| java.lang.Object
 java.util.AbstractMap
wicket.util.concurrent.ConcurrentHashMap
| ConcurrentHashMap | | public class ConcurrentHashMap extends AbstractMap implements Map,Cloneable,Serializable(Code) | | A version of Hashtable supporting concurrency for both retrievals and
updates:
- Retrievals
- Retrievals may overlap updates. (This is the same policy as
ConcurrentReaderHashMap.) Successful retrievals using get(key) and
containsKey(key) usually run without locking. Unsuccessful retrievals (i.e.,
when the key is not present) do involve brief synchronization (locking).
Because retrieval operations can ordinarily overlap with update operations
(i.e., put, remove, and their derivatives), retrievals can only be guaranteed
to return the results of the most recently completed operations
holding upon their onset. Retrieval operations may or may not return results
reflecting in-progress writing operations. However, the retrieval operations
do always return consistent results -- either those holding before any single
modification or after it, but never a nonsense result. For aggregate
operations such as putAll and clear, concurrent reads may reflect insertion
or removal of only some entries.
Iterators and Enumerations (i.e., those returned by keySet().iterator(),
entrySet().iterator(), values().iterator(), keys(), and elements()) return
elements reflecting the state of the hash table at some point at or since the
creation of the iterator/enumeration. They will return at most one instance
of each element (via next()/nextElement()), but might or might not reflect
puts and removes that have been processed since they were created. They do
not throw ConcurrentModificationException. However, these
iterators are designed to be used by only one thread at a time. Passing an
iterator across multiple threads may lead to unpredictable results if the
table is being concurrently modified.
- Updates
- This class supports a hard-wired preset concurrency
level of
32. This allows a maximum of 32 put and/or remove operations to proceed
concurrently. This level is an upper bound on concurrency, not a guarantee,
since it interacts with how well-strewn elements are across bins of the
table. (The preset value in part reflects the fact that even on large
multiprocessors, factors other than synchronization tend to be bottlenecks
when more than 32 threads concurrently attempt updates.) Additionally,
operations triggering internal resizing and clearing do not execute
concurrently with any operation.
There is NOT any support for locking the entire table to prevent
updates. This makes it imposssible, for example, to add an element only if it
is not already present, since another thread may be in the process of doing
the same thing. If you need such capabilities, consider instead using the
ConcurrentReaderHashMap class.
Because of how concurrency control is split up, the size() and isEmpty()
methods require accumulations across 32 control segments, and so might be
slightly slower than you expect.
This class may be used as a direct replacement for java.util.Hashtable in any
application that does not rely on the ability to lock the entire table to
prevent updates. As of this writing, it performs much faster than Hashtable
in typical multi-threaded applications with multiple readers and writers.
Like Hashtable but unlike java.util.HashMap, this class does NOT allow
null to be used as a key or value.
Implementation note: A slightly faster implementation of this class will be
possible once planned Java Memory Model revisions are in place.
[
Introduction to this package. ]
|
| Inner Class :final protected static class Segment implements Serializable | |
| Inner Class :protected static class Entry implements Map.Entry | |
| Inner Class :protected class KeyIterator extends HashIterator | |
| Inner Class :protected class ValueIterator extends HashIterator | |
| Field Summary | |
| final protected static int | CONCURRENCY_LEVEL
The number of concurrency control segments. | | final public static int | DEFAULT_INITIAL_CAPACITY
The default initial number of table slots for this table (32). | | final public static float | DEFAULT_LOAD_FACTOR
The default load factor for this table (0.75) Used when not otherwise
specified in constructor. | | final protected static int | SEGMENT_MASK
| | protected transient Set | entrySet
| | protected transient Set | keySet
| | final protected float | loadFactor
The load factor for the hash table. | | final protected Segment[] | segments
The array of concurrency control segments. | | protected transient Entry[] | table
The hash table data. | | protected int | threshold
Per-segment resize threshold. | | protected transient Collection | values
| | protected transient volatile int | votesForResize
Number of segments voting for resize. |
| Constructor Summary | |
| public | ConcurrentHashMap(int initialCapacity, float loadFactor)
Constructs a new, empty map with the specified initial capacity and the
specified load factor.
Parameters:
initialCapacity - the initial capacity. | | public | ConcurrentHashMap(int initialCapacity)
Constructs a new, empty map with the specified initial capacity and
default load factor. | | public | ConcurrentHashMap()
Constructs a new, empty map with a default initial capacity and default
load factor. | | public | ConcurrentHashMap(Map t)
Constructs a new map with the same mappings as the given map. |
| Method Summary | |
| protected static int | bitcount(int w)
Return the number of set bits in w. | | public void | clear()
Removes all mappings from this map. | | public Object | clone()
Returns a shallow copy of this ConcurrentHashMap instance: the
keys and values themselves are not cloned. | | public boolean | contains(Object value)
Tests 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.
