java.util.WeakHashMap Maven / Gradle / Ivy
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You 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 java.util;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
/**
* WeakHashMap is an implementation of Map with keys which are WeakReferences. A
* key/value mapping is removed when the key is no longer referenced. All
* optional operations (adding and removing) are supported. Keys and values can
* be any objects. Note that the garbage collector acts similar to a second
* thread on this collection, possibly removing keys.
*
* @since 1.2
* @see HashMap
* @see WeakReference
*/
public class WeakHashMap extends AbstractMap implements Map {
private static final int DEFAULT_SIZE = 16;
private final ReferenceQueue referenceQueue;
int elementCount;
Entry[] elementData;
private final int loadFactor;
private int threshold;
volatile int modCount;
// Simple utility method to isolate unchecked cast for array creation
@SuppressWarnings("unchecked")
private static Entry[] newEntryArray(int size) {
return new Entry[size];
}
private static final class Entry extends WeakReference implements
Map.Entry {
final int hash;
boolean isNull;
V value;
Entry next;
interface Type {
R get(Map.Entry entry);
}
Entry(K key, V object, ReferenceQueue queue) {
super(key, queue);
isNull = key == null;
hash = isNull ? 0 : Collections.secondaryHash(key);
value = object;
}
public K getKey() {
return super.get();
}
public V getValue() {
return value;
}
public V setValue(V object) {
V result = value;
value = object;
return result;
}
@Override
public boolean equals(Object other) {
if (!(other instanceof Map.Entry)) {
return false;
}
Map.Entry entry = (Map.Entry) other;
Object key = super.get();
return (key == null ? key == entry.getKey() : key.equals(entry
.getKey()))
&& (value == null ? value == entry.getValue() : value
.equals(entry.getValue()));
}
@Override
public int hashCode() {
return hash + (value == null ? 0 : value.hashCode());
}
@Override
public String toString() {
return super.get() + "=" + value;
}
}
class HashIterator implements Iterator {
private int position = 0, expectedModCount;
private Entry currentEntry, nextEntry;
private K nextKey;
final Entry.Type type;
HashIterator(Entry.Type type) {
this.type = type;
expectedModCount = modCount;
}
public boolean hasNext() {
if (nextEntry != null && (nextKey != null || nextEntry.isNull)) {
return true;
}
while (true) {
if (nextEntry == null) {
while (position < elementData.length) {
if ((nextEntry = elementData[position++]) != null) {
break;
}
}
if (nextEntry == null) {
return false;
}
}
// ensure key of next entry is not gc'ed
nextKey = nextEntry.get();
if (nextKey != null || nextEntry.isNull) {
return true;
}
nextEntry = nextEntry.next;
}
}
public R next() {
if (expectedModCount == modCount) {
if (hasNext()) {
currentEntry = nextEntry;
nextEntry = currentEntry.next;
R result = type.get(currentEntry);
// free the key
nextKey = null;
return result;
}
throw new NoSuchElementException();
}
throw new ConcurrentModificationException();
}
public void remove() {
if (expectedModCount == modCount) {
if (currentEntry != null) {
removeEntry(currentEntry);
currentEntry = null;
expectedModCount++;
// cannot poll() as that would change the expectedModCount
} else {
throw new IllegalStateException();
}
} else {
throw new ConcurrentModificationException();
}
}
}
/**
* Constructs a new empty {@code WeakHashMap} instance.
*/
public WeakHashMap() {
this(DEFAULT_SIZE);
}
/**
* Constructs a new {@code WeakHashMap} instance with the specified
* capacity.
*
* @param capacity
* the initial capacity of this map.
* @throws IllegalArgumentException
* if the capacity is less than zero.
*/
public WeakHashMap(int capacity) {
if (capacity < 0) {
throw new IllegalArgumentException("capacity < 0: " + capacity);
}
elementCount = 0;
elementData = newEntryArray(capacity == 0 ? 1 : capacity);
loadFactor = 7500; // Default load factor of 0.75
computeMaxSize();
referenceQueue = new ReferenceQueue();
}
/**
* Constructs a new {@code WeakHashMap} instance with the specified capacity
* and load factor.
*
* @param capacity
* the initial capacity of this map.
* @param loadFactor
* the initial load factor.
* @throws IllegalArgumentException
* if the capacity is less than zero or the load factor is less
* or equal to zero.
*/
public WeakHashMap(int capacity, float loadFactor) {
if (capacity < 0) {
throw new IllegalArgumentException("capacity < 0: " + capacity);
}
if (loadFactor <= 0) {
throw new IllegalArgumentException("loadFactor <= 0: " + loadFactor);
}
elementCount = 0;
elementData = newEntryArray(capacity == 0 ? 1 : capacity);
this.loadFactor = (int) (loadFactor * 10000);
computeMaxSize();
referenceQueue = new ReferenceQueue();
}
/**
* Constructs a new {@code WeakHashMap} instance containing the mappings
* from the specified map.
