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This artifact provides a single jar that contains all classes required to use remote EJB and JMS, including all dependencies. It is intended for use by those not using maven, maven users should just import the EJB and JMS BOM's instead (shaded JAR's cause lots of problems with maven, as it is very easy to inadvertently end up with different versions on classes on the class path).

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/*
 * Copyright (C) 2016 The Guava Authors
 *
 * Licensed 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 com.google.common.graph;

import static com.google.common.base.Preconditions.checkNotNull;

import java.util.Map;
import javax.annotation.CheckForNull;

/**
 * A {@link MapIteratorCache} that adds additional caching. In addition to the caching provided by
 * {@link MapIteratorCache}, this structure caches values for the two most recently retrieved keys.
 *
 * @author James Sexton
 */
@ElementTypesAreNonnullByDefault
final class MapRetrievalCache extends MapIteratorCache {
  // See the note about volatile in the superclass.
  @CheckForNull private transient volatile CacheEntry cacheEntry1;
  @CheckForNull private transient volatile CacheEntry cacheEntry2;

  MapRetrievalCache(Map backingMap) {
    super(backingMap);
  }

  @SuppressWarnings("unchecked") // Safe because we only cast if key is found in map.
  @Override
  @CheckForNull
  V get(Object key) {
    checkNotNull(key);
    V value = getIfCached(key);
    if (value != null) {
      return value;
    }

    value = getWithoutCaching(key);
    if (value != null) {
      addToCache((K) key, value);
    }
    return value;
  }

  // Internal methods (package-visible, but treat as only subclass-visible)

  @Override
  @CheckForNull
  V getIfCached(@CheckForNull Object key) {
    V value = super.getIfCached(key);
    if (value != null) {
      return value;
    }

    // Store a local reference to the cache entry. If the backing map is immutable, this,
    // in combination with immutable cache entries, will ensure a thread-safe cache.
    CacheEntry entry;

    // Check cache. We use == on purpose because it's cheaper and a cache miss is ok.
    entry = cacheEntry1;
    if (entry != null && entry.key == key) {
      return entry.value;
    }
    entry = cacheEntry2;
    if (entry != null && entry.key == key) {
      // Promote second cache entry to first so the access pattern
      // [K1, K2, K1, K3, K1, K4...] still hits the cache half the time.
      addToCache(entry);
      return entry.value;
    }
    return null;
  }

  @Override
  void clearCache() {
    super.clearCache();
    cacheEntry1 = null;
    cacheEntry2 = null;
  }

  private void addToCache(K key, V value) {
    addToCache(new CacheEntry(key, value));
  }

  private void addToCache(CacheEntry entry) {
    // Slide new entry into first cache position. Drop previous entry in second cache position.
    cacheEntry2 = cacheEntry1;
    cacheEntry1 = entry;
  }

  private static final class CacheEntry {
    final K key;
    final V value;

    CacheEntry(K key, V value) {
      this.key = key;
      this.value = value;
    }
  }
}