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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.io.Serializable;
20  import java.util.Comparator;
21  import java.util.Objects;
22  
23  /**
24   * Comparator for classes annotated with {@link Order}.
25   *
26   * @since 2.1
27   */
28  public class OrderComparator implements Comparator<Class<?>>, Serializable {
29  
30      private static final long serialVersionUID = 1L;
31      private static final Comparator<Class<?>> INSTANCE = new OrderComparator();
32  
33      /**
34       * Returns a singleton instance of this class.
35       *
36       * @return the singleton for this class.
37       */
38      public static Comparator<Class<?>> getInstance() {
39          return INSTANCE;
40      }
41  
42      @Override
43      public int compare(final Class<?> lhs, final Class<?> rhs) {
44          final Order lhsOrder = Objects.requireNonNull(lhs, "lhs").getAnnotation(Order.class);
45          final Order rhsOrder = Objects.requireNonNull(rhs, "rhs").getAnnotation(Order.class);
46          if (lhsOrder == null && rhsOrder == null) {
47              // both unannotated means equal priority
48              return 0;
49          }
50          // if only one class is @Order-annotated, then prefer that one
51          if (rhsOrder == null) {
52              return -1;
53          }
54          if (lhsOrder == null) {
55              return 1;
56          }
57          // larger value means lower priority
58          return Integer.signum(rhsOrder.value() - lhsOrder.value());
59      }
60  }