/*
    * Copyright (C) 2009 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.cache;
  
  import static com.google.common.base.Objects.firstNonNull;
  import static com.google.common.base.Preconditions.checkArgument;
  import static com.google.common.base.Preconditions.checkNotNull;
  import static com.google.common.base.Preconditions.checkState;
  
  import com.google.common.annotations.Beta;
  import com.google.common.annotations.GwtCompatible;
  import com.google.common.annotations.GwtIncompatible;
  import com.google.common.base.Ascii;
  import com.google.common.base.Equivalence;
  import com.google.common.base.Objects;
  import com.google.common.base.Supplier;
  import com.google.common.base.Suppliers;
  import com.google.common.base.Ticker;
  import com.google.common.cache.AbstractCache.SimpleStatsCounter;
  import com.google.common.cache.AbstractCache.StatsCounter;
  import com.google.common.cache.LocalCache.Strength;
  
  import java.lang.ref.SoftReference;
  import java.lang.ref.WeakReference;
  import java.util.ConcurrentModificationException;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.TimeUnit;
  import java.util.logging.Level;
  import java.util.logging.Logger;
  
  import javax.annotation.CheckReturnValue;
  
  /**
   * <p>A builder of {@link LoadingCache} and {@link Cache} instances having any combination of the
   * following features:
   *
   * <ul>
   * <li>automatic loading of entries into the cache
   * <li>least-recently-used eviction when a maximum size is exceeded
   * <li>time-based expiration of entries, measured since last access or last write
   * <li>keys automatically wrapped in {@linkplain WeakReference weak} references
   * <li>values automatically wrapped in {@linkplain WeakReference weak} or
   *     {@linkplain SoftReference soft} references
   * <li>notification of evicted (or otherwise removed) entries
   * <li>accumulation of cache access statistics
   * </ul>
   *
   * <p>These features are all optional; caches can be created using all or none of them. By default
   * cache instances created by {@code CacheBuilder} will not perform any type of eviction.
   *
   * <p>Usage example: <pre>   {@code
   *
   *   LoadingCache<Key, Graph> graphs = CacheBuilder.newBuilder()
   *       .maximumSize(10000)
   *       .expireAfterWrite(10, TimeUnit.MINUTES)
   *       .removalListener(MY_LISTENER)
   *       .build(
   *           new CacheLoader<Key, Graph>() {
   *             public Graph load(Key key) throws AnyException {
   *               return createExpensiveGraph(key);
   *             }
   *           });}</pre>
   *
   * <p>Or equivalently, <pre>   {@code
   *
   *   // In real life this would come from a command-line flag or config file
   *   String spec = "maximumSize=10000,expireAfterWrite=10m";
   *
   *   LoadingCache<Key, Graph> graphs = CacheBuilder.from(spec)
   *       .removalListener(MY_LISTENER)
   *       .build(
   *           new CacheLoader<Key, Graph>() {
   *             public Graph load(Key key) throws AnyException {
   *               return createExpensiveGraph(key);
   *             }
   *           });}</pre>
   *
   * <p>The returned cache is implemented as a hash table with similar performance characteristics to
   * {@link ConcurrentHashMap}. It implements all optional operations of the {@link LoadingCache} and
   * {@link Cache} interfaces. The {@code asMap} view (and its collection views) have <i>weakly
   * consistent iterators</i>. This means that they are safe for concurrent use, but if other threads
   * modify the cache after the iterator is created, it is undefined which of these changes, if any,
   * are reflected in that iterator. These iterators never throw {@link
   * ConcurrentModificationException}.
   *
  * <p><b>Note:</b> by default, the returned cache uses equality comparisons (the
  * {@link Object#equals equals} method) to determine equality for keys or values. However, if
  * {@link #weakKeys} was specified, the cache uses identity ({@code ==})
  * comparisons instead for keys. Likewise, if {@link #weakValues} or {@link #softValues} was
  * specified, the cache uses identity comparisons for values.
  *
  * <p>Entries are automatically evicted from the cache when any of
  * {@linkplain #maximumSize(long) maximumSize}, {@linkplain #maximumWeight(long) maximumWeight},
  * {@linkplain #expireAfterWrite expireAfterWrite},
  * {@linkplain #expireAfterAccess expireAfterAccess}, {@linkplain #weakKeys weakKeys},
  * {@linkplain #weakValues weakValues}, or {@linkplain #softValues softValues} are requested.
  *
  * <p>If {@linkplain #maximumSize(long) maximumSize} or
  * {@linkplain #maximumWeight(long) maximumWeight} is requested entries may be evicted on each cache
  * modification.
  *
  * <p>If {@linkplain #expireAfterWrite expireAfterWrite} or
  * {@linkplain #expireAfterAccess expireAfterAccess} is requested entries may be evicted on each
  * cache modification, on occasional cache accesses, or on calls to {@link Cache#cleanUp}. Expired
  * entries may be counted by {@link Cache#size}, but will never be visible to read or write
  * operations.
  *
  * <p>If {@linkplain #weakKeys weakKeys}, {@linkplain #weakValues weakValues}, or
  * {@linkplain #softValues softValues} are requested, it is possible for a key or value present in
  * the cache to be reclaimed by the garbage collector. Entries with reclaimed keys or values may be
  * removed from the cache on each cache modification, on occasional cache accesses, or on calls to
  * {@link Cache#cleanUp}; such entries may be counted in {@link Cache#size}, but will never be
  * visible to read or write operations.
  *
  * <p>Certain cache configurations will result in the accrual of periodic maintenance tasks which
  * will be performed during write operations, or during occasional read operations in the absence of
  * writes. The {@link Cache#cleanUp} method of the returned cache will also perform maintenance, but
  * calling it should not be necessary with a high throughput cache. Only caches built with
  * {@linkplain #removalListener removalListener}, {@linkplain #expireAfterWrite expireAfterWrite},
  * {@linkplain #expireAfterAccess expireAfterAccess}, {@linkplain #weakKeys weakKeys},
  * {@linkplain #weakValues weakValues}, or {@linkplain #softValues softValues} perform periodic
  * maintenance.
  *
  * <p>The caches produced by {@code CacheBuilder} are serializable, and the deserialized caches
  * retain all the configuration properties of the original cache. Note that the serialized form does
  * <i>not</i> include cache contents, but only configuration.
  *
  * <p>See the Guava User Guide article on <a href=
  * "http://code.google.com/p/guava-libraries/wiki/CachesExplained">caching</a> for a higher-level
  * explanation.
  *
  * @param <K> the base key type for all caches created by this builder
  * @param <V> the base value type for all caches created by this builder
  * @author Charles Fry
  * @author Kevin Bourrillion
  * @since 10.0
  */
 @GwtCompatible(emulated = true)
 public final class CacheBuilder<K, V> {
   private static final int DEFAULT_INITIAL_CAPACITY = 16;
   private static final int DEFAULT_CONCURRENCY_LEVEL = 4;
   private static final int DEFAULT_EXPIRATION_NANOS = 0;
   private static final int DEFAULT_REFRESH_NANOS = 0;
 
