https://github.com/eclipse-ee4j/orb-gmbal-pfl/pfl-basic: Basic functions (Eclipse Foundation)
  • Russell Gold 
/*
 * Copyright (c) 1997, 2018 Oracle and/or its affiliates. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Distribution License v. 1.0, which is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

package org.glassfish.pfl.basic.concurrent ;

import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.ArrayList;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;



A hashtable-based Map implementation with weak keys.
An entry in a WeakHashMap will automatically be removed when
its key is no longer in ordinary use.  More precisely, the presence of a
mapping for a given key will not prevent the key from being discarded by the
garbage collector, that is, made finalizable, finalized, and then reclaimed.
When a key has been discarded its entry is effectively removed from the map,
so this class behaves somewhat differently from other Map
implementations.

 Both null values and the null key are supported. This class has
performance characteristics similar to those of the HashMap
class, and has the same efficiency parameters of initial capacity
and load factor.

 Like most collection classes, this class is not synchronized.
A synchronized WeakHashMap may be constructed using the
Collections.synchronizedMap method.

 This class is intended primarily for use with key objects whose
equals methods test for object identity using the
== operator.  Once such a key is discarded it can never be
recreated, so it is impossible to do a lookup of that key in a
WeakHashMap at some later time and be surprised that its entry
has been removed.  This class will work perfectly well with key objects
whose equals methods are not based upon object identity, such
as String instances.  With such recreatable key objects,
however, the automatic removal of WeakHashMap entries whose
keys have been discarded may prove to be confusing.

 The behavior of the WeakHashMap class depends in part upon
the actions of the garbage collector, so several familiar (though not
required) Map invariants do not hold for this class.  Because
the garbage collector may discard keys at any time, a
WeakHashMap may behave as though an unknown thread is silently
removing entries.  In particular, even if you synchronize on a
WeakHashMap instance and invoke none of its mutator methods, it
is possible for the size method to return smaller values over
time, for the isEmpty method to return false and
then true, for the containsKey method to return
true and later false for a given key, for the
get method to return a value for a given key but later return
null, for the put method to return
null and the remove method to return
false for a key that previously appeared to be in the map, and
for successive examinations of the key set, the value collection, and
the entry set to yield successively smaller numbers of elements.

 Each key object in a WeakHashMap is stored indirectly as
the referent of a weak reference.  Therefore a key will automatically be
removed only after the weak references to it, both inside and outside of the
map, have been cleared by the garbage collector.

 Implementation note: The value objects in a
WeakHashMap are held by ordinary strong references.  Thus care
should be taken to ensure that value objects do not strongly refer to their
own keys, either directly or indirectly, since that will prevent the keys
from being discarded.  Note that a value object may refer indirectly to its
key via the WeakHashMap itself; that is, a value object may
strongly refer to some other key object whose associated value object, in
turn, strongly refers to the key of the first value object.  One way
to deal with this is to wrap values themselves within
WeakReferences before
inserting, as in: m.put(key, new WeakReference(value)),
and then unwrapping upon each get.

The iterators returned by the iterator method of the collections
returned by all of this class's "collection view methods" are
fail-fast: if the map is structurally modified at any time after the
iterator is created, in any way except through the iterator's own
remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. 
Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification.  Fail-fast iterators
throw ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness:  the fail-fast behavior of iterators
should be used only to detect bugs.

This class is a member of the

Java Collections Framework.

This is copied directly from JDK 6 to get a version with the fix
for but 6425537.

