File	Doc	Category	Size	Date	Package
ConcurrentHashMap.java	API Doc	Java SE 5 API	51723	Fri Aug 26 14:57:26 BST 2005	java.util.concurrent

ConcurrentHashMap

• java.lang.Object
  • java.util.AbstractMap

Fields Summary
private static final long	serialVersionUID
static int	DEFAULT_INITIAL_CAPACITY The default initial number of table slots for this table. Used when not otherwise specified in constructor.
static final int	MAXIMUM_CAPACITY 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 to ensure that entries are indexible using ints.
static final float	DEFAULT_LOAD_FACTOR The default load factor for this table. Used when not otherwise specified in constructor.
static final int	DEFAULT_SEGMENTS The default number of concurrency control segments.
static final int	MAX_SEGMENTS The maximum number of segments to allow; used to bound constructor arguments.
static final int	RETRIES_BEFORE_LOCK Number of unsynchronized retries in size and containsValue methods before resorting to locking. This is used to avoid unbounded retries if tables undergo continuous modification which would make it impossible to obtain an accurate result.
final int	segmentMask Mask value for indexing into segments. The upper bits of a key's hash code are used to choose the segment.
final int	segmentShift Shift value for indexing within segments.
final Segment[]	segments The segments, each of which is a specialized hash table
transient Set	keySet
transient Set	entrySet
transient Collection	values

Constructors Summary
public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) Creates a new, empty map with the specified initial capacity, load factor, and concurrency level. param initialCapacity the initial capacity. The implementation performs internal sizing to accommodate this many elements. param loadFactor the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold. param concurrencyLevel the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads. throws IllegalArgumentException if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive. if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) throw new IllegalArgumentException(); if (concurrencyLevel > MAX_SEGMENTS) concurrencyLevel = MAX_SEGMENTS; // Find power-of-two sizes best matching arguments int sshift = 0; int ssize = 1; while (ssize < concurrencyLevel) { ++sshift; ssize <<= 1; } segmentShift = 32 - sshift; segmentMask = ssize - 1; this.segments = new Segment[ssize]; if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; int c = initialCapacity / ssize; if (c * ssize < initialCapacity) ++c; int cap = 1; while (cap < c) cap <<= 1; for (int i = 0; i < this.segments.length; ++i) this.segments[i] = new Segment<K,V>(cap, loadFactor);
public ConcurrentHashMap(int initialCapacity) Creates a new, empty map with the specified initial capacity, and with default load factor and concurrencyLevel. param initialCapacity the initial capacity. The implementation performs internal sizing to accommodate this many elements. throws IllegalArgumentException if the initial capacity of elements is negative. this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_SEGMENTS);
public ConcurrentHashMap() Creates a new, empty map with a default initial capacity, load factor, and concurrencyLevel. this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_SEGMENTS);
public ConcurrentHashMap(Map t) Creates a new map with the same mappings as the given map. The map is created with a capacity of twice the number of mappings in the given map or 11 (whichever is greater), and a default load factor and concurrencyLevel. param t the map this(Math.max((int) (t.size() / DEFAULT_LOAD_FACTOR) + 1, 11), DEFAULT_LOAD_FACTOR, DEFAULT_SEGMENTS); putAll(t);