   static final Supplier<? extends StatsCounter> NULL_STATS_COUNTER = Suppliers.ofInstance(
       new StatsCounter() {
         @Override
         public void recordHits(int count) {}
 
         @Override
         public void recordMisses(int count) {}
 
         @Override
         public void recordLoadSuccess(long loadTime) {}
 
         @Override
         public void recordLoadException(long loadTime) {}
 
         @Override
         public void recordEviction() {}
 
         @Override
         public CacheStats snapshot() {
           return EMPTY_STATS;
         }
       });
   static final CacheStats EMPTY_STATS = new CacheStats(0, 0, 0, 0, 0, 0);
 
   static final Supplier<StatsCounter> CACHE_STATS_COUNTER =
       new Supplier<StatsCounter>() {
     @Override
     public StatsCounter get() {
       return new SimpleStatsCounter();
     }
   };
 
   enum NullListener implements RemovalListener<Object, Object> {
     INSTANCE;
 
     @Override
     public void onRemoval(RemovalNotification<Object, Object> notification) {}
   }
 
   enum OneWeigher implements Weigher<Object, Object> {
     INSTANCE;
 
     @Override
     public int weigh(Object key, Object value) {
       return 1;
     }
   }
 
   static final Ticker NULL_TICKER = new Ticker() {
     @Override
     public long read() {
       return 0;
     }
   };
 
   private static final Logger logger = Logger.getLogger(CacheBuilder.class.getName());
 
   static final int UNSET_INT = -1;
 
   boolean strictParsing = true;
 
   int initialCapacity = UNSET_INT;
   int concurrencyLevel = UNSET_INT;
   long maximumSize = UNSET_INT;
   long maximumWeight = UNSET_INT;
   Weigher<? super K, ? super V> weigher;
 
   Strength keyStrength;
   Strength valueStrength;
 
   long expireAfterWriteNanos = UNSET_INT;
   long expireAfterAccessNanos = UNSET_INT;
   long refreshNanos = UNSET_INT;
 
