K - the type of keys maintained by this mapV - the type of mapped valuespublic class ConcurrentReferenceHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>, Serializable
This table is designed around specific advanced use-cases. If there is any doubt whether this table is for
you, you most likely should be using ConcurrentHashMap instead.
This table supports strong, weak, and soft keys and values. By default keys are weak, and values are
strong. Such a configuration offers similar behavior to WeakHashMap, entries of this
table are periodically removed once their corresponding keys are no longer referenced outside of this
table. In other words, this table will not prevent a key from being discarded by the garbage collector.
Once a key has been discarded by the collector, the corresponding entry is no longer visible to this table;
however, the entry may occupy space until a future table operation decides to reclaim it. For this reason,
summary functions such as size and isEmpty might return a value greater than the observed
number of entries. In order to support a high level of concurrency, stale entries are only reclaimed during
blocking (usually mutating) operations.
Enabling soft keys allows entries in this table to remain until their space is absolutely needed by the garbage collector. This is unlike weak keys which can be reclaimed as soon as they are no longer referenced by a normal strong reference. The primary use case for soft keys is a cache, which ideally occupies memory that is not in use for as long as possible.
By default, values are held using a normal strong reference. This provides the commonly desired guarantee that a value will always have at least the same life-span as it's key. For this reason, care should be taken to ensure that a value never refers, either directly or indirectly, to its key, thereby preventing reclamation. If this is unavoidable, then it is recommended to use the same reference type in use for the key. However, it should be noted that non-strong values may disappear beforeQuery their corresponding key.
While this table does allow the use of both strong keys and values, it is recommended to use
ConcurrentHashMap for such a configuration, since it is optimized for that
case.
Just like ConcurrentHashMap, 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. 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
| Modifier and Type | Class and Description |
|---|---|
static class |
ConcurrentReferenceHashMap.Option |
static class |
ConcurrentReferenceHashMap.ReferenceType
An option specifying which Java reference type should be used to refer to a key and/or value.
|
AbstractMap.SimpleImmutableEntry<K,V>| Constructor and Description |
|---|
ConcurrentReferenceHashMap()
Creates a new, empty map with a default initial capacity (16), reference types (weak keys, strong
values), default load factor (0.75) and concurrencyLevel (16).
|
ConcurrentReferenceHashMap(int initialCapacity)
Creates a new, empty map with the specified initial capacity, and with default reference types (weak
keys, strong values), load factor (0.75) and concurrencyLevel (16).
|
ConcurrentReferenceHashMap(int initialCapacity,
ConcurrentReferenceHashMap.ReferenceType keyType,
ConcurrentReferenceHashMap.ReferenceType valueType)
Creates a new, empty map with the specified initial capacity, reference types and with default load
factor (0.75) and concurrencyLevel (16).
|
ConcurrentReferenceHashMap(int initialCapacity,
float loadFactor)
Creates a new, empty map with the specified initial capacity and load factor and with the default
reference types (weak keys, strong values), and concurrencyLevel (16).
|
ConcurrentReferenceHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel)
Creates a new, empty map with the specified initial capacity, load factor and concurrency level.
|
ConcurrentReferenceHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel,
ConcurrentReferenceHashMap.ReferenceType keyType,
ConcurrentReferenceHashMap.ReferenceType valueType,
EnumSet<ConcurrentReferenceHashMap.Option> options)
Creates a new, empty map with the specified initial capacity, reference types, load factor and
concurrency level.
|
ConcurrentReferenceHashMap(Map<? extends K,? extends V> m)
Creates a new map with the same mappings as the given map.
|
| Modifier and Type | Method and Description |
|---|---|
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. |
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. |
boolean |
isEmpty()
Returns true if this map contains no key-value mappings.
|
Enumeration<K> |
keys()
Returns an enumeration of the keys in this table.
|
Set<K> |
keySet()
Returns a
Set view of the keys contained in this map. |
void |
purgeStaleEntries()
Removes any stale entries whose keys have been finalized.
