net.sf.ehcache.util.concurrent
Class ConcurrentHashMap<K,V>

java.lang.Object
  net.sf.ehcache.util.concurrent.ConcurrentHashMap<K,V>

Type Parameters:

K - the type of keys maintained by this map

V - the type of mapped values

All Implemented Interfaces:

ConcurrentMap<K,V>, Map<K,V>

Direct Known Subclasses:

CountBasedBackEnd, PooledBasedBackEnd

public class ConcurrentHashMap<K,V>
extends Object
implements ConcurrentMap<K,V>

A hash table supporting full concurrency of retrievals and high expected concurrency for updates. This class is based on the same functional standard 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. (More formally, an update operation for a given key bears a happens-before relation with any (non-null) retrieval for that key reporting the updated value.) 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. Bear in mind that the results of aggregate status methods including size, isEmpty, and containsValue are typically useful only when a map is not undergoing concurrent updates in other threads. Otherwise the results of these methods reflect transient states that may be adequate for monitoring or estimation purposes, but not for program control.

The table is dynamically expanded when there are too many collisions (i.e., keys that have distinct hash codes but fall into the same slot modulo the table size), with the expected average effect of maintaining roughly two bins per mapping (corresponding to a 0.75 load factor threshold for resizing). There may be much variance around this average as mappings are added and removed, but overall, this maintains a commonly accepted time/space tradeoff for hash tables. However, resizing this or any other kind of hash table may be a relatively slow operation. When possible, it is a good idea to provide a size estimate as an optional initialCapacity constructor argument. An additional optional loadFactor constructor argument provides a further means of customizing initial table capacity by specifying the table density to be used in calculating the amount of space to allocate for the given number of elements. Also, for compatibility with previous versions of this class, constructors may optionally specify an expected concurrencyLevel as an additional hint for internal sizing. Note that using many keys with exactly the same hashCode() is a sure way to slow down performance of any hash table.

A Set projection of a ConcurrentHashMap may be created (using newKeySet() or newKeySet(int)), or viewed (using keySet(Object) when only keys are of interest, and the mapped values are (perhaps transiently) not used or all take the same mapping value.

A ConcurrentHashMap can be used as scalable frequency map (a form of histogram or multiset) by using LongAdder values and initializing via #computeIfAbsent. For example, to add a count to a ConcurrentHashMap<String,LongAdder> freqs, you can use freqs.computeIfAbsent(k -> new LongAdder()).increment();

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.

ConcurrentHashMaps support parallel operations using the ForkJoinPool#commonPool. (Tasks that may be used in other contexts are available in class ForkJoinTasks). These operations are designed to be safely, and often sensibly, applied even with maps that are being concurrently updated by other threads; for example, when computing a snapshot summary of the values in a shared registry. There are three kinds of operation, each with four forms, accepting functions with Keys, Values, Entries, and (Key, Value) arguments and/or return values. (The first three forms are also available via the keySet(), values() and entrySet() views). Because the elements of a ConcurrentHashMap are not ordered in any particular way, and may be processed in different orders in different parallel executions, the correctness of supplied functions should not depend on any ordering, or on any other objects or values that may transiently change while computation is in progress; and except for forEach actions, should ideally be side-effect-free.

• forEach: Perform a given action on each element. A variant form applies a given transformation on each element before performing the action.
• search: Return the first available non-null result of applying a given function on each element; skipping further search when a result is found.
• reduce: Accumulate each element. The supplied reduction function cannot rely on ordering (more formally, it should be both associative and commutative). There are five variants:
  • Plain reductions. (There is not a form of this method for (key, value) function arguments since there is no corresponding return type.)
  • Mapped reductions that accumulate the results of a given function applied to each element.
  • Reductions to scalar doubles, longs, and ints, using a given basis value.

The concurrency properties of bulk operations follow from those of ConcurrentHashMap: Any non-null result returned from get(key) and related access methods bears a happens-before relation with the associated insertion or update. The result of any bulk operation reflects the composition of these per-element relations (but is not necessarily atomic with respect to the map as a whole unless it is somehow known to be quiescent). Conversely, because keys and values in the map are never null, null serves as a reliable atomic indicator of the current lack of any result. To maintain this property, null serves as an implicit basis for all non-scalar reduction operations. For the double, long, and int versions, the basis should be one that, when combined with any other value, returns that other value (more formally, it should be the identity element for the reduction). Most common reductions have these properties; for example, computing a sum with basis 0 or a minimum with basis MAX_VALUE.

