com.wl4g.component.common.collection.ConcurrentReferenceHashMap Maven / Gradle / Ivy
/*
* Copyright 2017 ~ 2025 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.wl4g.component.common.collection;
import static com.wl4g.component.common.lang.Assert2.*;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.lang.reflect.Array;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collections;
import java.util.EnumSet;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.locks.ReentrantLock;
import com.wl4g.component.common.lang.ObjectUtils2;
/**
* A {@link ConcurrentHashMap} that uses {@link ReferenceType#SOFT soft} or
* {@linkplain ReferenceType#WEAK weak} references for both {@code keys} and
* {@code values}.
*
*
* This class can be used as an alternative to
* {@code Collections.synchronizedMap(new WeakHashMap>())} in
* order to support better performance when accessed concurrently. This
* implementation follows the same design constraints as
* {@link ConcurrentHashMap} with the exception that {@code null} values and
* {@code 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
* {@linkplain SoftReference soft entry references}.
*
* @author Phillip Webb
* @since 3.2
* @param
* the key type
* @param
* the value type
* @see {@link org.springframework.util.ConcurrentReferenceHashMap}
*/
public class ConcurrentReferenceHashMap extends AbstractMap implements ConcurrentMap {
private static final int DEFAULT_INITIAL_CAPACITY = 16;
private static final float DEFAULT_LOAD_FACTOR = 0.75f;
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
private static final ReferenceType DEFAULT_REFERENCE_TYPE = ReferenceType.SOFT;
private static final int MAXIMUM_CONCURRENCY_LEVEL = 1 << 16;
private static final int MAXIMUM_SEGMENT_SIZE = 1 << 30;
/**
* Array of segments indexed using the high order bits from the hash.
*/
private final Segment[] segments;
/**
* When the average number of references per table exceeds this value resize
* will be attempted.
*/
private final float loadFactor;
/**
* The reference type: SOFT or WEAK.
*/
private final ReferenceType referenceType;
/**
* The shift value used to calculate the size of the segments array and an
* index from the hash.
*/
private final int shift;
/**
* Late binding entry set.
*/
private Set> entrySet;
/**
* Create a new {@code ConcurrentReferenceHashMap} instance.
*/
public ConcurrentReferenceHashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL, DEFAULT_REFERENCE_TYPE);
}
/**
* Create a new {@code ConcurrentReferenceHashMap} instance.
*
* @param initialCapacity
* the initial capacity of the map
*/
public ConcurrentReferenceHashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL, DEFAULT_REFERENCE_TYPE);
}
/**
* Create a new {@code ConcurrentReferenceHashMap} instance.
*
* @param initialCapacity
* the initial capacity of the map
* @param loadFactor
* the load factor. When the average number of references per
* table exceeds this value resize will be attempted
*/
public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor) {
this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL, DEFAULT_REFERENCE_TYPE);
}
/**
* Create a new {@code ConcurrentReferenceHashMap} instance.
*
* @param initialCapacity
* the initial capacity of the map
* @param concurrencyLevel
* the expected number of threads that will concurrently write to
* the map
*/
public ConcurrentReferenceHashMap(int initialCapacity, int concurrencyLevel) {
this(initialCapacity, DEFAULT_LOAD_FACTOR, concurrencyLevel, DEFAULT_REFERENCE_TYPE);
}
/**
* Create a new {@code ConcurrentReferenceHashMap} instance.
*
* @param initialCapacity
* the initial capacity of the map
* @param referenceType
* the reference type used for entries (soft or weak)
*/
public ConcurrentReferenceHashMap(int initialCapacity, ReferenceType referenceType) {
this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL, referenceType);
}
/**
* Create a new {@code ConcurrentReferenceHashMap} instance.
*
* @param initialCapacity
* the initial capacity of the map
* @param loadFactor
* the load factor. When the average number of references per
* table exceeds this value, resize will be attempted.
* @param concurrencyLevel
* the expected number of threads that will concurrently write to
* the map
*/
public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
this(initialCapacity, loadFactor, concurrencyLevel, DEFAULT_REFERENCE_TYPE);
}
/**
* Create a new {@code ConcurrentReferenceHashMap} instance.
