/*
File: ConcurrentHashMap
Written by Doug Lea. Adapted from JDK1.2 HashMap.java and Hashtable.java
which carries the following copyright:
* Copyright 1997 by Sun Microsystems, Inc.,
* 901 San Antonio Road, Palo Alto, California, 94303, U.S.A.
* All rights reserved.
*
* This software is the confidential and proprietary information
* of Sun Microsystems, Inc. ("Confidential Information"). You
* shall not disclose such Confidential Information and shall use
* it only in accordance with the terms of the license agreement
* you entered into with Sun.
History:
Date Who What
26nov2000 dl Created, based on ConcurrentReaderHashMap
12jan2001 dl public release
*/
package EDU.oswego.cs.dl.util.concurrent;
import java.util.Map;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.AbstractCollection;
import java.util.Collection;
import java.util.Set;
import java.util.Iterator;
import java.util.Enumeration;
import java.util.NoSuchElementException;
import java.io.Serializable;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
/**
* A version of Hashtable supporting
* concurrency for both retrievals and updates:
*
* <dl>
* <dt> Retrievals
*
* <dd> Retrievals may overlap updates. (This is the same policy as
* ConcurrentReaderHashMap.) Successful retrievals using get(key) and
* containsKey(key) usually run without locking. Unsuccessful
* retrievals (i.e., when the key is not present) do involve brief
* synchronization (locking). Because retrieval operations can
* ordinarily overlap with update operations (i.e., put, remove, and
* their derivatives), retrievals can only be guaranteed to return the
* results of the most recently <em>completed</em> operations holding
* upon their onset. Retrieval operations may or may not return
* results reflecting in-progress writing operations. However, the
* retrieval operations do always return consistent results -- either
* those holding before any single modification or after it, but never
* a nonsense result. For aggregate operations such as putAll and
* clear, concurrent reads may reflect insertion or removal of only
* some entries.
* <p>
*
* Iterators and Enumerations (i.e., those returned by
* keySet().iterator(), entrySet().iterator(), values().iterator(),
* keys(), and elements()) return elements reflecting the state of the
* hash table at some point at or since the creation of the
* iterator/enumeration. They will return at most one instance of
* each element (via next()/nextElement()), but might or might not
* reflect puts and removes that have been processed since they were
* created. They do <em>not</em> throw ConcurrentModificationException.
* However, these iterators are designed to be used by only one
* thread at a time. Sharing an iterator across multiple threads may
* lead to unpredictable results if the table is being concurrently
* modified. <p>
*
*
* <dt> Updates
*
* <dd> This class supports a hard-wired preset <em>concurrency
* level</em> of 32. This allows a maximum of 32 put and/or remove
* operations to proceed concurrently. This level is an upper bound on
* concurrency, not a guarantee, since it interacts with how
* well-strewn elements are across bins of the table. (The preset
* value in part reflects the fact that even on large multiprocessors,
* factors other than synchronization tend to be bottlenecks when more
* than 32 threads concurrently attempt updates.)
* Additionally, operations triggering internal resizing and clearing
* do not execute concurrently with any operation.
* <p>
*
* There is <em>NOT</em> any support for locking the entire table to
* prevent updates. This makes it imposssible, for example, to
* add an element only if it is not already present, since another
* thread may be in the process of doing the same thing.
* If you need such capabilities, consider instead using the
* ConcurrentReaderHashMap class.
*
* </dl>
*
* Because of how concurrency control is split up, the
* size() and isEmpty() methods require accumulations across 32
* control segments, and so might be slightly slower than you expect.
* <p>
*
* This class may be used as a direct replacement for
* java.util.Hashtable in any application that does not rely
* on the ability to lock the entire table to prevent updates.
* As of this writing, it performs much faster than Hashtable in
* typical multi-threaded applications with multiple readers and writers.
* Like Hashtable but unlike java.util.HashMap,
* this class does NOT allow <tt>null</tt> to be used as a key or
* value.