Author:     Doug Lea,      Josh Bloch, 	Mark Reinhold
Type parameters:
  • <K> – the type of keys maintained by this map
  • <V> – the type of mapped values
See Also:
Version:	%I%, %G%
Since:	1.2
/**
 * A hashtable-based <tt>Map</tt> implementation with <em>weak keys</em>.
 * An entry in a <tt>WeakHashMap</tt> will automatically be removed when
 * its key is no longer in ordinary use.  More precisely, the presence of a
 * mapping for a given key will not prevent the key from being discarded by the
 * garbage collector, that is, made finalizable, finalized, and then reclaimed.
 * When a key has been discarded its entry is effectively removed from the map,
 * so this class behaves somewhat differently from other <tt>Map</tt>
 * implementations.
 *
 * <p> Both null values and the null key are supported. This class has
 * performance characteristics similar to those of the <tt>HashMap</tt>
 * class, and has the same efficiency parameters of <em>initial capacity</em>
 * and <em>load factor</em>.
 *
 * <p> Like most collection classes, this class is not synchronized.
 * A synchronized <tt>WeakHashMap</tt> may be constructed using the
 * Collections.synchronizedMap method.
 *
 * <p> This class is intended primarily for use with key objects whose
 * <tt>equals</tt> methods test for object identity using the
 * <tt>==</tt> operator.  Once such a key is discarded it can never be
 * recreated, so it is impossible to do a lookup of that key in a
 * <tt>WeakHashMap</tt> at some later time and be surprised that its entry
 * has been removed.  This class will work perfectly well with key objects
 * whose <tt>equals</tt> methods are not based upon object identity, such
 * as <tt>String</tt> instances.  With such recreatable key objects,
 * however, the automatic removal of <tt>WeakHashMap</tt> entries whose
 * keys have been discarded may prove to be confusing.
 *
 * <p> The behavior of the <tt>WeakHashMap</tt> class depends in part upon
 * the actions of the garbage collector, so several familiar (though not
 * required) <tt>Map</tt> invariants do not hold for this class.  Because
 * the garbage collector may discard keys at any time, a
 * <tt>WeakHashMap</tt> may behave as though an unknown thread is silently
 * removing entries.  In particular, even if you synchronize on a
 * <tt>WeakHashMap</tt> instance and invoke none of its mutator methods, it
 * is possible for the <tt>size</tt> method to return smaller values over
 * time, for the <tt>isEmpty</tt> method to return <tt>false</tt> and
 * then <tt>true</tt>, for the <tt>containsKey</tt> method to return
 * <tt>true</tt> and later <tt>false</tt> for a given key, for the
 * <tt>get</tt> method to return a value for a given key but later return
 * <tt>null</tt>, for the <tt>put</tt> method to return
 * <tt>null</tt> and the <tt>remove</tt> method to return
 * <tt>false</tt> for a key that previously appeared to be in the map, and
 * for successive examinations of the key set, the value collection, and
 * the entry set to yield successively smaller numbers of elements.
 *
 * <p> Each key object in a <tt>WeakHashMap</tt> is stored indirectly as
 * the referent of a weak reference.  Therefore a key will automatically be
 * removed only after the weak references to it, both inside and outside of the
 * map, have been cleared by the garbage collector.
 *
 * <p> <strong>Implementation note:</strong> The value objects in a
 * <tt>WeakHashMap</tt> are held by ordinary strong references.  Thus care
 * should be taken to ensure that value objects do not strongly refer to their
 * own keys, either directly or indirectly, since that will prevent the keys
 * from being discarded.  Note that a value object may refer indirectly to its
 * key via the <tt>WeakHashMap</tt> itself; that is, a value object may
 * strongly refer to some other key object whose associated value object, in
 * turn, strongly refers to the key of the first value object.  One way
 * to deal with this is to wrap values themselves within
 * <tt>WeakReferences</tt> before
 * inserting, as in: <tt>m.put(key, new WeakReference(value))</tt>,
 * and then unwrapping upon each <tt>get</tt>.
 *
 * <p>The iterators returned by the <tt>iterator</tt> method of the collections
 * returned by all of this class's "collection view methods" are
 * <i>fail-fast</i>: if the map is structurally modified at any time after the
 * iterator is created, in any way except through the iterator's own
 * <tt>remove</tt> method, the iterator will throw a {@link
 * ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:  <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * <p>This is copied directly from JDK 6 to get a version with the fix
 * for but 6425537.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 *
 * @version	%I%, %G%
 * @author      Doug Lea
 * @author      Josh Bloch
 * @author	Mark Reinhold
 * @since	1.2
 * @see		java.util.HashMap
 * @see		java.lang.ref.WeakReference
 */
public class WeakHashMapSafeReadLock<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V> {

    
The default initial capacity -- MUST be a power of two.

