org.compass.core.util.backport.java.util.concurrent
Class ConcurrentHashMap

java.lang.Object
  java.util.AbstractMap
      org.compass.core.util.backport.java.util.AbstractMap
          org.compass.core.util.backport.java.util.concurrent.ConcurrentHashMap

All Implemented Interfaces:

Serializable, Map, ConcurrentMap

public class ConcurrentHashMap
extends AbstractMap
implements ConcurrentMap, Serializable

A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates. This class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable. However, even though all operations are thread-safe, retrieval operations do not entail locking, and there is not any support for locking the entire table in a way that prevents all access. This class is fully interoperable with Hashtable in programs that rely on its thread safety but not on its synchronization details.

Retrieval operations (including get) generally do not block, so may overlap with update operations (including put and remove). Retrievals reflect the results of the most recently completed update operations holding upon their onset. For aggregate operations such as putAll and clear, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They do not throw ConcurrentModificationException. However, iterators are designed to be used by only one thread at a time.

The allowed concurrency among update operations is guided by the optional concurrencyLevel constructor argument (default 16), which is used as a hint for internal sizing. The table is internally partitioned to try to permit the indicated number of concurrent updates without contention. Because placement in hash tables is essentially random, the actual concurrency will vary. Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the table. Using a significantly higher value than you need can waste space and time, and a significantly lower value can lead to thread contention. But overestimates and underestimates within an order of magnitude do not usually have much noticeable impact. A value of one is appropriate when it is known that only one thread will modify and all others will only read. Also, resizing this or any other kind of hash table is a relatively slow operation, so, when possible, it is a good idea to provide estimates of expected table sizes in constructors.

This class and its views and iterators implement all of the optional methods of the Map and Iterator interfaces.

Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.

This class is a member of the Java Collections Framework.

Since:

1.5

Author:

Doug Lea

See Also:

Serialized Form

Nested Class Summary

(package private) class	ConcurrentHashMap.EntryIterator
(package private) class	ConcurrentHashMap.EntrySet
(package private) static class	ConcurrentHashMap.HashEntry ConcurrentHashMap list entry.
(package private) class	ConcurrentHashMap.HashIterator
(package private) class	ConcurrentHashMap.KeyIterator
(package private) class	ConcurrentHashMap.KeySet
(package private) static class	ConcurrentHashMap.Segment Segments are specialized versions of hash tables.
(package private) class	ConcurrentHashMap.ValueIterator
(package private) class	ConcurrentHashMap.Values
(package private) class	ConcurrentHashMap.WriteThroughEntry Custom Entry class used by EntryIterator.next(), that relays setValue changes to the underlying map.

Nested classes/interfaces inherited from class org.compass.core.util.backport.java.util.AbstractMap

AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry

Nested classes/interfaces inherited from interface java.util.Map

Map.Entry<K,V>

Field Summary

(package private) static int	DEFAULT_CONCURRENCY_LEVEL The default concurrency level for this table, used when not otherwise specified in a constructor.
(package private) static int	DEFAULT_INITIAL_CAPACITY The default initial capacity for this table, used when not otherwise specified in a constructor.
(package private) static float	DEFAULT_LOAD_FACTOR The default load factor for this table, used when not otherwise specified in a constructor.
(package private) Set	entrySet
(package private) Set	keySet
(package private) static int	MAX_SEGMENTS The maximum number of segments to allow; used to bound constructor arguments.
(package private) static int	MAXIMUM_CAPACITY The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments.
(package private) static int	RETRIES_BEFORE_LOCK Number of unsynchronized retries in size and containsValue methods before resorting to locking.
(package private) int	segmentMask Mask value for indexing into segments.
(package private) ConcurrentHashMap.Segment[]	segments The segments, each of which is a specialized hash table
(package private) int	segmentShift Shift value for indexing within segments.
(package private) Collection	values

Constructor Summary

ConcurrentHashMap()
Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).

ConcurrentHashMap(int initialCapacity)
Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).

ConcurrentHashMap(int initialCapacity, float loadFactor)
Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).

ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
Creates a new, empty map with the specified initial capacity, load factor and concurrency level.

ConcurrentHashMap(Map m)
Creates a new map with the same mappings as the given map.