* <p>
*
* Implementation note: A slightly
* faster implementation of this class will be possible once planned
* Java Memory Model revisions are in place.
*
* <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
**/
public class ConcurrentHashMap
extends AbstractMap
implements Map, Cloneable, Serializable {
/*
The basic strategy is an optimistic-style scheme based on
the guarantee that the hash table and its lists are always
kept in a consistent enough state to be read without locking:
* Read operations first proceed without locking, by traversing the
apparently correct list of the apparently correct bin. If an
entry is found, but not invalidated (value field null), it is
returned. If not found, operations must recheck (after a memory
barrier) to make sure they are using both the right list and
the right table (which can change under resizes). If
invalidated, reads must acquire main update lock to wait out
the update, and then re-traverse.
* All list additions are at the front of each bin, making it easy
to check changes, and also fast to traverse. Entry next
pointers are never assigned. Remove() builds new nodes when
necessary to preserve this.
* Remove() (also clear()) invalidates removed nodes to alert read
operations that they must wait out the full modifications.
* Locking for puts, removes (and, when necessary gets, etc)
is controlled by Segments, each covering a portion of the
table. During operations requiring global exclusivity (mainly
resize and clear), ALL of these locks are acquired at once.
Note that these segments are NOT contiguous -- they are based
on the least 5 bits of hashcodes. This ensures that the same
segment controls the same slots before and after resizing, which
is necessary for supporting concurrent retrievals. This
comes at the price of a mismatch of logical vs physical locality,
but this seems not to be a performance problem in practice.
*/
/**
* The hash table data.
*/
protected transient Entry[] table;
/**
* The number of concurrency control segments.
* The value can be at most 32 since ints are used
* as bitsets over segments. Emprically, it doesn't
* seem to pay to decrease it either, so the value should be at least 32.
* In other words, do not redefine this :-)
***/
protected static final int CONCURRENCY_LEVEL = 32;
/**
* Bookkeeping for each concurrency control segment.
* Each segment contains a local count of the number of
* elements in its region.
* However, the main use of a Segment is for its lock.
**/
protected final static class Segment {
/**
* The number of elements in this segment's region.
* It is always updated within synchronized blocks.
**/
protected int count;
/**
* Get the count under synch.
**/
protected synchronized int getCount() { return count; }
/**
* Force a synchronization
**/
protected synchronized void synch() {}
}
/**
* The array of concurrency control segments.
**/
protected final Segment[] segments = new Segment[CONCURRENCY_LEVEL];
/**
* The default initial number of table slots for this table (32).
* Used when not otherwise specified in constructor.
**/
public static int DEFAULT_INITIAL_CAPACITY = 32;
/**
* The minimum capacity, used if a lower value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two.
*/
private static final int MINIMUM_CAPACITY = 32;
/**
* 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.
*/
private static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The default load factor for this table (0.75)
* Used when not otherwise specified in constructor.
**/
public static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The load factor for the hash table.
*
* @serial
*/
protected final float loadFactor;
/**
* The table is rehashed when its size exceeds this threshold. (The
* value of this field is always (int)(capacity * loadFactor).)
*
* @serial
*/
protected int threshold;
/**
* Number of segments voting for resize. The table is
* doubled when 1/4 of the segments reach threshold.
* Volatile but updated without synch since this is just a heuristic.
**/
protected transient volatile int votesForResize;
/**
* Return the number of set bits in w.
* For a derivation of this algorithm, see
* "Algorithms and data structures with applications to
* graphics and geometry", by Jurg Nievergelt and Klaus Hinrichs,
* Prentice Hall, 1993.
* See also notes by Torsten Sillke at
* http://www.mathematik.uni-bielefeld.de/~sillke/PROBLEMS/bitcount
**/
protected static int bitcount(int w) {
w -= (0xaaaaaaaa & w) >>> 1;
w = (w & 0x33333333) + ((w >>> 2) & 0x33333333);
w = w + (w >>> 4) & 0x0f0f0f0f;
w += w >>> 8;
w += w >>> 16;
return w & 0xff;
}
/**
* Returns the appropriate capacity (power of two) for the specified
* initial capacity argument.
