org.jdiagnose.concurrent
Class ConcurrentHashMap

java.lang.Object
  java.util.AbstractMap
      org.jdiagnose.concurrent.ConcurrentHashMap

All Implemented Interfaces:

java.lang.Cloneable, java.util.Map, java.io.Serializable

public class ConcurrentHashMap

extends java.util.AbstractMap

implements java.util.Map, java.lang.Cloneable, java.io.Serializable

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. ]

See Also:

Serialized Form

Nested Class Summary

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

protected static int	CONCURRENCY_LEVEL The number of concurrency control segments.
static int	DEFAULT_INITIAL_CAPACITY The default initial number of table slots for this table (32).
static float	DEFAULT_LOAD_FACTOR The default load factor for this table (0.75) Used when not otherwise specified in constructor.
protected java.util.Set	entrySet
protected java.util.Set	keySet
protected float	loadFactor The load factor for the hash table.
protected static int	SEGMENT_MASK Mask value for indexing into segments
protected ConcurrentHashMap.Segment[]	segments The array of concurrency control segments.
protected ConcurrentHashMap.Entry[]	table The hash table data.
protected int	threshold Per-segment resize threshold.
protected java.util.Collection	values
protected int	votesForResize Number of segments voting for resize.

Constructor Summary

ConcurrentHashMap()
Constructs a new, empty map with a default initial capacity and default load factor.

ConcurrentHashMap(int initialCapacity)
Constructs a new, empty map with the specified initial capacity and default load factor.

ConcurrentHashMap(int initialCapacity, float loadFactor)
Constructs a new, empty map with the specified initial capacity and the specified load factor.

ConcurrentHashMap(java.util.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.
void	clear() Removes all mappings from this map.
java.lang.Object	clone() Returns a shallow copy of this ConcurrentHashMap instance: the keys and values themselves are not cloned.
boolean	contains(java.lang.Object value) Tests 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	elements() Returns an enumeration of the values in this table.
java.util.Set	entrySet() Returns a collection view of the mappings contained in this map.
protected boolean	eq(java.lang.Object x, java.lang.Object y) Check for equality of non-null references x and y.
java.lang.Object	get(java.lang.Object key) Returns the value to which the specified key is mapped in this table.
protected static int	hash(java.lang.Object x) Return hash code for Object x.
boolean	isEmpty() Returns true if this map contains no key-value mappings.
java.util.Enumeration	keys() Returns an enumeration of the keys in this table.
java.util.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
java.lang.Object	put(java.lang.Object key, java.lang.Object value) Maps the specified key to the specified value in this table.
void	putAll(java.util.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.
java.lang.Object	remove(java.lang.Object key) Removes the key (and its corresponding value) from this table.
protected java.lang.Object	remove(java.lang.Object key, java.lang.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.
java.util.Collection	values() Returns a collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap

equals, hashCode, toString

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Map

equals, hashCode

Field Detail

table

protected transient ConcurrentHashMap.Entry[] table

The hash table data.

CONCURRENCY_LEVEL

protected static final int CONCURRENCY_LEVEL

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 :-)

See Also:

Constant Field Values

SEGMENT_MASK

protected static final int SEGMENT_MASK

Mask value for indexing into segments

See Also:

Constant Field Values

segments

protected final ConcurrentHashMap.Segment[] segments

The array of concurrency control segments.

DEFAULT_INITIAL_CAPACITY

public static int DEFAULT_INITIAL_CAPACITY

The default initial number of table slots for this table (32). Used when not otherwise specified in constructor.

DEFAULT_LOAD_FACTOR

public static final float DEFAULT_LOAD_FACTOR

The default load factor for this table (0.75) Used when not otherwise specified in constructor.

See Also:

Constant Field Values

loadFactor

protected final float loadFactor

The load factor for the hash table.

threshold

protected int threshold

Per-segment resize threshold.

votesForResize

protected transient volatile int votesForResize

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.

keySet

protected transient java.util.Set keySet

entrySet

protected transient java.util.Set entrySet

values

protected transient java.util.Collection values

Constructor Detail

ConcurrentHashMap

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. 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:

java.lang.IllegalArgumentException - if the load factor is nonpositive.

