/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * 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 CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.commons.lang3.concurrent;
  
  import java.util.concurrent.atomic.AtomicReference;
  
  import org.apache.commons.lang3.function.FailableConsumer;
  import org.apache.commons.lang3.function.FailableSupplier;
  
  /**
   * A specialized implementation of the {@link ConcurrentInitializer} interface
   * based on an {@link AtomicReference} variable.
   *
   * <p>
   * This class maintains a member field of type {@link AtomicReference}. It
   * implements the following algorithm to create and initialize an object in its
   * {@link #get()} method:
   * </p>
   * <ul>
   * <li>First it is checked whether the {@link AtomicReference} variable contains
   * already a value. If this is the case, the value is directly returned.</li>
   * <li>Otherwise the {@link #initialize()} method is called. This method must be
   * defined in concrete subclasses to actually create the managed object.</li>
   * <li>After the object was created by {@link #initialize()} it is checked
   * whether the {@link AtomicReference} variable is still undefined. This has to
   * be done because in the meantime another thread may have initialized the
   * object. If the reference is still empty, the newly created object is stored
   * in it and returned by this method.</li>
   * <li>Otherwise the value stored in the {@link AtomicReference} is returned.</li>
   * </ul>
   * <p>
   * Because atomic variables are used this class does not need any
   * synchronization. So there is no danger of deadlock, and access to the managed
   * object is efficient. However, if multiple threads access the {@code
   * AtomicInitializer} object before it has been initialized almost at the same
   * time, it can happen that {@link #initialize()} is called multiple times. The
   * algorithm outlined above guarantees that {@link #get()} always returns the
   * same object though.
   * </p>
   * <p>
   * Compared with the {@link LazyInitializer} class, this class can be more
   * efficient because it does not need synchronization. The drawback is that the
   * {@link #initialize()} method can be called multiple times which may be
   * problematic if the creation of the managed object is expensive. As a rule of
   * thumb this initializer implementation is preferable if there are not too many
   * threads involved and the probability that multiple threads access an
   * uninitialized object is small. If there is high parallelism,
   * {@link LazyInitializer} is more appropriate.
   * </p>
   *
   * @since 3.0
   * @param <T> the type of the object managed by this initializer class
   */
  public class AtomicInitializer<T> extends AbstractConcurrentInitializer<T, ConcurrentException> {
  
      /**
       * Builds a new instance.
       *
       * @param <T> the type of the object managed by the initializer.
       * @param <I> the type of the initializer managed by this builder.
       * @since 3.14.0
       */
      public static class Builder<I extends AtomicInitializer<T>, T> extends AbstractBuilder<I, T, Builder<I, T>, ConcurrentException> {
  
          /**
           * Constructs a new instance.
           */
          public Builder() {
              // empty
          }
  
          @SuppressWarnings("unchecked")
          @Override
          public I get() {
              return (I) new AtomicInitializer(getInitializer(), getCloser());
          }
  
      }
  
      private static final Object NO_INIT = new Object();
  
      /**
       * Creates a new builder.
       *
       * @param <T> the type of object to build.
      * @return a new builder.
      * @since 3.14.0
      */
     public static <T> Builder<AtomicInitializer<T>, T> builder() {
         return new Builder<>();
     }
 
     /** Holds the reference to the managed object. */
     private final AtomicReference<T> reference = new AtomicReference<>(getNoInit());
 
     /**
      * Constructs a new instance.
      */
     public AtomicInitializer() {
         // empty
     }
 
     /**
      * Constructs a new instance.
      *
      * @param initializer the initializer supplier called by {@link #initialize()}.
      * @param closer the closer consumer called by {@link #close()}.
      */
     private AtomicInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
         super(initializer, closer);
     }
 
     /**
      * Returns the object managed by this initializer. The object is created if
      * it is not available yet and stored internally. This method always returns
      * the same object.
      *
      * @return the object created by this {@link AtomicInitializer}
      * @throws ConcurrentException if an error occurred during initialization of
      * the object
      */
     @Override
     public T get() throws ConcurrentException {
         T result = reference.get();
 
         if (result == getNoInit()) {
             result = initialize();
             if (!reference.compareAndSet(getNoInit(), result)) {
                 // another thread has initialized the reference
                 result = reference.get();
             }
         }
 
         return result;
     }
 
     /** Gets the internal no-init object cast for this instance. */
     @SuppressWarnings("unchecked")
     private T getNoInit() {
         return (T) NO_INIT;
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     protected ConcurrentException getTypedException(final Exception e) {
         return new ConcurrentException(e);
     }
 
     /**
      * Tests whether this instance is initialized. Once initialized, always returns true.
      *
      * @return whether this instance is initialized. Once initialized, always returns true.
      * @since 3.14.0
      */
     @Override
     public boolean isInitialized() {
         return reference.get() != NO_INIT;
     }
 }