*/
private int p2capacity(int initialCapacity) {
int cap = initialCapacity;
// Compute the appropriate capacity
int result;
if (cap > MAXIMUM_CAPACITY || cap < 0) {
result = MAXIMUM_CAPACITY;
} else {
result = MINIMUM_CAPACITY;
while (result < cap)
result <<= 1;
}
return result;
}
/**
* Return hash code for Object x. Since we are using power-of-two
* tables, it is worth the effort to improve hashcode via
* the same multiplicative scheme as used in IdentityHashMap.
*/
protected static int hash(Object x) {
int h = x.hashCode();
// Multiply by 127 (quickly, via shifts), and mix in some high
// bits to help guard against bunching of codes that are
// consecutive or equally spaced.
return ((h << 7) - h + (h >>> 9) + (h >>> 17));
}
/**
* Constructs a new, empty map with the specified initial
* capacity and the specified load factor.
*
* @param initialCapacity the initial capacity.
* The actual initial capacity is rounded to the nearest power of two.
* @param loadFactor the load factor threshold, used to control resizing.
* This value is used in an approximate way: When at least
* a quarter of the segments of the table reach per-segment threshold, or
* one of the segments itself exceeds overall threshold,
* the table is doubled.
* This will on average cause resizing when the table-wide
* load factor is slightly less than the threshold. If you'd like
* to avoid resizing, you can set this to a ridiculously large
* value.
* @throws IllegalArgumentException if the load factor is nonpositive.
*/
public ConcurrentHashMap(int initialCapacity,
float loadFactor) {
if (loadFactor <= 0)
throw new IllegalArgumentException("Illegal Load factor: "+
loadFactor);
this.loadFactor = loadFactor;
int cap = p2capacity(initialCapacity);
table = new Entry[cap];
threshold = (int)(cap * loadFactor / segments.length) + 1;
for (int i = 0; i < segments.length; ++i)
segments[i] = new Segment();
}
/**
* Constructs a new, empty map with the specified initial
* capacity and default load factor.
*
* @param initialCapacity the initial capacity of the
* ConcurrentHashMap.
* @throws IllegalArgumentException if the initial maximum number
* of elements is less
* than zero.
*/
public ConcurrentHashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs a new, empty map with a default initial capacity
* and default load factor.
*/
public ConcurrentHashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs a new map with the same mappings as the given map. The
* map is created with a capacity of twice the number of mappings in
* the given map or 32 (whichever is greater), and a default load factor.
*/
public ConcurrentHashMap(Map t) {
this(Math.max(2*t.size(), 32), DEFAULT_LOAD_FACTOR);
putAll(t);
}
/**
* Returns the number of key-value mappings in this map.
*
* @return the number of key-value mappings in this map.
*/
public int size() {
int c = 0;
for (int i = 0; i < segments.length; ++i)
c += segments[i].getCount();
return c;
}
/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*
* @return <tt>true</tt> if this map contains no key-value mappings.
*/
public boolean isEmpty() {
for (int i = 0; i < segments.length; ++i)
if (segments[i].getCount() != 0)
return false;
return true;
}
/**
* Returns the value to which the specified key is mapped in this table.
*
* @param key a key in the table.
* @return the value to which the key is mapped in this table;
* <code>null</code> if the key is not mapped to any value in
* this table.
* @exception NullPointerException if the key is
* <code>null</code>.
* @see #put(Object, Object)
*/
public Object get(Object key) {
int hash = hash(key); // throws null pointer exception if key null
/*
Start off at the apparently correct bin. If entry is found, we
need to check value to make sure it is valid. If
not found, we need a barrier to check if we are actually in
right bin.