Search and transformation functions provided as arguments should similarly return null to indicate the lack of any result (in which case it is not used). In the case of mapped reductions, this also enables transformations to serve as filters, returning null (or, in the case of primitive specializations, the identity basis) if the element should not be combined. You can create compound transformations and filterings by composing them yourself under this "null means there is nothing there now" rule before using them in search or reduce operations.

Methods accepting and/or returning Entry arguments maintain key-value associations. They may be useful for example when finding the key for the greatest value. Note that "plain" Entry arguments can be supplied using new AbstractMap.SimpleEntry(k,v).

Bulk operations may complete abruptly, throwing an exception encountered in the application of a supplied function. Bear in mind when handling such exceptions that other concurrently executing functions could also have thrown exceptions, or would have done so if the first exception had not occurred.

Parallel speedups for bulk operations compared to sequential processing are common but not guaranteed. Operations involving brief functions on small maps may execute more slowly than sequential loops if the underlying work to parallelize the computation is more expensive than the computation itself. Similarly, parallelization may not lead to much actual parallelism if all processors are busy performing unrelated tasks.

All arguments to all task methods must be non-null.

jsr166e note: During transition, this class uses nested functional interfaces with different names but the same forms as those expected for JDK8.

This class is a member of the Java Collections Framework.

Since:

1.5

Author:

Doug Lea

Nested Class Summary

static class	ConcurrentHashMap.EntrySetView<K,V> A view of a ConcurrentHashMap as a Set of (key, value) entries.
static class	ConcurrentHashMap.KeySetView<K,V> A view of a ConcurrentHashMap as a Set of keys, in which additions may optionally be enabled by mapping to a common value.
protected static class	ConcurrentHashMap.Node Key-value entry.
protected static interface	ConcurrentHashMap.RemovalCallback
static interface	ConcurrentHashMap.Spliterator<T> A partitionable iterator.
protected static class	ConcurrentHashMap.TreeNode Nodes for use in TreeBins
static class	ConcurrentHashMap.ValuesView<K,V> A view of a ConcurrentHashMap as a Collection of values, in which additions are disabled.

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

Map.Entry<K,V>

Field Summary

protected static ConcurrentHashMap.Node	FAKE_NODE
protected static ConcurrentHashMap.Node	FAKE_TREE_NODE

Constructor Summary

ConcurrentHashMap()
Creates a new, empty map with the default initial table size (16).

ConcurrentHashMap(int initialCapacity)
Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.

ConcurrentHashMap(int initialCapacity, float loadFactor)
Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).

ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).