Methods Summary
public void	clear() Removes all mappings from this map. for (int i = 0; i < segments.length; ++i) segments[i].clear();
public boolean	contains(java.lang.Object value) Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality to {@link #containsValue}, and exists solely to ensure full compatibility with class {@link java.util.Hashtable}, which supported this method prior to introduction of the Java Collections framework. param value a value to search for. return true if and only if some key maps to the value argument in this table as determined by the equals method; false otherwise. throws NullPointerException if the value is null. return containsValue(value);
public boolean	containsKey(java.lang.Object key) Tests if the specified object is a key in this table. param key possible key. return true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise. throws NullPointerException if the key is null. int hash = hash(key); // throws NullPointerException if key null return segmentFor(hash).containsKey(key, hash);
public boolean	containsValue(java.lang.Object value) Returns true if this map maps one or more keys to the specified value. Note: This method requires a full internal traversal of the hash table, and so is much slower than method containsKey. 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. throws NullPointerException if the value is null. if (value == null) throw new NullPointerException(); // See explanation of modCount use above final Segment[] segments = this.segments; int[] mc = new int[segments.length]; // Try a few times without locking for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) { int sum = 0; int mcsum = 0; for (int i = 0; i < segments.length; ++i) { int c = segments[i].count; mcsum += mc[i] = segments[i].modCount; if (segments[i].containsValue(value)) return true; } boolean cleanSweep = true; if (mcsum != 0) { for (int i = 0; i < segments.length; ++i) { int c = segments[i].count; if (mc[i] != segments[i].modCount) { cleanSweep = false; break; } } } if (cleanSweep) return false; } // Resort to locking all segments for (int i = 0; i < segments.length; ++i) segments[i].lock(); boolean found = false; try { for (int i = 0; i < segments.length; ++i) { if (segments[i].containsValue(value)) { found = true; break; } } } finally { for (int i = 0; i < segments.length; ++i) segments[i].unlock(); } return found;
public java.util.Enumeration	elements() Returns an enumeration of the values in this table. return an enumeration of the values in this table. see #values return new ValueIterator();
public java.util.Set	entrySet() Returns a collection view of the mappings contained in this map. Each element in the returned collection is a Map.Entry. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. 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. The view's returned iterator is a "weakly consistent" iterator that will never throw {@link java.util.ConcurrentModificationException}, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction. return a collection view of the mappings contained in this map. Set<Map.Entry<K,V>> es = entrySet; return (es != null) ? es : (entrySet = (Set<Map.Entry<K,V>>) (Set) new EntrySet());
public V	get(java.lang.Object key) Returns the value to which the specified key is mapped in this table. param key a key in the table. return the value to which the key is mapped in this table; null if the key is not mapped to any value in this table. throws NullPointerException if the key is null. int hash = hash(key); // throws NullPointerException if key null return segmentFor(hash).get(key, hash);
static int	hash(java.lang.Object x) Returns a hash code for non-null Object x. Uses the same hash code spreader as most other java.util hash tables. param x the object serving as a key return the hash code /* ---------------- Small Utilities -------------- */ int h = x.hashCode(); h += ~(h << 9); h ^= (h >>> 14); h += (h << 4); h ^= (h >>> 10); return h;
public boolean	isEmpty() final Segment[] segments = this.segments; /* * We keep track of per-segment modCounts to avoid ABA * problems in which an element in one segment was added and * in another removed during traversal, in which case the * table was never actually empty at any point. Note the * similar use of modCounts in the size() and containsValue() * methods, which are the only other methods also susceptible * to ABA problems. */ int[] mc = new int[segments.length]; int mcsum = 0; for (int i = 0; i < segments.length; ++i) { if (segments[i].count != 0) return false; else mcsum += mc[i] = segments[i].modCount; } // If mcsum happens to be zero, then we know we got a snapshot // before any modifications at all were made. This is // probably common enough to bother tracking. if (mcsum != 0) { for (int i = 0; i < segments.length; ++i) { if (segments[i].count != 0 || mc[i] != segments[i].modCount) return false; } } return true;
public java.util.Set	keySet() 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. The set supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations. The view's returned iterator is a "weakly consistent" iterator that will never throw {@link java.util.ConcurrentModificationException}, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction. return a set view of the keys contained in this map. Set<K> ks = keySet; return (ks != null) ? ks : (keySet = new KeySet());
public java.util.Enumeration	keys() Returns an enumeration of the keys in this table. return an enumeration of the keys in this table. see #keySet return new KeyIterator();
public V	put(K key, V value) Maps the specified key to the specified value in this table. Neither the key nor the value can be null. The value can be retrieved by calling the get method with a key that is equal to the original key. param key the table key. param value the value. return the previous value of the specified key in this table, or null if it did not have one. throws NullPointerException if the key or value is null. if (value == null) throw new NullPointerException(); int hash = hash(key); return segmentFor(hash).put(key, hash, value, false);