*/
Segment seg = segments[hash & (CONCURRENCY_LEVEL-1)];
Entry[] tab = table;
int index = hash & (tab.length - 1);
Entry first = tab[index];
Entry e = first;
for (;;) {
if (e == null) {
// If key apparently not there, check to
// make sure this was a valid read
synchronized(seg) { tab = table; }
if (first == tab[index])
return null;
else {
// Wrong list -- must restart traversal at new first
e = first = tab[index = hash & (tab.length-1)];
}
}
// checking for pointer equality first wins in most applications
else if (key == e.key || (e.hash == hash && key.equals(e.key))) {
Object value = e.value;
if (value != null)
return value;
// Entry was invalidated during deletion. But it could
// have been re-inserted, so we must retraverse.
synchronized(seg) { tab = table; }
e = first = tab[index = hash & (tab.length-1)];
}
else
e = e.next;
}
}
/**
* Tests if the specified object is a key in this table.
*
* @param key possible key.
* @return <code>true</code> if and only if the specified object
* is a key in this table, as determined by the
* <tt>equals</tt> method; <code>false</code> otherwise.
* @exception NullPointerException if the key is
* <code>null</code>.
* @see #contains(Object)
*/
public boolean containsKey(Object key) {
return get(key) != null;
}
/**
* Maps the specified <code>key</code> to the specified
* <code>value</code> in this table. Neither the key nor the
* value can be <code>null</code>. (Note that this policy is
* the same as for java.util.Hashtable, but unlike java.util.HashMap,
* which does accept nulls as valid keys and values.)<p>
*
* The value can be retrieved by calling the <code>get</code> method
* with a key that is equal to the original key.
*
* @param key the table key.
* @param value the value.
* @return the previous value of the specified key in this table,
* or <code>null</code> if it did not have one.
* @exception NullPointerException if the key or value is
* <code>null</code>.
* @see Object#equals(Object)
* @see #get(Object)
*/
public Object put(Object key, Object value) {
/*
Strategy:
* Start out the same way as get(), trying to find key
without lock.
* If not found, get lock, check that it is correct list,
and if so, prepend new node. If not correct, retry.
* If found, get lock to avoid races with other puts or
removes. If the node is in the midst of removal, retry.
* When the local count exceeds threshold, check to see
if other segments have reached threshold, and if so,
start global locking/resizing via resize().
*/
if (value == null)
throw new NullPointerException();
int hash = hash(key);
Segment seg = segments[hash & (CONCURRENCY_LEVEL-1)];
Entry[] tab = table;
int index = hash & (tab.length-1);
Entry first = tab[index];
Entry e = first;
int segcount = 0;
for (;;) {
if (e == null) {
synchronized(seg) {
tab = table;
// make sure we are adding to correct list
if (first == tab[index]) {
// Add to front of list
Entry newEntry = new Entry(hash, key, value, first);
tab[index] = newEntry;
if ((segcount = ++seg.count) < threshold)
return null;
else
break; // fall below to call resize outside lock
}
else { // retry
e = first = tab[index = hash & (tab.length-1)];
}
}
}
else if (key == e.key || (e.hash == hash && key.equals(e.key))) {
// synch to avoid race with remove and to
// ensure proper serialization of multiple replaces
synchronized(seg) {
tab = table;
Object oldValue = e.value;
if (first == tab[index] && oldValue != null) {
e.value = value;
return oldValue;
}
else {
e = first = tab[index = hash & (tab.length-1)];
}
}
}
else
e = e.next;
}
// Reach here only to check if should call resize, outside of seg lock
int bit = (1 << (hash & (CONCURRENCY_LEVEL-1)));
int votes = votesForResize;
if ((votes & bit) == 0)
votes = votesForResize |= bit;
// Attempt resize if 1/4 segs vote,
// or if this seg itself reaches overall threshold.
// (The latter check is just a safeguard to avoid pathological cases.)
else if (bitcount(votes) >= CONCURRENCY_LEVEL / 4 ||
segcount > CONCURRENCY_LEVEL * threshold)
resize(0, 0);
return null;
}
/**
* Gather all locks in order to call rehash, by
* recursing withing synch blocks for each segment index.
