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:

java.util.concurrent.ConcurrentMap<K,V>, java.util.Map<K,V>

public class ConcurrentReferenceHashMap<K,V>
extends java.util.AbstractMap<K,V>
implements java.util.concurrent.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 and Description
private class	ConcurrentReferenceHashMap.Entries Allows a task access to ConcurrentReferenceHashMap.Segment entries.
protected static class	ConcurrentReferenceHashMap.Entry<K,V> A single map entry.
private class	ConcurrentReferenceHashMap.EntryIterator Internal entry iterator implementation.
private class	ConcurrentReferenceHashMap.EntrySet Internal entry-set implementation.
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 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.
private static class	ConcurrentReferenceHashMap.SoftEntryReference<K,V> Internal ConcurrentReferenceHashMap.Reference implementation for SoftReferences.
private class	ConcurrentReferenceHashMap.Task<T> A task that can be run against a ConcurrentReferenceHashMap.Segment.
private static class	ConcurrentReferenceHashMap.TaskOption Various options supported by a Task.
private static class	ConcurrentReferenceHashMap.WeakEntryReference<K,V> Internal ConcurrentReferenceHashMap.Reference implementation for WeakReferences.

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

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

Field Summary

Fields

Modifier and Type	Field and Description
private static int	DEFAULT_CONCURRENCY_LEVEL
private static int	DEFAULT_INITIAL_CAPACITY
private static float	DEFAULT_LOAD_FACTOR
private static ConcurrentReferenceHashMap.ReferenceType	DEFAULT_REFERENCE_TYPE
private java.util.Set<java.util.Map.Entry<K,V>>	entrySet Late binding entry set.
private float	loadFactor When the average number of references per table exceeds this value resize will be attempted.
private static int	MAXIMUM_CONCURRENCY_LEVEL
private static int	MAXIMUM_SEGMENT_SIZE
private ConcurrentReferenceHashMap.ReferenceType	referenceType The reference type: SOFT or WEAK.
private ConcurrentReferenceHashMap.Segment[]	segments Array of segments indexed using the high order bits from the hash.
private int	shift The shift value used to calculate the size of the segments array and an index from the hash.

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(java.lang.Object key)
protected ConcurrentReferenceHashMap.ReferenceManager	createReferenceManager() Factory method that returns the ConcurrentReferenceHashMap.ReferenceManager.
private <T> T	doTask(java.lang.Object key, ConcurrentReferenceHashMap.Task<T> task)
java.util.Set<java.util.Map.Entry<K,V>>	entrySet()
V	get(java.lang.Object key)
private ConcurrentReferenceHashMap.Entry<K,V>	getEntryIfAvailable(java.lang.Object key)
protected int	getHash(java.lang.Object o) Get the hash for a given object, apply an additional hash function to reduce collisions.
protected float	getLoadFactor()
V	getOrDefault(java.lang.Object key, V defaultValue)
protected ConcurrentReferenceHashMap.Reference<K,V>	getReference(java.lang.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)
private ConcurrentReferenceHashMap.Segment	getSegmentForHash(int hash)
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)
private V	put(K key, V value, boolean overwriteExisting)
V	putIfAbsent(K key, V value)
V	remove(java.lang.Object key)
boolean	remove(java.lang.Object key, java.lang.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

Field Detail

DEFAULT_INITIAL_CAPACITY

private static final int DEFAULT_INITIAL_CAPACITY

See Also:

Constant Field Values

DEFAULT_LOAD_FACTOR

private static final float DEFAULT_LOAD_FACTOR

See Also:

Constant Field Values

DEFAULT_CONCURRENCY_LEVEL

private static final int DEFAULT_CONCURRENCY_LEVEL

See Also:

Constant Field Values

DEFAULT_REFERENCE_TYPE

private static final ConcurrentReferenceHashMap.ReferenceType DEFAULT_REFERENCE_TYPE

MAXIMUM_CONCURRENCY_LEVEL

private static final int MAXIMUM_CONCURRENCY_LEVEL

See Also:

Constant Field Values

MAXIMUM_SEGMENT_SIZE

private static final int MAXIMUM_SEGMENT_SIZE

See Also:

Constant Field Values

segments

private final ConcurrentReferenceHashMap.Segment[] segments

Array of segments indexed using the high order bits from the hash.

loadFactor

private final float loadFactor

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

referenceType

private final ConcurrentReferenceHashMap.ReferenceType referenceType

The reference type: SOFT or WEAK.

shift

private final int shift

The shift value used to calculate the size of the segments array and an index from the hash.

entrySet

@Nullable
private volatile java.util.Set<java.util.Map.Entry<K,V>> entrySet

Late binding entry set.

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(@Nullable
                      java.lang.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

@Nullable
public V get(@Nullable
                       java.lang.Object key)

Specified by:

get in interface java.util.Map<K,V>

Overrides:

get in class java.util.AbstractMap<K,V>

getOrDefault

@Nullable
public V getOrDefault(@Nullable
                                java.lang.Object key,
                                @Nullable
                                V defaultValue)

Specified by:

getOrDefault in interface java.util.concurrent.ConcurrentMap<K,V>

Specified by:

getOrDefault in interface java.util.Map<K,V>

containsKey

public boolean containsKey(@Nullable
                           java.lang.Object key)

Specified by:

containsKey in interface java.util.Map<K,V>

Overrides:

containsKey in class java.util.AbstractMap<K,V>

getEntryIfAvailable

@Nullable
private ConcurrentReferenceHashMap.Entry<K,V> getEntryIfAvailable(@Nullable
                                                                            java.lang.Object key)

getReference

@Nullable
protected final ConcurrentReferenceHashMap.Reference<K,V> getReference(@Nullable
                                                                                 java.lang.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

@Nullable
public V put(@Nullable
                       K key,
                       @Nullable
                       V value)

Specified by:

put in interface java.util.Map<K,V>

Overrides:

put in class java.util.AbstractMap<K,V>

putIfAbsent

@Nullable
public V putIfAbsent(@Nullable
                               K key,
                               @Nullable
                               V value)

Specified by:

putIfAbsent in interface java.util.concurrent.ConcurrentMap<K,V>

Specified by:

putIfAbsent in interface java.util.Map<K,V>

put

@Nullable
private V put(@Nullable
                        K key,
                        @Nullable
                        V value,
                        boolean overwriteExisting)

remove

@Nullable
public V remove(java.lang.Object key)

Specified by:

remove in interface java.util.Map<K,V>

Overrides:

remove in class java.util.AbstractMap<K,V>

remove

public boolean remove(java.lang.Object key,
                      java.lang.Object value)

Specified by:

remove in interface java.util.concurrent.ConcurrentMap<K,V>

Specified by:

remove in interface java.util.Map<K,V>

replace

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

Specified by:

replace in interface java.util.concurrent.ConcurrentMap<K,V>

Specified by:

replace in interface java.util.Map<K,V>

replace

@Nullable
public V replace(K key,
                           V value)

Specified by:

replace in interface java.util.concurrent.ConcurrentMap<K,V>

Specified by:

replace in interface java.util.Map<K,V>

clear

public void clear()

Specified by:

clear in interface java.util.Map<K,V>

Overrides:

clear in class java.util.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 java.util.Map<K,V>

Overrides:

size in class java.util.AbstractMap<K,V>

isEmpty

public boolean isEmpty()

Specified by:

isEmpty in interface java.util.Map<K,V>

Overrides:

isEmpty in class java.util.AbstractMap<K,V>

entrySet

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

Specified by:

entrySet in interface java.util.Map<K,V>

Specified by:

entrySet in class java.util.AbstractMap<K,V>

doTask

@Nullable
private <T> T doTask(@Nullable
                               java.lang.Object key,
                               ConcurrentReferenceHashMap.Task<T> task)

getSegmentForHash

private ConcurrentReferenceHashMap.Segment getSegmentForHash(int hash)

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)