   Equivalence<Object> keyEquivalence;
   Equivalence<Object> valueEquivalence;
 
   RemovalListener<? super K, ? super V> removalListener;
   Ticker ticker;
 
   Supplier<? extends StatsCounter> statsCounterSupplier = NULL_STATS_COUNTER;
 
   // TODO(fry): make constructor private and update tests to use newBuilder
   CacheBuilder() {}
 
   /**
    * Constructs a new {@code CacheBuilder} instance with default settings, including strong keys,
    * strong values, and no automatic eviction of any kind.
    */
   public static CacheBuilder<Object, Object> newBuilder() {
     return new CacheBuilder<Object, Object>();
   }
 
   /**
    * Constructs a new {@code CacheBuilder} instance with the settings specified in {@code spec}.
    *
    * @since 12.0
    */
   @Beta
   @GwtIncompatible("To be supported")
   public static CacheBuilder<Object, Object> from(CacheBuilderSpec spec) {
     return spec.toCacheBuilder()
         .lenientParsing();
   }
 
   /**
    * Constructs a new {@code CacheBuilder} instance with the settings specified in {@code spec}.
    * This is especially useful for command-line configuration of a {@code CacheBuilder}.
    *
    * @param spec a String in the format specified by {@link CacheBuilderSpec}
    * @since 12.0
    */
   @Beta
   @GwtIncompatible("To be supported")
   public static CacheBuilder<Object, Object> from(String spec) {
     return from(CacheBuilderSpec.parse(spec));
   }
 
   /**
    * Enables lenient parsing. Useful for tests and spec parsing.
    */
   @GwtIncompatible("To be supported")
   CacheBuilder<K, V> lenientParsing() {
     strictParsing = false;
     return this;
   }
 
   /**
    * Sets a custom {@code Equivalence} strategy for comparing keys.
    *
    * <p>By default, the cache uses {@link Equivalence#identity} to determine key equality when
    * {@link #weakKeys} is specified, and {@link Equivalence#equals()} otherwise.
    */
   @GwtIncompatible("To be supported")
   CacheBuilder<K, V> keyEquivalence(Equivalence<Object> equivalence) {
     checkState(keyEquivalence == null, "key equivalence was already set to %s", keyEquivalence);
     keyEquivalence = checkNotNull(equivalence);
     return this;
   }
 
   Equivalence<Object> getKeyEquivalence() {
     return firstNonNull(keyEquivalence, getKeyStrength().defaultEquivalence());
   }
 
   /**
    * Sets a custom {@code Equivalence} strategy for comparing values.
    *
    * <p>By default, the cache uses {@link Equivalence#identity} to determine value equality when
    * {@link #weakValues} or {@link #softValues} is specified, and {@link Equivalence#equals()}
    * otherwise.
    */
   @GwtIncompatible("To be supported")
   CacheBuilder<K, V> valueEquivalence(Equivalence<Object> equivalence) {
     checkState(valueEquivalence == null,
         "value equivalence was already set to %s", valueEquivalence);
     this.valueEquivalence = checkNotNull(equivalence);
     return this;
   }
 
   Equivalence<Object> getValueEquivalence() {
     return firstNonNull(valueEquivalence, getValueStrength().defaultEquivalence());
   }
 
   /**
    * Sets the minimum total size for the internal hash tables. For example, if the initial capacity
    * is {@code 60}, and the concurrency level is {@code 8}, then eight segments are created, each
    * having a hash table of size eight. Providing a large enough estimate at construction time
    * avoids the need for expensive resizing operations later, but setting this value unnecessarily
    * high wastes memory.
    *
    * @throws IllegalArgumentException if {@code initialCapacity} is negative
    * @throws IllegalStateException if an initial capacity was already set
    */
   public CacheBuilder<K, V> initialCapacity(int initialCapacity) {
     checkState(this.initialCapacity == UNSET_INT, "initial capacity was already set to %s",
         this.initialCapacity);
     checkArgument(initialCapacity >= 0);
     this.initialCapacity = initialCapacity;
     return this;
   }
 
   int getInitialCapacity() {
     return (initialCapacity == UNSET_INT) ? DEFAULT_INITIAL_CAPACITY : initialCapacity;
   }
 
