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java.lang.Object net.sf.ehcache.util.concurrent.ConcurrentHashMap<K,V>
K
- the type of keys maintained by this mapV
- the type of mapped valuespublic class ConcurrentHashMap<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.
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.
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 extends E>) ). |
|
ConcurrentHashMap.Spliterator<K> |
keySpliterator()
Returns a partitionable iterator of the keys in this map. |
|
long |
mappingCount()
Returns the number of mappings. |
|
static
|
newKeySet()
Creates a new Set backed by a ConcurrentHashMap
from the given type to Boolean.TRUE . |
|
static
|
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 |
---|
protected static final ConcurrentHashMap.Node FAKE_NODE
protected static final ConcurrentHashMap.Node FAKE_TREE_NODE
Constructor Detail |
---|
public ConcurrentHashMap()
public ConcurrentHashMap(int initialCapacity)
initialCapacity
- The implementation performs internal
sizing to accommodate this many elements.
IllegalArgumentException
- if the initial capacity of
elements is negativepublic ConcurrentHashMap(Map<? extends K,? extends V> m)
m
- the mappublic ConcurrentHashMap(int initialCapacity, float loadFactor)
initialCapacity
) and
initial table density (loadFactor
).
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
IllegalArgumentException
- if the initial capacity of
elements is negative or the load factor is nonpositivepublic ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
initialCapacity
), table
density (loadFactor
), and number of concurrently
updating threads (concurrencyLevel
).
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 sizeconcurrencyLevel
- the estimated number of concurrently
updating threads. The implementation may use this value as
a sizing hint.
IllegalArgumentException
- if the initial capacity is
negative or the load factor or concurrencyLevel are
nonpositiveMethod Detail |
---|
protected final Object internalPutIfAbsent(Object k, Object v, int size)
protected void setPoolAccessor(PoolAccessor poolAccessor)
public static <K> ConcurrentHashMap.KeySetView<K,Boolean> newKeySet()
Set
backed by a ConcurrentHashMap
from the given type to Boolean.TRUE
.
public static <K> ConcurrentHashMap.KeySetView<K,Boolean> newKeySet(int initialCapacity)
Set
backed by a ConcurrentHashMap
from the given type to Boolean.TRUE
.
initialCapacity
- The implementation performs internal
sizing to accommodate this many elements.
IllegalArgumentException
- if the initial capacity of
elements is negativepublic boolean isEmpty()
isEmpty
in interface Map<K,V>
public int size()
size
in interface Map<K,V>
public long mappingCount()
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.
public V get(Object key)
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.)
get
in interface Map<K,V>
NullPointerException
- if the specified key is null@Deprecated public void recalculateSize(K k)
public List<V> getRandomValues(int amount)
public V getValueOrDefault(Object key, V defaultValue)
key
- the keydefaultValue
- the value to return if this map contains
no mapping for the given key
NullPointerException
- if the specified key is nullpublic boolean containsKey(Object key)
containsKey
in interface Map<K,V>
key
- possible key
true
if and only if the specified object
is a key in this table, as determined by the
equals
method; false
otherwise
NullPointerException
- if the specified key is nullpublic boolean containsValue(Object value)
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
.
containsValue
in interface Map<K,V>
value
- value whose presence in this map is to be tested
true
if this map maps one or more keys to the
specified value
NullPointerException
- if the specified value is nullpublic boolean contains(Object value)
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.
value
- a value to search for
true
if and only if some key maps to the
value
argument in this table as
determined by the equals
method;
false
otherwise
NullPointerException
- if the specified value is nullpublic V put(K key, V value)
The value can be retrieved by calling the get
method
with a key that is equal to the original key.
put
in interface Map<K,V>
key
- key with which the specified value is to be associatedvalue
- value to be associated with the specified key
key
, or
null
if there was no mapping for key
NullPointerException
- if the specified key or value is nullpublic V putIfAbsent(K key, V value)
putIfAbsent
in interface ConcurrentMap<K,V>
null
if there was no mapping for the key
NullPointerException
- if the specified key or value is nullpublic void putAll(Map<? extends K,? extends V> m)
putAll
in interface Map<K,V>
m
- mappings to be stored in this mappublic V remove(Object key)
remove
in interface Map<K,V>
key
- the key that needs to be removed
key
, or
null
if there was no mapping for key
NullPointerException
- if the specified key is nullpublic boolean remove(Object key, Object value)
remove
in interface ConcurrentMap<K,V>
NullPointerException
- if the specified key is nullprotected 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)
public boolean replace(K key, V oldValue, V newValue)
replace
in interface ConcurrentMap<K,V>
NullPointerException
- if any of the arguments are nullpublic V replace(K key, V value)
replace
in interface ConcurrentMap<K,V>
null
if there was no mapping for the key
NullPointerException
- if the specified key or value is nullpublic void clear()
clear
in interface Map<K,V>
public ConcurrentHashMap.KeySetView<K,V> keySet()
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.
keySet
in interface Map<K,V>
public ConcurrentHashMap.KeySetView<K,V> keySet(V mappedValue)
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 extends E>)
). This is of
course only appropriate if it is acceptable to use the same
value for all additions from this view.
mappedValue
- the mapped value to use for any
additions.
NullPointerException
- if the mappedValue is nullpublic ConcurrentHashMap.ValuesView<K,V> values()
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.
values
in interface Map<K,V>
public Set<Map.Entry<K,V>> entrySet()
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.
entrySet
in interface Map<K,V>
public Enumeration<K> keys()
keySet()
public Enumeration<V> elements()
values()
public ConcurrentHashMap.Spliterator<K> keySpliterator()
public ConcurrentHashMap.Spliterator<V> valueSpliterator()
public ConcurrentHashMap.Spliterator<Map.Entry<K,V>> entrySpliterator()
public int hashCode()
Map
, i.e.,
the sum of, for each key-value pair in the map,
key.hashCode() ^ value.hashCode()
.
hashCode
in interface Map<K,V>
hashCode
in class Object
public String toString()
{}
"). 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.
toString
in class Object
public boolean equals(Object o)
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.
equals
in interface Map<K,V>
equals
in class Object
o
- object to be compared for equality with this map
true
if the specified object is equal to this map
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