Java/Collections Data Structure/SoftReference — различия между версиями
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Текущая версия на 07:26, 1 июня 2010
Содержание
- 1 An implementation of Set that manages a map of soft references to the set values.
- 2 A phantom reference is used to determine when an object is just about to be reclaimed.
- 3 A soft reference holds onto its referent until memory becomes low.
- 4 A weak reference is used to determine when an object is no longer being referenced.
- 5 Cache based on SoftReference
- 6 Soft ValueMap
- 7 Testing PhantomReference
- 8 Testing SoftReference
- 9 Testing WeakReference
An implementation of Set that manages a map of soft references to the set values.
/*
* JBoss, Home of Professional Open Source
* Copyright 2005, JBoss Inc., and individual contributors as indicated
* by the @authors tag. See the copyright.txt in the distribution for a
* full listing of individual contributors.
*
* This is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License as
* published by the Free Software Foundation; either version 2.1 of
* the License, or (at your option) any later version.
*
* This software is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this software; if not, write to the Free
* Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
* 02110-1301 USA, or see the FSF site: http://www.fsf.org.
*/
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.reflect.Array;
import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Set;
/**
* An implementation of Set that manages a map of soft references to the set
* values. The map is keyed by the value hashCode and so this is only useful for
* value whose hashCode is a valid identity representation (String, primative
* wrappers, etc).
*
* @author Scott.Stark@jboss.org
* @version $Revision: 2787 $
*/
@SuppressWarnings("unchecked")
public class SoftSet implements Set {
private HashMap map = new HashMap();
/** The queue of garbage collected soft references */
private ReferenceQueue gcqueue = new ReferenceQueue();
static class ComparableSoftReference extends SoftReference {
private Integer key;
ComparableSoftReference(Integer key, Object o, ReferenceQueue q) {
super(o, q);
this.key = key;
}
Integer getKey() {
return key;
}
}
static class ComparableSoftReferenceIterator implements Iterator {
Iterator theIter;
ComparableSoftReferenceIterator(Iterator theIter) {
this.theIter = theIter;
}
public boolean hasNext() {
return theIter.hasNext();
}
public Object next() {
ComparableSoftReference csr = (ComparableSoftReference) theIter.next();
return csr.get();
}
public void remove() {
theIter.remove();
}
}
/**
*
*/
public SoftSet() {
}
public int size() {
processQueue();
return map.size();
}
public boolean isEmpty() {
processQueue();
return map.isEmpty();
}
public boolean contains(Object o) {
processQueue();
Integer key = new Integer(o.hashCode());
boolean contains = map.containsKey(key);
return contains;
}
public Iterator iterator() {
processQueue();
Iterator theIter = map.values().iterator();
return new ComparableSoftReferenceIterator(theIter);
}
public Object[] toArray() {
processQueue();
return toArray(new Object[0]);
}
public Object[] toArray(Object[] a) {
processQueue();
int size = map.size();
Object[] array = {};
if (a.length >= size)
array = a;
Iterator iter = map.values().iterator();
int index = 0;
while (iter.hasNext()) {
ComparableSoftReference csr = (ComparableSoftReference) iter.next();
Object value = csr.get();
// Create the correct array type
if (array.length == 0) {
if (value == null) {
index++;
continue;
}
Array.newInstance(value.getClass(), size);
}
array[index] = value;
index++;
}
return array;
}
public boolean add(Object o) {
processQueue();
Integer key = new Integer(o.hashCode());
ComparableSoftReference sr = new ComparableSoftReference(key, o, gcqueue);
return map.put(key, sr) == null;
}
public boolean remove(Object o) {
processQueue();
Integer key = new Integer(o.hashCode());
return map.remove(key) != null;
}
public boolean containsAll(Collection c) {
processQueue();
Iterator iter = c.iterator();
boolean contains = true;
while (iter.hasNext()) {
Object value = iter.next();
Integer key = new Integer(value.hashCode());
contains &= map.containsKey(key);
}
return contains;
}
public boolean addAll(Collection c) {
processQueue();
Iterator iter = c.iterator();
boolean added = false;
while (iter.hasNext()) {
Object value = iter.next();
Integer key = new Integer(value.hashCode());
ComparableSoftReference sr = new ComparableSoftReference(key, value, gcqueue);
added |= map.put(key, sr) == null;
}
return added;
}
public boolean retainAll(Collection c) {
Iterator iter = iterator();
boolean removed = false;
while (iter.hasNext()) {
Object value = iter.next();
if (c.contains(value) == false) {
iter.remove();
removed = true;
}
}
return removed;
}
public boolean removeAll(Collection c) {
processQueue();
Iterator iter = c.iterator();
boolean removed = false;
while (iter.hasNext()) {
Object value = iter.next();
removed |= remove(value);
}
return removed;
}
public void clear() {
while (gcqueue.poll() != null)
;
map.clear();
}
public boolean equals(Object o) {
return map.equals(o);
}
public int hashCode() {
return map.hashCode();
}
/**
* Iterate through the gcqueue for for any cleared reference, remove the
* associated value from the underlying set.