ConcurrentHashMap(Map<? extends K,? extends V> 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<V>	elements() Returns an enumeration of the values in this table.
Set<Map.Entry<K,V>>	entrySet() Returns a Set view of the mappings contained in this map.
ConcurrentHashMap.Spliterator<Map.Entry<K,V>>	entrySpliterator() Returns a partitionable iterator of the entries in this map.
boolean	equals(Object o) Compares the specified object with this map for equality.
V	get(Object key) Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
List<V>	getRandomValues(int amount)
V	getValueOrDefault(Object key, V defaultValue) Returns the value to which the specified key is mapped, or the given defaultValue if this map contains no mapping for the key.
int	hashCode() Returns the hash code value for this Map, i.e., the sum of, for each key-value pair in the map, key.hashCode() ^ value.hashCode().
protected Object	internalPutIfAbsent(Object k, Object v, int size) Implementation for putIfAbsent
boolean	isEmpty()
Enumeration<K>	keys() Returns an enumeration of the keys in this table.
ConcurrentHashMap.KeySetView<K,V>	keySet() Returns a Set view of the keys contained in this map.
ConcurrentHashMap.KeySetView<K,V>	keySet(V mappedValue) Returns a Set view of the keys in this map, using the given common mapped value for any additions (i.e., Collection.add(E) and Collection.addAll(java.util.Collection)).
ConcurrentHashMap.Spliterator<K>	keySpliterator() Returns a partitionable iterator of the keys in this map.
long	mappingCount() Returns the number of mappings.
static <K> ConcurrentHashMap.KeySetView<K,Boolean>	newKeySet() Creates a new Set backed by a ConcurrentHashMap from the given type to Boolean.TRUE.
static <K> ConcurrentHashMap.KeySetView<K,Boolean>	newKeySet(int initialCapacity) Creates a new Set backed by a ConcurrentHashMap from the given type to Boolean.TRUE.
V	put(K key, V value) Maps the specified key to the specified value in this table.
void	putAll(Map<? extends K,? extends V> m) Copies all of the mappings from the specified map to this one.
V	putIfAbsent(K key, V value)
void	recalculateSize(K k) Deprecated.
V	remove(Object key) Removes the key (and its corresponding value) from this map.
boolean	remove(Object key, Object value)
protected boolean	remove(Object key, Object value, ConcurrentHashMap.RemovalCallback hook)
protected V	removeAndNotify(Object key, ConcurrentHashMap.RemovalCallback hook)
protected V	removeAndNotify(Object key, Object value, ConcurrentHashMap.RemovalCallback hook)
V	replace(K key, V value)
boolean	replace(K key, V oldValue, V newValue)
protected void	setPoolAccessor(PoolAccessor poolAccessor)
int	size()
String	toString() Returns a string representation of this map.
ConcurrentHashMap.ValuesView<K,V>	values() Returns a Collection view of the values contained in this map.
ConcurrentHashMap.Spliterator<V>	valueSpliterator() Returns a partitionable iterator of the values in this map.

Methods inherited from class java.lang.Object

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

Field Detail

FAKE_NODE

protected static final ConcurrentHashMap.Node FAKE_NODE

FAKE_TREE_NODE

protected static final ConcurrentHashMap.Node FAKE_TREE_NODE

Constructor Detail

ConcurrentHashMap

public ConcurrentHashMap()

Creates a new, empty map with the default initial table size (16).

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity)

Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.

Parameters:

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

Throws:

IllegalArgumentException - if the initial capacity of elements is negative

ConcurrentHashMap

public ConcurrentHashMap(Map<? extends K,? extends V> m)

Creates a new map with the same mappings as the given map.

Parameters:

m - the map

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity,
                         float loadFactor)

Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.

loadFactor - the load factor (table density) for establishing the initial table size

Throws:

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

Since:

1.6

ConcurrentHashMap

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

Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.

loadFactor - the load factor (table density) for establishing the initial table size

concurrencyLevel - the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint.

Throws:

IllegalArgumentException - if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive

Method Detail

internalPutIfAbsent

protected final Object internalPutIfAbsent(Object k,
                                           Object v,
                                           int size)

Implementation for putIfAbsent

setPoolAccessor

protected void setPoolAccessor(PoolAccessor poolAccessor)

newKeySet

public static <K> ConcurrentHashMap.KeySetView<K,Boolean> newKeySet()

Creates a new Set backed by a ConcurrentHashMap from the given type to Boolean.TRUE.

Returns:

the new set

newKeySet

public static <K> ConcurrentHashMap.KeySetView<K,Boolean> newKeySet(int initialCapacity)

Creates a new Set backed by a ConcurrentHashMap from the given type to Boolean.TRUE.

Parameters:

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

Returns:

the new set

Throws:

IllegalArgumentException - if the initial capacity of elements is negative

isEmpty

public boolean isEmpty()

Specified by:

isEmpty in interface Map<K,V>

size

public int size()

Specified by:

size in interface Map<K,V>

mappingCount

public long mappingCount()

Returns the number of mappings. This method should be used instead of size() because a ConcurrentHashMap may contain more mappings than can be represented as an int. The value returned is an estimate; the actual count may differ if there are concurrent insertions or removals.

Returns:

the number of mappings

get

public V 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<K,V>

Throws:

NullPointerException - if the specified key is null

recalculateSize

@Deprecated
public void recalculateSize(K k)

Deprecated.

getRandomValues

public List<V> getRandomValues(int amount)

getValueOrDefault

public V getValueOrDefault(Object key,
                           V defaultValue)

Returns the value to which the specified key is mapped, or the given defaultValue if this map contains no mapping for the key.

