Class BoundedConcurrentHashMap<K,V>
- Type Parameters:
K
- the type of keys maintained by this mapV
- the type of mapped values
- All Implemented Interfaces:
Serializable
,ConcurrentMap<K,
,V> Map<K,
V>
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.
This class is copied from Hibernate (which took it from Infinispan), and was originally written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to the public domain, as explained at http://creativecommons.org/licenses/publicdomain
Like Hashtable
but unlike HashMap
, this class
does not allow null to be used as a key or value.
- Author:
- Doug Lea
- See Also:
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Nested Class Summary
Modifier and TypeClassDescription(package private) final class
(package private) final class
static enum
static interface
static interface
private static class
ConcurrentHashMap list entry.(package private) class
(package private) final class
(package private) final class
(package private) static final class
private static final class
Adapted to Infinispan BoundedConcurrentHashMap using LIRS implementation ideas from Charles Fry ([email protected]) See http://code.google.com/p/concurrentlinkedhashmap/source/browse/trunk/src/test/java/com/googlecode/concurrentlinkedhashmap/caches/LirsMap.java for original sources(package private) static final class
(package private) static final class
(package private) static class
private static enum
(package private) static final class
Segments are specialized versions of hash tables.(package private) final class
(package private) final class
(package private) final class
Custom Entry class used by EntryIterator.next(), that relays setValue changes to the underlying map.Nested classes/interfaces inherited from class java.util.AbstractMap
AbstractMap.SimpleEntry<K extends Object,
V extends Object>, AbstractMap.SimpleImmutableEntry<K extends Object, V extends Object> -
Field Summary
Modifier and TypeFieldDescription(package private) static final int
The default concurrency level for this table, used when not otherwise specified in a constructor.(package private) static final float
The default load factor for this table, used when not otherwise specified in a constructor.(package private) static final int
The default initial capacity for this table, used when not otherwise specified in a constructor.(package private) static final int
The maximum number of segments to allow; used to bound constructor arguments.(package private) static final int
The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments.(package private) static final int
Number of unsynchronized retries in size and containsValue methods before resorting to locking.(package private) final int
Mask value for indexing into segments.(package private) final BoundedConcurrentHashMap.Segment<K,
V>[] The segments, each of which is a specialized hash table(package private) final int
Shift value for indexing within segments.private static final long
(package private) Collection<V>
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Constructor Summary
ConstructorDescriptionCreates a new, empty map with the default maximum capacityBoundedConcurrentHashMap
(int capacity) Creates a new, empty map with the specified maximum capacity, default concurrency level and LRU eviction policy.BoundedConcurrentHashMap
(int capacity, int concurrencyLevel) Creates a new, empty map with the specified maximum capacity, load factor, concurrency level and LRU eviction policy.BoundedConcurrentHashMap
(int capacity, int concurrencyLevel, BoundedConcurrentHashMap.Eviction evictionStrategy) Creates a new, empty map with the specified maximum capacity, load factor, concurrency level and eviction strategy.BoundedConcurrentHashMap
(int capacity, int concurrencyLevel, BoundedConcurrentHashMap.Eviction evictionStrategy, BoundedConcurrentHashMap.EvictionListener<K, V> evictionListener) Creates a new, empty map with the specified maximum capacity, load factor and concurrency level. -
Method Summary
Modifier and TypeMethodDescriptionvoid
clear()
Removes all of the mappings from this map.boolean
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.elements()
Returns an enumeration of the values in this table.entrySet()
Returns aSet
view of the mappings contained in this map.Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.private static int
hash
(int h) Applies a supplemental hash function to a given hashCode, which defends against poor quality hash functions.boolean
isEmpty()
Returns true if this map contains no key-value mappings.keys()
Returns an enumeration of the keys in this table.keySet()
Returns aSet
view of the keys contained in this map.Maps the specified key to the specified value in this table.void
Copies all of the mappings from the specified map to this one.putIfAbsent
(K key, V value) private void
Reconstitute the ConcurrentHashMap instance from a stream (i.e., deserialize it).Removes the key (and its corresponding value) from this map.boolean
boolean
(package private) final BoundedConcurrentHashMap.Segment<K,
V> segmentFor
(int hash) Returns the segment that should be used for key with given hashint
size()
Returns the number of key-value mappings in this map.values()
Returns aCollection
view of the values contained in this map.private void
Save the state of the ConcurrentHashMap instance to a stream (i.e., serialize it).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.concurrent.ConcurrentMap
compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, replaceAll
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Field Details
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serialVersionUID
private static final long serialVersionUID- See Also:
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DEFAULT_MAXIMUM_CAPACITY
static final int DEFAULT_MAXIMUM_CAPACITYThe default initial capacity for this table, used when not otherwise specified in a constructor.- See Also:
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DEFAULT_LOAD_FACTOR
static final float DEFAULT_LOAD_FACTORThe default load factor for this table, used when not otherwise specified in a constructor.- See Also:
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DEFAULT_CONCURRENCY_LEVEL
static final int DEFAULT_CONCURRENCY_LEVELThe default concurrency level for this table, used when not otherwise specified in a constructor.- See Also:
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MAXIMUM_CAPACITY
static final int MAXIMUM_CAPACITYThe 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:
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MAX_SEGMENTS
static final int MAX_SEGMENTSThe maximum number of segments to allow; used to bound constructor arguments.- See Also:
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RETRIES_BEFORE_LOCK
static final int RETRIES_BEFORE_LOCKNumber 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:
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segmentMask
final int segmentMaskMask value for indexing into segments. The upper bits of a key's hash code are used to choose the segment. -
segmentShift
final int segmentShiftShift value for indexing within segments. -
segments
The segments, each of which is a specialized hash table -
keySet
-
entrySet
-
values
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Constructor Details
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BoundedConcurrentHashMap
public BoundedConcurrentHashMap(int capacity, int concurrencyLevel, BoundedConcurrentHashMap.Eviction evictionStrategy, BoundedConcurrentHashMap.EvictionListener<K, V> evictionListener) Creates a new, empty map with the specified maximum capacity, load factor and concurrency level.- Parameters:
capacity
- is the upper bound capacity for the number of elements in this mapconcurrencyLevel
- the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.evictionStrategy
- the algorithm used to evict elements from this mapevictionListener
- the evicton listener callback to be notified about evicted elements- Throws:
IllegalArgumentException
- if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.
