WeakHashMap 源码分析

Posted 2020-12-24 zhuxudong

package java.util;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;


/**
 * 1）基于弱引用键实现 Map 接口的哈希表，对于给定的键，当内存紧张时，垃圾回收器会按需回收键值对。
 * 2）WeakHashMap 支持 null 键和 null 值
 * 3）WeakHashMap 是线程不同步的，可以通过
 * {@link Collections#synchronizedMap Collections.synchronizedMap}
 * 方法获取线程同步的 Map。
 */
public class WeakHashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V> {

    /**
     * The default initial capacity -- MUST be a power of two.
     * 默认的初始容量
     */
    private static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     * 默认的最大大小
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     * 默认的加载因子，当未在构造函数中指定时
     */
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     * 哈希表数组
     */
    Entry<K,V>[] table;

    /**
     * The number of key-value mappings contained in this weak hash map.
     * 当前总的键值对数目
     */
    private int size;

    /**
     * The next size value at which to resize (capacity * load factor).
     * 下一次扩容的阈值
     */
    private int threshold;

    /**
     * The load factor for the hash table.
     * WeakHashMap 的加载因子
     */
    private final float loadFactor;

    /**
     * Reference queue for cleared WeakEntries
     * 存放被清除条目的引用队列
     */
    private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

    /**
     * The number of times this WeakHashMap has been structurally modified.
     * Structural modifications are those that change the number of
     * mappings in the map or otherwise modify its internal structure
     * (e.g., rehash).  This field is used to make iterators on
     * Collection-views of the map fail-fast.
     * WeakHashMap 的结构化修改次数
     */
    int modCount;

    /**
     * 新建 bucket 数为 n 的哈希表
     * created by ZXD at 22 Jul 2018 T 13:02:58
     */
    @SuppressWarnings("unchecked")
    private Entry<K,V>[] newTable(int n) {
        return (Entry<K,V>[]) new Entry<?,?>[n];
    }

    /**
     * Constructs a new, empty {@code WeakHashMap} with the given initial
     * capacity and the given load factor.
     * 基于自定义的初始化容量 initialCapacity 和加载因子 loadFactor 创建 WeakHashMap 实例
     */
    public WeakHashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Initial Capacity: "+
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;

        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal Load factor: "+
                                               loadFactor);
        int capacity = 1;
        while (capacity < initialCapacity)
            capacity <<= 1;
        table = newTable(capacity);
        this.loadFactor = loadFactor;
        threshold = (int)(capacity * loadFactor);
    }

    /**
     * Constructs a new, empty {@code WeakHashMap} with the given initial
     * capacity and the default load factor (0.75).
     * 基于自定义的初始化容量 initialCapacity 和加载因子 0.75 创建 WeakHashMap 实例
     */
    public WeakHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**
     * Constructs a new, empty {@code WeakHashMap} with the default initial
     * capacity (16) and load factor (0.75).
     * 基于默认的初始化容量 16 和加载因子 0.75 创建 WeakHashMap 实例
     */
    public WeakHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
    }

    /**
     * Constructs a new {@code WeakHashMap} with the same mappings as the
     * specified map.  The {@code WeakHashMap} is created with the default
     * load factor (0.75) and an initial capacity sufficient to hold the
     * mappings in the specified map.
     * 基于默认的初始化容量 16 和加载因子 0.75 创建 WeakHashMap 实例，并将形参 map 中的条目都
     * 加入到 WeakHashMap 中。
     */
    public WeakHashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                DEFAULT_INITIAL_CAPACITY),
             DEFAULT_LOAD_FACTOR);
        putAll(m);
    }

    // internal utilities
    /**
     * Value representing null keys inside tables.
     * null 键的占位符键
     */
    private static final Object NULL_KEY = new Object();

    /**
     * Use NULL_KEY for key if it is null.
     * 将 null 键转换为占位符键
     */
    private static Object maskNull(Object key) {
        return (key == null) ? NULL_KEY : key;
    }

