org.springframework.util

Class ConcurrentReferenceHashMap<K,V>

java.lang.Object

java.util.AbstractMap<K,V>

org.springframework.util.ConcurrentReferenceHashMap<K,V>

Type Parameters:

K - The key type

V - The value type

All Implemented Interfaces:

ConcurrentMap<K,V>, Map<K,V>

public class ConcurrentReferenceHashMap<K,V>
extends AbstractMap<K,V>
implements ConcurrentMap<K,V>

A ConcurrentHashMap that uses soft or weak references for both keys and values.

This class can be used as an alternative to Collections.synchronizedMap(new WeakHashMap<K, Reference<V>>()) in order to support better performance when accessed concurrently. This implementation follows the same design constraints as ConcurrentHashMap with the exception that null values and null keys are supported.

NOTE: The use of references means that there is no guarantee that items placed into the map will be subsequently available. The garbage collector may discard references at any time, so it may appear that an unknown thread is silently removing entries.

If not explicitly specified, this implementation will use soft entry references.

Since:

3.2

Author:

Phillip Webb

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
protected static class	ConcurrentReferenceHashMap.Entry<K,V> A single map entry.
protected static interface	ConcurrentReferenceHashMap.Reference<K,V> A reference to an ConcurrentReferenceHashMap.Entry contained in the map.
protected class	ConcurrentReferenceHashMap.ReferenceManager Strategy class used to manage ConcurrentReferenceHashMap.References.
static class	ConcurrentReferenceHashMap.ReferenceType Various reference types supported by this map.
protected static class	ConcurrentReferenceHashMap.Restructure The types of restructuring that can be performed.
protected class	ConcurrentReferenceHashMap.Segment A single segment used to divide the map to allow better concurrent performance.

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

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

Constructor Summary

Constructors

Constructor and Description
ConcurrentReferenceHashMap() Create a new ConcurrentReferenceHashMap instance.
ConcurrentReferenceHashMap(int initialCapacity) Create a new ConcurrentReferenceHashMap instance.
ConcurrentReferenceHashMap(int initialCapacity, ConcurrentReferenceHashMap.ReferenceType referenceType) Create a new ConcurrentReferenceHashMap instance.
ConcurrentReferenceHashMap(int initialCapacity, float loadFactor) Create a new ConcurrentReferenceHashMap instance.
ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) Create a new ConcurrentReferenceHashMap instance.
ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel, ConcurrentReferenceHashMap.ReferenceType referenceType) Create a new ConcurrentReferenceHashMap instance.
ConcurrentReferenceHashMap(int initialCapacity, int concurrencyLevel) Create a new ConcurrentReferenceHashMap instance.

Method Summary

Modifier and Type	Method and Description
protected static int	calculateShift(int minimumValue, int maximumValue) Calculate a shift value that can be used to create a power-of-two value between the specified maximum and minimum values.
void	clear()
boolean	containsKey(Object key)
protected ConcurrentReferenceHashMap.ReferenceManager	createReferenceManager() Factory method that returns the ConcurrentReferenceHashMap.ReferenceManager.
Set<Map.Entry<K,V>>	entrySet()
V	get(Object key)
protected int	getHash(Object o) Get the hash for a given object, apply an additional hash function to reduce collisions.
protected float	getLoadFactor()
protected ConcurrentReferenceHashMap.Reference<K,V>	getReference(Object key, ConcurrentReferenceHashMap.Restructure restructure) Return a ConcurrentReferenceHashMap.Reference to the ConcurrentReferenceHashMap.Entry for the specified key, or null if not found.
protected ConcurrentReferenceHashMap.Segment	getSegment(int index)
protected int	getSegmentsSize()
void	purgeUnreferencedEntries() Remove any entries that have been garbage collected and are no longer referenced.
V	put(K key, V value)
V	putIfAbsent(K key, V value)
V	remove(Object key)
boolean	remove(Object key, Object value)
V	replace(K key, V value)
boolean	replace(K key, V oldValue, V newValue)
int	size()

Methods inherited from class java.util.AbstractMap

clone, containsValue, equals, hashCode, isEmpty, keySet, putAll, toString, values

Methods inherited from class java.lang.Object

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

Methods inherited from interface java.util.concurrent.ConcurrentMap

compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, replaceAll

Methods inherited from interface java.util.Map

containsValue, equals, hashCode, isEmpty, keySet, putAll, values

Constructor Detail

ConcurrentReferenceHashMap

public ConcurrentReferenceHashMap()

Create a new ConcurrentReferenceHashMap instance.

ConcurrentReferenceHashMap

public ConcurrentReferenceHashMap(int initialCapacity)

Create a new ConcurrentReferenceHashMap instance.

Parameters:

initialCapacity - the initial capacity of the map

ConcurrentReferenceHashMap

public ConcurrentReferenceHashMap(int initialCapacity,
                                  float loadFactor)

Create a new ConcurrentReferenceHashMap instance.

Parameters:

initialCapacity - the initial capacity of the map

loadFactor - the load factor. When the average number of references per table exceeds this value resize will be attempted

ConcurrentReferenceHashMap

public ConcurrentReferenceHashMap(int initialCapacity,
                                  int concurrencyLevel)

Create a new ConcurrentReferenceHashMap instance.