/**
     * The default initial capacity -- MUST be a power of two.
     */
    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.

/**
     * 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.
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;

    
The load fast used when none specified in constructor.

/**
     * The load fast used when none specified in constructor.
     */
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;

    
The table, resized as necessary. Length MUST Always be a power of two.

/**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    private Entry[] table;

    
The number of key-value mappings contained in this weak hash map.

/**
     * The number of key-value mappings contained in this weak hash map.
     */
    private int size;

    
The next size value at which to resize (capacity * load factor).

/**
     * The next size value at which to resize (capacity * load factor).
     */
    private int threshold;

    
The load factor for the hash table.

/**
     * The load factor for the hash table.
     */
    private final float loadFactor;

    
Reference queue for cleared WeakEntries

/**
     * Reference queue for cleared WeakEntries
     */
    private final ReferenceQueue<K> queue = new ReferenceQueue<K>();

    
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.

See Also:
  • ConcurrentModificationException
/**
     * 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.
     *
     * @see ConcurrentModificationException
     */
    private volatile int modCount;

    
Constructs a new, empty WeakHashMap with the given initial
capacity and the given load factor.

Params:
  • initialCapacity – The initial capacity of the WeakHashMap
  • loadFactor –      The load factor of the WeakHashMap
Throws:
/**
     * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
     * capacity and the given load factor.
     *
     * @param  initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
     * @param  loadFactor      The load factor of the <tt>WeakHashMap</tt>
     * @throws IllegalArgumentException if the initial capacity is negative,
     *         or if the load factor is nonpositive.
     */
    public WeakHashMapSafeReadLock(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 = new Entry[capacity];
        this.loadFactor = loadFactor;
        threshold = (int)(capacity * loadFactor);
    }

    
Constructs a new, empty WeakHashMap with the given initial
capacity and the default load factor (0.75).

Params:
  • initialCapacity – The initial capacity of the WeakHashMap
Throws:
/**
     * Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
     * capacity and the default load factor (0.75).
     *
     * @param  initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
     * @throws IllegalArgumentException if the initial capacity is negative
     */
    public WeakHashMapSafeReadLock(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    
Constructs a new, empty WeakHashMap with the default initial
capacity (16) and load factor (0.75).

/**
     * Constructs a new, empty <tt>WeakHashMap</tt> with the default initial
     * capacity (16) and load factor (0.75).
     */
    public WeakHashMapSafeReadLock() {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        threshold = DEFAULT_INITIAL_CAPACITY;
        table = new Entry[DEFAULT_INITIAL_CAPACITY];
    }

    
Constructs a new WeakHashMap with the same mappings as the
specified map.  The WeakHashMap is created with the default
load factor (0.75) and an initial capacity sufficient to hold the
mappings in the specified map.

Params:
  • m – the map whose mappings are to be placed in this map
Throws:
Since:	1.3
/**
     * Constructs a new <tt>WeakHashMap</tt> with the same mappings as the
     * specified map.  The <tt>WeakHashMap</tt> is created with the default
     * load factor (0.75) and an initial capacity sufficient to hold the
     * mappings in the specified map.
     *
     * @param   m the map whose mappings are to be placed in this map
     * @throws  NullPointerException if the specified map is null
     * @since	1.3
     */
    public WeakHashMapSafeReadLock(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, 16),
             DEFAULT_LOAD_FACTOR);
        putAll(m);
    }

    // internal utilities

    
Value representing null keys inside tables.

/**
     * Value representing null keys inside tables.
     */
    private static final Object NULL_KEY = new Object();

    
Use NULL_KEY for key if it is null.

/**
     * Use NULL_KEY for key if it is null.
     */
    private static Object maskNull(Object key) {
        return (key == null ? NULL_KEY : key);
    }

    
Returns internal representation of null key back to caller as null.

/**
     * Returns internal representation of null key back to caller as null.
     */
    private static <K> K unmaskNull(Object key) {
        return (K) (key == NULL_KEY ? null : key);
    }

    
Checks for equality of non-null reference x and possibly-null y.  By
default uses Object.equals.