    /**
     * Returns internal representation of null key back to caller as null.
     * 将占位符键转换为 null 键
     */
    static Object unmaskNull(Object key) {
        return (key == NULL_KEY) ? null : key;
    }

    /**
     * Checks for equality of non-null reference x and possibly-null y.  By
     * default uses Object.equals.
     */
    private static boolean eq(Object x, Object y) {
        return x == y || x.equals(y);
    }

    /**
     * Retrieve object hash code and applies a supplemental hash function to the
     * result hash, which defends against poor quality hash functions.  This is
     * critical because HashMap uses power-of-two length hash tables, that
     * otherwise encounter collisions for hashCodes that do not differ
     * in lower bits.
     */
    final int hash(Object k) {
        int h = k.hashCode();
        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

    /**
     * Returns index for hash code h.
     * 根据哈希码和表长度计算索引值
     */
    private static int indexFor(int h, int length) {
        return h & (length-1);
    }

    /**
     * Expunges stale entries from the table.
     * 从 WeakHashMap 中清除存在于引用队列中的所有键值对
     */
    private void expungeStaleEntries() {
        // 依次弹出引用队列中的所有条目
        for (Object x; (x = queue.poll()) != null; ) {
            synchronized (queue) {
                @SuppressWarnings("unchecked")
                    Entry<K,V> e = (Entry<K,V>) x; // 暂存当前键值对
                int i = indexFor(e.hash, table.length); // 计算键值对在哈希表中的索引

                Entry<K,V> prev = table[i]; // 获取 bucket 的首节点
                Entry<K,V> p = prev; // 暂存首节点
                while (p != null) {
                    Entry<K,V> next = p.next; // 获取后置节点
                    if (p == e) { // 当前节点匹配目标节点
                        if (prev == e) // 当前节点为首节点
                            table[i] = next; // 更新 bucket 的首节点为当前节点的后置节点
                        else
                            prev.next = next; //
                        // Must not null out e.next;
                        // stale entries may be in use by a HashIterator
                        e.value = null; // Help GC 将目标节点的值置为 null
                        size--;
                        break; // 一旦找到则跳出循环
                    }
                    prev = p; // 继续遍历下一个节点
                    p = next;
                }
            }
        }
    }

    /**
     * Returns the table after first expunging stale entries.
     * 每次在获取哈希表时，都会执行一次弱键清理
     */
    private Entry<K,V>[] getTable() {
        expungeStaleEntries();
        return table;
    }

    /**
     * Returns the number of key-value mappings in this map.
     * This result is a snapshot, and may not reflect unprocessed
     * entries that will be removed before next attempted access
     * because they are no longer referenced.
     * 执行一次弱键清理，并返回有效的总键值对数目
     */
    public int size() {
        if (size == 0)
            return 0;
        expungeStaleEntries();
        return size;
    }

    /**
     * Returns {@code true} if this map contains no key-value mappings.
     * This result is a snapshot, and may not reflect unprocessed
     * entries that will be removed before next attempted access
     * because they are no longer referenced.
     * 是否为空
     */
    public boolean isEmpty() {
        return size() == 0;
    }

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that
     * {@code Objects.equals(key, k)},
     * then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it‘s also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     * 根据指定的键读取值
     */
    public V get(Object key) {
        Object k = maskNull(key); // 暂存键
        int h = hash(k); // 计算键的哈希值
        Entry<K,V>[] tab = getTable(); // 获取哈希表
        int index = indexFor(h, tab.length); // 计算索引
        Entry<K,V> e = tab[index]; // 获取具体的 bucket
        while (e != null) { // 遍历这个 bucket 关联的单向链表，并查找相等的键
            if (e.hash == h && eq(k, e.get()))
                return e.value;
            e = e.next;
        }
        return null;
    }

    /**
     * Returns {@code true} if this map contains a mapping for the
     * specified key.
     * WeakHashMap 中是否包含指定的键
     */
    public boolean containsKey(Object key) {
        return getEntry(key) != null;
    }

