LazySupplier.java

  1. /*
  2.  * *********************************************************************************************************************
  3.  *
  4.  * TheseFoolishThings: Miscellaneous utilities
  5.  * http://tidalwave.it/projects/thesefoolishthings
  6.  *
  7.  * Copyright (C) 2009 - 2023 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
  12.  * the License. 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
  17.  * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the License for the
  18.  * specific language governing permissions and limitations under the License.
  19.  *
  20.  * *********************************************************************************************************************
  21.  *
  22.  * git clone https://bitbucket.org/tidalwave/thesefoolishthings-src
  23.  * git clone https://github.com/tidalwave-it/thesefoolishthings-src
  24.  *
  25.  * *********************************************************************************************************************
  26.  */
  27. package it.tidalwave.util;

  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.RequiredArgsConstructor;
  33. import lombok.ToString;

  34. /***********************************************************************************************************************
  35.  *
  36.  * A supplier of an object that is lazily evaluated (when its value is requested for the first time). It warranties
  37.  * that the real wrapped supplier is called only once.
  38.  *
  39.  * @author  Fabrizio Giudici
  40.  * @since   3.2-ALPHA-13
  41.  *
  42.  **********************************************************************************************************************/
  43. @ThreadSafe @RequiredArgsConstructor(staticName = "of") @ToString(of = "ref")
  44. public class LazySupplier<T> implements Supplier<T>
  45.   {
  46.     @Nonnull
  47.     private final Supplier<T> supplier;

  48.     @VisibleForTesting volatile T ref = null;

  49.     /** {@inheritDoc} */
  50.     @Override @Nonnull
  51.     public synchronized T get()
  52.       {
  53.         // AtomicReference.updateAndGet() not good because in case of contention it might call supplier multiple times
  54.         // We don't mess with double check for null since nowadays synchronized is fast
  55.         if (ref == null)
  56.           {
  57.             ref = supplier.get();
  58.           }

  59.         return ref;
  60.       }

  61.     public synchronized void clear()
  62.       {
  63.         ref = null;
  64.       }

  65.     public void set (@Nonnull final T ref)
  66.       {
  67.         this.ref = ref;
  68.       }
  69.   }