* Initial call MUST be resize(0, 0)
*/
protected void resize(int index, int votes) {
if (index < segments.length) {
Segment seg = segments[index];
synchronized(seg) {
votes += seg.count / threshold;
resize(index+1, votes);
}
}
else if (votes >= CONCURRENCY_LEVEL / 4) // avoid false alarm
rehash();
}
/**
* Rehashes the contents of this map into a new table
* with a larger capacity.
*/
protected void rehash() {
votesForResize = 0; // reset
Entry[] oldMap = table;
int oldCapacity = oldMap.length;
if (oldCapacity >= MAXIMUM_CAPACITY) return;
int newCapacity = oldCapacity << 1;
Entry[] newMap = new Entry[newCapacity];
/*
We need to guarantee that any existing reads of oldMap can
proceed. So we cannot yet null out each oldMap bin.
Because we are using power-of-two expansion, the elements
from each bin must either stay at same index, or move
to oldCapacity+index. We also minimize new node creation by
catching cases where old nodes can be reused because their
.next fields won't change. (This is checked only for sequences
of one and two. It is not worth checking longer ones.)
*/
for (int i = 0; i < oldCapacity ; ++i) {
Entry l = null;
Entry h = null;
Entry e = oldMap[i];
while (e != null) {
int hash = e.hash;
Entry next = e.next;
if ((hash & oldCapacity) == 0) { // stays at newMap[i]
if (l == null && // try to reuse node
(next == null ||
(next.next == null && (next.hash & oldCapacity) == 0))) {
l = e;
break;
}
l = new Entry(hash, e.key, e.value, l);
}
else { // moves to newMap[oldCapacity+i]
if (h == null &&
(next == null ||
(next.next == null && (next.hash & oldCapacity) != 0))) {
h = e;
break;
}
h = new Entry(hash, e.key, e.value, h);
}
e = next;
}
newMap[i] = l;
newMap[oldCapacity + i] = h;
}
table = newMap;
threshold = (int)(newCapacity * loadFactor / segments.length) + 1;
// System.out.println("size: " + size() + " cap " + newCapacity);
}
/**
* Removes the key (and its corresponding value) from this
* table. This method does nothing if the key is not in the table.
*
* @param key the key that needs to be removed.
* @return the value to which the key had been mapped in this table,
* or <code>null</code> if the key did not have a mapping.
* @exception NullPointerException if the key is
* <code>null</code>.
*/
public Object remove(Object key) {
return remove(key, null);
}
/**
* Removes the (key, value) pair from this
* table. This method does nothing if the key is not in the table,
* or if the key is associated with a different value. This method
* is needed by EntrySet.
*
* @param key the key that needs to be removed.
* @param value the associated value. If the value is null,
* it means "any value".
* @return the value to which the key had been mapped in this table,
* or <code>null</code> if the key did not have a mapping.
* @exception NullPointerException if the key is
* <code>null</code>.
*/
protected Object remove(Object key, Object value) {
int hash = hash(key);
Segment seg = segments[hash & (CONCURRENCY_LEVEL-1)];
/*
Removes are fully synchronized on seg because otherwise, they tend
to interact badly with ongoing puts.
Strategy:
Find the entry, then
1. Set value field to null, to force get() to retry
2. Rebuild the list without this entry.
All entries following removed node can stay in list, but
all preceeding ones need to be cloned. Traversals rely
on this strategy to ensure that elements will not be
repeated during iteration.
*/
synchronized(seg) {
Entry[] tab = table;
int index = hash & (tab.length-1);
Entry first = tab[index];
Entry e = first;
for (;;) {
if (e == null)
return null;
else if (key == e.key || (e.hash == hash && key.equals(e.key))) {
Object oldValue = e.value;
if (value != null && !value.equals(oldValue))
return null;
e.value = null;
seg.count--;
Entry head = e.next;
for (Entry p = first; p != e; p = p.next)
head = new Entry(p.hash, p.key, p.value, head);
tab[index] = head;
return oldValue;
}
else
e = e.next;
}
}
}
/**
* Returns <tt>true</tt> 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 <tt>containsKey</tt>.