*/
private void processQueue() {
ComparableSoftReference cr;
while ((cr = (ComparableSoftReference) gcqueue.poll()) != null) {
map.remove(cr.getKey());
}
}
}
A phantom reference is used to determine when an object is just about to be reclaimed.
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
public class Main {
public static void main(String[] argv) throws Exception {
ReferenceQueue rq = new ReferenceQueue();
PhantomReference<String> pr = new PhantomReference<String>("object", rq);
while (true) {
Reference r = rq.remove();
if (r == pr) {
// about to be reclaimed.
r.clear();
}
}
}
}
A soft reference holds onto its referent until memory becomes low.
import java.lang.ref.SoftReference;
public class Main {
public static void main(String[] argv) throws Exception {
SoftReference<String> sr = new SoftReference<String>("object");
Object o = sr.get();
if (o != null) {
System.out.println(o);
} else {
System.out.println("collected or has been reclaimed");
}
}
}
A weak reference is used to determine when an object is no longer being referenced.
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
public class Main {
public static void main(String[] argv) throws Exception {
ReferenceQueue rq = new ReferenceQueue();
WeakReference<String> wr = new WeakReference<String>("string", rq);
while (true) {
Reference r = rq.remove();
if (r == wr) {
System.out.println("no longer referenced");
}
}
}
}
Cache based on SoftReference
/* Copyright 2004 The Apache Software Foundation
*
* 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.
*/
// Revised from xmlbeans
import java.util.HashMap;
import java.lang.ref.SoftReference;
/**
* @author Cezar Andrei (cezar.andrei at bea.ru)
* Date: Apr 26, 2005
*/
public class SoftCache
{
private HashMap map = new HashMap();
public Object get(Object key)
{
SoftReference softRef = (SoftReference)map.get(key);
if (softRef==null)
return null;
return softRef.get();
}
public Object put(Object key, Object value)
{
SoftReference softRef = (SoftReference)map.put(key, new SoftReference(value));
if (softRef==null)
return null;
Object oldValue = softRef.get();
softRef.clear();
return oldValue;
}
public Object remove(Object key)
{
SoftReference softRef = (SoftReference)map.remove(key);
if (softRef==null)
return null;
Object oldValue = softRef.get();
softRef.clear();
return oldValue;
}
}
Soft ValueMap
/* Copyright (C) 2003 Vladimir Roubtsov. All rights reserved.
*
* This program and the accompanying materials are made available under
* the terms of the Common Public License v1.0 which accompanies this distribution,
* and is available at http://www.eclipse.org/legal/cpl-v10.html
*
* $Id: SoftValueMap.java,v 1.1.1.1 2004/05/09 16:57:55 vlad_r Exp $
*/
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.Collection;
import java.util.Map;
import java.util.Set;
// ----------------------------------------------------------------------------
/**
* MT-safety: an instance of this class is <I>not</I> safe for access from
* multiple concurrent threads [even if access is done by a single thread at a
* time]. The caller is expected to synchronize externally on an instance [the
* implementation does not do internal synchronization for the sake of efficiency].