public void	putAll(java.util.Map t) Copies all of the mappings from the specified map to this one. These mappings replace any mappings that this map had for any of the keys currently in the specified Map. param t Mappings to be stored in this map. for (Iterator<? extends Map.Entry<? extends K, ? extends V>> it = (Iterator<? extends Map.Entry<? extends K, ? extends V>>) t.entrySet().iterator(); it.hasNext(); ) { Entry<? extends K, ? extends V> e = it.next(); put(e.getKey(), e.getValue()); }
public V	putIfAbsent(K key, V value) If the specified key is not already associated with a value, associate it with the given value. This is equivalent to if (!map.containsKey(key)) return map.put(key, value); else return map.get(key); Except that the action is performed atomically. param key key with which the specified value is to be associated. param value value to be associated with the specified key. return previous value associated with specified key, or null if there was no mapping for key. throws NullPointerException if the specified key or value is null. if (value == null) throw new NullPointerException(); int hash = hash(key); return segmentFor(hash).put(key, hash, value, true);
private void	readObject(java.io.ObjectInputStream s) Reconstitute the ConcurrentHashMap instance from a stream (i.e., deserialize it). param s the stream s.defaultReadObject(); // Initialize each segment to be minimally sized, and let grow. for (int i = 0; i < segments.length; ++i) { segments[i].setTable(new HashEntry[1]); } // Read the keys and values, and put the mappings in the table for (;;) { K key = (K) s.readObject(); V value = (V) s.readObject(); if (key == null) break; put(key, value); }
public V	remove(java.lang.Object key) Removes the key (and its corresponding value) from this table. This method does nothing if the key is not in the table. param key the key that needs to be removed. return the value to which the key had been mapped in this table, or null if the key did not have a mapping. throws NullPointerException if the key is null. int hash = hash(key); return segmentFor(hash).remove(key, hash, null);
public boolean	remove(java.lang.Object key, java.lang.Object value) Remove entry for key only if currently mapped to given value. Acts as if (map.get(key).equals(value)) { map.remove(key); return true; } else return false; except that the action is performed atomically. param key key with which the specified value is associated. param value value associated with the specified key. return true if the value was removed throws NullPointerException if the specified key is null. int hash = hash(key); return segmentFor(hash).remove(key, hash, value) != null;
public boolean	replace(K key, V oldValue, V newValue) Replace entry for key only if currently mapped to given value. Acts as if (map.get(key).equals(oldValue)) { map.put(key, newValue); return true; } else return false; except that the action is performed atomically. param key key with which the specified value is associated. param oldValue value expected to be associated with the specified key. param newValue value to be associated with the specified key. return true if the value was replaced throws NullPointerException if the specified key or values are null. if (oldValue == null || newValue == null) throw new NullPointerException(); int hash = hash(key); return segmentFor(hash).replace(key, hash, oldValue, newValue);
public V	replace(K key, V value) Replace entry for key only if currently mapped to some value. Acts as if ((map.containsKey(key)) { return map.put(key, value); } else return null; except that the action is performed atomically. param key key with which the specified value is associated. param value value to be associated with the specified key. return previous value associated with specified key, or null if there was no mapping for key. throws NullPointerException if the specified key or value is null. if (value == null) throw new NullPointerException(); int hash = hash(key); return segmentFor(hash).replace(key, hash, value);
final java.util.concurrent.ConcurrentHashMap$Segment	segmentFor(int hash) Returns the segment that should be used for key with given hash param hash the hash code for the key return the segment return (Segment<K,V>) segments[(hash >>> segmentShift) & segmentMask];
public int	size() final Segment[] segments = this.segments; long sum = 0; long check = 0; int[] mc = new int[segments.length]; // Try a few times to get accurate count. On failure due to // continuous async changes in table, resort to locking. for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) { check = 0; sum = 0; int mcsum = 0; for (int i = 0; i < segments.length; ++i) { sum += segments[i].count; mcsum += mc[i] = segments[i].modCount; } if (mcsum != 0) { for (int i = 0; i < segments.length; ++i) { check += segments[i].count; if (mc[i] != segments[i].modCount) { check = -1; // force retry break; } } } if (check == sum) break; } if (check != sum) { // Resort to locking all segments sum = 0; for (int i = 0; i < segments.length; ++i) segments[i].lock(); for (int i = 0; i < segments.length; ++i) sum += segments[i].count; for (int i = 0; i < segments.length; ++i) segments[i].unlock(); } if (sum > Integer.MAX_VALUE) return Integer.MAX_VALUE; else return (int)sum;
public java.util.Collection	values() 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. The collection supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations. The view's returned iterator is a "weakly consistent" iterator that will never throw {@link java.util.ConcurrentModificationException}, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction. return a collection view of the values contained in this map. Collection<V> vs = values; return (vs != null) ? vs : (values = new Values());
private void	writeObject(java.io.ObjectOutputStream s) Save the state of the ConcurrentHashMap instance to a stream (i.e., serialize it). param s the stream serialData the key (Object) and value (Object) for each key-value mapping, followed by a null pair. The key-value mappings are emitted in no particular order. s.defaultWriteObject(); for (int k = 0; k < segments.length; ++k) { Segment<K,V> seg = (Segment<K,V>)segments[k]; seg.lock(); try { HashEntry[] tab = seg.table; for (int i = 0; i < tab.length; ++i) { for (HashEntry<K,V> e = (HashEntry<K,V>)tab[i]; e != null; e = e.next) { s.writeObject(e.key); s.writeObject(e.value); } } } finally { seg.unlock(); } } s.writeObject(null); s.writeObject(null);