    /**
     * Returns the entry associated with the specified key in this map.
     * Returns null if the map contains no mapping for this key.
     * 根据指定的键获取键值对
     */
    Entry<K,V> getEntry(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K,V>[] tab = getTable();
        int index = indexFor(h, tab.length);
        Entry<K,V> e = tab[index];
        while (e != null && !(e.hash == h && eq(k, e.get())))
            e = e.next;
        return e;
    }

    /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for this key, the old
     * value is replaced.
     * 如果键已经存在，则使用新值替换旧值，并返回旧值；
     * 否则插入新的键值对，并返回 null。
     */
    public V put(K key, V value) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K,V>[] tab = getTable();
        int i = indexFor(h, tab.length);
        // 遍历单向链表
        for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
            if (h == e.hash && eq(k, e.get())) { // 找到匹配的键值对
                V oldValue = e.value; // 暂存旧值
                if (value != oldValue) // 旧值和新值不相等，则替换为新值
                    e.value = value;
                return oldValue; // 返回旧值
            }
        }

        modCount++;
        Entry<K,V> e = tab[i]; // 获取单向链表首节点
        tab[i] = new Entry<>(k, value, queue, h, e); // 将新节点作为 bucket 首节点插入到哈希表中
        if (++size >= threshold) // 元素总数超出扩容阈值
            resize(tab.length * 2); // 扩容为原来的两倍
        return null;
    }

    /**
     * Rehashes the contents of this map into a new array with a
     * larger capacity.  This method is called automatically when the
     * number of keys in this map reaches its threshold.
     *
     * If current capacity is MAXIMUM_CAPACITY, this method does not
     * resize the map, but sets threshold to Integer.MAX_VALUE.
     * This has the effect of preventing future calls.
     * 当哈希表的容量为 1 << 30 时，不进行扩容，而是将阈值设置为 Integer.MAX_VALUE
     */
    void resize(int newCapacity) {
        Entry<K,V>[] oldTable = getTable(); // 暂存旧哈希表
        int oldCapacity = oldTable.length; // 暂存旧容量
        if (oldCapacity == MAXIMUM_CAPACITY) { // 如果旧容量达到 MAXIMUM_CAPACITY
            threshold = Integer.MAX_VALUE; // 更新扩容阈值为 Integer.MAX_VALUE
            return;
        }

        Entry<K,V>[] newTable = newTable(newCapacity); // 创建新容量的哈希表
        transfer(oldTable, newTable); // 迁移数据
        table = newTable; // 更新哈希表

        /*
         * If ignoring null elements and processing ref queue caused massive
         * shrinkage, then restore old table.  This should be rare, but avoids
         * unbounded expansion of garbage-filled tables.
         * 键值对总数大于等于扩容阈值的二分之一
         */
        if (size >= threshold / 2) {
            // 更新扩容阈值
            threshold = (int)(newCapacity * loadFactor);
        } else {
            // 根据引用队列清除无效的键值对
            expungeStaleEntries();
            // 回迁数据
            transfer(newTable, oldTable);
            table = oldTable;
        }
    }

    /** Transfers all entries from src to dest tables */
    private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest) {
        for (int j = 0; j < src.length; ++j) { // 遍历整个哈希表
            Entry<K,V> e = src[j]; // 暂存首节点
            src[j] = null; // 将原 bucket 置为 null
            while (e != null) { // 如果 bucket 不为 null
                Entry<K,V> next = e.next; // 暂存下一个节点
                Object key = e.get(); // 读取弱引用的键
                if (key == null) { // 键为 null，已经被回收
                    e.next = null;  // Help GC
                    e.value = null; //  "   "
                    size--; // 减小容量
                } else { // 键未被回收
                    // 基于哈希值和新的容量计算 index
                    int i = indexFor(e.hash, dest.length);
                    e.next = dest[i]; // 当前节点的后置节点设置为新哈希表的首节点
                    dest[i] = e; // 新 bucket 的首节点设置为当前节点
                }
                e = next; // 继续处理下一个节点
            }
        }
    }

