    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     */
    017    package org.apache.logging.log4j.core.config;
    018
    019    import java.util.Comparator;
    020
    021    import org.apache.logging.log4j.core.util.Assert;
    022
    023    /**
    024     * Comparator for classes annotated with {@link Order}.
    025     *
    026     * @since 2.1
    027     */
    028    public class OrderComparator implements Comparator<Class<?>> {
    029
    030        private static final Comparator<Class<?>> INSTANCE = new OrderComparator();
    031
    032        /**
    033         * Returns a singleton instance of this class.
    034         *
    035         * @return the singleton for this class.
    036         */
    037        public static Comparator<Class<?>> getInstance() {
    038            return INSTANCE;
    039        }
    040
    041        @Override
    042        public int compare(final Class<?> lhs, final Class<?> rhs) {
    043            final Order lhsOrder = Assert.requireNonNull(lhs, "lhs").getAnnotation(Order.class);
    044            final Order rhsOrder = Assert.requireNonNull(rhs, "rhs").getAnnotation(Order.class);
    045            if (lhsOrder == null && rhsOrder == null) {
    046                // both unannotated means equal priority
    047                return 0;
    048            }
    049            // if only one class is @Order-annotated, then prefer that one
    050            if (rhsOrder == null) {
    051                return -1;
    052            }
    053            if (lhsOrder == null) {
    054                return 1;
    055            }
    056            // larger value means lower priority
    057            return Integer.signum(rhsOrder.value() - lhsOrder.value());
    058        }
    059    }