* java.util.ConcurrentModificationException is not supported either.
*
* @author (C) 2002, Vlad Roubtsov
*/
public
final class SoftValueMap implements Map
{
// public: ................................................................
// TODO: for caching, does clearing of entries make sense? only to save
// entry memory -- which does not make sense if the set of key values is not
// growing over time... on the other hand, if the key set is unbounded,
// entry clearing is needed so that the hash table does not get polluted with
// empty-valued entries
// TODO: provide mode that disables entry clearing
// TODO: add shrinking rehashes (is it worth it?)
/**
* Equivalent to <CODE>SoftValueMap(1, 1)</CODE>.
*/
public SoftValueMap ()
{
this (1, 1);
}
/**
* Equivalent to <CODE>SoftValueMap(11, 0.75F, getClearCheckFrequency, putClearCheckFrequency)</CODE>.
*/
public SoftValueMap (final int readClearCheckFrequency, final int writeClearCheckFrequency)
{
this (11, 0.75F, readClearCheckFrequency, writeClearCheckFrequency);
}
/**
* Constructs a SoftValueMap with specified initial capacity, load factor,
* and cleared value removal frequencies.
*
* @param initialCapacity initial number of hash buckets in the table
* [may not be negative, 0 is equivalent to 1].
* @param loadFactor the load factor to use to determine rehashing points
* [must be in (0.0, 1.0] range].
* @param readClearCheckFrequency specifies that every readClearCheckFrequency
* {@link #get} should check for and remove all mappings whose soft values
* have been cleared by the garbage collector [may not be less than 1].
* @param writeClearCheckFrequency specifies that every writeClearCheckFrequency
* {@link #put} should check for and remove all mappings whose soft values
* have been cleared by the garbage collector [may not be less than 1].
*/
public SoftValueMap (int initialCapacity, final float loadFactor, final int readClearCheckFrequency, final int writeClearCheckFrequency)
{
if (initialCapacity < 0)
throw new IllegalArgumentException ("negative input: initialCapacity [" + initialCapacity + "]");
if ((loadFactor <= 0.0) || (loadFactor >= 1.0 + 1.0E-6))
throw new IllegalArgumentException ("loadFactor not in (0.0, 1.0] range: " + loadFactor);
if (readClearCheckFrequency < 1)
throw new IllegalArgumentException ("readClearCheckFrequency not in [1, +inf) range: " + readClearCheckFrequency);
if (writeClearCheckFrequency < 1)
throw new IllegalArgumentException ("writeClearCheckFrequency not in [1, +inf) range: " + writeClearCheckFrequency);
if (initialCapacity == 0) initialCapacity = 1;
m_valueReferenceQueue = new ReferenceQueue ();
m_loadFactor = loadFactor;
m_sizeThreshold = (int) (initialCapacity * loadFactor);
m_readClearCheckFrequency = readClearCheckFrequency;
m_writeClearCheckFrequency = writeClearCheckFrequency;
m_buckets = new SoftEntry [initialCapacity];
}
// unsupported operations:
public boolean equals (final Object rhs)
{
throw new UnsupportedOperationException ("not implemented: equals");
}
public int hashCode ()
{
throw new UnsupportedOperationException ("not implemented: hashCode");
}
/**
* Overrides Object.toString() for debug purposes.
*/
public String toString ()
{
final StringBuffer s = new StringBuffer ();
debugDump (s);
return s.toString ();
}
/**
* Returns the number of key-value mappings in this map. Some of the values
* may have been cleared already but not removed from the table.<P>
*
* <B>NOTE:</B> in contrast with the java.util.WeakHashMap implementation,
* this is a constant time operation.
*/
public int size ()
{
return m_size;
}
/**
* Returns "false" is this map contains key-value mappings (even if some of
* the values may have been cleared already but not removed from the table).<P>
*
* <B>NOTE:</B> in contrast with the java.util.WeakHashMap implementation,
* this is a constant time operation.
*/
public boolean isEmpty ()
{
return m_size == 0;
}
/**
* Returns the value that is mapped to a given "key". Returns
* null if (a) this key has never been mapped or (b) a previously mapped
* value has been cleared by the garbage collector and removed from the table.