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

    
Returns index for hash code h.

/**
     * Returns index for hash code h.
     */
    static int indexFor(int h, int length) {
        return h & (length-1);
    }

    
Expunges stale entries from the table.

/**
     * Expunges stale entries from the table.
     */
    @SuppressWarnings("unchecked")
    private void expungeStaleEntries() {
	Entry<K,V> e;
        while ( (e = (Entry<K,V>) queue.poll()) != null) {
            // Fix for 6425537: WeakHashMap should be safe for concurrent get
            synchronized( queue ) {
                int h = e.hash;
                int i = indexFor(h, table.length);

                Entry<K,V> prev = table[i];
                Entry<K,V> p = prev;
                while (p != null) {
                    Entry<K,V> next = p.next;
                    if (p == e) {
                        if (prev == e) {
                            table[i] = next;
                        } else {
                            prev.next = next;
                        }
                        e.next = null;  // Help GC
                        e.value = null; //  "   "
                        size--;
                        break;
                    }
                    prev = p;
                    p = next;
                }
            }
        }
    }

    
Returns the table after first expunging stale entries.

/**
     * Returns the table after first expunging stale entries.
     */
    private Entry[] 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.

/**
     * 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.
     */
    public int size() {
        if (size == 0) {
            return 0;
        }
        expungeStaleEntries();
        return size;
    }

    
Returns 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.

/**
     * Returns <tt>true</tt> 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.
     */
    @Override
    public boolean isEmpty() {
        return size() == 0;
    }

    static int computeHash( int h ) {
        int hash = h ^ (h >>> 20) ^ (h >>> 12) ;
        return hash ^ (hash >>> 7) ^ (hash >>> 4) ;
    }
    
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. 
More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null :
key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.) 
A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases. 
See Also:
/**
     * 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 (key==null ? k==null :
     * key.equals(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.
     *
     * @see #put(Object, Object)
     */
    @Override
    public V get(Object key) {
        Object k = maskNull(key);
        int h = computeHash(k.hashCode());
        Entry[] tab = getTable();
        int index = indexFor(h, tab.length);
        @SuppressWarnings("unchecked")
        Entry<K,V> e = tab[index];
        while (e != null) {
            if (e.hash == h && eq(k, e.get())) {
                return e.value;
            }
            e = e.next;
        }
        return null;
    }

    
Returns true if this map contains a mapping for the
specified key.

Params:
  • key –   The key whose presence in this map is to be tested
Returns:true if there is a mapping for key;
        false otherwise
/**
     * Returns <tt>true</tt> if this map contains a mapping for the
     * specified key.
     *
     * @param  key   The key whose presence in this map is to be tested
     * @return <tt>true</tt> if there is a mapping for <tt>key</tt>;
     *         <tt>false</tt> otherwise
     */
    @Override
    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.

/**
     * Returns the entry associated with the specified key in this map.
     * Returns null if the map contains no mapping for this key.
     */
    Entry<K,V> getEntry(Object key) {
        Object k = maskNull(key);
        int h = computeHash(k.hashCode());
        Entry[] tab = getTable();
        int index = indexFor(h, tab.length);
        @SuppressWarnings("unchecked")
        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.

Params:
  • key – key with which the specified value is to be associated.
  • value – value to be associated with the specified key.
Returns:the previous value associated with key, or
        null if there was no mapping for key.
        (A null return can also indicate that the map
        previously associated null with key.)
/**
     * 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.
     *
     * @param key key with which the specified value is to be associated.
     * @param value value to be associated with the specified key.
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    @Override
    public V put(K key, V value) {
        @SuppressWarnings("unchecked")
        K k = (K) maskNull(key);
        int h = computeHash(k.hashCode());
        Entry[] tab = getTable();
        int i = indexFor(h, tab.length);

        for (@SuppressWarnings("unchecked")
        Entry<K,V> e = tab[i]; e != null; e = e.next) {
            if (h == e.hash && eq(k, e.get())) {
                V oldValue = e.value;
                if (value != oldValue) {
                    e.value = value;
                }
                return oldValue;
            }
        }

        modCount++;
        @SuppressWarnings("unchecked")
	Entry<K,V> e = tab[i];
        tab[i] = new Entry<K,V>(k, value, queue, h, e);
        if (++size >= threshold) {
            resize(tab.length * 2);
        }
        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.

