manifold.util.concurrent.ConcurrentWeakHashMap Maven / Gradle / Ivy
/*
* Copyright (c) 2018 - Manifold Systems LLC
*
* 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 manifold.util.concurrent;
/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
import java.io.IOException;
import java.io.Serializable;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.locks.ReentrantLock;
/**
* A hash table with weak keys, full concurrency of retrievals, and
* adjustable expected concurrency for updates. Similar to
* {@link java.util.WeakHashMap}, entries of this table are periodically
* removed once their corresponding keys are no longer referenced outside of
* this table. In other words, this table will not prevent a key from being
* discarded by the garbage collector. Once a key has been discarded by the
* collector, the corresponding entry is no longer visible to this table;
* however, the entry may occupy space until a future table operation decides to
* reclaim it. For this reason, summary functions such as size and
* isEmpty might return a value greater than the observed number of
* entries. In order to support a high level of concurrency, stale entries are
* only reclaimed during blocking (usually mutating) operations.
*
* While keys in this table are only held using a weak reference, values are
* held using a normal strong reference. This provides the guarantee that a
* value will always have at least the same life-span as it's key. For this
* reason, care should be taken to ensure that a value never refers, either
* directly or indirectly, to its key, thereby preventing reclamation. If weak
* values are desired, one can simply use a {@link WeakReference} for the value
* type.
*
* Just like {@link java.util.concurrent.ConcurrentHashMap}, this class obeys the same
* functional specification as {@link Hashtable}, and includes
* versions of methods corresponding to each method of Hashtable.
* However, even though all operations are thread-safe, retrieval operations do
* not entail locking, and there is not any support for
* locking the entire table in a way that prevents all access. This class is
* fully interoperable with Hashtable in programs that rely on its
* thread safety but not on its synchronization details.
*
*
* Retrieval operations (including get) generally do not block, so
* may overlap with update operations (including put and
* remove). Retrievals reflect the results of the most recently
* completed update operations holding upon their onset. For
* aggregate operations such as putAll and clear,
* concurrent retrievals may reflect insertion or removal of only some entries.
* Similarly, Iterators and Enumerations return elements reflecting the state of
* the hash table at some point at or since the creation of the
* iterator/enumeration. They do not throw
* {@link ConcurrentModificationException}. However, iterators are designed to
* be used by only one thread at a time.
*
*
* The allowed concurrency among update operations is guided by the optional
* concurrencyLevel constructor argument (default 16),
* which is used as a hint for internal sizing. The table is internally
* partitioned to try to permit the indicated number of concurrent updates
* without contention. Because placement in hash tables is essentially random,
* the actual concurrency will vary. Ideally, you should choose a value to
* accommodate as many threads as will ever concurrently modify the table. Using
* a significantly higher value than you need can waste space and time, and a
* significantly lower value can lead to thread contention. But overestimates
* and underestimates within an order of magnitude do not usually have much
* noticeable impact. A value of one is appropriate when it is known that only
* one thread will modify and all others will only read. Also, resizing this or
* any other kind of hash table is a relatively slow operation, so, when
* possible, it is a good idea to provide estimates of expected table sizes in
* constructors.
*
*
* This class and its views and iterators implement all of the optional
* methods of the {@link Map} and {@link Iterator} interfaces.
*
*
* Like {@link Hashtable} but unlike {@link HashMap}, this class does
* not allow null to be used as a key or value.
*
*
* This class is a member of the
* Java Collections Framework.
*
* @param the type of keys maintained by this map
* @param the type of mapped values
*
* @author Doug Lea
* @author Jason T. Greene
*/
public class ConcurrentWeakHashMap extends AbstractMap
implements java.util.concurrent.ConcurrentMap, Serializable
{
private static final long serialVersionUID = 7249069246763182397L;
/*
* The basic strategy is to subdivide the table among Segments,
* each of which itself is a concurrently readable hash table.
*/
/* ---------------- Constants -------------- */
/**
* The default demo capacity for this table,
* used when not otherwise specified in a constructor.
*/
static final int DEFAULT_INITIAL_CAPACITY = 16;
/**
* The default load factor for this table, used when not
* otherwise specified in a constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The default concurrency level for this table, used when not
* otherwise specified in a constructor.
*/
static final int DEFAULT_CONCURRENCY_LEVEL = 16;
/**
* The maximum capacity, used if a higher value is implicitly
* specified by either of the constructors with arguments. MUST
* be a power of two <= 1<<30 to ensure that entries are indexable
* using ints.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The maximum number of segments to allow; used to bound
* constructor arguments.
*/
static final int MAX_SEGMENTS = 1 << 16; // slightly conservative
/**
* Number of unsynchronized retries in size and containsValue
* methods before resorting to locking. This is used to avoid
* unbounded retries if tables undergo continuous modification
* which would make it impossible to obtain an accurate result.
