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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
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 * 2 along with this work; if not, write to the Free Software Foundation,
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package java.lang.reflect;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Objects;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.function.BiFunction;
import java.util.function.Supplier;


Cache mapping pairs of (key, sub-key) -> value. Keys and values are weakly but sub-keys are strongly referenced. Keys are passed directly to get method which also takes a parameter. Sub-keys are calculated from keys and parameters using the subKeyFactory function passed to the constructor. Values are calculated from keys and parameters using the valueFactory function passed to the constructor. Keys can be null and are compared by identity while sub-keys returned by subKeyFactory or values returned by valueFactory can not be null. Sub-keys are compared using their Object.equals method. Entries are expunged from cache lazily on each invocation to get, containsValue or size methods when the WeakReferences to keys are cleared. Cleared WeakReferences to individual values don't cause expunging, but such entries are logically treated as non-existent and trigger re-evaluation of valueFactory on request for their key/subKey. 
Author:Peter Levart
Type parameters:
  • <K> – type of keys
  • <P> – type of parameters
  • <V> – type of values
/**
 * Cache mapping pairs of {@code (key, sub-key) -> value}. Keys and values are
 * weakly but sub-keys are strongly referenced.  Keys are passed directly to
 * {@link #get} method which also takes a {@code parameter}. Sub-keys are
 * calculated from keys and parameters using the {@code subKeyFactory} function
 * passed to the constructor. Values are calculated from keys and parameters
 * using the {@code valueFactory} function passed to the constructor.
 * Keys can be {@code null} and are compared by identity while sub-keys returned by
 * {@code subKeyFactory} or values returned by {@code valueFactory}
 * can not be null. Sub-keys are compared using their {@link #equals} method.
 * Entries are expunged from cache lazily on each invocation to {@link #get},
 * {@link #containsValue} or {@link #size} methods when the WeakReferences to
 * keys are cleared. Cleared WeakReferences to individual values don't cause
 * expunging, but such entries are logically treated as non-existent and
 * trigger re-evaluation of {@code valueFactory} on request for their
 * key/subKey.
 *
 * @author Peter Levart
 * @param <K> type of keys
 * @param <P> type of parameters
 * @param <V> type of values
 */
final class WeakCache<K, P, V> {

    private final ReferenceQueue<K> refQueue
        = new ReferenceQueue<>();
    // the key type is Object for supporting null key
    private final ConcurrentMap<Object, ConcurrentMap<Object, Supplier<V>>> map
        = new ConcurrentHashMap<>();
    private final ConcurrentMap<Supplier<V>, Boolean> reverseMap
        = new ConcurrentHashMap<>();
    private final BiFunction<K, P, ?> subKeyFactory;
    private final BiFunction<K, P, V> valueFactory;

    
Construct an instance of WeakCache 
Params:
  • subKeyFactory – a function mapping a pair of (key, parameter) -> sub-key
  • valueFactory –  a function mapping a pair of (key, parameter) -> value
Throws:
/**
     * Construct an instance of {@code WeakCache}
     *
     * @param subKeyFactory a function mapping a pair of
     *                      {@code (key, parameter) -> sub-key}
     * @param valueFactory  a function mapping a pair of
     *                      {@code (key, parameter) -> value}
     * @throws NullPointerException if {@code subKeyFactory} or
     *                              {@code valueFactory} is null.
     */
    public WeakCache(BiFunction<K, P, ?> subKeyFactory,
                     BiFunction<K, P, V> valueFactory) {
        this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
        this.valueFactory = Objects.requireNonNull(valueFactory);
    }

    
Look-up the value through the cache. This always evaluates the subKeyFactory function and optionally evaluates valueFactory function if there is no entry in the cache for given pair of (key, subKey) or the entry has already been cleared. 
Params:
  • key –       possibly null key
  • parameter – parameter used together with key to create sub-key and
                 value (should not be null)
Throws:
  • NullPointerException – if parameter passed in or sub-key calculated by subKeyFactory or value calculated by valueFactory is null.
Returns:the cached value (never null)
/**
     * Look-up the value through the cache. This always evaluates the
     * {@code subKeyFactory} function and optionally evaluates
     * {@code valueFactory} function if there is no entry in the cache for given
     * pair of (key, subKey) or the entry has already been cleared.
     *
     * @param key       possibly null key
     * @param parameter parameter used together with key to create sub-key and
     *                  value (should not be null)
     * @return the cached value (never null)
     * @throws NullPointerException if {@code parameter} passed in or
     *                              {@code sub-key} calculated by
     *                              {@code subKeyFactory} or {@code value}
     *                              calculated by {@code valueFactory} is null.
     */
    public V get(K key, P parameter) {
        Objects.requireNonNull(parameter);

        expungeStaleEntries();