Method Summary

void	clear() Removes all of the mappings from this map.
boolean	contains(Object value) Legacy method testing if some key maps into the specified value in this table.
boolean	containsKey(Object key) Tests if the specified object is a key in this table.
boolean	containsValue(Object value) Returns true if this map maps one or more keys to the specified value.
Enumeration	elements() Returns an enumeration of the values in this table.
Set	entrySet() Returns a Set view of the mappings contained in this map.
Object	get(Object key) Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
(package private) static int	hash(Object x) Returns a hash code for non-null Object x.
boolean	isEmpty() Returns true if this map contains no key-value mappings.
Enumeration	keys() Returns an enumeration of the keys in this table.
Set	keySet() Returns a Set view of the keys contained in this map.
Object	put(Object key, Object value) Maps the specified key to the specified value in this table.
void	putAll(Map m) Copies all of the mappings from the specified map to this one.
Object	putIfAbsent(Object key, Object value) If the specified key is not already associated with a value, associate it with the given value.
Object	remove(Object key) Removes the key (and its corresponding value) from this map.
boolean	remove(Object key, Object value) Removes the entry for a key only if currently mapped to a given value.
Object	replace(Object key, Object value) Replaces the entry for a key only if currently mapped to some value.
boolean	replace(Object key, Object oldValue, Object newValue) Replaces the entry for a key only if currently mapped to a given value.
(package private) ConcurrentHashMap.Segment	segmentFor(int hash) Returns the segment that should be used for key with given hash
int	size() Returns the number of key-value mappings in this map.
Collection	values() Returns a Collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap

clone, equals, hashCode, toString

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Map

equals, hashCode

Field Detail

DEFAULT_INITIAL_CAPACITY

static final int DEFAULT_INITIAL_CAPACITY

The default initial capacity for this table, used when not otherwise specified in a constructor.

See Also:

Constant Field Values

DEFAULT_LOAD_FACTOR

static final float DEFAULT_LOAD_FACTOR

The default load factor for this table, used when not otherwise specified in a constructor.

See Also:

Constant Field Values

DEFAULT_CONCURRENCY_LEVEL

static final int DEFAULT_CONCURRENCY_LEVEL

The default concurrency level for this table, used when not otherwise specified in a constructor.

See Also:

Constant Field Values

MAXIMUM_CAPACITY

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 indexable using ints.

See Also:

Constant Field Values

MAX_SEGMENTS

static final int MAX_SEGMENTS

The maximum number of segments to allow; used to bound constructor arguments.

See Also:

Constant Field Values

RETRIES_BEFORE_LOCK

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.

See Also:

Constant Field Values

segmentMask

final int segmentMask

Mask value for indexing into segments. The upper bits of a key's hash code are used to choose the segment.

segmentShift

final int segmentShift

Shift value for indexing within segments.

segments

final ConcurrentHashMap.Segment[] segments

The segments, each of which is a specialized hash table

keySet

transient Set keySet

entrySet

transient Set entrySet

values

transient Collection values

Constructor Detail

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity,
                         float loadFactor,
                         int concurrencyLevel)

Creates a new, empty map with the specified initial capacity, load factor and concurrency level.

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements.

loadFactor - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.

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.

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity,
                         float loadFactor)

Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).

Parameters:

initialCapacity - The implementation performs internal sizing to accommodate this many elements.

loadFactor - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.

Throws:

IllegalArgumentException - if the initial capacity of elements is negative or the load factor is nonpositive

Since:

1.6

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity)

Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements.

Throws:

IllegalArgumentException - if the initial capacity of elements is negative.

ConcurrentHashMap

public ConcurrentHashMap()

Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).

ConcurrentHashMap

public ConcurrentHashMap(Map m)

Creates a new map with the same mappings as the given map. The map is created with a capacity of 1.5 times the number of mappings in the given map or 16 (whichever is greater), and a default load factor (0.75) and concurrencyLevel (16).

Parameters:

m - the map

Method Detail

hash

static int hash(Object x)

Returns a hash code for non-null Object x. Uses the same hash code spreader as most other java.util hash tables.

Parameters:

x - the object serving as a key

Returns:

the hash code

segmentFor

final ConcurrentHashMap.Segment segmentFor(int hash)

Returns the segment that should be used for key with given hash

Parameters:

hash - the hash code for the key

Returns:

the segment

isEmpty

public boolean isEmpty()

Returns true if this map contains no key-value mappings.