*/
static final int RETRIES_BEFORE_LOCK = 2;
/* ---------------- Fields -------------- */
/**
* Mask value for indexing into segments. The upper bits of a
* key's hash code are used to choose the segment.
*/
final int segmentMask;
/**
* Shift value for indexing within segments.
*/
final int segmentShift;
/**
* The segments, each of which is a specialized hash table
*/
final Segment[] segments;
transient Set keySet;
transient Set> entrySet;
transient Collection values;
/* ---------------- Small Utilities -------------- */
/**
* Applies a supplemental hash function to a given hashCode, which
* defends against poor quality hash functions. This is critical
* because ConcurrentWeakHashMap uses power-of-two length hash tables,
* that otherwise encounter collisions for hashCodes that do not
* differ in lower or upper bits.
*/
private static int hash( int h )
{
// Spread bits to regularize both segment and index locations,
// using variant of single-word Wang/Jenkins hash.
h += (h << 15) ^ 0xffffcd7d;
h ^= (h >>> 10);
h += (h << 3);
h ^= (h >>> 6);
h += (h << 2) + (h << 14);
return h ^ (h >>> 16);
}
/**
* Returns the segment that should be used for key with given hash
*
* @param hash the hash code for the key
*
* @return the segment
*/
final Segment segmentFor( int hash )
{
return segments[(hash >>> segmentShift) & segmentMask];
}
/* ---------------- Inner Classes -------------- */
/**
* A weak-key reference which stores the key hash needed for reclamation.
*/
static final class WeakKeyReference extends WeakReference
{
final int hash;
WeakKeyReference( K key, int hash, ReferenceQueue refQueue )
{
super( key, refQueue );
this.hash = hash;
}
}
/**
* ConcurrentWeakHashMap list entry. Note that this is never exported
* out as a user-visible Map.Entry.
*
* Because the value field is volatile, not final, it is legal wrt
* the Java Memory Model for an unsynchronized reader to see null
* instead of demo value when read via a data race. Although a
* reordering leading to this is not likely to ever actually
* occur, the Segment.readValueUnderLock method is used as a
* backup in case a null (pre-initialized) value is ever seen in
* an unsynchronized access method.
*/
static final class HashEntry
{
final WeakReference keyRef;
final int hash;
volatile V value;
final HashEntry next;
HashEntry( K key, int hash, HashEntry next, V value, ReferenceQueue refQueue )
{
this.keyRef = new WeakKeyReference( key, hash, refQueue );
this.hash = hash;
this.next = next;
this.value = value;
}
@SuppressWarnings("unchecked")
static final HashEntry[] newArray( int i )
{
return new HashEntry[i];
}
}
/**
* Segments are specialized versions of hash tables. This
* subclasses from ReentrantLock opportunistically, just to
* simplify some locking and avoid separate construction.
*/
static final class Segment extends ReentrantLock implements Serializable
{
/*
* Segments maintain a table of entry lists that are ALWAYS
* kept in a consistent state, so can be read without locking.
* Next fields of nodes are immutable (final). All list
* additions are performed at the front of each bin. This
* makes it easy to check changes, and also fast to traverse.
* When nodes would otherwise be changed, new nodes are
* created to replace them. This works well for hash tables
* since the bin lists tend to be short. (The average length
* is less than two for the default load factor threshold.)
*
* Read operations can thus proceed without locking, but rely
* on selected uses of volatiles to ensure that completed
* write operations performed by other threads are
* noticed. For most purposes, the "count" field, tracking the
* number of elements, serves as that volatile variable
* ensuring visibility. This is convenient because this field
* needs to be read in many read operations anyway:
*
* - All (unsynchronized) read operations must first read the
* "count" field, and should not look at table entries if
* it is 0.
*
* - All (synchronized) write operations should write to
* the "count" field after structurally changing any bin.
* The operations must not take any action that could even
* momentarily cause a concurrent read operation to see
* inconsistent data. This is made easier by the nature of
* the read operations in Map. For example, no operation
* can reveal that the table has grown but the threshold
* has not yet been updated, so there are no atomicity
* requirements for this with respect to reads.
*
* As a guide, all critical volatile reads and writes to the
* count field are marked in code comments.
*/
private static final long serialVersionUID = 2249069246763182397L;
/**
* The number of elements in this segment's region.
*/
transient volatile int count;
/**
* Number of updates that alter the size of the table. This is
* used during bulk-read methods to make sure they see a
* consistent snapshot: If modCounts change during a traversal
* of segments computing size or checking containsValue, then
* we might have an inconsistent view of state so (usually)
* must retry.
*/
transient int modCount;
/**
* The table is rehashed when its size exceeds this threshold.
* (The value of this field is always (int)(capacity *
* loadFactor).)
*/
transient int threshold;
/**
* The per-segment table.
*/
transient volatile HashEntry[] table;
/**
* The load factor for the hash table. Even though this value
* is same for all segments, it is replicated to avoid needing
* links to outer object.