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BoundedConcurrentHashMap
public BoundedConcurrentHashMap(int capacity, int concurrencyLevel) Creates a new, empty map with the specified maximum capacity, load factor, concurrency level and LRU eviction policy.- Parameters:
capacity
- is the upper bound capacity for the number of elements in this mapconcurrencyLevel
- 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.
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BoundedConcurrentHashMap
public BoundedConcurrentHashMap(int capacity, int concurrencyLevel, BoundedConcurrentHashMap.Eviction evictionStrategy) Creates a new, empty map with the specified maximum capacity, load factor, concurrency level and eviction strategy.- Parameters:
capacity
- is the upper bound capacity for the number of elements in this mapconcurrencyLevel
- the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.evictionStrategy
- the algorithm used to evict elements from this map- Throws:
IllegalArgumentException
- if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.
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BoundedConcurrentHashMap
public BoundedConcurrentHashMap(int capacity) Creates a new, empty map with the specified maximum capacity, default concurrency level and LRU eviction policy.- Parameters:
capacity
- is the upper bound capacity for the number of elements in this map- Throws:
IllegalArgumentException
- if the initial capacity of elements is negative or the load factor is nonpositive- Since:
- 1.6
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BoundedConcurrentHashMap
public BoundedConcurrentHashMap()Creates a new, empty map with the default maximum capacity
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Method Details
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hash
private static int hash(int h) Applies a supplemental hash function to a given hashCode, which defends against poor quality hash functions. This is critical because ConcurrentHashMap uses power-of-two length hash tables, that otherwise encounter collisions for hashCodes that do not differ in lower or upper bits. -
segmentFor
Returns the segment that should be used for key with given hash- Parameters:
hash
- the hash code for the key- Returns:
- the segment
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isEmpty
public boolean isEmpty()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. -
get
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.More formally, if this map contains a mapping from a key
k
to a valuev
such thatkey.equals(k)
, then this method returnsv
; otherwise it returnsnull
. (There can be at most one such mapping.)- Specified by:
get
in interfaceMap<K,
V> - Overrides:
get
in classAbstractMap<K,
V> - Throws:
NullPointerException
- if the specified key is null
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containsKey
Tests if the specified object is a key in this table.- Specified by:
containsKey
in interfaceMap<K,
V> - Overrides:
containsKey
in classAbstractMap<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
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containsValue
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 interfaceMap<K,
V> - Overrides:
containsValue
in classAbstractMap<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
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contains
Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality tocontainsValue(java.lang.Object)
, and exists solely to ensure full compatibility with classHashtable
, 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
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put
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 interfaceMap<K,
V> - Overrides:
put
in classAbstractMap<K,
V> - Parameters:
key
- key with which the specified value is to be associatedvalue
- 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
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putIfAbsent
- Specified by:
putIfAbsent
in interfaceConcurrentMap<K,
V> - Specified by:
putIfAbsent
in interfaceMap<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
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putAll
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. -
remove
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 interfaceMap<K,
V> - Overrides:
remove
in classAbstractMap<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
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remove
- Specified by:
remove
in interfaceConcurrentMap<K,
V> - Specified by:
remove
in interfaceMap<K,
V> - Throws:
NullPointerException
- if the specified key is null
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replace
- Specified by:
replace
in interfaceConcurrentMap<K,
V> - Specified by:
replace
in interfaceMap<K,
V> - Throws:
NullPointerException
- if any of the arguments are null
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replace
- Specified by:
replace
in interfaceConcurrentMap<K,
V> - Specified by:
replace
in interfaceMap<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
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clear
public void clear()Removes all of the mappings from this map. -
keySet
Returns aSet
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. -
values
Returns aCollection
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. -
entrySet
Returns aSet
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. -
keys
Returns an enumeration of the keys in this table.- Returns:
- an enumeration of the keys in this table
- See Also:
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elements
Returns an enumeration of the values in this table.- Returns:
- an enumeration of the values in this table
- See Also:
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writeObject
Save the state of the ConcurrentHashMap instance to a stream (i.e., serialize it).- Parameters:
s
- the stream- Throws:
IOException
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readObject
Reconstitute the ConcurrentHashMap instance from a stream (i.e., deserialize it).- Parameters:
s
- the stream- Throws:
IOException
ClassNotFoundException
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