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    }