*
* @param value value whose presence in this map is to be tested.
* @return <tt>true</tt> if this map maps one or more keys to the
* specified value.
* @exception NullPointerException if the value is <code>null</code>.
*/
public boolean containsValue(Object value) {
if (value == null) throw new NullPointerException();
for (int s = 0; s < segments.length; ++s) {
Segment seg = segments[s];
Entry[] tab;
synchronized(seg) { tab = table; }
for (int i = s; i < tab.length; i+= segments.length) {
for (Entry e = tab[i]; e != null; e = e.next) {
Object v = e.value;
if (v != null && value.equals(v))
return true;
}
}
}
return false;
}
/**
* Tests if some key maps into the specified value in this table.
* This operation is more expensive than the <code>containsKey</code>
* method.<p>
*
* Note that this method is identical in functionality to containsValue,
* (which is part of the Map interface in the collections framework).
*
* @param value a value to search for.
* @return <code>true</code> if and only if some key maps to the
* <code>value</code> argument in this table as
* determined by the <tt>equals</tt> method;
* <code>false</code> otherwise.
* @exception NullPointerException if the value is <code>null</code>.
* @see #containsKey(Object)
* @see #containsValue(Object)
* @see Map
*/
public boolean contains(Object value) {
return containsValue(value);
}
/**
* 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 t Mappings to be stored in this map.
*/
public void putAll(Map t) {
for (Iterator it = t.entrySet().iterator(); it.hasNext();) {
Map.Entry entry = (Map.Entry) it.next();
put(entry.getKey(), entry.getValue());
}
}
/**
* Removes all mappings from this map.
*/
public void clear() {
/* We don't need all locks at once so long as locks
are obtained in low to high order
*/
for (int s = 0; s < segments.length; ++s) {
Segment seg = segments[s];
synchronized(seg) {
Entry[] tab = table;
for (int i = s; i < tab.length; i+= segments.length) {
for (Entry e = tab[i]; e != null; e = e.next)
e.value = null;
tab[i] = null;
seg.count = 0;
}
}
}
}
/**
* Returns a shallow copy of this
* <tt>ConcurrentHashMap</tt> instance: the keys and
* values themselves are not cloned.
*
* @return a shallow copy of this map.
*/
public Object clone() {
try {
ConcurrentHashMap t = (ConcurrentHashMap)super.clone();
t.keySet = null;
t.entrySet = null;
t.values = null;
// get current capacity. It is OK if this changes
// by the time putAll is called. It is only used
// to help avoid resizings.
int cap;
synchronized(segments[0]) { cap = table.length; }
t.table = new Entry[cap];
t.threshold = (int)(cap * loadFactor / segments.length) + 1;
for (int i = 0; i < t.segments.length; ++i)
t.segments[i] = new Segment();
t.putAll(this);
return t;
}
catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}
// Views
protected transient Set keySet = null;
protected transient Set entrySet = null;
protected transient Collection values = null;
/**
* Returns a 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 <tt>Iterator.remove</tt>,
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and
* <tt>clear</tt> operations. It does not support the <tt>add</tt> or
* <tt>addAll</tt> operations.
*
* @return a set view of the keys contained in this map.
*/
public Set keySet() {
Set ks = keySet;
if (ks != null)
return ks;
else {
return keySet = new AbstractSet() {
public Iterator iterator() {
return new KeyIterator();
}
public int size() {
return ConcurrentHashMap.this.size();
}
public boolean contains(Object o) {
return ConcurrentHashMap.this.containsKey(o);
}
public boolean remove(Object o) {
return ConcurrentHashMap.this.remove(o) != null;
}
public void clear() {
ConcurrentHashMap.this.clear();
}
};
}
}
/**
* Returns a 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
* <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations.