   /**
    * Guides the allowed concurrency among update operations. Used as a hint for internal sizing. The
    * table is internally partitioned to try to permit the indicated number of concurrent updates
    * without contention. Because assignment of entries to these partitions is not necessarily
    * uniform, the actual concurrency observed may vary. Ideally, you should choose a value to
    * accommodate as many threads as will ever concurrently modify the table. Using a significantly
    * higher value than you need can waste space and time, and a significantly lower value can lead
    * to thread contention. But overestimates and underestimates within an order of magnitude do not
    * usually have much noticeable impact. A value of one permits only one thread to modify the cache
    * at a time, but since read operations and cache loading computations can proceed concurrently,
    * this still yields higher concurrency than full synchronization.
    *
    * <p> Defaults to 4. <b>Note:</b>The default may change in the future. If you care about this
    * value, you should always choose it explicitly.
    *
    * <p>The current implementation uses the concurrency level to create a fixed number of hashtable
    * segments, each governed by its own write lock. The segment lock is taken once for each explicit
    * write, and twice for each cache loading computation (once prior to loading the new value,
    * and once after loading completes). Much internal cache management is performed at the segment
    * granularity. For example, access queues and write queues are kept per segment when they are
    * required by the selected eviction algorithm. As such, when writing unit tests it is not
    * uncommon to specify {@code concurrencyLevel(1)} in order to achieve more deterministic eviction
    * behavior.
    *
    * <p>Note that future implementations may abandon segment locking in favor of more advanced
    * concurrency controls.
    *
    * @throws IllegalArgumentException if {@code concurrencyLevel} is nonpositive
    * @throws IllegalStateException if a concurrency level was already set
    */
   public CacheBuilder<K, V> concurrencyLevel(int concurrencyLevel) {
     checkState(this.concurrencyLevel == UNSET_INT, "concurrency level was already set to %s",
         this.concurrencyLevel);
     checkArgument(concurrencyLevel > 0);
     this.concurrencyLevel = concurrencyLevel;
     return this;
   }
 
   int getConcurrencyLevel() {
     return (concurrencyLevel == UNSET_INT) ? DEFAULT_CONCURRENCY_LEVEL : concurrencyLevel;
   }
 
   /**
    * Specifies the maximum number of entries the cache may contain. Note that the cache <b>may evict
    * an entry before this limit is exceeded</b>. As the cache size grows close to the maximum, the
    * cache evicts entries that are less likely to be used again. For example, the cache may evict an
    * entry because it hasn't been used recently or very often.
    *
    * <p>When {@code size} is zero, elements will be evicted immediately after being loaded into the
    * cache. This can be useful in testing, or to disable caching temporarily without a code change.
    *
    * <p>This feature cannot be used in conjunction with {@link #maximumWeight}.
    *
    * @param size the maximum size of the cache
    * @throws IllegalArgumentException if {@code size} is negative
    * @throws IllegalStateException if a maximum size or weight was already set
    */
   public CacheBuilder<K, V> maximumSize(long size) {
     checkState(this.maximumSize == UNSET_INT, "maximum size was already set to %s",
         this.maximumSize);
     checkState(this.maximumWeight == UNSET_INT, "maximum weight was already set to %s",
         this.maximumWeight);
     checkState(this.weigher == null, "maximum size can not be combined with weigher");
     checkArgument(size >= 0, "maximum size must not be negative");
     this.maximumSize = size;
     return this;
   }
 
   /**
    * Specifies the maximum weight of entries the cache may contain. Weight is determined using the
    * {@link Weigher} specified with {@link #weigher}, and use of this method requires a
    * corresponding call to {@link #weigher} prior to calling {@link #build}.
    *
    * <p>Note that the cache <b>may evict an entry before this limit is exceeded</b>. As the cache
    * size grows close to the maximum, the cache evicts entries that are less likely to be used
    * again. For example, the cache may evict an entry because it hasn't been used recently or very
    * often.
    *
    * <p>When {@code weight} is zero, elements will be evicted immediately after being loaded into
    * cache. This can be useful in testing, or to disable caching temporarily without a code
    * change.
    *
    * <p>Note that weight is only used to determine whether the cache is over capacity; it has no
    * effect on selecting which entry should be evicted next.
    *
    * <p>This feature cannot be used in conjunction with {@link #maximumSize}.
    *
    * @param weight the maximum total weight of entries the cache may contain
    * @throws IllegalArgumentException if {@code weight} is negative
    * @throws IllegalStateException if a maximum weight or size was already set
    * @since 11.0
    */
   @GwtIncompatible("To be supported")
   public CacheBuilder<K, V> maximumWeight(long weight) {
     checkState(this.maximumWeight == UNSET_INT, "maximum weight was already set to %s",
         this.maximumWeight);
     checkState(this.maximumSize == UNSET_INT, "maximum size was already set to %s",
         this.maximumSize);
     this.maximumWeight = weight;
     checkArgument(weight >= 0, "maximum weight must not be negative");
     return this;
   }
 