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity)

Constructs a new, empty map with the specified initial capacity and default load factor.

Parameters:

initialCapacity - the initial capacity of the ConcurrentHashMap.

Throws:

java.lang.IllegalArgumentException - if the initial maximum number of elements is less than zero.

ConcurrentHashMap

public ConcurrentHashMap()

Constructs a new, empty map with a default initial capacity and default load factor.

ConcurrentHashMap

public ConcurrentHashMap(java.util.Map t)

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.

Method Detail

bitcount

protected static int bitcount(int w)

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

hash

protected static int hash(java.lang.Object x)

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.

eq

protected boolean eq(java.lang.Object x,
                     java.lang.Object y)

Check for equality of non-null references x and y.

newTable

protected ConcurrentHashMap.Entry[] newTable(int capacity)

Create table array and set the per-segment threshold

size

public int size()

Returns the number of key-value mappings in this map.

Specified by:

size in interface java.util.Map

Returns:

the number of key-value mappings in this map.

isEmpty

public boolean isEmpty()

Returns true if this map contains no key-value mappings.

Specified by:

isEmpty in interface java.util.Map

Returns:

true if this map contains no key-value mappings.

get

public java.lang.Object get(java.lang.Object key)

Returns the value to which the specified key is mapped in this table.

Specified by:

get in interface java.util.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:

java.lang.NullPointerException - if the key is null.

See Also:

put(Object, Object)

containsKey

public boolean containsKey(java.lang.Object key)

Tests if the specified object is a key in this table.

Specified by:

containsKey in interface java.util.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:

java.lang.NullPointerException - if the key is null.

See Also:

contains(Object)

put

public java.lang.Object put(java.lang.Object key,
                            java.lang.Object value)

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.

Specified by:

put in interface java.util.Map

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:

java.lang.NullPointerException - if the key or value is null.

See Also:

Object.equals(Object), get(Object)

resize

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.

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.

rehash

protected void rehash()

Rehashes the contents of this map into a new table with a larger capacity.

remove

public java.lang.Object remove(java.lang.Object key)

Removes the key (and its corresponding value) from this table. This method does nothing if the key is not in the table.

Specified by:

remove in interface java.util.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:

java.lang.NullPointerException - if the key is null.

remove

protected java.lang.Object remove(java.lang.Object key,
                                  java.lang.Object value)

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.

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:

java.lang.NullPointerException - if the key is null.

containsValue

public boolean containsValue(java.lang.Object value)

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.

Specified by:

containsValue in interface java.util.Map

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:

java.lang.NullPointerException - if the value is null.

contains

public boolean contains(java.lang.Object value)

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.

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:

java.lang.NullPointerException - if the value is null.

See Also:

containsKey(Object), containsValue(Object), Map

putAll

public void putAll(java.util.Map t)

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.

Specified by:

putAll in interface java.util.Map

Parameters:

t - Mappings to be stored in this map.

clear

public void clear()

Removes all mappings from this map.

Specified by:

clear in interface java.util.Map

clone

public java.lang.Object clone()

Returns a shallow copy of this ConcurrentHashMap instance: the keys and values themselves are not cloned.

Returns:

a shallow copy of this map.

keySet

public java.util.Set keySet()

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.

Specified by:

keySet in interface java.util.Map

Returns:

a set view of the keys contained in this map.

values

public java.util.Collection values()

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.

Specified by:

values in interface java.util.Map

Returns:

a collection view of the values contained in this map.

entrySet

public java.util.Set entrySet()

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.

Specified by:

entrySet in interface java.util.Map

Returns:

a collection view of the mappings contained in this map.

keys

public java.util.Enumeration keys()

Returns an enumeration of the keys in this table.

Returns:

an enumeration of the keys in this table.

See Also:

Enumeration, elements(), keySet(), Map

elements

public java.util.Enumeration elements()

Returns an enumeration of the values in this table. Use the Enumeration methods on the returned object to fetch the elements sequentially.

Returns:

an enumeration of the values in this table.

See Also:

Enumeration, keys(), values(), Map