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1   /*
2    * Copyright (c) 2001-2017, Zoltan Farkas All Rights Reserved.
3    *
4    * This library is free software; you can redistribute it and/or
5    * modify it under the terms of the GNU Lesser General Public
6    * License as published by the Free Software Foundation; either
7    * version 2.1 of the License, or (at your option) any later version.
8    *
9    * This library is distributed in the hope that it will be useful,
10   * but WITHOUT ANY WARRANTY; without even the implied warranty of
11   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12   * GNU General Public License for more details.
13   *
14   * You should have received a copy of the GNU Lesser General Public
15   * License along with this program; if not, write to the Free Software
16   * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
17   *
18   * Additionally licensed with:
19   *
20   * Licensed under the Apache License, Version 2.0 (the "License");
21   * you may not use this file except in compliance with the License.
22   * You may obtain a copy of the License at
23   *
24   *      http://www.apache.org/licenses/LICENSE-2.0
25   *
26   * Unless required by applicable law or agreed to in writing, software
27   * distributed under the License is distributed on an "AS IS" BASIS,
28   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
29   * See the License for the specific language governing permissions and
30   * limitations under the License.
31   */
32  package org.spf4j.base;
33  
34  import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
35  import java.util.concurrent.Callable;
36  
37  /**
38   * use Callables.memorize to create a instance.
39   * @author zoly
40   */
41  @SuppressFBWarnings("NOS_NON_OWNED_SYNCHRONIZATION")
42  public final class MemorizedCallable<V> implements Callable<V> {
43  
44      private static final Object NOTHING = new Object();
45  
46      private volatile V value;
47  
48      private final Callable<V> callable;
49  
50      MemorizedCallable(final Callable<V> callable) {
51          this.callable = callable;
52          value = (V) NOTHING;
53      }
54  
55      @Override
56      public V call() throws Exception {
57          V result = value;
58          if (result == NOTHING) {
59              synchronized (this) {
60                  result = value;
61                  if (result == NOTHING) {
62                      result = callable.call();
63                      value = result;
64                  }
65              }
66          }
67          return result;
68      }
69  
70      public V clear() {
71        V result = value;
72        value = (V) NOTHING;
73        return result;
74      }
75  
76      public NullAbleOptional<V> getIfMemorized() {
77        V rv = value;
78        if (rv == NOTHING) {
79          return NullAbleOptional.EMPTY;
80        } else {
81          return NullAbleOptional.of(rv);
82        }
83      }
84  
85      public void set(final V v) {
86        this.value = v;
87      }
88  
89      @Override
90      public String toString() {
91          return "MemorizedCallable{" + "value=" + value + '}';
92      }
93  
94  }