All Downloads are FREE. Search and download functionalities are using the official Maven repository.

com.oracle.coherence.common.collections.WeakIdentityHashMap Maven / Gradle / Ivy

There is a newer version: 24.03
Show newest version
/*
 * Copyright (c) 2000, 2020, Oracle and/or its affiliates.
 *
 * Licensed under the Universal Permissive License v 1.0 as shown at
 * http://oss.oracle.com/licenses/upl.
 */
package com.oracle.coherence.common.collections;

// Note: the following is a copy-and-paste of 1.7 java.util.WeakHashMap, modified only to change it to be identity based.
//       this implementation has also been stripped of any Java internal class usage

import java.util.*;
import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;


/**
 * Equivalent of {@link java.util.WeakHashMap} but where key equality is based solely on key identity.
 */
public class WeakIdentityHashMap
    extends AbstractMap
    implements Map {

    /**
     * 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[] 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 queue = new ReferenceQueue<>();

    /**
     * The number of times this WeakIdentityHashMap 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;

    /**
     * The default threshold of map capacity above which alternative hashing is
     * used for String keys. Alternative hashing reduces the incidence of
     * collisions due to weak hash code calculation for String keys.
     * 

* This value may be overridden by defining the system property * {@code jdk.map.althashing.threshold}. A property value of {@code 1} * forces alternative hashing to be used at all times whereas * {@code -1} value ensures that alternative hashing is never used. */ static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE; /** * holds values which can't be initialized until after VM is booted. */ private static class Holder { /** * Table capacity above which to switch to use alternative hashing. */ static final int ALTERNATIVE_HASHING_THRESHOLD = ALTERNATIVE_HASHING_THRESHOLD_DEFAULT; } /** * If {@code true} then perform alternate hashing to reduce the incidence of * collisions due to weak hash code calculation. */ transient boolean useAltHashing; @SuppressWarnings("unchecked") private Entry[] newTable(int n) { return (Entry[]) new Entry[n]; } /** * Constructs a new, empty WeakIdentityHashMap with the given initial * capacity and the given load factor. * * @param initialCapacity The initial capacity of the WeakIdentityHashMap * @param loadFactor The load factor of the WeakIdentityHashMap * @throws IllegalArgumentException if the initial capacity is negative, * or if the load factor is nonpositive. */ public WeakIdentityHashMap(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); useAltHashing = capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD; } /** * Constructs a new, empty WeakIdentityHashMap with the given initial * capacity and the default load factor (0.75). * * @param initialCapacity The initial capacity of the WeakIdentityHashMap * @throws IllegalArgumentException if the initial capacity is negative */ public WeakIdentityHashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR); } /** * Constructs a new, empty WeakIdentityHashMap with the default initial * capacity (16) and load factor (0.75). */ public WeakIdentityHashMap() { this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR); } /** * Constructs a new WeakIdentityHashMap with the same mappings as the * specified map. The WeakIdentityHashMap 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 WeakIdentityHashMap(Map 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; } /** * 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. */ static int hash(Object k) { int h = System.identityHashCode(k); // 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 e = (Entry) x; int i = indexFor(e.hash, table.length); Entry prev = table[i]; Entry p = prev; while (p != null) { Entry 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[] 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 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. * *

More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code (key==k}, * then this method returns {@code v}; otherwise * it returns {@code null}. (There can be at most one such mapping.) * *