*
* @param key mapping key [may not be null].
*
* @return Object value mapping for "key" [can be null].
*/
public Object get (final Object key)
{
if (key == null) throw new IllegalArgumentException ("null input: key");
if ((++ m_readAccessCount % m_readClearCheckFrequency) == 0) removeClearedValues ();
// index into the corresponding hash bucket:
final int keyHashCode = key.hashCode ();
final SoftEntry [] buckets = m_buckets;
final int bucketIndex = (keyHashCode & 0x7FFFFFFF) % buckets.length;
Object result = null;
// traverse the singly-linked list of entries in the bucket:
for (SoftEntry entry = buckets [bucketIndex]; entry != null; entry = entry.m_next)
{
final Object entryKey = entry.m_key;
if (IDENTITY_OPTIMIZATION)
{
// note: this uses an early identity comparison opimization, making this a bit
// faster for table keys that do not override equals() [Thread, etc]
if ((key == entryKey) || ((keyHashCode == entryKey.hashCode ()) && key.equals (entryKey)))
{
final Reference ref = entry.m_softValue;
result = ref.get (); // may return null to the caller
// [see comment for ENQUEUE_FOUND_CLEARED_ENTRIES]
if (ENQUEUE_FOUND_CLEARED_ENTRIES && (result == null))
{
ref.enqueue ();
}
return result;
}
}
else
{
if ((keyHashCode == entryKey.hashCode ()) && key.equals (entryKey))
{
final Reference ref = entry.m_softValue;
result = ref.get (); // may return null to the caller
// [see comment for ENQUEUE_FOUND_CLEARED_ENTRIES]
if (ENQUEUE_FOUND_CLEARED_ENTRIES && (result == null))
{
ref.enqueue ();
}
return result;
}
}
}
return null;
}
/**
* Updates the table to map "key" to "value". Any existing mapping is overwritten.
*
* @param key mapping key [may not be null].
* @param value mapping value [may not be null].
*
* @return Object previous value mapping for "key" [null if no previous mapping
* existed or its value has been cleared by the garbage collector and removed from the table].
*/
public Object put (final Object key, final Object value)
{
if (key == null) throw new IllegalArgumentException ("null input: key");
if (value == null) throw new IllegalArgumentException ("null input: value");
if ((++ m_writeAccessCount % m_writeClearCheckFrequency) == 0) removeClearedValues ();
SoftEntry currentKeyEntry = null;
// detect if "key" is already in the table [in which case, set "currentKeyEntry" to point to its entry]:
// index into the corresponding hash bucket:
final int keyHashCode = key.hashCode ();
SoftEntry [] buckets = m_buckets;
int bucketIndex = (keyHashCode & 0x7FFFFFFF) % buckets.length;
// traverse the singly-linked list of entries in the bucket:
for (SoftEntry entry = buckets [bucketIndex]; entry != null; entry = entry.m_next)
{
final Object entryKey = entry.m_key;
if (IDENTITY_OPTIMIZATION)
{
// note: this uses an early identity comparison opimization, making this a bit
// faster for table keys that do not override equals() [Thread, etc]
if ((key == entryKey) || ((keyHashCode == entryKey.hashCode ()) && key.equals (entryKey)))
{
currentKeyEntry = entry;
break;
}
}
else
{
if ((keyHashCode == entryKey.hashCode ()) && key.equals (entryKey))
{
currentKeyEntry = entry;
break;
}
}
}
if (currentKeyEntry != null)
{
// replace the current value:
final IndexedSoftReference ref = currentKeyEntry.m_softValue;
final Object currentKeyValue = ref.get (); // can be null already [no need to work around the get() bug, though]
if (currentKeyValue == null) ref.m_bucketIndex = -1; // disable removal by removeClearedValues() [need to do this because of the identity comparison there]
currentKeyEntry.m_softValue = new IndexedSoftReference (value, m_valueReferenceQueue, bucketIndex);
return currentKeyValue; // may return null to the caller
}
else
{
// add a new entry:
if (m_size >= m_sizeThreshold) rehash ();
// recompute the hash bucket index:
buckets = m_buckets;