    /**
     * Copies all of the mappings from the specified map to this map.
     * These mappings will replace any mappings that this map had for any
     * of the keys currently in the specified map.
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        int numKeysToBeAdded = m.size();
        if (numKeysToBeAdded == 0)
            return;

        /*
         * Expand the map if the map if the number of mappings to be added
         * is greater than or equal to threshold.  This is conservative; the
         * obvious condition is (m.size() + size) >= threshold, but this
         * condition could result in a map with twice the appropriate capacity,
         * if the keys to be added overlap with the keys already in this map.
         * By using the conservative calculation, we subject ourself
         * to at most one extra resize.
         */
        if (numKeysToBeAdded > threshold) {
            int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
            if (targetCapacity > MAXIMUM_CAPACITY)
                targetCapacity = MAXIMUM_CAPACITY;
            int newCapacity = table.length;
            while (newCapacity < targetCapacity)
                newCapacity <<= 1;
            if (newCapacity > table.length)
                resize(newCapacity);
        }

        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

    /**
     * Removes the mapping for a key from this weak hash map if it is present.
     * More formally, if this map contains a mapping from key {@code k} to
     * value {@code v} such that <code>(key==null ?  k==null :
     * key.equals(k))</code>, that mapping is removed.  (The map can contain
     * at most one such mapping.)
     *
     * <p>Returns the value to which this map previously associated the key,
     * or {@code null} if the map contained no mapping for the key.  A
     * return value of {@code null} does not <i>necessarily</i> indicate
     * that the map contained no mapping for the key; it‘s also possible
     * that the map explicitly mapped the key to {@code null}.
     *
     * <p>The map will not contain a mapping for the specified key once the
     * call returns.
     * 根据指定的键移除条目
     */
    public V remove(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K,V>[] tab = getTable();
        int i = indexFor(h, tab.length);
        Entry<K,V> prev = tab[i];
        Entry<K,V> e = prev;

        while (e != null) {
            Entry<K,V> next = e.next;
            if (h == e.hash && eq(k, e.get())) { // 找到目标键
                modCount++;
                size--;
                if (prev == e)
                    tab[i] = next;
                else
                    prev.next = next;
                return e.value;
            }
            prev = e;
            e = next;
        }

        return null;
    }

    /** Special version of remove needed by Entry set */
    boolean removeMapping(Object o) {
        if (!(o instanceof Map.Entry))
            return false;
        Entry<K,V>[] tab = getTable();
        Map.Entry<?,?> entry = (Map.Entry<?,?>)o;
        Object k = maskNull(entry.getKey());
        int h = hash(k);
        int i = indexFor(h, tab.length);
        Entry<K,V> prev = tab[i];
        Entry<K,V> e = prev;

        while (e != null) {
            Entry<K,V> next = e.next;
            if (h == e.hash && e.equals(entry)) {
                modCount++;
                size--;
                if (prev == e)
                    tab[i] = next;
                else
                    prev.next = next;
                return true;
            }
            prev = e;
            e = next;
        }

        return false;
    }

    /**
     * Removes all of the mappings from this map.
     * The map will be empty after this call returns.
     */
    public void clear() {
        // 清除引用队列
        while (queue.poll() != null)
            ;

        modCount++;
        Arrays.fill(table, null); // 将哈希表的 bucket 都置为 null
        size = 0;

        // Allocation of array may have caused GC, which may have caused
        // additional entries to go stale.  Removing these entries from the
        // reference queue will make them eligible for reclamation.
        // 再次清除引用队列
        while (queue.poll() != null)
            ;
    }

    /**
     * Returns {@code true} if this map maps one or more keys to the
     * specified value.
     * WeakHashMap 是否包含指定的值
     */
    public boolean containsValue(Object value) {
        if (value==null)
            return containsNullValue();

