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package java.util;
import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
import java.util.concurrent.ThreadLocalRandom;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
/**
* Hash table based implementation of the <tt>Map interface, with
* <em>weak keys.
* An entry in a <tt>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 <tt>Map
* implementations.
*
* <p> Both null values and the null key are supported. This class has
* performance characteristics similar to those of the <tt>HashMap
* class, and has the same efficiency parameters of <em>initial capacity
* and <em>load factor.
*
* <p> Like most collection classes, this class is not synchronized.
* A synchronized <tt>WeakHashMap may be constructed using the
* {@link Collections#synchronizedMap Collections.synchronizedMap}
* method.
*
* <p> This class is intended primarily for use with key objects whose
* <tt>equals methods test for object identity using the
* <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 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 methods are not based upon object identity, such
* as <tt>String instances. With such recreatable key objects,
* however, the automatic removal of <tt>WeakHashMap entries whose
* keys have been discarded may prove to be confusing.
*
* <p> The behavior of the WeakHashMap class depends in part upon
* the actions of the garbage collector, so several familiar (though not
* required) <tt>Map invariants do not hold for this class. Because
* the garbage collector may discard keys at any time, a
* <tt>WeakHashMap may behave as though an unknown thread is silently
* removing entries. In particular, even if you synchronize on a
* <tt>WeakHashMap instance and invoke none of its mutator methods, it
* is possible for the <tt>size method to return smaller values over
* time, for the <tt>isEmpty method to return false and
* then <tt>true, for the containsKey method to return
* <tt>true and later false for a given key, for the
* <tt>get method to return a value for a given key but later return
* <tt>null, for the put method to return
* <tt>null and the remove method to return
* <tt>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.
*
* <p> 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.
*
* <p> Implementation note: The value objects in a
* <tt>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 <tt>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. If the values
* in the map do not rely on the map holding strong references to them, one way
* to deal with this is to wrap values themselves within
* <tt>WeakReferences before
* inserting, as in: <tt>m.put(key, new WeakReference(value)),
* and then unwrapping upon each <tt>get.
*
* <p>The iterators returned by the iterator method of the collections
* returned by all of this class's "collection view methods" are
* <i>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
* <tt>remove 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 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>.
*
* @param <K> the type of keys maintained by this map
* @param <V> the type of mapped values
*
* @author Doug Lea
* @author Josh Bloch
* @author Mark Reinhold
* @since 1.2
* @see java.util.HashMap
* @see java.lang.ref.WeakReference
*/
public class WeakHashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V> {
/**
* 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.
*/
private static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor 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.
*/
Entry<K,V>[] table;
/**
* 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).
*/
private int threshold;
/**
* The load factor for the hash table.
*/
private final float loadFactor;
/**
* Reference queue for cleared WeakEntries
*/
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.
*
* @see ConcurrentModificationException
*/
int modCount;
@SuppressWarnings("unchecked")
private Entry<K,V>[] newTable(int n) {
return (Entry<K,V>[]) new Entry[n];
}
/**
* Constructs a new, empty <tt>WeakHashMap with the given initial
* capacity and the given load factor.
*
* @param initialCapacity The initial capacity of the <tt>WeakHashMap
* @param loadFactor The load factor of the <tt>WeakHashMap
* @throws IllegalArgumentException if the initial capacity is negative,
* or if the load factor is nonpositive.
*/
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 <tt>WeakHashMap with the given initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity The initial capacity of the <tt>WeakHashMap
* @throws IllegalArgumentException if the initial capacity is negative
*/
public WeakHashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs a new, empty <tt>WeakHashMap with the default initial
* capacity (16) and load factor (0.75).
*/
public WeakHashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs a new <tt>WeakHashMap with the same mappings as the
* specified map. The <tt>WeakHashMap 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 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.
*/
private static final Object NULL_KEY = new Object();
/**
* 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.
*/
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.
*/
private static int indexFor(int h, int length) {
return h & (length-1);
}
/**
* Expunges stale entries from the table.
*/
private void expungeStaleEntries() {
for (Object x; (x = queue.poll()) != null; ) {
synchronized (queue) {
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry) x;
int i = indexFor(e.hash, 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;
// Must not null out e.next;
// stale entries may be in use by a HashIterator
e.value = null; // Help GC
size--;
break;
}
prev = p;
p = next;
}
}
}
}
/**
* Returns the table after first expunging stale entries.
*/
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.
*/
public int size() {
if (size == 0)
return 0;
expungeStaleEntries();
return size;
}
/**
* Returns <tt>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.
*/
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 (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 necessarily
* 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)
*/
public V get(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) {
if (e.hash == h && eq(k, e.get()))
return e.value;
e = e.next;
}
return null;
}
/**
* Returns <tt>true 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 if there is a mapping for key;
* <tt>false otherwise
*/
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.
*/
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.
*
* @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, or
* <tt>null if there was no mapping for key.
* (A <tt>null return can also indicate that the map
* previously associated <tt>null with key.)
*/
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);
for (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++;
Entry<K,V> e = tab[i];
tab[i] = new Entry<>(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.
*
* @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<K,V>[] oldTable = getTable();
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
Entry<K,V>[] newTable = newTable(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 */
private void transfer(Entry<K,V>[] src, Entry[] dest) {
for (int j = 0; j < src.length; ++j) {
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.
*
* @param m mappings to be stored in this map.
* @throws NullPointerException if the specified map is null.
*/
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 <tt>k to
* value <tt>v such that (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 if the map contained no mapping for the key. A
* return value of <tt>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 <tt>null.
*
* <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, or
* <tt>null if there was no mapping for key
*/
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())) {
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() {
// clear out ref queue. We don't need to expunge entries
// since table is getting cleared.
while (queue.poll() != null)
;
modCount++;
Arrays.fill(table, 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 <tt>true 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 if this map maps one or more keys to the
* specified value
*/
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
*/
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