        Object cacheKey = CacheKey.valueOf(key, refQueue);

        // lazily install the 2nd level valuesMap for the particular cacheKey
        ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
        if (valuesMap == null) {
            ConcurrentMap<Object, Supplier<V>> oldValuesMap
                = map.putIfAbsent(cacheKey,
                                  valuesMap = new ConcurrentHashMap<>());
            if (oldValuesMap != null) {
                valuesMap = oldValuesMap;
            }
        }

        // create subKey and retrieve the possible Supplier<V> stored by that
        // subKey from valuesMap
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {
                // supplier might be a Factory or a CacheValue<V> instance
                V value = supplier.get();
                if (value != null) {
                    return value;
                }
            }
            // else no supplier in cache
            // or a supplier that returned null (could be a cleared CacheValue
            // or a Factory that wasn't successful in installing the CacheValue)

            // lazily construct a Factory
            if (factory == null) {
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) {
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    // successfully installed Factory
                    supplier = factory;
                }
                // else retry with winning supplier
            } else {
                if (valuesMap.replace(subKey, supplier, factory)) {
                    // successfully replaced
                    // cleared CacheEntry / unsuccessful Factory
                    // with our Factory
                    supplier = factory;
                } else {
                    // retry with current supplier
                    supplier = valuesMap.get(subKey);
                }
            }
        }
    }

    
Checks whether the specified non-null value is already present in this WeakCache. The check is made using identity comparison regardless of whether value's class overrides Object.equals or not. 
Params:
  • value – the non-null value to check
Throws:
Returns:true if given value is already cached
/**
     * Checks whether the specified non-null value is already present in this
     * {@code WeakCache}. The check is made using identity comparison regardless
     * of whether value's class overrides {@link Object#equals} or not.
     *
     * @param value the non-null value to check
     * @return true if given {@code value} is already cached
     * @throws NullPointerException if value is null
     */
    public boolean containsValue(V value) {
        Objects.requireNonNull(value);

        expungeStaleEntries();
        return reverseMap.containsKey(new LookupValue<>(value));
    }

    
Returns the current number of cached entries that
can decrease over time when keys/values are GC-ed.

/**
     * Returns the current number of cached entries that
     * can decrease over time when keys/values are GC-ed.
     */
    public int size() {
        expungeStaleEntries();
        return reverseMap.size();
    }

    private void expungeStaleEntries() {
        CacheKey<K> cacheKey;
        while ((cacheKey = (CacheKey<K>)refQueue.poll()) != null) {
            cacheKey.expungeFrom(map, reverseMap);
        }
    }

    
A factory Supplier that implements the lazy synchronized construction of the value and installment of it into the cache. 
/**
     * A factory {@link Supplier} that implements the lazy synchronized
     * construction of the value and installment of it into the cache.
     */
    private final class Factory implements Supplier<V> {

        private final K key;
        private final P parameter;
        private final Object subKey;
        private final ConcurrentMap<Object, Supplier<V>> valuesMap;

        Factory(K key, P parameter, Object subKey,
                ConcurrentMap<Object, Supplier<V>> valuesMap) {
            this.key = key;
            this.parameter = parameter;
            this.subKey = subKey;
            this.valuesMap = valuesMap;
        }

        @Override
        public synchronized V get() { // serialize access
            // re-check
            Supplier<V> supplier = valuesMap.get(subKey);
            if (supplier != this) {
                // something changed while we were waiting:
                // might be that we were replaced by a CacheValue
                // or were removed because of failure ->
                // return null to signal WeakCache.get() to retry
                // the loop
                return null;
            }
            // else still us (supplier == this)

            // create new value
            V value = null;
            try {
                value = Objects.requireNonNull(valueFactory.apply(key, parameter));
            } finally {
                if (value == null) { // remove us on failure
                    valuesMap.remove(subKey, this);
                }
            }
            // the only path to reach here is with non-null value
            assert value != null;