*
* @serial
*/
final float loadFactor;
/**
* The collected weak-key reference queue for this segment.
* This should be (re)initialized whenever table is assigned,
*/
transient volatile ReferenceQueue refQueue;
Segment( int initialCapacity, float lf )
{
loadFactor = lf;
setTable( HashEntry.newArray( initialCapacity ) );
}
@SuppressWarnings("unchecked")
static final Segment[] newArray( int i )
{
return new Segment[i];
}
/**
* Sets table to new HashEntry array.
* Call only while holding lock or in constructor.
*/
void setTable( HashEntry[] newTable )
{
threshold = (int)(newTable.length * loadFactor);
table = newTable;
refQueue = new ReferenceQueue();
}
/**
* Returns properly casted first entry of bin for given hash.
*/
HashEntry getFirst( int hash )
{
HashEntry[] tab = table;
return tab[hash & (tab.length - 1)];
}
/**
* Reads value field of an entry under lock. Called if value
* field ever appears to be null. This is possible only if a
* compiler happens to reorder a HashEntry initialization with
* its table assignment, which is legal under memory model
* but is not known to ever occur.
*/
V readValueUnderLock( HashEntry e )
{
lock();
try
{
removeStale();
return e.value;
}
finally
{
unlock();
}
}
/* Specialized implementations of map methods */
V get( Object key, int hash )
{
if( count != 0 )
{ // read-volatile
HashEntry e = getFirst( hash );
while( e != null )
{
if( e.hash == hash && key.equals( e.keyRef.get() ) )
{
V v = e.value;
if( v != null )
{
return v;
}
return readValueUnderLock( e ); // recheck
}
e = e.next;
}
}
return null;
}
boolean containsKey( Object key, int hash )
{
if( count != 0 )
{ // read-volatile
HashEntry e = getFirst( hash );
while( e != null )
{
if( e.hash == hash && key.equals( e.keyRef.get() ) )
{
return true;
}
e = e.next;
}
}
return false;
}
boolean containsValue( Object value )
{
if( count != 0 )
{ // read-volatile
HashEntry[] tab = table;
int len = tab.length;
for( int i = 0; i < len; i++ )
{
for( HashEntry e = tab[i]; e != null; e = e.next )
{
V v = e.value;
if( v == null ) // recheck
{
v = readValueUnderLock( e );
}
if( value.equals( v ) )
{
return true;
}
}
}
}
return false;
}
boolean replace( K key, int hash, V oldValue, V newValue )
{
lock();
try
{
removeStale();
HashEntry e = getFirst( hash );
while( e != null && (e.hash != hash || !key.equals( e.keyRef.get() )) )
{
e = e.next;
}
boolean replaced = false;
if( e != null && oldValue.equals( e.value ) )
{
replaced = true;
e.value = newValue;
}
return replaced;
}
finally
{
unlock();
}
}
V replace( K key, int hash, V newValue )
{
lock();
try
{
removeStale();
HashEntry e = getFirst( hash );
while( e != null && (e.hash != hash || !key.equals( e.keyRef.get() )) )
{
e = e.next;
}
V oldValue = null;
if( e != null )
{
oldValue = e.value;
e.value = newValue;
}
return oldValue;
}
finally
{
unlock();
}
}
V put( K key, int hash, V value, boolean onlyIfAbsent )
{
lock();
try
{
removeStale();
int c = count;
if( c++ > threshold )
{// ensure capacity
int reduced = rehash();
if( reduced > 0 ) // adjust from possible weak cleanups
{
count = (c -= reduced) - 1; // write-volatile
}
}
HashEntry[] tab = table;
int index = hash & (tab.length - 1);
HashEntry first = tab[index];
HashEntry e = first;
while( e != null && (e.hash != hash || !key.equals( e.keyRef.get() )) )
{
e = e.next;
}
V oldValue;
if( e != null )
{
oldValue = e.value;
if( !onlyIfAbsent )
{
e.value = value;
}
}
else
{
oldValue = null;
++modCount;
tab[index] = new HashEntry( key, hash, first, value, refQueue );
count = c; // write-volatile
}
return oldValue;
}
finally
{
unlock();
}
}
int rehash()
{
HashEntry[] oldTable = table;
int oldCapacity = oldTable.length;
if( oldCapacity >= MAXIMUM_CAPACITY )
{
return 0;
}
/*
* Reclassify nodes in each list to new Map. Because we are
* using power-of-two expansion, the elements from each bin
* must either stay at same index, or move with a power of two
* offset. We eliminate unnecessary node creation by catching
* cases where old nodes can be reused because their next
* fields won't change. Statistically, at the default
* threshold, only about one-sixth of them need cloning when
* a table doubles. The nodes they replace will be garbage
* collectable as soon as they are no longer referenced by any
* reader thread that may be in the midst of traversing table
* right now.
