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1   /*
2    * *************************************************************************************************************************************************************
3    *
4    * TheseFoolishThings: Miscellaneous utilities
5    * http://tidalwave.it/projects/thesefoolishthings
6    *
7    * Copyright (C) 2009 - 2025 by Tidalwave s.a.s. (http://tidalwave.it)
8    *
9    * *************************************************************************************************************************************************************
10   *
11   * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License.
12   * You may obtain a copy of the License at
13   *
14   *     http://www.apache.org/licenses/LICENSE-2.0
15   *
16   * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
17   * CONDITIONS OF ANY KIND, either express or implied.  See the License for the specific language governing permissions and limitations under the License.
18   *
19   * *************************************************************************************************************************************************************
20   *
21   * git clone https://bitbucket.org/tidalwave/thesefoolishthings-src
22   * git clone https://github.com/tidalwave-it/thesefoolishthings-src
23   *
24   * *************************************************************************************************************************************************************
25   */
26  package it.tidalwave.util;
27  
28  import javax.annotation.Nonnull;
29  import javax.annotation.concurrent.ThreadSafe;
30  import java.util.function.Supplier;
31  import it.tidalwave.util.annotation.VisibleForTesting;
32  import lombok.EqualsAndHashCode;
33  import lombok.Getter;
34  import lombok.RequiredArgsConstructor;
35  
36  /***************************************************************************************************************************************************************
37   *
38   * A supplier of an object that is lazily evaluated (when its value is requested for the first time). It warranties
39   * that the real wrapped supplier is called only once.
40   *
41   * @author  Fabrizio Giudici
42   * @since   3.2-ALPHA-13
43   *
44   **************************************************************************************************************************************************************/
45  @ThreadSafe @RequiredArgsConstructor(staticName = "of") @EqualsAndHashCode(of = {"ref", "initialized"})
46  public class LazySupplier<T> implements Supplier<T>
47    {
48      @Nonnull
49      private final Supplier<T> supplier;
50  
51      @VisibleForTesting volatile T ref = null;
52  
53      @Getter
54      @VisibleForTesting volatile boolean initialized;
55  
56      /** {@inheritDoc} */
57      @Override @Nonnull
58      public synchronized T get()
59        {
60          // AtomicReference.updateAndGet() not good because in case of contention it might call supplier multiple times
61          // We don't mess with double check for null since nowadays synchronized is fast
62          if (ref == null)
63            {
64              ref = supplier.get();
65              initialized = true;
66            }
67  
68          return ref;
69        }
70  
71      public synchronized void clear()
72        {
73          ref = null;
74        }
75  
76      public void set (@Nonnull final T ref)
77        {
78          this.ref = ref;
79        }
80  
81      @Nonnull
82      public String toString()
83        {
84          return String.format("LazySupplier(%s)", (ref == null && !initialized) ? "<not initialised>" : ref);
85        }
86    }