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[] tab = getTable(); int index = indexFor(h, tab.length); Entry 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. * * @param key The key whose presence in this map is to be tested * @return true if there is a mapping for key; * 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 getEntry(Object key) { Object k = maskNull(key); int h = hash(k); Entry[] tab = getTable(); int index = indexFor(h, tab.length); Entry 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 key, or * null if there was no mapping for key. * (A null return can also indicate that the map * previously associated null with key.) */ public V put(K key, V value) { Object k = maskNull(key); int h = hash(k); Entry[] tab = getTable(); int i = indexFor(h, tab.length); for (Entry 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 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[] oldTable = getTable(); int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { threshold = Integer.MAX_VALUE; return; } Entry[] newTable = newTable(newCapacity); boolean oldAltHashing = useAltHashing; useAltHashing |= newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD; boolean rehash = oldAltHashing ^ useAltHashing; transfer(oldTable, newTable, rehash); 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, false); table = oldTable; } } /** Transfers all entries from src to dest tables */ private void transfer(Entry[] src, Entry[] dest, boolean rehash) { for (int j = 0; j < src.length; ++j) { Entry e = src[j]; src[j] = null; while (e != null) { Entry next = e.next; Object key = e.get(); if (key == null) { e.next = null; // Help GC e.value = null; // " " size--; } else { if (rehash) { e.hash = hash(key); } 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 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 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==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. * * @param key key whose mapping is to be removed from the map * @return the previous value associated with key, or * null if there was no mapping for key */ public V remove(Object key) { Object k = maskNull(key); int h = hash(k); Entry[] tab = getTable(); int i = indexFor(h, tab.length); Entry prev = tab[i]; Entry e = prev; while (e != null) { Entry 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[] tab = getTable(); Map.Entry entry = (Map.Entry)o; Object k = maskNull(entry.getKey()); int h = hash(k); int i = indexFor(h, tab.length); Entry prev = tab[i]; Entry e = prev; while (e != null) { Entry 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++; java.util.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 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 true 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 */ 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. */ private static class Entry extends WeakReference implements Map.Entry { V value; int hash; Entry next; /** * Creates new entry. */ Entry(Object key, V value, ReferenceQueue queue, int hash, Entry next) { super(key, queue); this.value = value; this.hash = hash; this.next = next; } @SuppressWarnings("unchecked") public K getKey() { return (K) WeakIdentityHashMap.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) { 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 ((k==null ? 0 : hash(k)) ^ (v==null ? 0 : v.hashCode())); } public String toString() { return getKey() + "=" + getValue(); } } private abstract class HashIterator implements Iterator { private int index; private Entry entry = null; private Entry lastReturned = null; private int expectedModCount = modCount; /** * Strong reference needed to avoid disappearance of key * between hasNext and next */ private Object nextKey = null; /** * Strong reference needed to avoid disappearance of key * between nextEntry() and any use of the entry */ private Object currentKey = null; HashIterator() { index = isEmpty() ? 0 : table.length; } public boolean hasNext() { Entry[] t = table; while (nextKey == null) { Entry 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 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(); WeakIdentityHashMap.this.remove(currentKey); expectedModCount = modCount; lastReturned = null; currentKey = null; } } private class ValueIterator extends HashIterator { public V next() { return nextEntry().value; } } private class KeyIterator extends HashIterator { public K next() { return nextEntry().getKey(); } } private class EntryIterator extends HashIterator> { public Map.Entry next() { return nextEntry(); } } // Views private transient Set> entrySet = null; /** * 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 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. */ public Set keySet() { Set ks = keySet; return (ks != null ? ks : (keySet = new KeySet())); } protected transient Set keySet; private class KeySet extends AbstractSet { public Iterator iterator() { return new KeyIterator(); } public int size() { return WeakIdentityHashMap.this.size(); } public boolean contains(Object o) { return containsKey(o); } public boolean remove(Object o) { if (containsKey(o)) { WeakIdentityHashMap.this.remove(o); return true; } else return false; } public void clear() { WeakIdentityHashMap.this.clear(); } } /** * 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 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. */ public Collection values() { Collection vs = values; return (vs != null) ? vs : (values = new Values()); } protected transient Collection values; private class Values extends AbstractCollection { public Iterator iterator() { return new ValueIterator(); } public int size() { return WeakIdentityHashMap.this.size(); } public boolean contains(Object o) { return containsValue(o); } public void clear() { WeakIdentityHashMap.this.clear(); } } /** * 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 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. */ public Set> entrySet() { Set> es = entrySet; return es != null ? es : (entrySet = new EntrySet()); } private class EntrySet extends AbstractSet> { public Iterator> 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); } public boolean remove(Object o) { return removeMapping(o); } public int size() { return WeakIdentityHashMap.this.size(); } public void clear() { WeakIdentityHashMap.this.clear(); } private List> deepCopy() { List> list = new ArrayList<>(size()); for (Map.Entry e : this) list.add(new AbstractMap.SimpleEntry<>(e)); return list; } public Object[] toArray() { return deepCopy().toArray(); } public T[] toArray(T[] a) { return deepCopy().toArray(a); } } }