Parameters:

initialCapacity - the initial capacity of the map

concurrencyLevel - the expected number of threads that will concurrently write to the map

ConcurrentReferenceHashMap

public ConcurrentReferenceHashMap(int initialCapacity,
                                  ConcurrentReferenceHashMap.ReferenceType referenceType)

Create a new ConcurrentReferenceHashMap instance.

Parameters:

initialCapacity - the initial capacity of the map

referenceType - the reference type used for entries (soft or weak)

ConcurrentReferenceHashMap

public ConcurrentReferenceHashMap(int initialCapacity,
                                  float loadFactor,
                                  int concurrencyLevel)

Create a new ConcurrentReferenceHashMap instance.

Parameters:

initialCapacity - the initial capacity of the map

loadFactor - the load factor. When the average number of references per table exceeds this value, resize will be attempted.

concurrencyLevel - the expected number of threads that will concurrently write to the map

ConcurrentReferenceHashMap

public ConcurrentReferenceHashMap(int initialCapacity,
                                  float loadFactor,
                                  int concurrencyLevel,
                                  ConcurrentReferenceHashMap.ReferenceType referenceType)

Create a new ConcurrentReferenceHashMap instance.

Parameters:

initialCapacity - the initial capacity of the map

loadFactor - the load factor. When the average number of references per table exceeds this value, resize will be attempted.

concurrencyLevel - the expected number of threads that will concurrently write to the map

referenceType - the reference type used for entries (soft or weak)

Method Detail

getLoadFactor

protected final float getLoadFactor()

getSegmentsSize

protected final int getSegmentsSize()

getSegment

protected final ConcurrentReferenceHashMap.Segment getSegment(int index)

createReferenceManager

protected ConcurrentReferenceHashMap.ReferenceManager createReferenceManager()

Factory method that returns the ConcurrentReferenceHashMap.ReferenceManager. This method will be called once for each ConcurrentReferenceHashMap.Segment.

Returns:

a new reference manager

getHash

protected int getHash(Object o)

Get the hash for a given object, apply an additional hash function to reduce collisions. This implementation uses the same Wang/Jenkins algorithm as ConcurrentHashMap. Subclasses can override to provide alternative hashing.

Parameters:

o - the object to hash (may be null)

Returns:

the resulting hash code

get

public V get(Object key)

Specified by:

get in interface Map<K,V>

Overrides:

get in class AbstractMap<K,V>

containsKey

public boolean containsKey(Object key)

Specified by:

containsKey in interface Map<K,V>

Overrides:

containsKey in class AbstractMap<K,V>

getReference

protected final ConcurrentReferenceHashMap.Reference<K,V> getReference(Object key,
                                                                       ConcurrentReferenceHashMap.Restructure restructure)

Return a ConcurrentReferenceHashMap.Reference to the ConcurrentReferenceHashMap.Entry for the specified key, or null if not found.

Parameters:

key - the key (can be null)

restructure - types of restructure allowed during this call

Returns:

the reference, or null if not found

put

public V put(K key,
             V value)

Specified by:

put in interface Map<K,V>

Overrides:

put in class AbstractMap<K,V>

putIfAbsent

public V putIfAbsent(K key,
                     V value)

Specified by:

putIfAbsent in interface ConcurrentMap<K,V>

Specified by:

putIfAbsent in interface Map<K,V>

remove

public V remove(Object key)

Specified by:

remove in interface Map<K,V>

Overrides:

remove in class AbstractMap<K,V>

remove

public boolean remove(Object key,
                      Object value)

Specified by:

remove in interface ConcurrentMap<K,V>

Specified by:

remove in interface Map<K,V>

replace

public boolean replace(K key,
                       V oldValue,
                       V newValue)

Specified by:

replace in interface ConcurrentMap<K,V>

Specified by:

replace in interface Map<K,V>

replace

public V replace(K key,
                 V value)

Specified by:

replace in interface ConcurrentMap<K,V>

Specified by:

replace in interface Map<K,V>

clear

public void clear()

Specified by:

clear in interface Map<K,V>

Overrides:

clear in class AbstractMap<K,V>

purgeUnreferencedEntries

public void purgeUnreferencedEntries()

Remove any entries that have been garbage collected and are no longer referenced. Under normal circumstances garbage collected entries are automatically purged as items are added or removed from the Map. This method can be used to force a purge, and is useful when the Map is read frequently but updated less often.

size

public int size()

Specified by:

size in interface Map<K,V>

Overrides:

size in class AbstractMap<K,V>

entrySet

public Set<Map.Entry<K,V>> entrySet()

Specified by:

entrySet in interface Map<K,V>

Specified by:

entrySet in class AbstractMap<K,V>

calculateShift

protected static int calculateShift(int minimumValue,
                                    int maximumValue)

Calculate a shift value that can be used to create a power-of-two value between the specified maximum and minimum values.

Parameters:

minimumValue - the minimum value

maximumValue - the maximum value

Returns:

the calculated shift (use 1 << shift to obtain a value)