*
* @param initialCapacity
* the initial capacity of the map
* @param loadFactor
* the load factor. When the average number of references per
* table exceeds this value, resize will be attempted.
* @param concurrencyLevel
* the expected number of threads that will concurrently write to
* the map
* @param referenceType
* the reference type used for entries (soft or weak)
*/
@SuppressWarnings("unchecked")
public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel, ReferenceType referenceType) {
isTrue(initialCapacity >= 0, "Initial capacity must not be negative");
isTrue(loadFactor > 0f, "Load factor must be positive");
isTrue(concurrencyLevel > 0, "Concurrency level must be positive");
notNull(referenceType, "Reference type must not be null");
this.loadFactor = loadFactor;
this.shift = calculateShift(concurrencyLevel, MAXIMUM_CONCURRENCY_LEVEL);
int size = 1 << this.shift;
this.referenceType = referenceType;
int roundedUpSegmentCapacity = (int) ((initialCapacity + size - 1L) / size);
this.segments = (Segment[]) Array.newInstance(Segment.class, size);
for (int i = 0; i < this.segments.length; i++) {
this.segments[i] = new Segment(roundedUpSegmentCapacity);
}
}
protected final float getLoadFactor() {
return this.loadFactor;
}
protected final int getSegmentsSize() {
return this.segments.length;
}
protected final Segment getSegment(int index) {
return this.segments[index];
}
/**
* Factory method that returns the {@link ReferenceManager}. This method
* will be called once for each {@link Segment}.
*
* @return a new reference manager
*/
protected ReferenceManager createReferenceManager() {
return new ReferenceManager();
}
/**
* Get the hash for a given object, apply an additional hash function to
* reduce collisions. This implementation uses the same Wang/Jenkins
* algorithm as {@link ConcurrentHashMap}. Subclasses can override to
* provide alternative hashing.
*
* @param o
* the object to hash (may be null)
* @return the resulting hash code
*/
protected int getHash(Object o) {
int hash = o == null ? 0 : o.hashCode();
hash += (hash << 15) ^ 0xffffcd7d;
hash ^= (hash >>> 10);
hash += (hash << 3);
hash ^= (hash >>> 6);
hash += (hash << 2) + (hash << 14);
hash ^= (hash >>> 16);
return hash;
}
@Override
public V get(Object key) {
Entry entry = getEntryIfAvailable(key);
return (entry != null ? entry.getValue() : null);
}
@Override
public V getOrDefault(Object key, V defaultValue) {
Entry entry = getEntryIfAvailable(key);
return (entry != null ? entry.getValue() : defaultValue);
}
@Override
public boolean containsKey(Object key) {
Entry entry = getEntryIfAvailable(key);
return (entry != null && ObjectUtils2.nullSafeEquals(entry.getKey(), key));
}
private Entry getEntryIfAvailable(Object key) {
Reference reference = getReference(key, Restructure.WHEN_NECESSARY);
return (reference != null ? reference.get() : null);
}
/**
* Return a {@link Reference} to the {@link Entry} for the specified
* {@code key}, or {@code null} if not found.