|
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) |
V |
remove(Object key)
Removes the key (and its corresponding value) from this map.
|
boolean |
remove(Object key,
Object value) |
V |
replace(K key,
V value) |
boolean |
replace(K key,
V oldValue,
V newValue) |
int |
size()
Returns the number of key-value mappings in this map.
|
Collection<V> |
values()
Returns a
Collection view of the values contained in this map. |
clone, equals, hashCode, toStringfinalize, getClass, notify, notifyAll, wait, wait, waitcompute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, replaceAllpublic ConcurrentReferenceHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel,
ConcurrentReferenceHashMap.ReferenceType keyType,
ConcurrentReferenceHashMap.ReferenceType valueType,
EnumSet<ConcurrentReferenceHashMap.Option> options)
Behavioral changing options such as ConcurrentReferenceHashMap.Option.IDENTITY_COMPARISONS can also be specified.
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.keyType - the reference type to use for keysvalueType - the reference type to use for valuesoptions - the behavioral optionsIllegalArgumentException - if the initial capacity is negative or the load factor or
concurrencyLevel are nonpositive.public ConcurrentReferenceHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel)
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.IllegalArgumentException - if the initial capacity is negative or the load factor or
concurrencyLevel are nonpositive.public ConcurrentReferenceHashMap(int initialCapacity,
float loadFactor)
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.IllegalArgumentException - if the initial capacity of elements is negative or the load factor is
nonpositivepublic ConcurrentReferenceHashMap(int initialCapacity,
ConcurrentReferenceHashMap.ReferenceType keyType,
ConcurrentReferenceHashMap.ReferenceType valueType)
initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate
this many elements.keyType - the reference type to use for keysvalueType - the reference type to use for valuesIllegalArgumentException - if the initial capacity of elements is negative.public ConcurrentReferenceHashMap(int initialCapacity)
initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate
this many elements.IllegalArgumentException - if the initial capacity of elements is negative.public ConcurrentReferenceHashMap()
public ConcurrentReferenceHashMap(Map<? extends K,? extends V> m)
m - the mappublic boolean isEmpty()
public int size()
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>get in class AbstractMap<K,V>NullPointerException - if the specified key is nullpublic boolean containsKey(Object key)
containsKey in interface Map<K,V>containsKey in class AbstractMap<K,V>key - possible keyNullPointerException - if the specified key is nullpublic boolean containsValue(Object value)
containsValue in interface Map<K,V>containsValue in class AbstractMap<K,V>value - value whose presence in this map is to be testedpublic boolean contains(Object value)
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.value - a value to search forNullPointerException - 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>put in class AbstractMap<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keyNullPointerException - if the specified key or value is nullpublic V putIfAbsent(K key, V value)
putIfAbsent in interface ConcurrentMap<K,V>putIfAbsent in interface Map<K,V>NullPointerException - if the specified key or value is nullpublic void putAll(Map<? extends K,? extends V> m)
public V remove(Object key)
remove in interface Map<K,V>remove in class AbstractMap<K,V>key - the key that needs to be removedNullPointerException - if the specified key is nullpublic boolean remove(Object key, Object value)
remove in interface ConcurrentMap<K,V>remove in interface Map<K,V>NullPointerException - if the specified key is nullpublic boolean replace(K key, V oldValue, V newValue)
replace in interface ConcurrentMap<K,V>replace in interface Map<K,V>NullPointerException - if any of the arguments are nullpublic V replace(K key, V value)
replace in interface ConcurrentMap<K,V>replace in interface Map<K,V>NullPointerException - if the specified key or value is nullpublic void clear()
public void purgeStaleEntries()
Note: this method will acquire locks, one at a time, across all segments of this table, so if it is to be used, it should be used sparingly.
public Set<K> 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. 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.
public Collection<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. 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.
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.
public Enumeration<K> keys()
keySet()public Enumeration<V> elements()
values()Copyright © 2020. All rights reserved.