            // wrap value with CacheValue (WeakReference)
            CacheValue<V> cacheValue = new CacheValue<>(value);

            // put into reverseMap
            reverseMap.put(cacheValue, Boolean.TRUE);

            // try replacing us with CacheValue (this should always succeed)
            if (!valuesMap.replace(subKey, this, cacheValue)) {
                throw new AssertionError("Should not reach here");
            }

            // successfully replaced us with new CacheValue -> return the value
            // wrapped by it
            return value;
        }
    }

    
Common type of value suppliers that are holding a referent. The Object.equals and Object.hashCode of implementations is defined to compare the referent by identity. 
/**
     * Common type of value suppliers that are holding a referent.
     * The {@link #equals} and {@link #hashCode} of implementations is defined
     * to compare the referent by identity.
     */
    private interface Value<V> extends Supplier<V> {}

    
An optimized Value used to look-up the value in WeakCache.containsValue method so that we are not constructing the whole CacheValue just to look-up the referent. 
/**
     * An optimized {@link Value} used to look-up the value in
     * {@link WeakCache#containsValue} method so that we are not
     * constructing the whole {@link CacheValue} just to look-up the referent.
     */
    private static final class LookupValue<V> implements Value<V> {
        private final V value;

        LookupValue(V value) {
            this.value = value;
        }

        @Override
        public V get() {
            return value;
        }

        @Override
        public int hashCode() {
            return System.identityHashCode(value); // compare by identity
        }

        @Override
        public boolean equals(Object obj) {
            return obj == this ||
                   obj instanceof Value &&
                   this.value == ((Value<?>) obj).get();  // compare by identity
        }
    }

    
A Value that weakly references the referent. 
/**
     * A {@link Value} that weakly references the referent.
     */
    private static final class CacheValue<V>
        extends WeakReference<V> implements Value<V>
    {
        private final int hash;

        CacheValue(V value) {
            super(value);
            this.hash = System.identityHashCode(value); // compare by identity
        }

        @Override
        public int hashCode() {
            return hash;
        }

        @Override
        public boolean equals(Object obj) {
            V value;
            return obj == this ||
                   obj instanceof Value &&
                   // cleared CacheValue is only equal to itself
                   (value = get()) != null &&
                   value == ((Value<?>) obj).get(); // compare by identity
        }
    }

    
CacheKey containing a weakly referenced key. It registers itself with the refQueue so that it can be used to expunge the entry when the WeakReference is cleared. 
/**
     * CacheKey containing a weakly referenced {@code key}. It registers
     * itself with the {@code refQueue} so that it can be used to expunge
     * the entry when the {@link WeakReference} is cleared.
     */
    private static final class CacheKey<K> extends WeakReference<K> {

        // a replacement for null keys
        private static final Object NULL_KEY = new Object();

        static <K> Object valueOf(K key, ReferenceQueue<K> refQueue) {
            return key == null
                   // null key means we can't weakly reference it,
                   // so we use a NULL_KEY singleton as cache key
                   ? NULL_KEY
                   // non-null key requires wrapping with a WeakReference
                   : new CacheKey<>(key, refQueue);
        }

        private final int hash;

        private CacheKey(K key, ReferenceQueue<K> refQueue) {
            super(key, refQueue);
            this.hash = System.identityHashCode(key);  // compare by identity
        }

        @Override
        public int hashCode() {
            return hash;
        }

        @Override
        public boolean equals(Object obj) {
            K key;
            return obj == this ||
                   obj != null &&
                   obj.getClass() == this.getClass() &&
                   // cleared CacheKey is only equal to itself
                   (key = this.get()) != null &&
                   // compare key by identity
                   key == ((CacheKey<K>) obj).get();
        }

        void expungeFrom(ConcurrentMap<?, ? extends ConcurrentMap<?, ?>> map,
                         ConcurrentMap<?, Boolean> reverseMap) {
            // removing just by key is always safe here because after a CacheKey
            // is cleared and enqueue-ed it is only equal to itself
            // (see equals method)...
            ConcurrentMap<?, ?> valuesMap = map.remove(this);
            // remove also from reverseMap if needed
            if (valuesMap != null) {
                for (Object cacheValue : valuesMap.values()) {
                    reverseMap.remove(cacheValue);
                }
            }
        }
    }
}