*/
HashEntry[] newTable = HashEntry.newArray( oldCapacity << 1 );
threshold = (int)(newTable.length * loadFactor);
int sizeMask = newTable.length - 1;
int reduce = 0;
for( int i = 0; i < oldCapacity; i++ )
{
// We need to guarantee that any existing reads of old Map can
// proceed. So we cannot yet null out each bin.
HashEntry e = oldTable[i];
if( e != null )
{
HashEntry next = e.next;
int idx = e.hash & sizeMask;
// Single node on list
if( next == null )
{
newTable[idx] = e;
}
else
{
// Reuse trailing consecutive sequence at same slot
HashEntry lastRun = e;
int lastIdx = idx;
for( HashEntry last = next; last != null; last = last.next )
{
int k = last.hash & sizeMask;
if( k != lastIdx )
{
lastIdx = k;
lastRun = last;
}
}
newTable[lastIdx] = lastRun;
// Clone all remaining nodes
for( HashEntry p = e; p != lastRun; p = p.next )
{
// Skip GC'd weak refs
K key = p.keyRef.get();
if( key == null )
{
reduce++;
continue;
}
int k = p.hash & sizeMask;
HashEntry n = newTable[k];
newTable[k] = new HashEntry( key, p.hash, n, p.value, refQueue );
}
}
}
}
table = newTable;
return reduce;
}
/**
* Remove; match on key only if value null, else match both.
*/
V remove( Object key, int hash, Object value, boolean weakRemove )
{
lock();
try
{
if( !weakRemove )
{
removeStale();
}
int c = count - 1;
HashEntry[] tab = table;
int index = hash & (tab.length - 1);
HashEntry first = tab[index];
HashEntry e = first;
// a weak remove operation compares the WeakReference instance
while( e != null && (!weakRemove || key != e.keyRef)
&& (e.hash != hash || !key.equals( e.keyRef.get() )) )
{
e = e.next;
}
V oldValue = null;
if( e != null )
{
V v = e.value;
if( value == null || value.equals( v ) )
{
oldValue = v;
// All entries following removed node can stay
// in list, but all preceding ones need to be
// cloned.
++modCount;
HashEntry newFirst = e.next;
for( HashEntry p = first; p != e; p = p.next )
{
K pKey = p.keyRef.get();
if( pKey == null )
{ // Skip GC'd keys
c--;
continue;
}
newFirst = new HashEntry( pKey, p.hash, newFirst, p.value, refQueue );
}
tab[index] = newFirst;
count = c; // write-volatile
}
}
return oldValue;
}
finally
{
unlock();
}
}
@SuppressWarnings("unchecked")
void removeStale()
{
WeakKeyReference ref;
while( (ref = (WeakKeyReference)refQueue.poll()) != null )
{
remove( ref, ref.hash, null, true );
}
}
void clear()
{
if( count != 0 )
{
lock();
try
{
HashEntry[] tab = table;
for( int i = 0; i < tab.length; i++ )
{
tab[i] = null;
}
++modCount;
// replace the reference queue to avoid unnecessary stale cleanups
refQueue = new ReferenceQueue();
count = 0; // write-volatile
}
finally
{
unlock();
}
}
}
}
/* ---------------- Public operations -------------- */
/**
* Creates a new, empty map with the specified demo
* capacity, load factor and concurrency level.
*
* @param initialCapacity the demo capacity. The implementation
* performs internal sizing to accommodate this many elements.
* @param loadFactor the load factor threshold, used to control resizing.
* Resizing may be performed when the average number of elements per
* bin exceeds this threshold.
* @param concurrencyLevel the estimated number of concurrently
* updating threads. The implementation performs internal sizing
* to try to accommodate this many threads.
*
* @throws IllegalArgumentException if the demo capacity is
* negative or the load factor or concurrencyLevel are
* nonpositive.
*/
public ConcurrentWeakHashMap( int initialCapacity, float loadFactor, int concurrencyLevel )
{
if( !(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0 )
{
throw new IllegalArgumentException();
}
if( concurrencyLevel > MAX_SEGMENTS )
{
concurrencyLevel = MAX_SEGMENTS;
}
// Find power-of-two sizes best matching arguments
int sshift = 0;
int ssize = 1;
while( ssize < concurrencyLevel )
{
++sshift;
ssize <<= 1;
}
segmentShift = 32 - sshift;
segmentMask = ssize - 1;
this.segments = Segment.newArray( ssize );
if( initialCapacity > MAXIMUM_CAPACITY )
{
initialCapacity = MAXIMUM_CAPACITY;
}
int c = initialCapacity / ssize;
if( c * ssize < initialCapacity )
{
++c;
}
int cap = 1;
while( cap < c )
{
cap <<= 1;
}
for( int i = 0; i < this.segments.length; ++i )
{
this.segments[i] = new Segment( cap, loadFactor );
}
}
/**
* Creates a new, empty map with the specified demo capacity
* and load factor and with the default concurrencyLevel (16).
