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, Juergen Hoeller

Nested Class Summary

Nested Classes
Modifier and Type	Class	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.Reference`s.
`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 extends Object,V extends Object>, AbstractMap.SimpleImmutableEntry<K extends Object,V extends Object>

Constructor Summary

Constructors
Constructor	Description
`ConcurrentReferenceHashMap()`	Create a new `ConcurrentReferenceHashMap` instance.
`ConcurrentReferenceHashMap(int initialCapacity)`	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.
`ConcurrentReferenceHashMap(int initialCapacity, ConcurrentReferenceHashMap.ReferenceType referenceType)`	Create a new `ConcurrentReferenceHashMap` instance.

Method Summary

All Methods Static Methods Instance Methods Concrete Methods
Modifier and Type	Method	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()`
`V`	`getOrDefault(Object key, V defaultValue)`
`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()`
`boolean`	`isEmpty()`
`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, 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, merge, replaceAll

Methods inherited from interface java.util.Map
containsValue, equals, hashCode, 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>
  - getOrDefault
```
public V getOrDefault(Object key,
                      V defaultValue)
```
    Specified by:
    getOrDefault in interface ConcurrentMap<K,V>
    Specified by:
    getOrDefault in interface Map<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>
  - isEmpty
```
public boolean isEmpty()
```
    Specified by:
    isEmpty in interface Map<K,V>
    Overrides:
    isEmpty 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)

Class ConcurrentReferenceHashMap<K,​V>

Nested Class Summary

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

Constructor Summary

Method Summary

Methods inherited from class java.util.AbstractMap

Methods inherited from class java.lang.Object

Methods inherited from interface java.util.concurrent.ConcurrentMap

Methods inherited from interface java.util.Map

Constructor Detail

ConcurrentReferenceHashMap

ConcurrentReferenceHashMap

ConcurrentReferenceHashMap

ConcurrentReferenceHashMap

ConcurrentReferenceHashMap

ConcurrentReferenceHashMap

ConcurrentReferenceHashMap

Method Detail

getLoadFactor

getSegmentsSize

getSegment

createReferenceManager

getHash

get

getOrDefault

containsKey

getReference

put

putIfAbsent

remove

remove

replace

replace

clear

purgeUnreferencedEntries

size

isEmpty

entrySet

calculateShift

Class ConcurrentReferenceHashMap<K,V>