*
* @param key
* the key (can be {@code null})
* @param restructure
* types of restructure allowed during this call
* @return the reference, or {@code null} if not found
*/
protected final Reference getReference(Object key, Restructure restructure) {
int hash = getHash(key);
return getSegmentForHash(hash).getReference(key, hash, restructure);
}
@Override
public V put(K key, V value) {
return put(key, value, true);
}
@Override
public V putIfAbsent(K key, V value) {
return put(key, value, false);
}
private V put(final K key, final V value, final boolean overwriteExisting) {
return doTask(key, new Task(TaskOption.RESTRUCTURE_BEFORE, TaskOption.RESIZE) {
@Override
protected V execute(Reference reference, Entry entry, Entries entries) {
if (entry != null) {
V previousValue = entry.getValue();
if (overwriteExisting) {
entry.setValue(value);
}
return previousValue;
}
entries.add(value);
return null;
}
});
}
@Override
public V remove(Object key) {
return doTask(key, new Task(TaskOption.RESTRUCTURE_AFTER, TaskOption.SKIP_IF_EMPTY) {
@Override
protected V execute(Reference reference, Entry entry) {
if (entry != null) {
reference.release();
return entry.value;
}
return null;
}
});
}
@Override
public boolean remove(Object key, final Object value) {
return doTask(key, new Task(TaskOption.RESTRUCTURE_AFTER, TaskOption.SKIP_IF_EMPTY) {
@Override
protected Boolean execute(Reference reference, Entry entry) {
if (entry != null && ObjectUtils2.nullSafeEquals(entry.getValue(), value)) {
reference.release();
return true;
}
return false;
}
});
}
@Override
public boolean replace(K key, final V oldValue, final V newValue) {
return doTask(key, new Task(TaskOption.RESTRUCTURE_BEFORE, TaskOption.SKIP_IF_EMPTY) {
@Override
protected Boolean execute(Reference reference, Entry entry) {
if (entry != null && ObjectUtils2.nullSafeEquals(entry.getValue(), oldValue)) {
entry.setValue(newValue);
return true;
}
return false;
}
});
}
@Override
public V replace(K key, final V value) {
return doTask(key, new Task(TaskOption.RESTRUCTURE_BEFORE, TaskOption.SKIP_IF_EMPTY) {
@Override
protected V execute(Reference reference, Entry entry) {
if (entry != null) {
V previousValue = entry.getValue();
entry.setValue(value);
return previousValue;
}
return null;
}
});
}
@Override
public void clear() {
for (Segment segment : this.segments) {
segment.clear();
}
}
/**
* 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.
*/
public void purgeUnreferencedEntries() {
for (Segment segment : this.segments) {
segment.restructureIfNecessary(false);
}
}
@Override
public int size() {
int size = 0;
for (Segment segment : this.segments) {
size += segment.getCount();
}
return size;
}
@Override
public Set> entrySet() {
if (this.entrySet == null) {
this.entrySet = new EntrySet();
}
return this.entrySet;
}
private T doTask(Object key, Task task) {
int hash = getHash(key);
return getSegmentForHash(hash).doTask(hash, key, task);
}
private Segment getSegmentForHash(int hash) {
return this.segments[(hash >>> (32 - this.shift)) & (this.segments.length - 1)];
}
/**
* Calculate a shift value that can be used to create a power-of-two value
* between the specified maximum and minimum values.
*
* @param minimumValue
* the minimum value
* @param maximumValue
* the maximum value
* @return the calculated shift (use {@code 1 << shift} to obtain a value)
*/
protected static int calculateShift(int minimumValue, int maximumValue) {
int shift = 0;
int value = 1;
while (value < minimumValue && value < maximumValue) {
value <<= 1;
shift++;
}
return shift;
}
/**
* Various reference types supported by this map.
*/
public enum ReferenceType {
/** Use {@link SoftReference}s */
SOFT,
/** Use {@link WeakReference}s */
WEAK
}
/**
* A single segment used to divide the map to allow better concurrent
* performance.
*/
@SuppressWarnings("serial")
protected final class Segment extends ReentrantLock {
private final ReferenceManager referenceManager;
private final int initialSize;
/**
* Array of references indexed using the low order bits from the hash.
* This property should only be set along with {@code resizeThreshold}.
*/
private volatile Reference[] references;
/**
* The total number of references contained in this segment. This
* includes chained references and references that have been garbage
* collected but not purged.
*/
private volatile int count = 0;
/**
* The threshold when resizing of the references should occur. When
* {@code count} exceeds this value references will be resized.
*/
private int resizeThreshold;
public Segment(int initialCapacity) {
this.referenceManager = createReferenceManager();
this.initialSize = 1 << calculateShift(initialCapacity, MAXIMUM_SEGMENT_SIZE);
setReferences(createReferenceArray(this.initialSize));
}
public Reference getReference(Object key, int hash, Restructure restructure) {
if (restructure == Restructure.WHEN_NECESSARY) {
restructureIfNecessary(false);
}
if (this.count == 0) {
return null;
}
// Use a local copy to protect against other threads writing
Reference[] references = this.references;
int index = getIndex(hash, references);
Reference head = references[index];
return findInChain(head, key, hash);
}
/**
* Apply an update operation to this segment. The segment will be locked
* during the update.