*
* @param initialCapacity The implementation performs internal
* sizing to accommodate this many elements.
* @param loadFactor the load factor threshold, used to control resizing.
* Resizing may be performed when the average number of elements per
* bin exceeds this threshold.
*
* @throws IllegalArgumentException if the demo capacity of
* elements is negative or the load factor is nonpositive
* @since 1.6
*/
public ConcurrentWeakHashMap( int initialCapacity, float loadFactor )
{
this( initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL );
}
/**
* Creates a new, empty map with the specified demo capacity,
* and with default load factor (0.75) and concurrencyLevel (16).
*
* @param initialCapacity the demo capacity. The implementation
* performs internal sizing to accommodate this many elements.
*
* @throws IllegalArgumentException if the demo capacity of
* elements is negative.
*/
public ConcurrentWeakHashMap( int initialCapacity )
{
this( initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL );
}
/**
* Creates a new, empty map with a default demo capacity (16),
* load factor (0.75) and concurrencyLevel (16).
*/
public ConcurrentWeakHashMap()
{
this( DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL );
}
/**
* Creates a new map with the same mappings as the given map.
* The map is created with a capacity of 1.5 times the number
* of mappings in the given map or 16 (whichever is greater),
* and a default load factor (0.75) and concurrencyLevel (16).
*
* @param m the map
*/
public ConcurrentWeakHashMap( Map m )
{
this( Math.max( (int)(m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY ), DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL );
putAll( m );
}
/**
* Returns true if this map contains no key-value mappings.
*
* @return true if this map contains no key-value mappings
*/
public boolean isEmpty()
{
final Segment[] segments = this.segments;
/*
* We keep track of per-segment modCounts to avoid ABA
* problems in which an element in one segment was added and
* in another removed during traversal, in which case the
* table was never actually empty at any point. Note the
* similar use of modCounts in the size() and containsValue()
* methods, which are the only other methods also susceptible
* to ABA problems.
*/
int[] mc = new int[segments.length];
int mcsum = 0;
for( int i = 0; i < segments.length; ++i )
{
if( segments[i].count != 0 )
{
return false;
}
else
{
mcsum += mc[i] = segments[i].modCount;
}
}
// If mcsum happens to be zero, then we know we got a snapshot
// before any modifications at all were made. This is
// probably common enough to bother tracking.
if( mcsum != 0 )
{
for( int i = 0; i < segments.length; ++i )
{
if( segments[i].count != 0 || mc[i] != segments[i].modCount )
{
return false;
}
}
}
return true;
}
/**
* Returns the number of key-value mappings in this map. If the
* map contains more than Integer.MAX_VALUE elements, returns
* Integer.MAX_VALUE.
*
* @return the number of key-value mappings in this map
*/
public int size()
{
final Segment[] segments = this.segments;
long sum = 0;
long check = 0;
int[] mc = new int[segments.length];
// Try a few times to get accurate count. On failure due to
// continuous async changes in table, resort to locking.
for( int k = 0; k < RETRIES_BEFORE_LOCK; ++k )
{
check = 0;
sum = 0;
int mcsum = 0;
for( int i = 0; i < segments.length; ++i )
{
sum += segments[i].count;
mcsum += mc[i] = segments[i].modCount;
}
if( mcsum != 0 )
{
for( int i = 0; i < segments.length; ++i )
{
check += segments[i].count;
if( mc[i] != segments[i].modCount )
{
check = -1; // force retry
break;
}
}
}
if( check == sum )
{
break;
}
}
if( check != sum )
{ // Resort to locking all segments
sum = 0;
for( int i = 0; i < segments.length; ++i )
{
segments[i].lock();
}
for( int i = 0; i < segments.length; ++i )
{
sum += segments[i].count;
}
for( int i = 0; i < segments.length; ++i )
{
segments[i].unlock();
}
}
if( sum > Integer.MAX_VALUE )
{
return Integer.MAX_VALUE;
}
else
{
return (int)sum;
}
}
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
*
More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code key.equals(k)},
* then this method returns {@code v}; otherwise it returns
* {@code null}. (There can be at most one such mapping.)
*
* @throws NullPointerException if the specified key is null
*/
public V get( Object key )
{
int hash = hash( key.hashCode() );
return segmentFor( hash ).get( key, hash );
}
/**
* Tests if the specified object is a key in this table.
*
* @param key possible key
*
* @return true if and only if the specified object
* is a key in this table, as determined by the
* equals method; false otherwise.
*
* @throws NullPointerException if the specified key is null
*/
public boolean containsKey( Object key )
{
int hash = hash( key.hashCode() );
return segmentFor( hash ).containsKey( key, hash );
}
/**
* Returns true if this map maps one or more keys to the
* specified value. Note: This method requires a full internal
* traversal of the hash table, and so is much slower than
* method containsKey.
