org.apache.openjpa.lib.util.concurrent
Class ConcurrentHashMap

java.lang.Object
  java.util.AbstractMap
      org.apache.openjpa.lib.util.concurrent.ConcurrentHashMap

All Implemented Interfaces:

Serializable, Cloneable, Map, ConcurrentMap, SizedMap

public class ConcurrentHashMap
extends AbstractMap
implements ConcurrentMap, SizedMap, Cloneable, Serializable

This class implements a HashMap which has limited synchronization. In particular mutators are generally synchronized while accessors are generally not. Additionally the Iterators returned by this class are not "fail-fast", but instead try to continue to iterate over the data structure after changes have been made. The synchronization semantics are built right in to the implementation rather than using a delegating wrapper like the other collection classes do because it wasn't clear to me that the how the two should be seperated or that it would be useful to do so. This can probably be a topic for further debate in the future. This class is based heavily on the HashMap class in the Java collections package.

See Also:

Serialized Form

Nested Class Summary

protected static class

ConcurrentHashMap.Entry

Nested classes/interfaces inherited from class java.util.AbstractMap

AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>

Field Summary

(package private) static double[]

RANDOMS Cache of random numbers used in "random" methods, since generating them is expensive.

Constructor Summary

ConcurrentHashMap()
Constructs an empty ConcurrentHashMap with the default initial capacity(16) and the default load factor(0.75).

ConcurrentHashMap(int initialCapacity)
Constructs an empty ConcurrentHashMap with the specified initial capacity and the default load factor(0.75).

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

ConcurrentHashMap(Map m)
Constructs a new ConcurrentHashMap with the same mappings as the specified Map.

Method Summary

int	capacity() Returns the current capacity of backing table in this map.
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	containsKey(Object key) Returns true if this map contains a mapping for the specified key.
boolean	containsValue(Object value) Returns true if this map maps one or more keys to the specified value.
protected ConcurrentHashMap.Entry	createEntry(int h, Object k, Object v, ConcurrentHashMap.Entry n)
Set	entrySet() Returns a collection view of the mappings contained in this map.
Object	get(Object key) Returns the value to which the specified key is mapped in this identity hash map, or null if the map contains no mapping for this key.
protected ConcurrentHashMap.Entry	getEntry(Object key) Returns the entry associated with the specified key in the ConcurrentHashMap.
int	getMaxSize() The maximum number of entries, or Integer.MAX_VALUE for no limit.
boolean	isEmpty() Returns true if this map contains no key-value mappings.
boolean	isFull() Whether the map is full.
Set	keySet() Returns a set view of the keys contained in this map.
float	loadFactor() Returns the load factor for this map.
void	overflowRemoved(Object key, Object value) Overridable callback for when an overflow entry is automatically removed.
Object	put(Object key, Object value) Associates the specified value with the specified key in this map.
void	putAll(Map t) Copies all of the mappings from the specified map to this map These mappings will replace any mappings that this map had for any of the keys currently in the specified map.
Object	putIfAbsent(Object key, Object value)
Iterator	randomEntryIterator() Iterate over map entries, beginning at an arbitrary (not strictly random) entry.
Object	remove(Object key) Removes the mapping for this key from this map if present.
Map.Entry	removeRandom() Remove an arbitrary(not strictly random) entry from the map.
void	setMaxSize(int maxSize) The maximum number of entries, or Integer.MAX_VALUE for no limit.
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.

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

RANDOMS

static final double[] RANDOMS

Cache of random numbers used in "random" methods, since generating them is expensive. We hope each map changes enough between cycling through this list that the overall effect is random enough.

Constructor Detail

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity,
                         float loadFactor)

Constructs an empty ConcurrentHashMap with the specified initial capacity and load factor.

Parameters:

initialCapacity - The initial capacity.

loadFactor - The load factor.

Throws:

IllegalArgumentException - if the initial capacity is negative or the load factor is nonpositive.

ConcurrentHashMap

public ConcurrentHashMap(int initialCapacity)

Constructs an empty ConcurrentHashMap with the specified initial capacity and the default load factor(0.75).

Parameters:

initialCapacity - the initial capacity.

Throws:

IllegalArgumentException - if the initial capacity is negative.

ConcurrentHashMap

public ConcurrentHashMap()

Constructs an empty ConcurrentHashMap with the default initial capacity(16) and the default load factor(0.75).

ConcurrentHashMap

public ConcurrentHashMap(Map m)

Constructs a new ConcurrentHashMap with the same mappings as the specified Map. The ConcurrentHashMap is created with default load factor(0.75) and an initial capacity sufficient to hold the mappings in the specified Map.

Parameters:

m - the map whose mappings are to be placed in this map.

Throws:

NullPointerException - if the specified map is null.

Method Detail

capacity

public final int capacity()

Returns the current capacity of backing table in this map.