bucketIndex = (keyHashCode & 0x7FFFFFFF) % buckets.length;
final SoftEntry bucketListHead = buckets [bucketIndex];
final SoftEntry newEntry = new SoftEntry (m_valueReferenceQueue, key, value, bucketListHead, bucketIndex);
buckets [bucketIndex] = newEntry;
++ m_size;
return null;
}
}
public Object remove (final Object key)
{
if (key == null) throw new IllegalArgumentException ("null input: key");
if ((++ m_writeAccessCount % m_writeClearCheckFrequency) == 0) removeClearedValues ();
// index into the corresponding hash bucket:
final int keyHashCode = key.hashCode ();
final SoftEntry [] buckets = m_buckets;
final int bucketIndex = (keyHashCode & 0x7FFFFFFF) % buckets.length;
Object result = null;
// traverse the singly-linked list of entries in the bucket:
for (SoftEntry entry = buckets [bucketIndex], prev = null; entry != null; prev = entry, entry = entry.m_next)
{
final Object entryKey = entry.m_key;
if ((IDENTITY_OPTIMIZATION && (entryKey == key)) || ((keyHashCode == entryKey.hashCode ()) && key.equals (entryKey)))
{
if (prev == null) // head of the list
{
buckets [bucketIndex] = entry.m_next;
}
else
{
prev.m_next = entry.m_next;
}
final IndexedSoftReference ref = entry.m_softValue;
result = ref.get (); // can be null already [however, no need to work around 4485942]
// [regardless of whether the value has been enqueued or not, disable its processing by removeClearedValues() since the entire entry is removed here]
ref.m_bucketIndex = -1;
// help GC:
entry.m_softValue = null;
entry.m_key = null;
entry.m_next = null;
entry = null;
-- m_size;
break;
}
}
return result;
}
public void clear ()
{
final SoftEntry [] buckets = m_buckets;
for (int b = 0, bLimit = buckets.length; b < bLimit; ++ b)
{
for (SoftEntry entry = buckets [b]; entry != null; )
{
final SoftEntry next = entry.m_next; // remember next pointer because we are going to reuse this entry
// [regardless of whether the value has been enqueued or not, disable its processing by removeClearedValues()]
entry.m_softValue.m_bucketIndex = -1;
// help GC:
entry.m_softValue = null;
entry.m_next = null;
entry.m_key = null;
entry = next;
}
buckets [b] = null;
}
m_size = 0;
m_readAccessCount = 0;
m_writeAccessCount = 0;
}
// unsupported operations:
public boolean containsKey (final Object key)
{
throw new UnsupportedOperationException ("not implemented: containsKey");
}
public boolean containsValue (final Object value)
{
throw new UnsupportedOperationException ("not implemented: containsValue");
}
public void putAll (final Map map)
{
throw new UnsupportedOperationException ("not implemented: putAll");
}
public Set keySet ()
{
throw new UnsupportedOperationException ("not implemented: keySet");
}
public Set entrySet ()
{
throw new UnsupportedOperationException ("not implemented: entrySet");
}
public Collection values ()
{
throw new UnsupportedOperationException ("not implemented: values");
}
// protected: .............................................................
// package: ...............................................................
void debugDump (final StringBuffer out)
{
if (out != null)
{
out.append (getClass ().getName ().concat ("@").concat (Integer.toHexString (System.identityHashCode (this)))); out.append (EOL);
out.append ("size = " + m_size + ", bucket table size = " + m_buckets.length + ", load factor = " + m_loadFactor + EOL);
out.append ("size threshold = " + m_sizeThreshold + ", get clear frequency = " + m_readClearCheckFrequency + ", put clear frequency = " + m_writeClearCheckFrequency + EOL);
out.append ("get count: " + m_readAccessCount + ", put count: " + m_writeAccessCount + EOL);
}
}
// private: ...............................................................
/**
* An extension of WeakReference that can store an index of the bucket it
* is associated with.