   /**
    * Specifies the weigher to use in determining the weight of entries. Entry weight is taken
    * into consideration by {@link #maximumWeight(long)} when determining which entries to evict, and
    * use of this method requires a corresponding call to {@link #maximumWeight(long)} prior to
    * calling {@link #build}. Weights are measured and recorded when entries are inserted into the
    * cache, and are thus effectively static during the lifetime of a cache entry.
    *
    * <p>When the weight of an entry is zero it will not be considered for size-based eviction
    * (though it still may be evicted by other means).
    *
    * <p><b>Important note:</b> Instead of returning <em>this</em> as a {@code CacheBuilder}
    * instance, this method returns {@code CacheBuilder<K1, V1>}. From this point on, either the
    * original reference or the returned reference may be used to complete configuration and build
    * the cache, but only the "generic" one is type-safe. That is, it will properly prevent you from
    * building caches whose key or value types are incompatible with the types accepted by the
    * weigher already provided; the {@code CacheBuilder} type cannot do this. For best results,
    * simply use the standard method-chaining idiom, as illustrated in the documentation at top,
    * configuring a {@code CacheBuilder} and building your {@link Cache} all in a single statement.
    *
    * <p><b>Warning:</b> if you ignore the above advice, and use this {@code CacheBuilder} to build
    * a cache whose key or value type is incompatible with the weigher, you will likely experience
    * a {@link ClassCastException} at some <i>undefined</i> point in the future.
    *
    * @param weigher the weigher to use in calculating the weight of cache entries
    * @throws IllegalArgumentException if {@code size} is negative
    * @throws IllegalStateException if a maximum size was already set
    * @since 11.0
    */
   @GwtIncompatible("To be supported")
   public <K1 extends K, V1 extends V> CacheBuilder<K1, V1> weigher(
       Weigher<? super K1, ? super V1> weigher) {
     checkState(this.weigher == null);
     if (strictParsing) {
       checkState(this.maximumSize == UNSET_INT, "weigher can not be combined with maximum size",
           this.maximumSize);
     }
 
     // safely limiting the kinds of caches this can produce
     @SuppressWarnings("unchecked")
     CacheBuilder<K1, V1> me = (CacheBuilder<K1, V1>) this;
     me.weigher = checkNotNull(weigher);
     return me;
   }
 
   long getMaximumWeight() {
     if (expireAfterWriteNanos == 0 || expireAfterAccessNanos == 0) {
       return 0;
     }
     return (weigher == null) ? maximumSize : maximumWeight;
   }
 
   // Make a safe contravariant cast now so we don't have to do it over and over.
   @SuppressWarnings("unchecked")
   <K1 extends K, V1 extends V> Weigher<K1, V1> getWeigher() {
     return (Weigher<K1, V1>) Objects.firstNonNull(weigher, OneWeigher.INSTANCE);
   }
 
   /**
    * Specifies that each key (not value) stored in the cache should be wrapped in a {@link
    * WeakReference} (by default, strong references are used).
    *
    * <p><b>Warning:</b> when this method is used, the resulting cache will use identity ({@code ==})
    * comparison to determine equality of keys.
    *
    * <p>Entries with keys that have been garbage collected may be counted in {@link Cache#size},
    * but will never be visible to read or write operations; such entries are cleaned up as part of
    * the routine maintenance described in the class javadoc.
    *
    * @throws IllegalStateException if the key strength was already set
    */
   @GwtIncompatible("java.lang.ref.WeakReference")
   public CacheBuilder<K, V> weakKeys() {
     return setKeyStrength(Strength.WEAK);
   }
 
   CacheBuilder<K, V> setKeyStrength(Strength strength) {
     checkState(keyStrength == null, "Key strength was already set to %s", keyStrength);
     keyStrength = checkNotNull(strength);
     return this;
   }
 
   Strength getKeyStrength() {
     return firstNonNull(keyStrength, Strength.STRONG);
   }
 
   /**
    * Specifies that each value (not key) stored in the cache should be wrapped in a
    * {@link WeakReference} (by default, strong references are used).
    *
    * <p>Weak values will be garbage collected once they are weakly reachable. This makes them a poor
    * candidate for caching; consider {@link #softValues} instead.
    *
    * <p><b>Note:</b> when this method is used, the resulting cache will use identity ({@code ==})
    * comparison to determine equality of values.
    *
    * <p>Entries with values that have been garbage collected may be counted in {@link Cache#size},
    * but will never be visible to read or write operations; such entries are cleaned up as part of
    * the routine maintenance described in the class javadoc.
    *
    * @throws IllegalStateException if the value strength was already set
    */
   @GwtIncompatible("java.lang.ref.WeakReference")
   public CacheBuilder<K, V> weakValues() {
     return setValueStrength(Strength.WEAK);
   }
 
