@SuppressWarnings("all") public class ConcurrentReferenceHashMap<K, V> extends AbstractMap implements Serializable
An advanced hash table supporting configurable garbage collection semantics of keys and values, optional referential-equality, full concurrency of retrievals, and adjustable expected concurrency for updates.
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 its 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 you 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 they 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 that you 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.
K - the type of keys maintained by this mapV - the type of mapped values| Modifiers | Name | Description |
|---|---|---|
protected class |
ConcurrentReferenceHashMap.HashIterator |
|
protected static class |
ConcurrentReferenceHashMap.InitializableEntry |
|
static enum |
ConcurrentReferenceHashMap.Option |
Behavior-changing configuration options for the map |
static enum |
ConcurrentReferenceHashMap.ReferenceType |
An option specifying which Java reference type should be used to refer to a key and/or value. |
protected static class |
ConcurrentReferenceHashMap.SimpleEntry |
|
protected class |
ConcurrentReferenceHashMap.WriteThroughEntry |
Custom Entry class used by EntryIterator.next(), that relays setValue changes to the underlying map. |
| Constructor and description |
|---|
ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel, ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType, EnumSet<Option> options)Creates a new, empty map with the specified initial capacity, reference types, load factor, and concurrency level. |
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)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, ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType)Creates a new, empty map with the specified initial capacity, reference types, and with a default load factor (0.75) and concurrencyLevel (16). |
ConcurrentReferenceHashMap(int initialCapacity, ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType, EnumSet<Option> options)Creates a new, empty map with the specified initial capacity, reference types, and with a default load factor (0.75) and concurrencyLevel (16). |
ConcurrentReferenceHashMap(ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType)Creates a new, empty reference map with the specified key and value reference types. |
ConcurrentReferenceHashMap(ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType, EnumSet<Option> options)Creates a new, empty reference map with the specified reference types and behavioral options. |
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()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(Map<? extends K, ? extends V> m)Creates a new map with the same mappings as the given map. |
| Type Params | Return Type | Name and description |
|---|---|---|
|
public V |
apply(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) |
|
public V |
applyIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)
|
|
public V |
applyIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) |
|
public Set<Entry<K, V>> |
cachedEntrySet() |
|
public void |
clear()Removes all of the mappings from this map. |
|
public boolean |
contains(Object value)Legacy method testing if some key maps into the specified value in this table. |
|
public boolean |
containsKey(Object key)Tests if the specified object is a key in this table. |
|
public boolean |
containsValue(Object value)Returns true if this map maps one or more keys to the specified value. |
|
public Enumeration<V> |
elements()Returns an enumeration of the values in this table. |
|
public Set<Entry<K, V>> |
entrySet()Returns a Set view of the mappings contained in this map. |
|
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. |
|
protected int |
hashOf(Object key) |
|
public boolean |
isEmpty()Returns true if this map contains no key-value mappings. |
|
public Set<K> |
keySet()Returns a Set view of the keys contained in this map. |
|
public Enumeration<K> |
keys()Returns an enumeration of the keys in this table. |
|
public void |
purgeStaleEntries()Removes any stale entries whose keys have been finalized. |
|
public V |
put(K key, V value)Maps the specified key to the specified value in this table. |
|
public void |
putAll(Map<? extends K, ? extends V> m)Copies all of the mappings from the specified map to this one. |
|
public V |
putIfAbsent(K key, V value){@inheritDoc} |
|
public V |
remove(Object key)Removes the key (and its corresponding value) from this map. |
|
public boolean |
remove(Object key, Object value){@inheritDoc} |
|
public boolean |
replace(K key, V oldValue, V newValue){@inheritDoc} |
|
public V |
replace(K key, V value){@inheritDoc} |
|
public int |
size()Returns the number of key-value mappings in this map. |
|
public Collection<V> |
values()Returns a Collection view of the values contained in this map. |
| Methods inherited from class | Name |
|---|---|
class AbstractMap |
remove, get, put, equals, toString, values, hashCode, clear, isEmpty, size, entrySet, putAll, containsKey, keySet, containsValue, wait, wait, wait, getClass, notify, notifyAll, remove, replace, replace, replaceAll, merge, putIfAbsent, compute, forEach, computeIfAbsent, getOrDefault, computeIfPresent |
Creates a new, empty map with the specified initial capacity, reference types, load factor, and concurrency level.
Behavioral changing options such as Option#IDENTITY_COMPARISONS#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 optionsCreates a new, empty map with the specified initial capacity, load factor, and concurrency level.
initialCapacity - the initial capacity. The implementation
performs internal sizing to accommodate this number of 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.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).
initialCapacity - The implementation performs internal
sizing to accommodate this number of 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.Creates a new, empty map with the specified initial capacity, reference types, and with a default load factor (0.75) and concurrencyLevel (16).
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 valuesCreates a new, empty map with the specified initial capacity, reference types, and with a default load factor (0.75) and concurrencyLevel (16). Behavioral changing options such as Option#IDENTITY_COMPARISONS#IDENTITY_COMPARISONS can also be specified.
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 valuesoptions - the behavioral optionsCreates a new, empty reference map with the specified key and value reference types.
keyType - the reference type to use for keysvalueType - the reference type to use for valuesCreates a new, empty reference map with the specified reference types and behavioral options.
keyType - the reference type to use for keysvalueType - the reference type to use for valuesCreates 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).
initialCapacity - the initial capacity. The implementation
performs internal sizing to accommodate this many elements.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).
Creates a new map with the same mappings as the given map. The map is created with a capacity of 1.5 times the number of mappings in the given map or 16 (whichever is greater), and a default load factor (0.75) and concurrencyLevel (16).
m - the mapmap, then returning the current value or null if now
absent:
if (map.get(key) == null) {
V newValue = mappingFunction.apply(key);
if (newValue != null)
return map.putIfAbsent(key, newValue);
}
The default implementation may retry these steps when multiple threads attempt updates including potentially calling the mapping function multiple times.
This implementation assumes that the ConcurrentMap cannot contain null
values and get() returning null unambiguously means the key is
absent. Implementations which support null values must
override this default implementation.
Removes all of the mappings from this map.
Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality to containsValue, 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 forTests if the specified object is a key in this table.
key - possible keyReturns 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, therefore it is much slower than the method containsKey.
value - value whose presence in this map is to be testedReturns an enumeration of the values in this table.
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 is guaranteed to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.
Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.
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.)
Returns true if this map contains no key-value mappings.
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. 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.
Returns an enumeration of the keys in this table.
Removes any stale entries whose keys have been finalized. Use of this method is normally not necessary since stale entries are automatically removed lazily, when blocking operations are required. However, there are some cases where this operation should be performed eagerly, such as cleaning up old references to a ClassLoader in a multi-classloader environment.
Note: this method will acquire locks one at a time across all segments of this table, so this method should be used sparingly.
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.
key - key with which the specified value is to be associatedvalue - value to be associated with the specified keyCopies 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.
m - mappings to be stored in this map{@inheritDoc}
Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.
key - the key that needs to be removed{@inheritDoc}
{@inheritDoc}
{@inheritDoc}
Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
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. 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.
Copyright © 2003-2022 The Apache Software Foundation. All rights reserved.