Class ClockMap<K,V>

java.lang.Object

java.util.AbstractMap<K,V>

org.apache.cayenne.util.ClockMap<K,V>

All Implemented Interfaces:

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

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

A bounded concurrent map that evicts the entries least recently read. It is a ConcurrentHashMap with the CLOCK ("second chance") replacement policy on top: each entry carries a "referenced" bit that a read sets, and once the map outgrows its maximum size, a sweep walks the entries, clearing the bit of those that were read since its last pass and removing those that were not. The policy approximates LRU, with a read costing one hash lookup and, at most once per sweep revolution, one bit write: no allocation, no queue and no linked list is touched on a read.

The sweep runs on the thread that inserts past the limit and is budgeted, so that no single insert pays for a whole revolution. The map may therefore briefly hold a few entries more than its maximum size. A single sweep runs at a time; inserts that find it running skip it, and the next insert catches up.

Iteration views are weakly consistent, like those of ConcurrentHashMap, and do not count as reads.

Since:

5.0

Nested Class Summary

Nested classes/interfaces inherited from class AbstractMap

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

Constructor Summary

Constructors

Constructor	Description
ClockMap(int maxSize)

Method Summary

Modifier and Type	Method	Description
void	clear()
boolean	containsKey(Object key)
Set<Map.Entry<K,V>>	entrySet()
V	get(Object key)
boolean	isEmpty()
Set<K>	keySet()
V	merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)	Atomic, like ConcurrentHashMap.merge(Object, Object, BiFunction): the remapping function runs under the lock of the key's hash bin and must be short and free of side effects on this map.
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 AbstractMap

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

Methods inherited from class Object

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

Methods inherited from interface ConcurrentMap

compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, replaceAll

Methods inherited from interface Map

containsValue, equals, hashCode, putAll, values

Constructor Details

ClockMap

public ClockMap(int maxSize)

Parameters:

maxSize - the number of entries past which inserts evict. Zero or less keeps the map empty.

Method Details

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>

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 V replace(K key,
 V value)

Specified by:

replace in interface ConcurrentMap<K,V>

Specified by:

replace 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>

merge

public V merge(K key,
 V value,
 BiFunction<? super V, ? super V, ? extends V> remappingFunction)

Atomic, like ConcurrentHashMap.merge(Object, Object, BiFunction): the remapping function runs under the lock of the key's hash bin and must be short and free of side effects on this map.

Specified by:

merge in interface ConcurrentMap<K,V>

Specified by:

merge in interface Map<K,V>

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>

clear

public void clear()

Specified by:

clear in interface Map<K,V>

Overrides:

clear in class AbstractMap<K,V>

keySet

public Set<K> keySet()

Specified by:

keySet in interface Map<K,V>

Overrides:

keySet 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>