   /**
    * Specifies that each value (not key) stored in the cache should be wrapped in a
    * {@link SoftReference} (by default, strong references are used). Softly-referenced objects will
    * be garbage-collected in a <i>globally</i> least-recently-used manner, in response to memory
    * demand.
    *
    * <p><b>Warning:</b> in most circumstances it is better to set a per-cache {@linkplain
    * #maximumSize(long) maximum size} instead of using soft references. You should only use this
    * method if you are well familiar with the practical consequences of soft references.
    *
    * <p><b>Note:</b> when this method is used, the resulting cache will use identity ({@code ==})
    * comparison to determine equality of values.
    *
    * <p>Entries with values that have been garbage collected may be counted in {@link Cache#size},
    * but will never be visible to read or write operations; such entries are cleaned up as part of
    * the routine maintenance described in the class javadoc.
    *
    * @throws IllegalStateException if the value strength was already set
    */
   @GwtIncompatible("java.lang.ref.SoftReference")
   public CacheBuilder<K, V> softValues() {
     return setValueStrength(Strength.SOFT);
   }
 
   CacheBuilder<K, V> setValueStrength(Strength strength) {
     checkState(valueStrength == null, "Value strength was already set to %s", valueStrength);
     valueStrength = checkNotNull(strength);
     return this;
   }
 
   Strength getValueStrength() {
     return firstNonNull(valueStrength, Strength.STRONG);
   }
 
   /**
    * Specifies that each entry should be automatically removed from the cache once a fixed duration
    * has elapsed after the entry's creation, or the most recent replacement of its value.
    *
    * <p>When {@code duration} is zero, this method hands off to
    * {@link #maximumSize(long) maximumSize}{@code (0)}, ignoring any otherwise-specificed maximum
    * size or weight. This can be useful in testing, or to disable caching temporarily without a code
    * change.
    *
    * <p>Expired entries may be counted in {@link Cache#size}, but will never be visible to read or
    * write operations. Expired entries are cleaned up as part of the routine maintenance described
    * in the class javadoc.
    *
    * @param duration the length of time after an entry is created that it should be automatically
    *     removed
    * @param unit the unit that {@code duration} is expressed in
    * @throws IllegalArgumentException if {@code duration} is negative
    * @throws IllegalStateException if the time to live or time to idle was already set
    */
   public CacheBuilder<K, V> expireAfterWrite(long duration, TimeUnit unit) {
     checkState(expireAfterWriteNanos == UNSET_INT, "expireAfterWrite was already set to %s ns",
         expireAfterWriteNanos);
     checkArgument(duration >= 0, "duration cannot be negative: %s %s", duration, unit);
     this.expireAfterWriteNanos = unit.toNanos(duration);
     return this;
   }
 
   long getExpireAfterWriteNanos() {
     return (expireAfterWriteNanos == UNSET_INT) ? DEFAULT_EXPIRATION_NANOS : expireAfterWriteNanos;
   }
 
   /**
    * Specifies that each entry should be automatically removed from the cache once a fixed duration
    * has elapsed after the entry's creation, the most recent replacement of its value, or its last
    * access. Access time is reset by all cache read and write operations (including
    * {@code Cache.asMap().get(Object)} and {@code Cache.asMap().put(K, V)}), but not by operations
    * on the collection-views of {@link Cache#asMap}.
    *
    * <p>When {@code duration} is zero, this method hands off to
    * {@link #maximumSize(long) maximumSize}{@code (0)}, ignoring any otherwise-specificed maximum
    * size or weight. This can be useful in testing, or to disable caching temporarily without a code
    * change.
    *
    * <p>Expired entries may be counted in {@link Cache#size}, but will never be visible to read or
    * write operations. Expired entries are cleaned up as part of the routine maintenance described
    * in the class javadoc.
    *
    * @param duration the length of time after an entry is last accessed that it should be
    *     automatically removed
    * @param unit the unit that {@code duration} is expressed in
    * @throws IllegalArgumentException if {@code duration} is negative
    * @throws IllegalStateException if the time to idle or time to live was already set
    */
   public CacheBuilder<K, V> expireAfterAccess(long duration, TimeUnit unit) {
     checkState(expireAfterAccessNanos == UNSET_INT, "expireAfterAccess was already set to %s ns",
         expireAfterAccessNanos);
     checkArgument(duration >= 0, "duration cannot be negative: %s %s", duration, unit);
     this.expireAfterAccessNanos = unit.toNanos(duration);
     return this;
   }
 
   long getExpireAfterAccessNanos() {
     return (expireAfterAccessNanos == UNSET_INT)
         ? DEFAULT_EXPIRATION_NANOS : expireAfterAccessNanos;
   }
 