*
* @param map
* the mappings to add.
*/
public WeakHashMap(Map map) {
this(map.size() < 6 ? 11 : map.size() * 2);
putAllImpl(map);
}
/**
* Removes all mappings from this map, leaving it empty.
*
* @see #isEmpty()
* @see #size()
*/
@Override
public void clear() {
if (elementCount > 0) {
elementCount = 0;
Arrays.fill(elementData, null);
modCount++;
while (referenceQueue.poll() != null) {
// do nothing
}
}
}
private void computeMaxSize() {
threshold = (int) ((long) elementData.length * loadFactor / 10000);
}
/**
* Returns whether this map contains the specified key.
*
* @param key
* the key to search for.
* @return {@code true} if this map contains the specified key,
* {@code false} otherwise.
*/
@Override
public boolean containsKey(Object key) {
return getEntry(key) != null;
}
/**
* Returns a set containing all of the mappings in this map. Each mapping is
* an instance of {@link Map.Entry}. As the set is backed by this map,
* changes in one will be reflected in the other. It does not support adding
* operations.
*
* @return a set of the mappings.
*/
@Override
public Set> entrySet() {
poll();
return new AbstractSet>() {
@Override
public int size() {
return WeakHashMap.this.size();
}
@Override
public void clear() {
WeakHashMap.this.clear();
}
@Override
public boolean remove(Object object) {
if (contains(object)) {
WeakHashMap.this
.remove(((Map.Entry) object).getKey());
return true;
}
return false;
}
@Override
public boolean contains(Object object) {
if (object instanceof Map.Entry) {
Entry entry = getEntry(((Map.Entry) object)
.getKey());
if (entry != null) {
Object key = entry.get();
if (key != null || entry.isNull) {
return object.equals(entry);
}
}
}
return false;
}
@Override
public Iterator> iterator() {
return new HashIterator>(
new Entry.Type, K, V>() {
public Map.Entry get(Map.Entry entry) {
return entry;
}
});
}
};
}
/**
* Returns a set of the keys contained in this map. The set is backed by
* this map so changes to one are reflected by the other. The set does not
* support adding.
*
* @return a set of the keys.
*/
@Override
public Set keySet() {
poll();
if (keySet == null) {
keySet = new AbstractSet() {
@Override
public boolean contains(Object object) {
return containsKey(object);
}
@Override
public int size() {
return WeakHashMap.this.size();
}
@Override
public void clear() {
WeakHashMap.this.clear();
}
@Override
public boolean remove(Object key) {
if (containsKey(key)) {
WeakHashMap.this.remove(key);
return true;
}
return false;
}
@Override
public Iterator iterator() {
return new HashIterator(new Entry.Type() {
public K get(Map.Entry entry) {
return entry.getKey();
}
});
}
};
}
return keySet;
}
/**
* Returns a collection of the values contained in this map. The collection
* is backed by this map so changes to one are reflected by the other. The
* collection supports remove, removeAll, retainAll and clear operations,
* and it does not support add or addAll operations.
*
* This method returns a collection which is the subclass of
* AbstractCollection. The iterator method of this subclass returns a
* "wrapper object" over the iterator of map's entrySet(). The size method
* wraps the map's size method and the contains method wraps the map's
* containsValue method.
*
* The collection is created when this method is called at first time and
* returned in response to all subsequent calls. This method may return
* different Collection when multiple calls to this method, since it has no
* synchronization performed.
*
* @return a collection of the values contained in this map.
*/
@Override
public Collection values() {
poll();
if (valuesCollection == null) {
valuesCollection = new AbstractCollection() {
@Override
public int size() {
return WeakHashMap.this.size();
}
@Override
public void clear() {
WeakHashMap.this.clear();
}
@Override
public boolean contains(Object object) {
return containsValue(object);
}
@Override
public Iterator iterator() {
return new HashIterator(new Entry.Type() {
public V get(Map.Entry entry) {
return entry.getValue();
}
});
}
};
}
return valuesCollection;
}
/**
* Returns the value of the mapping with the specified key.
*
* @param key
* the key.
* @return the value of the mapping with the specified key, or {@code null}
* if no mapping for the specified key is found.
*/
@Override
public V get(Object key) {
poll();
if (key != null) {
int index = (Collections.secondaryHash(key) & 0x7FFFFFFF) % elementData.length;
Entry entry = elementData[index];
while (entry != null) {
if (key.equals(entry.get())) {
return entry.value;
}
entry = entry.next;
}
return null;
}
Entry entry = elementData[0];
while (entry != null) {
if (entry.isNull) {
return entry.value;
}
entry = entry.next;
}
return null;
}
Entry getEntry(Object key) {
poll();
if (key != null) {
int index = (Collections.secondaryHash(key) & 0x7FFFFFFF) % elementData.length;
Entry entry = elementData[index];
while (entry != null) {
if (key.equals(entry.get())) {
return entry;
}
entry = entry.next;
}
return null;
}
Entry entry = elementData[0];
while (entry != null) {
if (entry.isNull) {
return entry;
}
entry = entry.next;
}
return null;
}
/**
* Returns whether this map contains the specified value.
