) resource).closeDocument(document, Lock.READ_LOCK);
      }
    }
  }