Params:
  • newCapacity – the new capacity, MUST be a power of two;
       must be greater than current capacity unless current
       capacity is MAXIMUM_CAPACITY (in which case value
       is irrelevant).
/**
     * 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.
     *
     * @param newCapacity the new capacity, MUST be a power of two;
     *        must be greater than current capacity unless current
     *        capacity is MAXIMUM_CAPACITY (in which case value
     *        is irrelevant).
     */
    void resize(int newCapacity) {
        Entry[] oldTable = getTable();
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        transfer(oldTable, newTable);
        table = newTable;

        /*
         * 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.
         */
        if (size >= threshold / 2) {
            threshold = (int)(newCapacity * loadFactor);
        } else {
            expungeStaleEntries();
            transfer(newTable, oldTable);
            table = oldTable;
        }
    }

    
Transfers all entries from src to dest tables 
/** Transfers all entries from src to dest tables */
    @SuppressWarnings("unchecked")
    private void transfer(Entry[] src, Entry[] dest) {
        for (int j = 0; j < src.length; ++j) {
            @SuppressWarnings("unchecked")
            Entry<K,V> e = src[j];
            src[j] = null;
            while (e != null) {
                Entry<K,V> next = e.next;
                Object key = e.get();
                if (key == null) {
                    e.next = null;  // Help GC
                    e.value = null; //  "   "
                    size--;
                } else {
                    int i = indexFor(e.hash, dest.length);
                    e.next = dest[i];
                    dest[i] = e;
                }
                e = next;
            }
        }
    }

    
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.

Params:
  • m – mappings to be stored in this map.
Throws:
/**
     * 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.
     *
     * @param m mappings to be stored in this map.
     * @throws  NullPointerException if the specified map is null.
     */
    @Override
    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 k to
value v such that (key==null ?  k==null :
key.equals(k)), that mapping is removed.  (The map can contain
at most one such mapping.)

Returns the value to which this map previously associated the key,
or null if the map contained no mapping for the key.  A
return value of null does not necessarily indicate
that the map contained no mapping for the key; it's also possible
that the map explicitly mapped the key to null.

The map will not contain a mapping for the specified key once the
call returns.

Params:
  • key – key whose mapping is to be removed from the map
Returns:the previous value associated with key, or
        null if there was no mapping for key
/**
     * 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 <tt>k</tt> to
     * value <tt>v</tt> 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 <tt>null</tt> if the map contained no mapping for the key.  A
     * return value of <tt>null</tt> 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 <tt>null</tt>.
     *
     * <p>The map will not contain a mapping for the specified key once the
     * call returns.
     *
     * @param key key whose mapping is to be removed from the map
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>
     */
    @Override
    public V remove(Object key) {
        Object k = maskNull(key);
        int h = computeHash(k.hashCode());
        Entry[] tab = getTable();
        int i = indexFor(h, tab.length);
        @SuppressWarnings("unchecked")
        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())) {
                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 
/** Special version of remove needed by Entry set */
    Entry<K,V> removeMapping(Object o) {
        if (!(o instanceof Map.Entry)) {
            return null;
        }
        Entry[] tab = getTable();
        Map.Entry entry = (Map.Entry)o;
        Object k = maskNull(entry.getKey());
        int h = computeHash(k.hashCode());
        int i = indexFor(h, tab.length);
        @SuppressWarnings("unchecked")
        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 e;
            }
            prev = e;
            e = next;
        }

        return null;
    }

    
Removes all of the mappings from this map.
The map will be empty after this call returns.

/**
     * Removes all of the mappings from this map.
     * The map will be empty after this call returns.
     */
    public void clear() {
        // clear out ref queue. We don't need to expunge entries
        // since table is getting cleared.
        while (queue.poll() != null)
            ;

        modCount++;
        Entry[] tab = table;
        for (int i = 0; i < tab.length; ++i)
            tab[i] = 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.
        while (queue.poll() != null)
            ;
    }

    
Returns true if this map maps one or more keys to the
specified value.

