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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements. See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache license, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License. You may obtain a copy of the License at
8    *
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the license for the specific language governing permissions and
15   * limitations under the license.
16   */
17  package org.apache.logging.log4j.core.config;
18  
19  import java.util.Comparator;
20  import java.util.Objects;
21  
22  /**
23   * Comparator for classes annotated with {@link Order}.
24   *
25   * @since 2.1
26   */
27  public class OrderComparator implements Comparator<Class<?>> {
28  
29      private static final Comparator<Class<?>> INSTANCE = new OrderComparator();
30  
31      /**
32       * Returns a singleton instance of this class.
33       *
34       * @return the singleton for this class.
35       */
36      public static Comparator<Class<?>> getInstance() {
37          return INSTANCE;
38      }
39  
40      @Override
41      public int compare(final Class<?> lhs, final Class<?> rhs) {
42          final Order lhsOrder = Objects.requireNonNull(lhs, "lhs").getAnnotation(Order.class);
43          final Order rhsOrder = Objects.requireNonNull(rhs, "rhs").getAnnotation(Order.class);
44          if (lhsOrder == null && rhsOrder == null) {
45              // both unannotated means equal priority
46              return 0;
47          }
48          // if only one class is @Order-annotated, then prefer that one
49          if (rhsOrder == null) {
50              return -1;
51          }
52          if (lhsOrder == null) {
53              return 1;
54          }
55          // larger value means lower priority
56          return Integer.signum(rhsOrder.value() - lhsOrder.value());
57      }
58  }