*
* @param hash
* the hash of the key
* @param key
* the key
* @param task
* the update operation
* @return the result of the operation
*/
public T doTask(final int hash, final Object key, final Task task) {
boolean resize = task.hasOption(TaskOption.RESIZE);
if (task.hasOption(TaskOption.RESTRUCTURE_BEFORE)) {
restructureIfNecessary(resize);
}
if (task.hasOption(TaskOption.SKIP_IF_EMPTY) && this.count == 0) {
return task.execute(null, null, null);
}
lock();
try {
final int index = getIndex(hash, this.references);
final Reference head = this.references[index];
Reference reference = findInChain(head, key, hash);
Entry entry = (reference != null ? reference.get() : null);
Entries entries = new Entries() {
@Override
public void add(V value) {
@SuppressWarnings("unchecked")
Entry newEntry = new Entry((K) key, value);
Reference newReference = Segment.this.referenceManager.createReference(newEntry, hash, head);
Segment.this.references[index] = newReference;
Segment.this.count++;
}
};
return task.execute(reference, entry, entries);
} finally {
unlock();
if (task.hasOption(TaskOption.RESTRUCTURE_AFTER)) {
restructureIfNecessary(resize);
}
}
}
/**
* Clear all items from this segment.
*/
public void clear() {
if (this.count == 0) {
return;
}
lock();
try {
setReferences(createReferenceArray(this.initialSize));
this.count = 0;
} finally {
unlock();
}
}
/**
* Restructure the underlying data structure when it becomes necessary.
* This method can increase the size of the references table as well as
* purge any references that have been garbage collected.
*
* @param allowResize
* if resizing is permitted
*/
protected final void restructureIfNecessary(boolean allowResize) {
boolean needsResize = ((this.count > 0) && (this.count >= this.resizeThreshold));
Reference reference = this.referenceManager.pollForPurge();
if ((reference != null) || (needsResize && allowResize)) {
lock();
try {
int countAfterRestructure = this.count;
Set> toPurge = Collections.emptySet();
if (reference != null) {
toPurge = new HashSet>();
while (reference != null) {
toPurge.add(reference);
reference = this.referenceManager.pollForPurge();
}
}
countAfterRestructure -= toPurge.size();
// Recalculate taking into account count inside lock and
// items that
// will be purged
needsResize = (countAfterRestructure > 0 && countAfterRestructure >= this.resizeThreshold);
boolean resizing = false;
int restructureSize = this.references.length;
if (allowResize && needsResize && restructureSize < MAXIMUM_SEGMENT_SIZE) {
restructureSize <<= 1;
resizing = true;
}
// Either create a new table or reuse the existing one
Reference[] restructured = (resizing ? createReferenceArray(restructureSize) : this.references);
// Restructure
for (int i = 0; i < this.references.length; i++) {
reference = this.references[i];
if (!resizing) {
restructured[i] = null;
}
while (reference != null) {
if (!toPurge.contains(reference) && (reference.get() != null)) {
int index = getIndex(reference.getHash(), restructured);
restructured[index] = this.referenceManager.createReference(reference.get(), reference.getHash(),
restructured[index]);
}
reference = reference.getNext();
}
}
// Replace volatile members
if (resizing) {
setReferences(restructured);
}
this.count = Math.max(countAfterRestructure, 0);
} finally {
unlock();
}
}
}
private Reference findInChain(Reference reference, Object key, int hash) {
Reference currRef = reference;
while (currRef != null) {
if (currRef.getHash() == hash) {
Entry entry = currRef.get();
if (entry != null) {
K entryKey = entry.getKey();
if (ObjectUtils2.nullSafeEquals(entryKey, key)) {
return currRef;
}
}
}
currRef = currRef.getNext();
}
return null;
}
@SuppressWarnings("unchecked")
private Reference[] createReferenceArray(int size) {
return (Reference[]) Array.newInstance(Reference.class, size);
}
private int getIndex(int hash, Reference[] references) {
return (hash & (references.length - 1));
}
/**
* Replace the references with a new value, recalculating the
* resizeThreshold.