        Entry<K,V>[] tab = getTable();
        for (int i = tab.length; i-- > 0;)
            for (Entry<K,V> e = tab[i]; e != null; e = e.next)
                if (value.equals(e.value))
                    return true;
        return false;
    }

    /**
     * Special-case code for containsValue with null argument
     * 是否包含 null 值
     */
    private boolean containsNullValue() {
        Entry<K,V>[] tab = getTable();
        for (int i = tab.length; i-- > 0;)
            for (Entry<K,V> e = tab[i]; e != null; e = e.next)
                if (e.value==null)
                    return true;
        return false;
    }

    /**
     * The entries in this hash table extend WeakReference, using its main ref
     * field as the key.
     */
    private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
        V value;
        final int hash;
        Entry<K,V> next;

        /**
         * Creates new entry.
         */
        Entry(Object key, V value,
              ReferenceQueue<Object> queue,
              int hash, Entry<K,V> next) {
            super(key, queue);
            this.value = value;
            this.hash  = hash;
            this.next  = next;
        }

        @SuppressWarnings("unchecked")
        public K getKey() {
            return (K) WeakHashMap.unmaskNull(get());
        }

        public V getValue() {
            return value;
        }

        public V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        public boolean equals(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
            K k1 = getKey();
            Object k2 = e.getKey();
            if (k1 == k2 || (k1 != null && k1.equals(k2))) {
                V v1 = getValue();
                Object v2 = e.getValue();
                if (v1 == v2 || (v1 != null && v1.equals(v2)))
                    return true;
            }
            return false;
        }

        public int hashCode() {
            K k = getKey();
            V v = getValue();
            return Objects.hashCode(k) ^ Objects.hashCode(v);
        }

        public String toString() {
            return getKey() + "=" + getValue();
        }
    }

    private abstract class HashIterator<T> implements Iterator<T> {
        private int index;
        private Entry<K,V> entry;
        private Entry<K,V> lastReturned;
        private int expectedModCount = modCount;

        /**
         * Strong reference needed to avoid disappearance of key
         * between hasNext and next
         */
        private Object nextKey;

        /**
         * Strong reference needed to avoid disappearance of key
         * between nextEntry() and any use of the entry
         */
        private Object currentKey;

        HashIterator() {
            index = isEmpty() ? 0 : table.length;
        }

        public boolean hasNext() {
            Entry<K,V>[] t = table;

            while (nextKey == null) {
                Entry<K,V> e = entry;
                int i = index;
                while (e == null && i > 0)
                    e = t[--i];
                entry = e;
                index = i;
                if (e == null) {
                    currentKey = null;
                    return false;
                }
                nextKey = e.get(); // hold on to key in strong ref
                if (nextKey == null)
                    entry = entry.next;
            }
            return true;
        }

        /** The common parts of next() across different types of iterators */
        protected Entry<K,V> nextEntry() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (nextKey == null && !hasNext())
                throw new NoSuchElementException();

            lastReturned = entry;
            entry = entry.next;
            currentKey = nextKey;
            nextKey = null;
            return lastReturned;
        }

        public void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();

            WeakHashMap.this.remove(currentKey);
            expectedModCount = modCount;
            lastReturned = null;
            currentKey = null;
        }

    }

    private class ValueIterator extends HashIterator<V> {
        public V next() {
            return nextEntry().value;
        }
    }

    private class KeyIterator extends HashIterator<K> {
        public K next() {
            return nextEntry().getKey();
        }
    }

    private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
        public Map.Entry<K,V> next() {
            return nextEntry();
        }
    }

    // Views

    private transient Set<Map.Entry<K,V>> entrySet;