*/
static class IndexedSoftReference extends SoftReference
{
IndexedSoftReference (final Object referent, ReferenceQueue queue, final int bucketIndex)
{
super (referent, queue);
m_bucketIndex = bucketIndex;
}
int m_bucketIndex;
} // end of nested class
/**
* The structure used for chaining colliding keys.
*/
static class SoftEntry
{
SoftEntry (final ReferenceQueue valueReferenceQueue, final Object key, Object value, final SoftEntry next, final int bucketIndex)
{
m_key = key;
m_softValue = new IndexedSoftReference (value, valueReferenceQueue, bucketIndex); // ... do not retain a strong reference to the value
value = null;
m_next = next;
}
IndexedSoftReference m_softValue; // soft reference to the value [never null]
Object m_key; // strong reference to the key [never null]
SoftEntry m_next; // singly-linked list link
} // end of nested class
/**
* Re-hashes the table into a new array of buckets. During the process
* cleared value entries are discarded, making for another efficient cleared
* value removal method.
*/
private void rehash ()
{
// TODO: it is possible to run this method twice, first time using the 2*k+1 prime sequencer for newBucketCount
// and then with that value reduced to actually shrink capacity. As it is right now, the bucket table can
// only grow in size
final SoftEntry [] buckets = m_buckets;
final int newBucketCount = (m_buckets.length << 1) + 1;
final SoftEntry [] newBuckets = new SoftEntry [newBucketCount];
int newSize = 0;
// rehash all entry chains in every bucket:
for (int b = 0, bLimit = buckets.length; b < bLimit; ++ b)
{
for (SoftEntry entry = buckets [b]; entry != null; )
{
final SoftEntry next = entry.m_next; // remember next pointer because we are going to reuse this entry
IndexedSoftReference ref = entry.m_softValue; // get the soft value reference
Object entryValue = ref.get (); // convert the soft reference to a local strong one
// skip entries whose keys have been cleared: this also saves on future removeClearedValues() work
if (entryValue != null)
{
// [assertion: "softValue" couldn"t have been enqueued already and can"t be enqueued until strong reference in "entryKey" is nulled out]
// index into the corresponding new hash bucket:
final int entryKeyHashCode = entry.m_key.hashCode ();
final int newBucketIndex = (entryKeyHashCode & 0x7FFFFFFF) % newBucketCount;
final SoftEntry bucketListHead = newBuckets [newBucketIndex];
entry.m_next = bucketListHead;
newBuckets [newBucketIndex] = entry;
// adjust bucket index:
ref.m_bucketIndex = newBucketIndex;
++ newSize;
entryValue = null;
}
else
{
// ["softValue" may or may not have been enqueued already]
// adjust bucket index:
// [regardless of whether "softValue" has been enqueued or not, disable its removal by removeClearedValues() since the buckets get restructured]
ref.m_bucketIndex = -1;
}
entry = next;
}
}
if (DEBUG)
{
if (m_size > newSize) System.out.println ("DEBUG: rehash() cleared " + (m_size - newSize) + " values, new size = " + newSize);
}
m_size = newSize;
m_sizeThreshold = (int) (newBucketCount * m_loadFactor);
m_buckets = newBuckets;
}
/**
* Removes all entries whose soft values have been cleared _and_ enqueued.
* See comments below for why this is safe wrt to rehash().