Params:
  • value – value whose presence in this map is to be tested
Returns:true if this map maps one or more keys to the
        specified value
/**
     * Returns <tt>true</tt> if this map maps one or more keys to the
     * specified value.
     *
     * @param value value whose presence in this map is to be tested
     * @return <tt>true</tt> if this map maps one or more keys to the
     *         specified value
     */
    public boolean containsValue(Object value) {
	if (value==null)
            return containsNullValue();

	Entry[] tab = getTable();
        for (int i = tab.length ; i-- > 0 ;)
            for (Entry 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

/**
     * Special-case code for containsValue with null argument
     */
    private boolean containsNullValue() {
	Entry[] tab = getTable();
        for (int i = tab.length ; i-- > 0 ;)
            for (Entry 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.

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

        
Creates new entry.

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

        public K getKey() {
            return WeakHashMapSafeReadLock.<K>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;
            Object k1 = getKey();
            Object k2 = e.getKey();
            if (k1 == k2 || (k1 != null && k1.equals(k2))) {
                Object v1 = getValue();
                Object v2 = e.getValue();
                if (v1 == v2 || (v1 != null && v1.equals(v2)))
                    return true;
            }
            return false;
        }

        public int hashCode() {
            Object k = getKey();
            Object v = getValue();
            return  ((k==null ? 0 : k.hashCode()) ^
                     (v==null ? 0 : v.hashCode()));
        }

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

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

        
Strong reference needed to avoid disappearance of key
between hasNext and next

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

        
Strong reference needed to avoid disappearance of key
between nextEntry() and any use of the entry

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

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

        public boolean hasNext() {
            Entry[] 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 
/** 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();

            WeakHashMapSafeReadLock.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 = null;

    private transient volatile Set<K> myKeySet = null ;
    private transient volatile Collection<V> myValues = null ;

    
Returns a 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 remove operation), the results of
the iteration are undefined.  The set supports element removal,
which removes the corresponding mapping from the map, via the
Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations.  It does not support the add or addAll
operations.

/**
     * 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 <tt>remove</tt> operation), the results of
     * the iteration are undefined.  The set supports element removal,
     * which removes the corresponding mapping from the map, via the
     * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
     * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
     * operations.
     */
    public Set<K> keySet() {
        Set<K> ks = myKeySet;
        return (ks != null ? ks : (myKeySet = new KeySet()));
    }

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

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

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

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

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

    
Returns a 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 remove operation),
the results of the iteration are undefined.  The collection
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Collection.remove, removeAll,
retainAll and clear operations.  It does not
support the add or addAll operations.

/**
     * 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 <tt>remove</tt> operation),
     * the results of the iteration are undefined.  The collection
     * supports element removal, which removes the corresponding
     * mapping from the map, via the <tt>Iterator.remove</tt>,
     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
     * support the <tt>add</tt> or <tt>addAll</tt> operations.
     */
    public Collection<V> values() {
        Collection<V> vs = myValues;
        return (vs != null ?  vs : (myValues = new Values()));
    }

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

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

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

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

    
Returns a 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 remove operation, or through the
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 Iterator.remove,
Set.remove, removeAll, retainAll and
clear operations.  It does not support the
add or addAll operations.

/**
     * 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 <tt>remove</tt> operation, or through the
     * <tt>setValue</tt> 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 <tt>Iterator.remove</tt>,
     * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
     * <tt>clear</tt> operations.  It does not support the
     * <tt>add</tt> or <tt>addAll</tt> 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 candidate = getEntry(e.getKey());
            return candidate != null && candidate.equals(e);
        }

        @Override
        public boolean remove(Object o) {
            return removeMapping(o) != null;
        }

        @Override
        public int size() {
            return WeakHashMapSafeReadLock.this.size();
        }

        @Override
        public void clear() {
            WeakHashMapSafeReadLock.this.clear();
        }

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

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

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