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
// ----------------------------------------------------------------------------
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