001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements. See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache license, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License. You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the license for the specific language governing permissions and
015 * limitations under the license.
016 */
017package org.apache.logging.log4j.core.config;
018
019import java.util.Comparator;
020import java.util.Objects;
021
022/**
023 * Comparator for classes annotated with {@link Order}.
024 *
025 * @since 2.1
026 */
027public class OrderComparator implements Comparator<Class<?>> {
028
029    private static final Comparator<Class<?>> INSTANCE = new OrderComparator();
030
031    /**
032     * Returns a singleton instance of this class.
033     *
034     * @return the singleton for this class.
035     */
036    public static Comparator<Class<?>> getInstance() {
037        return INSTANCE;
038    }
039
040    @Override
041    public int compare(final Class<?> lhs, final Class<?> rhs) {
042        final Order lhsOrder = Objects.requireNonNull(lhs, "lhs").getAnnotation(Order.class);
043        final Order rhsOrder = Objects.requireNonNull(rhs, "rhs").getAnnotation(Order.class);
044        if (lhsOrder == null && rhsOrder == null) {
045            // both unannotated means equal priority
046            return 0;
047        }
048        // if only one class is @Order-annotated, then prefer that one
049        if (rhsOrder == null) {
050            return -1;
051        }
052        if (lhsOrder == null) {
053            return 1;
054        }
055        // larger value means lower priority
056        return Integer.signum(rhsOrder.value() - lhsOrder.value());
057    }
058}