*
* @param value value whose presence in this map is to be tested
*
* @return true if this map maps one or more keys to the
* specified value
*
* @throws NullPointerException if the specified value is null
*/
public boolean containsValue( Object value )
{
if( value == null )
{
throw new NullPointerException();
}
// See explanation of modCount use above
final Segment[] segments = this.segments;
int[] mc = new int[segments.length];
// Try a few times without locking
for( int k = 0; k < RETRIES_BEFORE_LOCK; ++k )
{
int sum = 0;
int mcsum = 0;
for( int i = 0; i < segments.length; ++i )
{
int c = segments[i].count;
mcsum += mc[i] = segments[i].modCount;
if( segments[i].containsValue( value ) )
{
return true;
}
}
boolean cleanSweep = true;
if( mcsum != 0 )
{
for( int i = 0; i < segments.length; ++i )
{
int c = segments[i].count;
if( mc[i] != segments[i].modCount )
{
cleanSweep = false;
break;
}
}
}
if( cleanSweep )
{
return false;
}
}
// Resort to locking all segments
for( int i = 0; i < segments.length; ++i )
{
segments[i].lock();
}
boolean found = false;
try
{
for( int i = 0; i < segments.length; ++i )
{
if( segments[i].containsValue( value ) )
{
found = true;
break;
}
}
}
finally
{
for( int i = 0; i < segments.length; ++i )
{
segments[i].unlock();
}
}
return found;
}
/**
* Legacy method testing if some key maps into the specified value
* in this table. This method is identical in functionality to
* {@link #containsValue}, and exists solely to ensure
* full compatibility with class {@link Hashtable},
* which supported this method prior to introduction of the
* Java Collections framework.
*
* @param value a value to search for
*
* @return true if and only if some key maps to the
* value argument in this table as
* determined by the equals method;
* false otherwise
*
* @throws NullPointerException if the specified value is null
*/
public boolean contains( Object value )
{
return containsValue( value );
}
/**
* Maps the specified key to the specified value in this table.
* Neither the key nor the value can be null.
*
*
The value can be retrieved by calling the get method
* with a key that is equal to the original key.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
*
* @return the previous value associated with key, or
* null if there was no mapping for key
*
* @throws NullPointerException if the specified key or value is null
*/
public V put( K key, V value )
{
if( value == null )
{
throw new NullPointerException();
}
int hash = hash( key.hashCode() );
return segmentFor( hash ).put( key, hash, value, false );
}
/**
* {@inheritDoc}
*
* @return the previous value associated with the specified key,
* or null if there was no mapping for the key
*
* @throws NullPointerException if the specified key or value is null
*/
public V putIfAbsent( K key, V value )
{
if( value == null )
{
throw new NullPointerException();
}
int hash = hash( key.hashCode() );
return segmentFor( hash ).put( key, hash, value, true );
}
/**
* Copies all of the mappings from the specified map to this one.
* These mappings replace any mappings that this map had for any of the
* keys currently in the specified map.
*
* @param m mappings to be stored in this map
*/
public void putAll( Map m )
{
for( Entry e : m.entrySet() )
{
put( e.getKey(), e.getValue() );
}
}
/**
* Removes the key (and its corresponding value) from this map.
* This method does nothing if the key is not in the map.
*
* @param key the key that needs to be removed
*
* @return the previous value associated with key, or
* null if there was no mapping for key
*
* @throws NullPointerException if the specified key is null
*/
public V remove( Object key )
{
int hash = hash( key.hashCode() );
return segmentFor( hash ).remove( key, hash, null, false );
}
/**
* {@inheritDoc}
*
* @throws NullPointerException if the specified key is null
*/
public boolean remove( Object key, Object value )
{
int hash = hash( key.hashCode() );
if( value == null )
{
return false;
}
return segmentFor( hash ).remove( key, hash, value, false ) != null;
}
/**
* {@inheritDoc}
*
* @throws NullPointerException if any of the arguments are null
*/
public boolean replace( K key, V oldValue, V newValue )
{
if( oldValue == null || newValue == null )
{
throw new NullPointerException();
}
int hash = hash( key.hashCode() );
return segmentFor( hash ).replace( key, hash, oldValue, newValue );
}
/**
* {@inheritDoc}
*
* @return the previous value associated with the specified key,
* or null if there was no mapping for the key
*
* @throws NullPointerException if the specified key or value is null
*/
public V replace( K key, V value )
{
if( value == null )
{
throw new NullPointerException();
}
int hash = hash( key.hashCode() );
return segmentFor( hash ).replace( key, hash, value );
}
/**
* Removes all of the mappings from this map.
*/
public void clear()
{
for( int i = 0; i < segments.length; ++i )
{
segments[i].clear();
}
}
/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. The set supports element
* removal, which removes the corresponding mapping from this map,
* via the Iterator.remove, Set.remove,
* removeAll, retainAll, and clear
* operations. It does not support the add or
* addAll operations.