Specified by:

isEmpty in interface Map

Overrides:

isEmpty in class AbstractMap

Returns:

true if this map contains no key-value mappings

size

public int size()

Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Specified by:

size in interface Map

Overrides:

size in class AbstractMap

Returns:

the number of key-value mappings in this map

get

public Object get(Object key)

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.equals(k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

Specified by:

get in interface Map

Overrides:

get in class AbstractMap

Throws:

NullPointerException - if the specified key is null

containsKey

public boolean containsKey(Object key)

Tests if the specified object is a key in this table.

Specified by:

containsKey in interface Map

Overrides:

containsKey in class AbstractMap

Parameters:

key - possible key

Returns:

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 specified key is null

containsValue

public boolean containsValue(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.

Specified by:

containsValue in interface Map

Overrides:

containsValue in class AbstractMap

Parameters:

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

Throws:

NullPointerException - if the specified value is null

contains

public boolean contains(Object value)

Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality to containsValue(java.lang.Object), and exists solely to ensure full compatibility with class Hashtable, which supported this method prior to introduction of the Java Collections framework.

Parameters:

value - a value to search for

Returns:

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 specified value is null

put

public Object put(Object key,
                  Object 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.

Specified by:

put in interface Map

Overrides:

put in class AbstractMap

Parameters:

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

Throws:

NullPointerException - if the specified key or value is null

putIfAbsent

public Object putIfAbsent(Object key,
                          Object 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.

Specified by:

putIfAbsent in interface ConcurrentMap

Parameters:

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 the specified key, or null if there was no mapping for the key

Throws:

NullPointerException - if the specified key or value is null

putAll

public void putAll(Map m)

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.

Specified by:

putAll in interface Map

Overrides:

putAll in class AbstractMap

Parameters:

m - mappings to be stored in this map

remove

public Object remove(Object key)

Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.

Specified by:

remove in interface Map

Overrides:

remove in class AbstractMap

Parameters:

key - the key that needs to be removed

Returns:

the previous value associated with key, or null if there was no mapping for key

Throws:

NullPointerException - if the specified key is null

remove

public boolean remove(Object key,
                      Object value)

Removes the entry for a key only if currently mapped to a given value. This is equivalent to

   if (map.containsKey(key) && map.get(key).equals(value)) {
       map.remove(key);
       return true;
   } else return false;

except that the action is performed atomically.

Specified by:

remove in interface ConcurrentMap

Parameters:

key - key with which the specified value is associated

value - value expected to be associated with the specified key

Returns:

true if the value was removed

Throws:

NullPointerException - if the specified key is null

replace

public boolean replace(Object key,
                       Object oldValue,
                       Object newValue)

Replaces the entry for a key only if currently mapped to a given value. This is equivalent to

   if (map.containsKey(key) && map.get(key).equals(oldValue)) {
       map.put(key, newValue);
       return true;
   } else return false;

except that the action is performed atomically.

Specified by:

replace in interface ConcurrentMap

Parameters:

key - key with which the specified value is associated

oldValue - value expected to be associated with the specified key

newValue - value to be associated with the specified key

Returns:

true if the value was replaced

Throws:

NullPointerException - if any of the arguments are null

replace

public Object replace(Object key,
                      Object value)

Replaces the entry for a key only if currently mapped to some value. This is equivalent to

   if (map.containsKey(key)) {
       return map.put(key, value);
   } else return null;

except that the action is performed atomically.

Specified by:

replace in interface ConcurrentMap

Parameters:

key - key with which the specified value is associated

value - value to be associated with the specified key

Returns:

the previous value associated with the specified key, or null if there was no mapping for the key

Throws:

NullPointerException - if the specified key or value is null

clear

public void clear()

Removes all of the mappings from this map.

Specified by:

clear in interface Map

Overrides:

clear in class AbstractMap

keySet

public 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 iterator is a "weakly consistent" iterator that will never throw 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.

Specified by:

keySet in interface Map

Overrides:

keySet in class AbstractMap

values

public 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 iterator is a "weakly consistent" iterator that will never throw 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.

Specified by:

values in interface Map

Overrides:

values in class AbstractMap

entrySet

public Set entrySet()

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. 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.

The view's iterator is a "weakly consistent" iterator that will never throw 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.

Specified by:

entrySet in interface Map

Specified by:

entrySet in class AbstractMap

keys

public Enumeration keys()

Returns an enumeration of the keys in this table.

Returns:

an enumeration of the keys in this table

See Also:

keySet

elements

public Enumeration elements()

Returns an enumeration of the values in this table.

Returns:

an enumeration of the values in this table

See Also:

values