   /**
    * Specifies that active entries are eligible for automatic refresh once a fixed duration has
    * elapsed after the entry's creation, or the most recent replacement of its value. The semantics
    * of refreshes are specified in {@link LoadingCache#refresh}, and are performed by calling
    * {@link CacheLoader#reload}.
    *
    * <p>As the default implementation of {@link CacheLoader#reload} is synchronous, it is
    * recommended that users of this method override {@link CacheLoader#reload} with an asynchronous
    * implementation; otherwise refreshes will be performed during unrelated cache read and write
    * operations.
    *
    * <p>Currently automatic refreshes are performed when the first stale request for an entry
    * occurs. The request triggering refresh will make a blocking call to {@link CacheLoader#reload}
    * and immediately return the new value if the returned future is complete, and the old value
    * otherwise.
    *
    * <p><b>Note:</b> <i>all exceptions thrown during refresh will be logged and then swallowed</i>.
    *
    * @param duration the length of time after an entry is created that it should be considered
    *     stale, and thus eligible for refresh
    * @param unit the unit that {@code duration} is expressed in
    * @throws IllegalArgumentException if {@code duration} is negative
    * @throws IllegalStateException if the refresh interval was already set
    * @since 11.0
    */
   @Beta
   @GwtIncompatible("To be supported (synchronously).")
   public CacheBuilder<K, V> refreshAfterWrite(long duration, TimeUnit unit) {
     checkNotNull(unit);
     checkState(refreshNanos == UNSET_INT, "refresh was already set to %s ns", refreshNanos);
     checkArgument(duration > 0, "duration must be positive: %s %s", duration, unit);
     this.refreshNanos = unit.toNanos(duration);
     return this;
   }
 
   long getRefreshNanos() {
     return (refreshNanos == UNSET_INT) ? DEFAULT_REFRESH_NANOS : refreshNanos;
   }
 
   /**
    * Specifies a nanosecond-precision time source for use in determining when entries should be
    * expired. By default, {@link System#nanoTime} is used.
    *
    * <p>The primary intent of this method is to facilitate testing of caches which have been
    * configured with {@link #expireAfterWrite} or {@link #expireAfterAccess}.
    *
    * @throws IllegalStateException if a ticker was already set
    */
   public CacheBuilder<K, V> ticker(Ticker ticker) {
     checkState(this.ticker == null);
     this.ticker = checkNotNull(ticker);
     return this;
   }
 
   Ticker getTicker(boolean recordsTime) {
     if (ticker != null) {
       return ticker;
     }
     return recordsTime ? Ticker.systemTicker() : NULL_TICKER;
   }
 
   /**
    * Specifies a listener instance that caches should notify each time an entry is removed for any
    * {@linkplain RemovalCause reason}. Each cache created by this builder will invoke this listener
    * as part of the routine maintenance described in the class documentation above.
    *
    * <p><b>Warning:</b> after invoking this method, do not continue to use <i>this</i> cache
    * builder reference; instead use the reference this method <i>returns</i>. At runtime, these
    * point to the same instance, but only the returned reference has the correct generic type
    * information so as to ensure type safety. For best results, use the standard method-chaining
    * idiom illustrated in the class documentation above, configuring a builder and building your
    * cache in a single statement. Failure to heed this advice can result in a {@link
    * ClassCastException} being thrown by a cache operation at some <i>undefined</i> point in the
    * future.
    *
    * <p><b>Warning:</b> any exception thrown by {@code listener} will <i>not</i> be propagated to
    * the {@code Cache} user, only logged via a {@link Logger}.
    *
    * @return the cache builder reference that should be used instead of {@code this} for any
    *     remaining configuration and cache building
    * @throws IllegalStateException if a removal listener was already set
    */
   @CheckReturnValue
   public <K1 extends K, V1 extends V> CacheBuilder<K1, V1> removalListener(
       RemovalListener<? super K1, ? super V1> listener) {
     checkState(this.removalListener == null);
 
     // safely limiting the kinds of caches this can produce
     @SuppressWarnings("unchecked")
     CacheBuilder<K1, V1> me = (CacheBuilder<K1, V1>) this;
     me.removalListener = checkNotNull(listener);
     return me;
   }
 