Returns:

the current capacity of backing table in this map.

loadFactor

public final float loadFactor()

Returns the load factor for this map.

Returns:

the load factor for this map.

getMaxSize

public int getMaxSize()

Description copied from interface: SizedMap

The maximum number of entries, or Integer.MAX_VALUE for no limit.

Specified by:

getMaxSize in interface SizedMap

setMaxSize

public void setMaxSize(int maxSize)

Description copied from interface: SizedMap

The maximum number of entries, or Integer.MAX_VALUE for no limit.

Specified by:

setMaxSize in interface SizedMap

isFull

public boolean isFull()

Description copied from interface: SizedMap

Whether the map is full.

Specified by:

isFull in interface SizedMap

overflowRemoved

public void overflowRemoved(Object key,
                            Object value)

Description copied from interface: SizedMap

Overridable callback for when an overflow entry is automatically removed.

Specified by:

overflowRemoved in interface SizedMap

size

public final int size()

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

Specified by:

size in interface Map

Overrides:

size in class AbstractMap

Returns:

the number of key-value mappings in this map.

isEmpty

public final boolean isEmpty()

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

Specified by:

isEmpty in interface Map

Overrides:

isEmpty in class AbstractMap

Returns:

true if this map contains no key-value mappings.

get

public Object get(Object key)

Returns the value to which the specified key is mapped in this identity hash map, or null if the map contains no mapping for this key. A return value of null does not necessarily indicate that the map contains no mapping for the key; it is also possible that the map explicitly maps the key to null. The containsKey method may be used to distinguish these two cases.

Specified by:

get in interface Map

Overrides:

get in class AbstractMap

Parameters:

key - the key whose associated value is to be returned.

Returns:

the value to which this map maps the specified key, or null if the map contains no mapping for this key.

See Also:

put(Object, Object)

containsKey

public final boolean containsKey(Object key)

Returns true if this map contains a mapping for the specified key.

Specified by:

containsKey in interface Map

Overrides:

containsKey in class AbstractMap

Parameters:

key - The key whose presence in this map is to be tested

Returns:

true if this map contains a mapping for the specified key.

getEntry

protected ConcurrentHashMap.Entry getEntry(Object key)

Returns the entry associated with the specified key in the ConcurrentHashMap. Returns null if the ConcurrentHashMap contains no mapping for this key.

put

public Object put(Object key,
                  Object value)

Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.

Specified by:

put in interface Map

Overrides:

put in class AbstractMap

Parameters:

key - key with which the specified value is to be associated.

value - value to be associated with the specified key.

Returns:

previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the ConcurrentHashMap previously associated null with the specified key.

putIfAbsent

public Object putIfAbsent(Object key,
                          Object value)

putAll

public final void putAll(Map t)

Copies all of the mappings from the specified map to this map These mappings will replace any mappings that this map had for any of the keys currently in the specified map.

Specified by:

putAll in interface Map

Overrides:

putAll in class AbstractMap

Parameters:

t - mappings to be stored in this map.

Throws:

NullPointerException - if the specified map is null.

remove

public Object remove(Object key)

Removes the mapping for this key from this map if present.

Specified by:

remove in interface Map

Overrides:

remove in class AbstractMap

Parameters:

key - key whose mapping is to be removed from the map.

Returns:

previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key.

clear

public void clear()

Removes all mappings from this map.

Specified by:

clear in interface Map

Overrides:

clear in class AbstractMap

removeRandom

public Map.Entry removeRandom()

Description copied from interface: ConcurrentMap

Remove an arbitrary(not strictly random) entry from the map. This allows implementation of concurrent caches with size ceilings.

Specified by:

removeRandom in interface ConcurrentMap

Returns:

the removed entry, or null if map is empty

randomEntryIterator

public Iterator randomEntryIterator()

Description copied from interface: ConcurrentMap

Iterate over map entries, beginning at an arbitrary (not strictly random) entry.

Specified by:

randomEntryIterator in interface ConcurrentMap

containsValue

public final boolean containsValue(Object value)

Returns true if this map maps one or more keys to the specified value.

Specified by:

containsValue in interface Map

Overrides:

containsValue in class AbstractMap

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.

clone

public final Object clone()

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

Overrides:

clone in class AbstractMap

Returns:

a shallow copy of this map.

createEntry

protected ConcurrentHashMap.Entry createEntry(int h,
                                              Object k,
                                              Object v,
                                              ConcurrentHashMap.Entry n)

keySet

public final 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 Map

Overrides:

keySet in class AbstractMap

Returns:

a set view of the keys contained in this map.

values

public final 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 Map

Overrides:

values in class AbstractMap

Returns:

a collection view of the values contained in this map.

entrySet

public final 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 Map

Specified by:

entrySet in class AbstractMap

Returns:

a collection view of the mappings contained in this map.

See Also:

ConcurrentHashMap.Entry