K - the type of keys maintained by this mapV - the type of mapped valuespublic final class ConcurrentReferenceHashMap<K,V>
extends java.util.AbstractMap<K,V>
implements java.util.concurrent.ConcurrentMap<K,V>, java.io.Serializable
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 before 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.
| 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.
|
| 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,
java.util.EnumSet<ConcurrentReferenceHashMap.Option> options)
Creates a new, empty map with the specified initial
capacity, reference types, load factor and concurrency level.
|
ConcurrentReferenceHashMap(java.util.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(java.lang.Object value)
Legacy method testing if some key maps into the specified value
in this table.
|
boolean |
containsKey(java.lang.Object key)
Tests if the specified object is a key in this table.
|
boolean |
containsValue(java.lang.Object value)
Returns true if this map maps one or more keys to the
specified value.
|
java.util.Enumeration<V> |
elements()
Returns an enumeration of the values in this table.
|
java.util.Set<java.util.Map.Entry<K,V>> |
entrySet()
Returns a
Set view of the mappings contained in this map. |
V |
get(java.lang.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.
|
java.util.Enumeration<K> |
keys()
Returns an enumeration of the keys in this table.
|
java.util.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(java.util.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(java.lang.Object key)
Removes the key (and its corresponding value) from this map.
|
boolean |
remove(java.lang.Object key,
java.lang.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.
|
java.util.Collection<V> |
values()
Returns a
Collection view of the values contained in this map. |
finalize, getClass, notify, notifyAll, wait, wait, waitpublic ConcurrentReferenceHashMap(int initialCapacity,
float loadFactor,
int concurrencyLevel,
ConcurrentReferenceHashMap.ReferenceType keyType,
ConcurrentReferenceHashMap.ReferenceType valueType,
java.util.EnumSet<ConcurrentReferenceHashMap.Option> options)
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 optionsjava.lang.IllegalArgumentException - 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.java.lang.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.java.lang.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 valuesjava.lang.IllegalArgumentException - 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.java.lang.IllegalArgumentException - if the initial capacity of
elements is negative.public ConcurrentReferenceHashMap()
public ConcurrentReferenceHashMap(java.util.Map<? extends K,? extends V> m)
m - the mappublic boolean isEmpty()
public int size()
public V get(java.lang.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.)
public boolean containsKey(java.lang.Object key)
containsKey in interface java.util.Map<K,V>containsKey in class java.util.AbstractMap<K,V>key - possible keyjava.lang.NullPointerException - if the specified key is nullpublic boolean containsValue(java.lang.Object value)
containsValue in interface java.util.Map<K,V>containsValue in class java.util.AbstractMap<K,V>value - value whose presence in this map is to be testedjava.lang.NullPointerException - if the specified value is nullpublic boolean contains(java.lang.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 forjava.lang.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 java.util.Map<K,V>put in class java.util.AbstractMap<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keyjava.lang.NullPointerException - if the specified key or value is nullpublic V putIfAbsent(K key, V value)
putIfAbsent in interface java.util.concurrent.ConcurrentMap<K,V>putIfAbsent in interface java.util.Map<K,V>java.lang.NullPointerException - if the specified key or value is nullpublic void putAll(java.util.Map<? extends K,? extends V> m)
public V remove(java.lang.Object key)
remove in interface java.util.Map<K,V>remove in class java.util.AbstractMap<K,V>key - the key that needs to be removedjava.lang.NullPointerException - if the specified key is nullpublic boolean remove(java.lang.Object key,
java.lang.Object value)
public V replace(K key, V value)
replace in interface java.util.concurrent.ConcurrentMap<K,V>replace in interface java.util.Map<K,V>java.lang.NullPointerException - if the specified key or value is nullpublic void clear()
public void purgeStaleEntries()
public java.util.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 java.util.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 java.util.Set<java.util.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 java.util.Enumeration<K> keys()
keySet()