*
*
The view's iterator is a "weakly consistent" iterator
* that will never throw {@link ConcurrentModificationException},
* and guarantees to traverse elements as they existed upon
* construction of the iterator, and may (but is not guaranteed to)
* reflect any modifications subsequent to construction.
*/
public Set keySet()
{
Set ks = keySet;
return (ks != null) ? ks : (keySet = new KeySet());
}
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. The collection
* supports element removal, which removes the corresponding
* mapping from this map, via the Iterator.remove,
* Collection.remove, removeAll,
* retainAll, and clear operations. It does not
* support the add or addAll operations.
*
*
The view's iterator is a "weakly consistent" iterator
* that will never throw {@link ConcurrentModificationException},
* and guarantees to traverse elements as they existed upon
* construction of the iterator, and may (but is not guaranteed to)
* reflect any modifications subsequent to construction.
*/
public Collection values()
{
Collection vs = values;
return (vs != null) ? vs : (values = new Values());
}
/**
* Returns a {@link Set} view of the mappings contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. The set supports element
* removal, which removes the corresponding mapping from the map,
* via the Iterator.remove, Set.remove,
* removeAll, retainAll, and clear
* operations. It does not support the add or
* addAll operations.
*
*
The view's iterator is a "weakly consistent" iterator
* that will never throw {@link ConcurrentModificationException},
* and guarantees to traverse elements as they existed upon
* construction of the iterator, and may (but is not guaranteed to)
* reflect any modifications subsequent to construction.
*/
public Set> entrySet()
{
Set> es = entrySet;
return (es != null) ? es : (entrySet = new EntrySet());
}
/**
* Returns an enumeration of the keys in this table.
*
* @return an enumeration of the keys in this table
*
* @see #keySet()
*/
public Enumeration keys()
{
return new KeyIterator();
}
/**
* Returns an enumeration of the values in this table.
*
* @return an enumeration of the values in this table
*
* @see #values()
*/
public Enumeration elements()
{
return new ValueIterator();
}
/* ---------------- Iterator Support -------------- */
abstract class HashIterator
{
int nextSegmentIndex;
int nextTableIndex;
HashEntry[] currentTable;
HashEntry nextEntry;
HashEntry lastReturned;
K currentKey; // Strong reference to weak key (prevents gc)
HashIterator()
{
nextSegmentIndex = segments.length - 1;
nextTableIndex = -1;
advance();
}
public boolean hasMoreElements()
{
return hasNext();
}
final void advance()
{
if( nextEntry != null && (nextEntry = nextEntry.next) != null )
{
return;
}
while( nextTableIndex >= 0 )
{
if( (nextEntry = currentTable[nextTableIndex--]) != null )
{
return;
}
}
while( nextSegmentIndex >= 0 )
{
Segment seg = segments[nextSegmentIndex--];
if( seg.count != 0 )
{
currentTable = seg.table;
for( int j = currentTable.length - 1; j >= 0; --j )
{
if( (nextEntry = currentTable[j]) != null )
{
nextTableIndex = j - 1;
return;
}
}
}
}
}
public boolean hasNext()
{
while( nextEntry != null )
{
if( nextEntry.keyRef.get() != null )
{
return true;
}
advance();
}
return false;
}
HashEntry nextEntry()
{
do
{
if( nextEntry == null )
{
throw new NoSuchElementException();
}
lastReturned = nextEntry;
currentKey = lastReturned.keyRef.get();
advance();
} while( currentKey == null ); // Skip GC'd keys
return lastReturned;
}
public void remove()
{
if( lastReturned == null )
{
throw new IllegalStateException();
}
ConcurrentWeakHashMap.this.remove( currentKey );
lastReturned = null;
}
}
final class KeyIterator extends HashIterator implements Iterator, Enumeration
{
public K next()
{
return super.nextEntry().keyRef.get();
}
public K nextElement()
{
return super.nextEntry().keyRef.get();
}
}
final class ValueIterator extends HashIterator implements Iterator, Enumeration
{
public V next()
{
return super.nextEntry().value;
}
public V nextElement()
{
return super.nextEntry().value;
}
}
/*
* This class is needed for JDK5 compatibility.
*/
static class SimpleEntry implements Entry, Serializable
{
private static final long serialVersionUID = -8499721149061103585L;
private final K key;
private V value;
public SimpleEntry( K key, V value )
{
this.key = key;
this.value = value;
}
public SimpleEntry( Entry entry )
{
this.key = entry.getKey();
this.value = entry.getValue();
}
public K getKey()
{
return key;
}
public V getValue()
{
return value;
}
public V setValue( V value )
{
V oldValue = this.value;
this.value = value;
return oldValue;
}
public boolean equals( Object o )
{
if( !(o instanceof Map.Entry) )
{
return false;
}
@SuppressWarnings("unchecked")
Entry e = (Entry)o;
return eq( key, e.getKey() ) && eq( value, e.getValue() );
}
public int hashCode()
{
return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode());
}
public String toString()
{
return key + "=" + value;
}
private static boolean eq( Object o1, Object o2 )
{
return o1 == null ? o2 == null : o1.equals( o2 );
}
}
/**
* Custom Entry class used by EntryIterator.next(), that relays setValue
* changes to the underlying map.