    /**
     * Returns a {@link Set} view of the keys contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  If the map is modified
     * while an iteration over the set is in progress (except through
     * the iterator‘s own {@code remove} operation), the results of
     * the iteration are undefined.  The set supports element removal,
     * which removes the corresponding mapping from the map, via the
     * {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.  It does not support the {@code add} or {@code addAll}
     * operations.
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks == null) {
            ks = new KeySet();
            keySet = ks;
        }
        return ks;
    }

    private class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return new KeyIterator();
        }

        public int size() {
            return WeakHashMap.this.size();
        }

        public boolean contains(Object o) {
            return containsKey(o);
        }

        public boolean remove(Object o) {
            if (containsKey(o)) {
                WeakHashMap.this.remove(o);
                return true;
            }
            else
                return false;
        }

        public void clear() {
            WeakHashMap.this.clear();
        }

        public Spliterator<K> spliterator() {
            return new KeySpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
        }
    }

    /**
     * Returns a {@link Collection} view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are
     * reflected in the collection, and vice-versa.  If the map is
     * modified while an iteration over the collection is in progress
     * (except through the iterator‘s own {@code remove} operation),
     * the results of the iteration are undefined.  The collection
     * supports element removal, which removes the corresponding
     * mapping from the map, via the {@code Iterator.remove},
     * {@code Collection.remove}, {@code removeAll},
     * {@code retainAll} and {@code clear} operations.  It does not
     * support the {@code add} or {@code addAll} operations.
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return vs;
    }

    private class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator();
        }

        public int size() {
            return WeakHashMap.this.size();
        }

        public boolean contains(Object o) {
            return containsValue(o);
        }

        public void clear() {
            WeakHashMap.this.clear();
        }

        public Spliterator<V> spliterator() {
            return new ValueSpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
        }
    }

    /**
     * Returns a {@link Set} view of the mappings contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  If the map is modified
     * while an iteration over the set is in progress (except through
     * the iterator‘s own {@code remove} operation, or through the
     * {@code setValue} operation on a map entry returned by the
     * iterator) the results of the iteration are undefined.  The set
     * supports element removal, which removes the corresponding
     * mapping from the map, via the {@code Iterator.remove},
     * {@code Set.remove}, {@code removeAll}, {@code retainAll} and
     * {@code clear} operations.  It does not support the
     * {@code add} or {@code addAll} operations.
     */
    public Set<Map.Entry<K,V>> entrySet() {
        Set<Map.Entry<K,V>> es = entrySet;
        return es != null ? es : (entrySet = new EntrySet());
    }

    private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        public Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator();
        }

        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
            Entry<K,V> candidate = getEntry(e.getKey());
            return candidate != null && candidate.equals(e);
        }

        public boolean remove(Object o) {
            return removeMapping(o);
        }

        public int size() {
            return WeakHashMap.this.size();
        }

        public void clear() {
            WeakHashMap.this.clear();
        }

        private List<Map.Entry<K,V>> deepCopy() {
            List<Map.Entry<K,V>> list = new ArrayList<>(size());
            for (Map.Entry<K,V> e : this)
                list.add(new AbstractMap.SimpleEntry<>(e));
            return list;
        }

        public Object[] toArray() {
            return deepCopy().toArray();
        }

        public <T> T[] toArray(T[] a) {
            return deepCopy().toArray(a);
        }

        public Spliterator<Map.Entry<K,V>> spliterator() {
            return new EntrySpliterator<>(WeakHashMap.this, 0, -1, 0, 0);
        }
    }

    @SuppressWarnings("unchecked")
    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
        Objects.requireNonNull(action);
        int expectedModCount = modCount;

        Entry<K, V>[] tab = getTable();
        for (Entry<K, V> entry : tab) {
            while (entry != null) {
                Object key = entry.get();
                if (key != null) {
                    action.accept((K)WeakHashMap.unmaskNull(key), entry.value);
                }
                entry = entry.next;

                if (expectedModCount != modCount) {
                    throw new ConcurrentModificationException();
                }
            }
        }
    }

    @SuppressWarnings("unchecked")
    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        Objects.requireNonNull(function);
        int expectedModCount = modCount;

