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1   /*
2    * *************************************************************************************************************************************************************
3    *
4    * TheseFoolishThings: Miscellaneous utilities
5    * http://tidalwave.it/projects/thesefoolishthings
6    *
7    * Copyright (C) 2009 - 2026 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.concurrent.ThreadSafe;
29  import jakarta.annotation.Nonnull;
30  import java.util.Optional;
31  import java.util.concurrent.atomic.AtomicBoolean;
32  import java.util.concurrent.atomic.AtomicReference;
33  import java.util.function.Supplier;
34  import it.tidalwave.util.annotation.VisibleForTesting;
35  import lombok.EqualsAndHashCode;
36  import lombok.RequiredArgsConstructor;
37  
38  /***************************************************************************************************************************************************************
39   * A supplier of an object that is lazily evaluated (when its value is requested for the first time). It warranties
40   * that the real wrapped supplier is called only once.
41   * @author  Fabrizio Giudici
42   * @since   3.2-ALPHA-13
43   **************************************************************************************************************************************************************/
44  /* @ThreadSafe */ @RequiredArgsConstructor(staticName = "of") @EqualsAndHashCode(of = {"ref", "initialized"})
45  public class LazySupplier<T> implements Supplier<T>
46    {
47      @Nonnull
48      private final Supplier<T> supplier;
49  
50      @VisibleForTesting final AtomicReference<T> ref = new AtomicReference<>(null);
51  
52      @VisibleForTesting final AtomicBoolean initialized = new AtomicBoolean(false);
53  
54      /***********************************************************************************************************************************************************
55       * {@inheritDoc}
56       **********************************************************************************************************************************************************/
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.get() == null)
63            {
64              ref.set(supplier.get());
65              initialized.set(true);
66            }
67  
68          return ref.get();
69        }
70  
71      /***********************************************************************************************************************************************************
72       * Returns the value without computing it: unlike {@link #get()}, this method never calls the wrapped supplier, so it can be used to find out whether the
73       * value is available without the side effects of computing it. The returned {@link Optional} contains the value if it has been computed and is not null.
74       * @return  the value, if available, or an empty {@code Optional}
75       * @since   5.0-ALPHA-4
76       **********************************************************************************************************************************************************/
77      @Nonnull
78      public synchronized Optional<T> peek()
79        {
80          return initialized.get() ? Optional.ofNullable(ref.get()) : Optional.empty();
81        }
82  
83      /***********************************************************************************************************************************************************
84       * Clears the supplier.
85       **********************************************************************************************************************************************************/
86      public synchronized void clear()
87        {
88          ref.set(null);
89        }
90  
91      /***********************************************************************************************************************************************************
92       * Changes the reference.
93       * @param   ref   the new reference
94       **********************************************************************************************************************************************************/
95      public synchronized void set (@Nonnull final T ref)
96        {
97          this.ref.set(ref);
98          initialized.set(true);
99        }
100 
101     /***********************************************************************************************************************************************************
102      * {@inheritDoc}
103      **********************************************************************************************************************************************************/
104     @Nonnull
105     public String toString()
106       {
107         return String.format("LazySupplier(%s)", (ref.get() == null && !initialized.get()) ? "<not initialised>" : ref.get());
108       }
109   }