*
* @param value
* the value to search for.
* @return {@code true} if this map contains the specified value,
* {@code false} otherwise.
*/
@Override
public boolean containsValue(Object value) {
poll();
if (value != null) {
for (int i = elementData.length; --i >= 0;) {
Entry entry = elementData[i];
while (entry != null) {
K key = entry.get();
if ((key != null || entry.isNull)
&& value.equals(entry.value)) {
return true;
}
entry = entry.next;
}
}
} else {
for (int i = elementData.length; --i >= 0;) {
Entry entry = elementData[i];
while (entry != null) {
K key = entry.get();
if ((key != null || entry.isNull) && entry.value == null) {
return true;
}
entry = entry.next;
}
}
}
return false;
}
/**
* Returns the number of elements in this map.
*
* @return the number of elements in this map.
*/
@Override
public boolean isEmpty() {
return size() == 0;
}
@SuppressWarnings("unchecked")
void poll() {
Entry toRemove;
while ((toRemove = (Entry) referenceQueue.poll()) != null) {
removeEntry(toRemove);
}
}
void removeEntry(Entry toRemove) {
Entry entry, last = null;
int index = (toRemove.hash & 0x7FFFFFFF) % elementData.length;
entry = elementData[index];
// Ignore queued entries which cannot be found, the user could
// have removed them before they were queued, i.e. using clear()
while (entry != null) {
if (toRemove == entry) {
modCount++;
if (last == null) {
elementData[index] = entry.next;
} else {
last.next = entry.next;
}
elementCount--;
break;
}
last = entry;
entry = entry.next;
}
}
/**
* Maps the specified key to the specified value.
*
* @param key
* the key.
* @param value
* the value.
* @return the value of any previous mapping with the specified key or
* {@code null} if there was no mapping.
*/
@Override
public V put(K key, V value) {
poll();
int index = 0;
Entry entry;
if (key != null) {
index = (Collections.secondaryHash(key) & 0x7FFFFFFF) % elementData.length;
entry = elementData[index];
while (entry != null && !key.equals(entry.get())) {
entry = entry.next;
}
} else {
entry = elementData[0];
while (entry != null && !entry.isNull) {
entry = entry.next;
}
}
if (entry == null) {
modCount++;
if (++elementCount > threshold) {
rehash();
index = key == null ? 0 : (Collections.secondaryHash(key) & 0x7FFFFFFF)
% elementData.length;
}
entry = new Entry(key, value, referenceQueue);
entry.next = elementData[index];
elementData[index] = entry;
return null;
}
V result = entry.value;
entry.value = value;
return result;
}
private void rehash() {
int length = elementData.length * 2;
if (length == 0) {
length = 1;
}
Entry[] newData = newEntryArray(length);
for (int i = 0; i < elementData.length; i++) {
Entry entry = elementData[i];
while (entry != null) {
int index = entry.isNull ? 0 : (entry.hash & 0x7FFFFFFF)
% length;
Entry next = entry.next;
entry.next = newData[index];
newData[index] = entry;
entry = next;
}
}
elementData = newData;
computeMaxSize();
}
/**
* Copies all the mappings in the given map to this map. These mappings will
* replace all mappings that this map had for any of the keys currently in
* the given map.
*
* @param map
* the map to copy mappings from.
* @throws NullPointerException
* if {@code map} is {@code null}.
*/
@Override
public void putAll(Map map) {
putAllImpl(map);
}
/**
* Removes the mapping with the specified key from this map.
*
* @param key
* the key of the mapping to remove.
* @return the value of the removed mapping or {@code null} if no mapping
* for the specified key was found.
*/
@Override
public V remove(Object key) {
poll();
int index = 0;
Entry entry, last = null;
if (key != null) {
index = (Collections.secondaryHash(key) & 0x7FFFFFFF) % elementData.length;
entry = elementData[index];
while (entry != null && !key.equals(entry.get())) {
last = entry;
entry = entry.next;
}
} else {
entry = elementData[0];
while (entry != null && !entry.isNull) {
last = entry;
entry = entry.next;
}
}
if (entry != null) {
modCount++;
if (last == null) {
elementData[index] = entry.next;
} else {
last.next = entry.next;
}
elementCount--;
return entry.value;
}
return null;
}
/**
* Returns the number of elements in this map.
*
* @return the number of elements in this map.
*/
@Override
public int size() {
poll();
return elementCount;
}
private void putAllImpl(Map map) {
if (map.entrySet() != null) {
super.putAll(map);
}
}
}