Parameters:

key - the key

defaultValue - the value to return if this map contains no mapping for the given key

Returns:

the mapping for the key, if present; else the defaultValue

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<K,V>

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 may require a full traversal of the map, and is much slower than method containsKey.

Specified by:

containsValue in interface Map<K,V>

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 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 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.

Specified by:

put in interface Map<K,V>

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 V putIfAbsent(K key,
                     V value)

Specified by:

putIfAbsent in interface ConcurrentMap<K,V>

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<? extends K,? extends V> 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<K,V>

Parameters:

m - mappings to be stored in this map

remove

public V 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<K,V>

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)

Specified by:

remove in interface ConcurrentMap<K,V>

Throws:

NullPointerException - if the specified key is null

remove

protected boolean remove(Object key,
                         Object value,
                         ConcurrentHashMap.RemovalCallback hook)

removeAndNotify

protected V removeAndNotify(Object key,
                            ConcurrentHashMap.RemovalCallback hook)

removeAndNotify

protected V removeAndNotify(Object key,
                            Object value,
                            ConcurrentHashMap.RemovalCallback hook)

replace

public boolean replace(K key,
                       V oldValue,
                       V newValue)

Specified by:

replace in interface ConcurrentMap<K,V>

Throws:

NullPointerException - if any of the arguments are null

replace

public V replace(K key,
                 V value)

Specified by:

replace in interface ConcurrentMap<K,V>

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<K,V>

keySet

public ConcurrentHashMap.KeySetView<K,V> 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.

Specified by:

keySet in interface Map<K,V>

Returns:

the set view

keySet

public ConcurrentHashMap.KeySetView<K,V> keySet(V mappedValue)

Returns a Set view of the keys in this map, using the given common mapped value for any additions (i.e., Collection.add(E) and Collection.addAll(java.util.Collection)). This is of course only appropriate if it is acceptable to use the same value for all additions from this view.

Parameters:

mappedValue - the mapped value to use for any additions.

Returns:

the set view

Throws:

NullPointerException - if the mappedValue is null

values

public ConcurrentHashMap.ValuesView<K,V> 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.

Specified by:

values in interface Map<K,V>

entrySet

public Set<Map.Entry<K,V>> 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<K,V>

keys

public Enumeration<K> 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<V> elements()

Returns an enumeration of the values in this table.

Returns:

an enumeration of the values in this table

See Also:

values()

keySpliterator

public ConcurrentHashMap.Spliterator<K> keySpliterator()

Returns a partitionable iterator of the keys in this map.

Returns:

a partitionable iterator of the keys in this map

valueSpliterator

public ConcurrentHashMap.Spliterator<V> valueSpliterator()

Returns a partitionable iterator of the values in this map.

Returns:

a partitionable iterator of the values in this map

entrySpliterator

public ConcurrentHashMap.Spliterator<Map.Entry<K,V>> entrySpliterator()

Returns a partitionable iterator of the entries in this map.

Returns:

a partitionable iterator of the entries in this map

hashCode

public int hashCode()

Returns the hash code value for this Map, i.e., the sum of, for each key-value pair in the map, key.hashCode() ^ value.hashCode().

Specified by:

hashCode in interface Map<K,V>

Overrides:

hashCode in class Object

Returns:

the hash code value for this map

toString

public String toString()

Returns a string representation of this map. The string representation consists of a list of key-value mappings (in no particular order) enclosed in braces ("{}"). Adjacent mappings are separated by the characters ", " (comma and space). Each key-value mapping is rendered as the key followed by an equals sign ("=") followed by the associated value.

Overrides:

toString in class Object

Returns:

a string representation of this map

equals

public boolean equals(Object o)

Compares the specified object with this map for equality. Returns true if the given object is a map with the same mappings as this map. This operation may return misleading results if either map is concurrently modified during execution of this method.

Specified by:

equals in interface Map<K,V>

Overrides:

equals in class Object

Parameters:

o - object to be compared for equality with this map

Returns:

true if the specified object is equal to this map