*
* @param references
* the new references
*/
private void setReferences(Reference[] references) {
this.references = references;
this.resizeThreshold = (int) (references.length * getLoadFactor());
}
/**
* Return the size of the current references array.
*/
public final int getSize() {
return this.references.length;
}
/**
* Return the total number of references in this segment.
*/
public final int getCount() {
return this.count;
}
}
/**
* A reference to an {@link Entry} contained in the map. Implementations are
* usually wrappers around specific Java reference implementations (e.g.,
* {@link SoftReference}).
*/
protected interface Reference {
/**
* Return the referenced entry, or {@code null} if the entry is no
* longer available.
*/
Entry get();
/**
* Return the hash for the reference.
*/
int getHash();
/**
* Return the next reference in the chain, or {@code null} if none.
*/
Reference getNext();
/**
* Release this entry and ensure that it will be returned from
* {@code ReferenceManager#pollForPurge()}.
*/
void release();
}
/**
* A single map entry.
*/
protected static final class Entry implements Map.Entry {
private final K key;
private volatile V value;
public Entry(K key, V value) {
this.key = key;
this.value = value;
}
@Override
public K getKey() {
return this.key;
}
@Override
public V getValue() {
return this.value;
}
@Override
public V setValue(V value) {
V previous = this.value;
this.value = value;
return previous;
}
@Override
public String toString() {
return (this.key + "=" + this.value);
}
@Override
@SuppressWarnings("rawtypes")
public final boolean equals(Object other) {
if (this == other) {
return true;
}
if (!(other instanceof Map.Entry)) {
return false;
}
Map.Entry otherEntry = (Map.Entry) other;
return (ObjectUtils2.nullSafeEquals(getKey(), otherEntry.getKey())
&& ObjectUtils2.nullSafeEquals(getValue(), otherEntry.getValue()));
}
@Override
public final int hashCode() {
return (ObjectUtils2.nullSafeHashCode(this.key) ^ ObjectUtils2.nullSafeHashCode(this.value));
}
}
/**
* A task that can be {@link Segment#doTask run} against a {@link Segment}.
*/
private abstract class Task {
private final EnumSet options;
public Task(TaskOption... options) {
this.options = (options.length == 0 ? EnumSet.noneOf(TaskOption.class) : EnumSet.of(options[0], options));
}
public boolean hasOption(TaskOption option) {
return this.options.contains(option);
}
/**
* Execute the task.
*
* @param reference
* the found reference or {@code null}
* @param entry
* the found entry or {@code null}
* @param entries
* access to the underlying entries
* @return the result of the task
* @see #execute(Reference, Entry)
*/
protected T execute(Reference reference, Entry entry, Entries entries) {
return execute(reference, entry);
}
/**
* Convenience method that can be used for tasks that do not need access
* to {@link Entries}.
*
* @param reference
* the found reference or {@code null}
* @param entry
* the found entry or {@code null}
* @return the result of the task
* @see #execute(Reference, Entry, Entries)
*/
protected T execute(Reference reference, Entry entry) {
return null;
}
}
/**
* Various options supported by a {@code Task}.
*/
private enum TaskOption {
RESTRUCTURE_BEFORE, RESTRUCTURE_AFTER, SKIP_IF_EMPTY, RESIZE
}
/**
* Allows a task access to {@link Segment} entries.
*/
private abstract class Entries {
/**
* Add a new entry with the specified value.
*
* @param value
* the value to add
*/
public abstract void add(V value);
}
/**
* Internal entry-set implementation.
*/
private class EntrySet extends AbstractSet> {
@Override
public Iterator> iterator() {
return new EntryIterator();
}
@Override
public boolean contains(Object o) {
if (o != null && o instanceof Map.Entry) {
Map.Entry entry = (java.util.Map.Entry) o;
Reference reference = ConcurrentReferenceHashMap.this.getReference(entry.getKey(), Restructure.NEVER);
Entry other = (reference != null ? reference.get() : null);
if (other != null) {
return ObjectUtils2.nullSafeEquals(entry.getValue(), other.getValue());
}
}
return false;
}
@Override
public boolean remove(Object o) {
if (o instanceof Map.Entry) {
Map.Entry entry = (Map.Entry) o;
return ConcurrentReferenceHashMap.this.remove(entry.getKey(), entry.getValue());
}
return false;
}
@Override
public int size() {
return ConcurrentReferenceHashMap.this.size();
}
@Override
public void clear() {
ConcurrentReferenceHashMap.this.clear();
}
}
/**
* Internal entry iterator implementation.