   // Make a safe contravariant cast now so we don't have to do it over and over.
   @SuppressWarnings("unchecked")
   <K1 extends K, V1 extends V> RemovalListener<K1, V1> getRemovalListener() {
     return (RemovalListener<K1, V1>) Objects.firstNonNull(removalListener, NullListener.INSTANCE);
   }
 
   /**
    * Enable the accumulation of {@link CacheStats} during the operation of the cache. Without this
    * {@link Cache#stats} will return zero for all statistics. Note that recording stats requires
    * bookkeeping to be performed with each operation, and thus imposes a performance penalty on
    * cache operation.
    *
    * @since 12.0 (previously, stats collection was automatic)
    */
   public CacheBuilder<K, V> recordStats() {
     statsCounterSupplier = CACHE_STATS_COUNTER;
     return this;
   }
   
   boolean isRecordingStats() {
     return statsCounterSupplier == CACHE_STATS_COUNTER;
   }
 
   Supplier<? extends StatsCounter> getStatsCounterSupplier() {
     return statsCounterSupplier;
   }
 
   /**
    * Builds a cache, which either returns an already-loaded value for a given key or atomically
    * computes or retrieves it using the supplied {@code CacheLoader}. If another thread is currently
    * loading the value for this key, simply waits for that thread to finish and returns its
    * loaded value. Note that multiple threads can concurrently load values for distinct keys.
    *
    * <p>This method does not alter the state of this {@code CacheBuilder} instance, so it can be
    * invoked again to create multiple independent caches.
    *
    * @param loader the cache loader used to obtain new values
    * @return a cache having the requested features
    */
   public <K1 extends K, V1 extends V> LoadingCache<K1, V1> build(
       CacheLoader<? super K1, V1> loader) {
     checkWeightWithWeigher();
     return new LocalCache.LocalLoadingCache<K1, V1>(this, loader);
   }
 
   /**
    * Builds a cache which does not automatically load values when keys are requested.
    *
    * <p>Consider {@link #build(CacheLoader)} instead, if it is feasible to implement a
    * {@code CacheLoader}.
    *
    * <p>This method does not alter the state of this {@code CacheBuilder} instance, so it can be
    * invoked again to create multiple independent caches.
    *
    * @return a cache having the requested features
    * @since 11.0
    */
   public <K1 extends K, V1 extends V> Cache<K1, V1> build() {
     checkWeightWithWeigher();
     checkNonLoadingCache();
     return new LocalCache.LocalManualCache<K1, V1>(this);
   }
 
   private void checkNonLoadingCache() {
     checkState(refreshNanos == UNSET_INT, "refreshAfterWrite requires a LoadingCache");
   }
 
   private void checkWeightWithWeigher() {
     if (weigher == null) {
       checkState(maximumWeight == UNSET_INT, "maximumWeight requires weigher");
     } else {
       if (strictParsing) {
         checkState(maximumWeight != UNSET_INT, "weigher requires maximumWeight");
       } else {
         if (maximumWeight == UNSET_INT) {
           logger.log(Level.WARNING, "ignoring weigher specified without maximumWeight");
         }
       }
     }
   }
 
   /**
    * Returns a string representation for this CacheBuilder instance. The exact form of the returned
    * string is not specified.
    */
   @Override
   public String toString() {
     Objects.ToStringHelper s = Objects.toStringHelper(this);
     if (initialCapacity != UNSET_INT) {
       s.add("initialCapacity", initialCapacity);
     }
     if (concurrencyLevel != UNSET_INT) {
       s.add("concurrencyLevel", concurrencyLevel);
     }
     if (maximumSize != UNSET_INT) {
       s.add("maximumSize", maximumSize);
     }
     if (maximumWeight != UNSET_INT) {
       s.add("maximumWeight", maximumWeight);
     }
     if (expireAfterWriteNanos != UNSET_INT) {
       s.add("expireAfterWrite", expireAfterWriteNanos + "ns");
     }
     if (expireAfterAccessNanos != UNSET_INT) {
       s.add("expireAfterAccess", expireAfterAccessNanos + "ns");
     }
     if (keyStrength != null) {
       s.add("keyStrength", Ascii.toLowerCase(keyStrength.toString()));
     }
     if (valueStrength != null) {
       s.add("valueStrength", Ascii.toLowerCase(valueStrength.toString()));
     }
     if (keyEquivalence != null) {
       s.addValue("keyEquivalence");
     }
     if (valueEquivalence != null) {
       s.addValue("valueEquivalence");
     }
     if (removalListener != null) {
       s.addValue("removalListener");
     }
     return s.toString();
   }
 }