        Entry<K, V>[] tab = getTable();;
        for (Entry<K, V> entry : tab) {
            while (entry != null) {
                Object key = entry.get();
                if (key != null) {
                    entry.value = function.apply((K)WeakHashMap.unmaskNull(key), entry.value);
                }
                entry = entry.next;

                if (expectedModCount != modCount) {
                    throw new ConcurrentModificationException();
                }
            }
        }
    }

    /**
     * Similar form as other hash Spliterators, but skips dead
     * elements.
     */
    static class WeakHashMapSpliterator<K,V> {
        final WeakHashMap<K,V> map;
        WeakHashMap.Entry<K,V> current; // current node
        int index;             // current index, modified on advance/split
        int fence;             // -1 until first use; then one past last index
        int est;               // size estimate
        int expectedModCount;  // for comodification checks

        WeakHashMapSpliterator(WeakHashMap<K,V> m, int origin,
                               int fence, int est,
                               int expectedModCount) {
            this.map = m;
            this.index = origin;
            this.fence = fence;
            this.est = est;
            this.expectedModCount = expectedModCount;
        }

        final int getFence() { // initialize fence and size on first use
            int hi;
            if ((hi = fence) < 0) {
                WeakHashMap<K,V> m = map;
                est = m.size();
                expectedModCount = m.modCount;
                hi = fence = m.table.length;
            }
            return hi;
        }

        public final long estimateSize() {
            getFence(); // force init
            return (long) est;
        }
    }

    static final class KeySpliterator<K,V>
        extends WeakHashMapSpliterator<K,V>
        implements Spliterator<K> {
        KeySpliterator(WeakHashMap<K,V> m, int origin, int fence, int est,
                       int expectedModCount) {
            super(m, origin, fence, est, expectedModCount);
        }

        public KeySpliterator<K,V> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null :
                new KeySpliterator<>(map, lo, index = mid, est >>>= 1,
                                     expectedModCount);
        }

        public void forEachRemaining(Consumer<? super K> action) {
            int i, hi, mc;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap<K,V> m = map;
            WeakHashMap.Entry<K,V>[] tab = m.table;
            if ((hi = fence) < 0) {
                mc = expectedModCount = m.modCount;
                hi = fence = tab.length;
            }
            else
                mc = expectedModCount;
            if (tab.length >= hi && (i = index) >= 0 &&
                (i < (index = hi) || current != null)) {
                WeakHashMap.Entry<K,V> p = current;
                current = null; // exhaust
                do {
                    if (p == null)
                        p = tab[i++];
                    else {
                        Object x = p.get();
                        p = p.next;
                        if (x != null) {
                            @SuppressWarnings("unchecked") K k =
                                (K) WeakHashMap.unmaskNull(x);
                            action.accept(k);
                        }
                    }
                } while (p != null || i < hi);
            }
            if (m.modCount != mc)
                throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super K> action) {
            int hi;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap.Entry<K,V>[] tab = map.table;
            if (tab.length >= (hi = getFence()) && index >= 0) {
                while (current != null || index < hi) {
                    if (current == null)
                        current = tab[index++];
                    else {
                        Object x = current.get();
                        current = current.next;
                        if (x != null) {
                            @SuppressWarnings("unchecked") K k =
                                (K) WeakHashMap.unmaskNull(x);
                            action.accept(k);
                            if (map.modCount != expectedModCount)
                                throw new ConcurrentModificationException();
                            return true;
                        }
                    }
                }
            }
            return false;
        }

        public int characteristics() {
            return Spliterator.DISTINCT;
        }
    }

    static final class ValueSpliterator<K,V>
        extends WeakHashMapSpliterator<K,V>
        implements Spliterator<V> {
        ValueSpliterator(WeakHashMap<K,V> m, int origin, int fence, int est,
                         int expectedModCount) {
            super(m, origin, fence, est, expectedModCount);
        }

        public ValueSpliterator<K,V> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null :
                new ValueSpliterator<>(map, lo, index = mid, est >>>= 1,
                                       expectedModCount);
        }