Parameters:
key - possible key. | | public boolean | containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value. | | public Enumeration | elements()
Returns an enumeration of the values in this table. | | public Set | entrySet()
Returns a collection view of the mappings contained in this map. | | protected boolean | eq(Object x, Object y)
Check for equality of non-null references x and y. | | public Object | get(Object key)
Returns the value to which the specified key is mapped in this table.
Parameters:
key - a key in the table. | | protected static int | hash(Object x)
Return hash code for Object x. | | public boolean | isEmpty()
Returns true if this map contains no key-value mappings. | | public Set | keySet()
Returns a set view of the keys contained in this map. | | public Enumeration | keys()
Returns an enumeration of the keys in this table. | | protected Entry[] | newTable(int capacity)
| | public Object | put(Object key, Object value)
Maps the specified key to the specified value
in this table. | | public void | putAll(Map t)
Copies all of the mappings from the specified map to this one. | | protected void | rehash()
Rehashes the contents of this map into a new table with a larger
capacity. | | public Object | remove(Object key)
Removes the key (and its corresponding value) from this table. | | protected Object | remove(Object key, Object value)
Removes the (key, value) pair from this table. | | protected void | resize(int index, Entry[] assumedTab)
Gather all locks in order to call rehash, by recursing within synch
blocks for each segment index.
Parameters:
index - the current segment. | | public int | size()
Returns the number of key-value mappings in this map. | | public Collection | values()
Returns a collection view of the values contained in this map. |
| CONCURRENCY_LEVEL | | final protected static int CONCURRENCY_LEVEL(Code) | | The number of concurrency control segments. The value can be at most 32
since ints are used as bitsets over segments. Emprically, it doesn't seem
to pay to decrease it either, so the value should be at least 32. In
other words, do not redefine this :-)
|
| DEFAULT_INITIAL_CAPACITY | | final public static int DEFAULT_INITIAL_CAPACITY(Code) | | The default initial number of table slots for this table (32). Used when
not otherwise specified in constructor.
|
| DEFAULT_LOAD_FACTOR | | final public static float DEFAULT_LOAD_FACTOR(Code) | | The default load factor for this table (0.75) Used when not otherwise
specified in constructor.
|
| SEGMENT_MASK | | final protected static int SEGMENT_MASK(Code) | | Mask value for indexing into segments
|
| entrySet | | protected transient Set entrySet(Code) | | |
| loadFactor | | final protected float loadFactor(Code) | | The load factor for the hash table.
|
| segments | | final protected Segment[] segments(Code) | | The array of concurrency control segments.
|
| table | | protected transient Entry[] table(Code) | | The hash table data.
|
| threshold | | protected int threshold(Code) | | Per-segment resize threshold.
|
| votesForResize | | protected transient volatile int votesForResize(Code) | | Number of segments voting for resize. The table is doubled when 1/4 of
the segments reach threshold. Volatile but updated without synch since
this is just a heuristic.
|
| ConcurrentHashMap | | public ConcurrentHashMap(int initialCapacity, float loadFactor)(Code) | | Constructs a new, empty map with the specified initial capacity and the
specified load factor.
Parameters:
initialCapacity - the initial capacity. The actual initial capacity is roundedto the nearest power of two.
Parameters:
loadFactor - the load factor threshold, used to control resizing. Thisvalue is used in an approximate way: When at least a quarterof the segments of the table reach per-segment threshold, orone of the segments itself exceeds overall threshold, thetable is doubled. This will on average cause resizing when thetable-wide load factor is slightly less than the threshold. Ifyou'd like to avoid resizing, you can set this to aridiculously large value.
throws:
IllegalArgumentException - if the load factor is nonpositive. |
| ConcurrentHashMap | | public ConcurrentHashMap(int initialCapacity)(Code) | | Constructs a new, empty map with the specified initial capacity and
default load factor.
Parameters:
initialCapacity - the initial capacity of the ConcurrentHashMap.
throws:
IllegalArgumentException - if the initial maximum number of elements is less than zero. |
| ConcurrentHashMap | | public ConcurrentHashMap()(Code) | | Constructs a new, empty map with a default initial capacity and default
load factor.
|
| ConcurrentHashMap | | public ConcurrentHashMap(Map t)(Code) | | Constructs a new map with the same mappings as the given map. The map is
created with a capacity of twice the number of mappings in the given map
or 32 (whichever is greater), and a default load factor.
Parameters:
t - map to copy |
| bitcount | | protected static int bitcount(int w)(Code) | | Return the number of set bits in w. For a derivation of this algorithm,
see "Algorithms and data structures with applications to graphics and
geometry", by Jurg Nievergelt and Klaus Hinrichs, Prentice Hall, 1993.