*/
private void removeClearedValues ()
{
int count = 0;
next: for (Reference _ref; (_ref = m_valueReferenceQueue.poll ()) != null; )
{
// remove entry containing "_ref" using its bucket index and identity comparison:
// index into the corresponding hash bucket:
final int bucketIndex = ((IndexedSoftReference) _ref).m_bucketIndex;
if (bucketIndex >= 0) // skip keys that were already removed by rehash()
{
// [assertion: this reference was not cleared when the last rehash() ran and so its m_bucketIndex is correct]
// traverse the singly-linked list of entries in the bucket:
for (SoftEntry entry = m_buckets [bucketIndex], prev = null; entry != null; prev = entry, entry = entry.m_next)
{
if (entry.m_softValue == _ref)
{
if (prev == null) // head of the list
{
m_buckets [bucketIndex] = entry.m_next;
}
else
{
prev.m_next = entry.m_next;
}
entry.m_softValue = null;
entry.m_key = null;
entry.m_next = null;
entry = null;
-- m_size;
if (DEBUG) ++ count;
continue next;
}
}
// no match found this can happen if a soft value got replaced by a put
final StringBuffer msg = new StringBuffer ("removeClearedValues(): soft reference [" + _ref + "] did not match within bucket #" + bucketIndex + EOL);
debugDump (msg);
throw new Error (msg.toString ());
}
// else: it has already been removed by rehash() or other methods
}
if (DEBUG)
{
if (count > 0) System.out.println ("DEBUG: removeClearedValues() cleared " + count + " keys, new size = " + m_size);
}
}
private final ReferenceQueue m_valueReferenceQueue; // reference queue for all references used by SoftEntry objects used by this table
private final float m_loadFactor; // determines the setting of m_sizeThreshold
private final int m_readClearCheckFrequency, m_writeClearCheckFrequency; // parameters determining frequency of running removeClearedKeys() by get() and put()/remove(), respectively
private SoftEntry [] m_buckets; // table of buckets
private int m_size; // number of values in the table, not cleared as of last check
private int m_sizeThreshold; // size threshold for rehashing
private int m_readAccessCount, m_writeAccessCount;
private static final String EOL = System.getProperty ("line.separator", "\n");
private static final boolean IDENTITY_OPTIMIZATION = true;
// setting this to "true" is an optimization and a workaround for bug 4485942:
private static final boolean ENQUEUE_FOUND_CLEARED_ENTRIES = true;
private static final boolean DEBUG = false;
} // end of class
// ----------------------------------------------------------------------------
Testing PhantomReference
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
ReferenceQueue referenceQueue = new ReferenceQueue();
Object object = new Object() {
public String toString() {
return "Referenced Object";
}
};
Object data = new Object() {
public String toString() {
return "Data";
}
};
HashMap map = new HashMap();
Reference reference = null;
System.out.println("Testing PhantomReference.");
reference = new PhantomReference(object, referenceQueue);
map.put(reference, data);
System.out.println(reference.get());
System.out.println(map.get(reference));
System.out.println(reference.isEnqueued());
System.gc();
System.out.println(reference.get());
System.out.println(map.get(reference));
System.out.println(reference.isEnqueued());
object = null;
data = null;
System.gc();
System.out.println(reference.get());
System.out.println(map.get(reference));
System.out.println(reference.isEnqueued());
}
}
Testing SoftReference
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
ReferenceQueue referenceQueue = new ReferenceQueue();
Object object = new Object() {
public String toString() {
return "Referenced Object";
}
};
Object data = new Object() {
public String toString() {
return "Data";
}
};
HashMap map = new HashMap();
Reference reference = new SoftReference(object, referenceQueue);
map.put(reference, data);
System.out.println(reference.get());
System.out.println(map.get(reference));
System.out.println(reference.isEnqueued());
System.gc();
System.out.println(reference.get());
System.out.println(map.get(reference));
System.out.println(reference.isEnqueued());
object = null;
data = null;
System.gc();
System.out.println(reference.get());
System.out.println(map.get(reference));
System.out.println(reference.isEnqueued());
}
}
Testing WeakReference
import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
ReferenceQueue referenceQueue = new ReferenceQueue();
Object object = new Object() {
public String toString() {
return "Referenced Object";
}
};
Object data = new Object() {
public String toString() {
return "Data";
}
};
HashMap map = new HashMap();
Reference reference = null;
System.out.println("Testing WeakReference.");
reference = new WeakReference(object, referenceQueue);
map.put(reference, data);
System.out.println(reference.get());
System.out.println(map.get(reference));
System.out.println(reference.isEnqueued());
System.gc();
System.out.println(reference.get());
System.out.println(map.get(reference));
System.out.println(reference.isEnqueued());
object = null;
data = null;
System.gc();
System.out.println(reference.get());
System.out.println(map.get(reference));
System.out.println(reference.isEnqueued());
}
}
