LazySupplier.java

  1. /*
  2.  * *************************************************************************************************************************************************************
  3.  *
  4.  * TheseFoolishThings: Miscellaneous utilities
  5.  * http://tidalwave.it/projects/thesefoolishthings
  6.  *
  7.  * Copyright (C) 2009 - 2024 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. import javax.annotation.Nonnull;
  28. import javax.annotation.concurrent.ThreadSafe;
  29. import java.util.function.Supplier;
  30. import it.tidalwave.util.annotation.VisibleForTesting;
  31. import lombok.EqualsAndHashCode;
  32. import lombok.Getter;
  33. import lombok.RequiredArgsConstructor;

  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") @EqualsAndHashCode(of = {"ref", "initialized"})
  44. public class LazySupplier<T> implements Supplier<T>
  45.   {
  46.     @Nonnull
  47.     private final Supplier<T> supplier;

  48.     @VisibleForTesting volatile T ref = null;

  49.     @Getter
  50.     @VisibleForTesting volatile boolean initialized;

  51.     /** {@inheritDoc} */
  52.     @Override @Nonnull
  53.     public synchronized T get()
  54.       {
  55.         // AtomicReference.updateAndGet() not good because in case of contention it might call supplier multiple times
  56.         // We don't mess with double check for null since nowadays synchronized is fast
  57.         if (ref == null)
  58.           {
  59.             ref = supplier.get();
  60.             initialized = true;
  61.           }

  62.         return ref;
  63.       }

  64.     public synchronized void clear()
  65.       {
  66.         ref = null;
  67.       }

  68.     public void set (@Nonnull final T ref)
  69.       {
  70.         this.ref = ref;
  71.       }

  72.     @Nonnull
  73.     public String toString()
  74.       {
  75.         return String.format("LazySupplier(%s)", (ref == null && !initialized) ? "<not initialised>" : ref);
  76.       }
  77.   }