See also notes by Torsten Sillke at
http://www.mathematik.uni-bielefeld.de/~sillke/PROBLEMS/bitcount
Parameters:
w - arg number of set bits |
| clear | | public void clear()(Code) | | Removes all mappings from this map.
|
| clone | | public Object clone()(Code) | | Returns a shallow copy of this ConcurrentHashMap instance: the
keys and values themselves are not cloned.
a shallow copy of this map. |
| contains | | public boolean contains(Object value)(Code) | | Tests if some key maps into the specified value in this table. This
operation is more expensive than the containsKey method.
Note that this method is identical in functionality to containsValue,
(which is part of the Map interface in the collections framework).
Parameters:
value - a value to search for. true if and only if some key maps to thevalue argument in this table as determined by theequals method; false otherwise.
exception:
NullPointerException - if the value is null.
See Also: ConcurrentHashMap.containsKey(Object)
See Also: ConcurrentHashMap.containsValue(Object)
See Also: Map |
| containsKey | | public boolean containsKey(Object key)(Code) | | Tests if the specified object is a key in this table.
Parameters:
key - possible key. true if and only if the specified object is a keyin this table, as determined by the equals method;false otherwise.
exception:
NullPointerException - if the key is null.
See Also: ConcurrentHashMap.contains(Object) |
| containsValue | | public boolean containsValue(Object value)(Code) | | 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.
Parameters:
value - value whose presence in this map is to be tested. true if this map maps one or more keys to thespecified value.
exception:
NullPointerException - if the value is null. |
| entrySet | | public Set entrySet()(Code) | | Returns a collection view of the mappings contained in this map. Each
element in the returned collection is a Map.Entry. 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 the map, via the
Iterator.remove, Collection.remove,
removeAll, retainAll, and clear
operations. It does not support the add or addAll
operations.
a collection view of the mappings contained in this map. |
| eq | | protected boolean eq(Object x, Object y)(Code) | | Check for equality of non-null references x and y.
Parameters:
x - ref
Parameters:
y - ref is equal |
| hash | | protected static int hash(Object x)(Code) | | Return hash code for Object x. Since we are using power-of-two tables, it
is worth the effort to improve hashcode via the same multiplicative
scheme as used in IdentityHashMap.
Parameters:
x - hash code |
| isEmpty | | public boolean isEmpty()(Code) | | Returns true if this map contains no key-value mappings.
true if this map contains no key-value mappings. |
| keySet | | public Set keySet()(Code) | | 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.
a set view of the keys contained in this map. |
| newTable | | protected Entry[] newTable(int capacity)(Code) | | Create table array and set the per-segment threshold *
Parameters:
capacity - table array |
| put | | public Object put(Object key, Object value)(Code) | | Maps the specified key to the specified value
in this table. Neither the key nor the value can be null.
(Note that this policy is the same as for java.util.Hashtable, but unlike
java.util.HashMap, which does accept nulls as valid keys and values.)
The value can be retrieved by calling the get method with
a key that is equal to the original key.
Parameters:
key - the table key.
Parameters:
value - the value. the previous value of the specified key in this table, ornull if it did not have one.
exception:
NullPointerException - if the key or value is null.
See Also: Object.equals(Object)
See Also: ConcurrentHashMap.get(Object) |
| putAll | | public void putAll(Map t)(Code) | | 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.
Parameters:
t - Mappings to be stored in this map. |
| rehash | | protected void rehash()(Code) | | Rehashes the contents of this map into a new table with a larger
capacity.
|
| remove | | public Object remove(Object key)(Code) | | Removes the key (and its corresponding value) from this table. This
method does nothing if the key is not in the table.
Parameters:
key - the key that needs to be removed. the value to which the key had been mapped in this table, ornull if the key did not have a mapping.
exception:
NullPointerException - if the key is null. |
| remove | | protected Object remove(Object key, Object value)(Code) | | Removes the (key, value) pair from this table. This method does nothing
if the key is not in the table, or if the key is associated with a
different value. This method is needed by EntrySet.
Parameters:
key - the key that needs to be removed.
Parameters:
value - the associated value. If the value is null, it means "anyvalue". the value to which the key had been mapped in this table, ornull if the key did not have a mapping.
exception:
NullPointerException - if the key is null. |
| resize | | protected void resize(int index, Entry[] assumedTab)(Code) | | Gather all locks in order to call rehash, by recursing within synch
blocks for each segment index.
Parameters:
index - the current segment. initially call value must be 0
Parameters:
assumedTab - the state of table on first call to resize. If this changes onany call, the attempt is aborted because the table has alreadybeen resized by another thread. |
| size | | public int size()(Code) | | Returns the number of key-value mappings in this map.
the number of key-value mappings in this map. |
| values | | public Collection values()(Code) | | 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.
a collection view of the values contained in this map. |
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