EDU.oswego.cs.dl.util.concurrent
public class ConcurrentHashMap extends AbstractMap implements Map, Cloneable, Serializable
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.
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.
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.
Nested Class Summary | |
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protected static class | ConcurrentHashMap.Entry
ConcurrentHashMap collision list entry. |
protected class | ConcurrentHashMap.HashIterator |
protected class | ConcurrentHashMap.KeyIterator |
protected static class | ConcurrentHashMap.Segment
Bookkeeping for each concurrency control segment.
|
protected class | ConcurrentHashMap.ValueIterator |
Field Summary | |
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protected static int | CONCURRENCY_LEVEL
The number of concurrency control segments.
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static int | DEFAULT_INITIAL_CAPACITY
The default initial number of table slots for this table (32).
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static float | DEFAULT_LOAD_FACTOR
The default load factor for this table (0.75)
Used when not otherwise specified in constructor.
|
protected Set | entrySet |
protected Set | keySet |
protected float | loadFactor
The load factor for the hash table.
|
protected ConcurrentHashMap.Segment[] | segments
The array of concurrency control segments.
|
protected static int | SEGMENT_MASK
Mask value for indexing into segments
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protected ConcurrentHashMap.Entry[] | table
The hash table data. |
protected int | threshold
Per-segment resize threshold.
|
protected Collection | values |
protected int | votesForResize
Number of segments voting for resize. |
Constructor Summary | |
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ConcurrentHashMap(int initialCapacity, float loadFactor)
Constructs a new, empty map with the specified initial
capacity and the specified load factor.
| |
ConcurrentHashMap(int initialCapacity)
Constructs a new, empty map with the specified initial
capacity and default load factor.
| |
ConcurrentHashMap()
Constructs a new, empty map with a default initial capacity
and default load factor. | |
ConcurrentHashMap(Map t)
Constructs a new map with the same mappings as the given map. |
Method Summary | |
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protected static int | bitcount(int w)
Return the number of set bits in w.
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void | clear()
Removes all mappings from this map. |
Object | clone()
Returns a shallow copy of this
ConcurrentHashMap instance: the keys and
values themselves are not cloned.
|
boolean | contains(Object value)
Tests 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 | elements()
Returns an enumeration of the values in this table.
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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.
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Object | get(Object key)
Returns the value to which the specified key is mapped in this table.
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protected static int | hash(Object x)
Return hash code for Object x. |
boolean | isEmpty()
Returns true if this map contains no key-value mappings.
|
Enumeration | keys()
Returns an enumeration of the keys in this table.
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Set | keySet()
Returns a set view of the keys contained in this map. |
protected ConcurrentHashMap.Entry[] | newTable(int capacity) Create table array and set the per-segment threshold * |
Object | put(Object key, Object value)
Maps the specified key to the specified
value in this table. |
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. |
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, ConcurrentHashMap.Entry[] assumedTab)
Gather all locks in order to call rehash, by
recursing within synch blocks for each segment index. |
int | size()
Returns the number of key-value mappings in this map.
|
Collection | values()
Returns a collection view of the values contained in this map. |
Serial:
Serial:
Parameters: initialCapacity the initial capacity. The actual initial capacity is rounded to the nearest power of two. loadFactor the load factor threshold, used to control resizing. This value is used in an approximate way: When at least a quarter of the segments of the table reach per-segment threshold, or one of the segments itself exceeds overall threshold, the table is doubled. This will on average cause resizing when the table-wide load factor is slightly less than the threshold. If you'd like to avoid resizing, you can set this to a ridiculously large value.
Throws: IllegalArgumentException if the load factor is nonpositive.
Parameters: initialCapacity the initial capacity of the ConcurrentHashMap.
Throws: IllegalArgumentException if the initial maximum number of elements is less than zero.
Returns: a shallow copy of this map.
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.
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 value is null
.
See Also: containsKey containsValue Map
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 key is
null
.
See Also: contains
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 value is null
.
Returns: an enumeration of the values in this table.
Returns: a collection view of the mappings contained in this map.
Parameters: key a key in the table.
Returns: the value to which the key is mapped in this table;
null
if the key is not mapped to any value in
this table.
Throws: NullPointerException if the key is
null
.
See Also: ConcurrentHashMap
Returns: true if this map contains no key-value mappings.
Returns: an enumeration of the keys in this table.
Returns: a set view of the keys contained in this map.
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. value the value.
Returns: the previous value of the specified key in this table,
or null
if it did not have one.
Throws: NullPointerException if the key or value is
null
.
See Also: Object#equals(Object) get
Parameters: t Mappings to be stored in this map.
Parameters: key the key that needs to be removed.
Returns: the value to which the key had been mapped in this table,
or null
if the key did not have a mapping.
Throws: NullPointerException if the key is
null
.
Parameters: key the key that needs to be removed. value the associated value. If the value is null, it means "any value".
Returns: the value to which the key had been mapped in this table,
or null
if the key did not have a mapping.
Throws: NullPointerException if the key is
null
.
Parameters: index the current segment. initially call value must be 0 assumedTab the state of table on first call to resize. If this changes on any call, the attempt is aborted because the table has already been resized by another thread.
Returns: the number of key-value mappings in this map.
Returns: a collection view of the values contained in this map.