*/
private class EntryIterator implements Iterator> {
private int segmentIndex;
private int referenceIndex;
private Reference[] references;
private Reference reference;
private Entry next;
private Entry last;
public EntryIterator() {
moveToNextSegment();
}
@Override
public boolean hasNext() {
getNextIfNecessary();
return (this.next != null);
}
@Override
public Entry next() {
getNextIfNecessary();
if (this.next == null) {
throw new NoSuchElementException();
}
this.last = this.next;
this.next = null;
return this.last;
}
private void getNextIfNecessary() {
while (this.next == null) {
moveToNextReference();
if (this.reference == null) {
return;
}
this.next = this.reference.get();
}
}
private void moveToNextReference() {
if (this.reference != null) {
this.reference = this.reference.getNext();
}
while (this.reference == null && this.references != null) {
if (this.referenceIndex >= this.references.length) {
moveToNextSegment();
this.referenceIndex = 0;
} else {
this.reference = this.references[this.referenceIndex];
this.referenceIndex++;
}
}
}
private void moveToNextSegment() {
this.reference = null;
this.references = null;
if (this.segmentIndex < ConcurrentReferenceHashMap.this.segments.length) {
this.references = ConcurrentReferenceHashMap.this.segments[this.segmentIndex].references;
this.segmentIndex++;
}
}
@Override
public void remove() {
state(this.last != null, "No element to remove");
ConcurrentReferenceHashMap.this.remove(this.last.getKey());
}
}
/**
* The types of restructuring that can be performed.
*/
protected enum Restructure {
WHEN_NECESSARY, NEVER
}
/**
* Strategy class used to manage {@link Reference}s. This class can be
* overridden if alternative reference types need to be supported.
*/
protected class ReferenceManager {
private final ReferenceQueue> queue = new ReferenceQueue>();
/**
* Factory method used to create a new {@link Reference}.
*
* @param entry
* the entry contained in the reference
* @param hash
* the hash
* @param next
* the next reference in the chain, or {@code null} if none
* @return a new {@link Reference}
*/
public Reference createReference(Entry entry, int hash, Reference next) {
if (ConcurrentReferenceHashMap.this.referenceType == ReferenceType.WEAK) {
return new WeakEntryReference(entry, hash, next, this.queue);
}
return new SoftEntryReference(entry, hash, next, this.queue);
}
/**
* Return any reference that has been garbage collected and can be
* purged from the underlying structure or {@code null} if no references
* need purging. This method must be thread safe and ideally should not
* block when returning {@code null}. References should be returned once
* and only once.
*
* @return a reference to purge or {@code null}
*/
@SuppressWarnings("unchecked")
public Reference pollForPurge() {
return (Reference) this.queue.poll();
}
}
/**
* Internal {@link Reference} implementation for {@link SoftReference}s.
*/
private static final class SoftEntryReference extends SoftReference> implements Reference {
private final int hash;
private final Reference nextReference;
public SoftEntryReference(Entry entry, int hash, Reference next, ReferenceQueue> queue) {
super(entry, queue);
this.hash = hash;
this.nextReference = next;
}
@Override
public int getHash() {
return this.hash;
}
@Override
public Reference getNext() {
return this.nextReference;
}
@Override
public void release() {
enqueue();
clear();
}
}
/**
* Internal {@link Reference} implementation for {@link WeakReference}s.
*/
private static final class WeakEntryReference extends WeakReference> implements Reference {
private final int hash;
private final Reference nextReference;
public WeakEntryReference(Entry entry, int hash, Reference next, ReferenceQueue> queue) {
super(entry, queue);
this.hash = hash;
this.nextReference = next;
}
@Override
public int getHash() {
return this.hash;
}
@Override
public Reference getNext() {
return this.nextReference;
}
@Override
public void release() {
enqueue();
clear();
}
}
}