*/
final class WriteThroughEntry extends SimpleEntry
{
private static final long serialVersionUID = -7900634345345313646L;
WriteThroughEntry( K k, V v )
{
super( k, v );
}
/**
* Set our entry's value and write through to the map. The
* value to return is somewhat arbitrary here. Since a
* WriteThroughEntry does not necessarily track asynchronous
* changes, the most recent "previous" value could be
* different from what we return (or could even have been
* removed in which case the put will re-establish). We do not
* and cannot guarantee more.
*/
public V setValue( V value )
{
if( value == null )
{
throw new NullPointerException();
}
V v = super.setValue( value );
ConcurrentWeakHashMap.this.put( getKey(), value );
return v;
}
}
final class EntryIterator extends HashIterator implements Iterator>
{
public Entry next()
{
HashEntry e = super.nextEntry();
return new WriteThroughEntry( e.keyRef.get(), e.value );
}
}
final class KeySet extends AbstractSet
{
public Iterator iterator()
{
return new KeyIterator();
}
public int size()
{
return ConcurrentWeakHashMap.this.size();
}
public boolean isEmpty()
{
return ConcurrentWeakHashMap.this.isEmpty();
}
public boolean contains( Object o )
{
return ConcurrentWeakHashMap.this.containsKey( o );
}
public boolean remove( Object o )
{
return ConcurrentWeakHashMap.this.remove( o ) != null;
}
public void clear()
{
ConcurrentWeakHashMap.this.clear();
}
}
final class Values extends AbstractCollection
{
public Iterator iterator()
{
return new ValueIterator();
}
public int size()
{
return ConcurrentWeakHashMap.this.size();
}
public boolean isEmpty()
{
return ConcurrentWeakHashMap.this.isEmpty();
}
public boolean contains( Object o )
{
return ConcurrentWeakHashMap.this.containsValue( o );
}
public void clear()
{
ConcurrentWeakHashMap.this.clear();
}
}
final class EntrySet extends AbstractSet>
{
public Iterator> iterator()
{
return new EntryIterator();
}
public boolean contains( Object o )
{
if( !(o instanceof Map.Entry) )
{
return false;
}
Entry e = (Entry)o;
V v = ConcurrentWeakHashMap.this.get( e.getKey() );
return v != null && v.equals( e.getValue() );
}
public boolean remove( Object o )
{
if( !(o instanceof Map.Entry) )
{
return false;
}
Entry e = (Entry)o;
return ConcurrentWeakHashMap.this.remove( e.getKey(), e.getValue() );
}
public int size()
{
return ConcurrentWeakHashMap.this.size();
}
public boolean isEmpty()
{
return ConcurrentWeakHashMap.this.isEmpty();
}
public void clear()
{
ConcurrentWeakHashMap.this.clear();
}
}
/* ---------------- Serialization Support -------------- */
/**
* Save the state of the ConcurrentWeakHashMap instance to a
* stream (i.e., serialize it).
*
* @param s the stream
*
* @serialData the key (Object) and value (Object)
* for each key-value mapping, followed by a null pair.
* The key-value mappings are emitted in no particular order.
*/
private void writeObject( java.io.ObjectOutputStream s ) throws IOException
{
s.defaultWriteObject();
for( int k = 0; k < segments.length; ++k )
{
Segment seg = segments[k];
seg.lock();
try
{
HashEntry[] tab = seg.table;
for( int i = 0; i < tab.length; ++i )
{
for( HashEntry e = tab[i]; e != null; e = e.next )
{
K key = e.keyRef.get();
if( key == null ) // Skip GC'd keys
{
continue;
}
s.writeObject( key );
s.writeObject( e.value );
}
}
}
finally
{
seg.unlock();
}
}
s.writeObject( null );
s.writeObject( null );
}
/**
* Reconstitute the ConcurrentWeakHashMap instance from a
* stream (i.e., deserialize it).
*
* @param s the stream
*/
@SuppressWarnings("unchecked")
private void readObject( java.io.ObjectInputStream s ) throws IOException, ClassNotFoundException
{
s.defaultReadObject();
// Initialize each segment to be minimally sized, and let grow.
for( int i = 0; i < segments.length; ++i )
{
segments[i].setTable( new HashEntry[1] );
}
// Read the keys and values, and put the mappings in the table
for( ; ; )
{
K key = (K)s.readObject();
V value = (V)s.readObject();
if( key == null )
{
break;
}
put( key, value );
}
}
}