        public void forEachRemaining(Consumer<? super V> action) {
            int i, hi, mc;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap<K,V> m = map;
            WeakHashMap.Entry<K,V>[] tab = m.table;
            if ((hi = fence) < 0) {
                mc = expectedModCount = m.modCount;
                hi = fence = tab.length;
            }
            else
                mc = expectedModCount;
            if (tab.length >= hi && (i = index) >= 0 &&
                (i < (index = hi) || current != null)) {
                WeakHashMap.Entry<K,V> p = current;
                current = null; // exhaust
                do {
                    if (p == null)
                        p = tab[i++];
                    else {
                        Object x = p.get();
                        V v = p.value;
                        p = p.next;
                        if (x != null)
                            action.accept(v);
                    }
                } while (p != null || i < hi);
            }
            if (m.modCount != mc)
                throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super V> action) {
            int hi;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap.Entry<K,V>[] tab = map.table;
            if (tab.length >= (hi = getFence()) && index >= 0) {
                while (current != null || index < hi) {
                    if (current == null)
                        current = tab[index++];
                    else {
                        Object x = current.get();
                        V v = current.value;
                        current = current.next;
                        if (x != null) {
                            action.accept(v);
                            if (map.modCount != expectedModCount)
                                throw new ConcurrentModificationException();
                            return true;
                        }
                    }
                }
            }
            return false;
        }

        public int characteristics() {
            return 0;
        }
    }

    static final class EntrySpliterator<K,V>
        extends WeakHashMapSpliterator<K,V>
        implements Spliterator<Map.Entry<K,V>> {
        EntrySpliterator(WeakHashMap<K,V> m, int origin, int fence, int est,
                       int expectedModCount) {
            super(m, origin, fence, est, expectedModCount);
        }

        public EntrySpliterator<K,V> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null :
                new EntrySpliterator<>(map, lo, index = mid, est >>>= 1,
                                       expectedModCount);
        }


        public void forEachRemaining(Consumer<? super Map.Entry<K, V>> action) {
            int i, hi, mc;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap<K,V> m = map;
            WeakHashMap.Entry<K,V>[] tab = m.table;
            if ((hi = fence) < 0) {
                mc = expectedModCount = m.modCount;
                hi = fence = tab.length;
            }
            else
                mc = expectedModCount;
            if (tab.length >= hi && (i = index) >= 0 &&
                (i < (index = hi) || current != null)) {
                WeakHashMap.Entry<K,V> p = current;
                current = null; // exhaust
                do {
                    if (p == null)
                        p = tab[i++];
                    else {
                        Object x = p.get();
                        V v = p.value;
                        p = p.next;
                        if (x != null) {
                            @SuppressWarnings("unchecked") K k =
                                (K) WeakHashMap.unmaskNull(x);
                            action.accept
                                (new AbstractMap.SimpleImmutableEntry<>(k, v));
                        }
                    }
                } while (p != null || i < hi);
            }
            if (m.modCount != mc)
                throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super Map.Entry<K,V>> action) {
            int hi;
            if (action == null)
                throw new NullPointerException();
            WeakHashMap.Entry<K,V>[] tab = map.table;
            if (tab.length >= (hi = getFence()) && index >= 0) {
                while (current != null || index < hi) {
                    if (current == null)
                        current = tab[index++];
                    else {
                        Object x = current.get();
                        V v = current.value;
                        current = current.next;
                        if (x != null) {
                            @SuppressWarnings("unchecked") K k =
                                (K) WeakHashMap.unmaskNull(x);
                            action.accept
                                (new AbstractMap.SimpleImmutableEntry<>(k, v));
                            if (map.modCount != expectedModCount)
                                throw new ConcurrentModificationException();
                            return true;
                        }
                    }
                }
            }
            return false;
        }

        public int characteristics() {
            return Spliterator.DISTINCT;
        }
    }

}