* It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a collection view of the values contained in this map.
*/
public Collection values() {
Collection vs = values;
if (vs != null)
return vs;
else {
return values = new AbstractCollection() {
public Iterator iterator() {
return new ValueIterator();
}
public int size() {
return ConcurrentHashMap.this.size();
}
public boolean contains(Object o) {
return ConcurrentHashMap.this.containsValue(o);
}
public void clear() {
ConcurrentHashMap.this.clear();
}
};
}
}
/**
* Returns a collection view of the mappings contained in this map. Each
* element in the returned collection is a <tt>Map.Entry</tt>. 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 the map, via the
* <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations.
* It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a collection view of the mappings contained in this map.
* @see Map.Entry
*/
public Set entrySet() {
Set es = entrySet;
if (es != null)
return es;
else {
return entrySet = new AbstractSet() {
public Iterator iterator() {
return new HashIterator();
}
public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
Object key = entry.getKey();
Object v = ConcurrentHashMap.this.get(key);
return v != null && v.equals(entry.getValue());
}
public boolean remove(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
Object k = entry.getKey();
Object v = entry.getValue();
return ConcurrentHashMap.this.remove(k, v) != null;
}
public int size() {
return ConcurrentHashMap.this.size();
}
public void clear() {
ConcurrentHashMap.this.clear();
}
};
}
}
/**
* Returns an enumeration of the keys in this table.
*
* @return an enumeration of the keys in this table.
* @see Enumeration
* @see #elements()
* @see #keySet()
* @see Map
*/
public Enumeration keys() {
return new KeyIterator();
}
/**
* Returns an enumeration of the values in this table.
* Use the Enumeration methods on the returned object to fetch the elements
* sequentially.
*
* @return an enumeration of the values in this table.
* @see java.util.Enumeration
* @see #keys()
* @see #values()
* @see Map
*/
public Enumeration elements() {
return new ValueIterator();
}
/**
* ConcurrentHashMap collision list entry.
*/
protected static class Entry implements Map.Entry {
/*
The use of volatile for value field ensures that
we can detect status changes without synchronization.
The other fields are never changed, and are
marked as final.
*/
protected final int hash;
protected final Object key;
protected final Entry next;
protected volatile Object value;
Entry(int hash, Object key, Object value, Entry next) {
this.hash = hash;
this.key = key;
this.next = next;
this.value = value;
}
// Map.Entry Ops
public Object getKey() {
return key;
}
/**
* Get the value. Note: In an entrySet or entrySet.iterator,
* unless you can guarantee lack of concurrent modification,
* <tt>getValue</tt> <em>might</em> return null, reflecting the
* fact that the entry has been concurrently removed. However,
* there are no assurances that concurrent removals will be
* reflected using this method.
*
* @return the current value, or null if the entry has been
* detectably removed.
**/
public Object getValue() {
return value;
}
/**
* Set the value of this entry. Note: In an entrySet or
* entrySet.iterator), unless you can guarantee lack of concurrent
* modification, <tt>setValue</tt> is not strictly guaranteed to
* actually replace the value field obtained via the <tt>get</tt>
* operation of the underlying hash table in multithreaded
* applications. If iterator-wide synchronization is not used,
* and any other concurrent <tt>put</tt> or <tt>remove</tt>
* operations occur, sometimes even to <em>other</em> entries,
* then this change is not guaranteed to be reflected in the hash
* table. (It might, or it might not. There are no assurances
* either way.)
*
* @param value the new value.
* @return the previous value, or null if entry has been detectably
* removed.
* @exception NullPointerException if the value is <code>null</code>.
*
**/
public Object setValue(Object value) {
if (value == null)
throw new NullPointerException();
Object oldValue = this.value;
this.value = value;
return oldValue;
}
public boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
if (!key.equals(e.getKey()))
return false;
Object v = value;
return (v == null) ? e.getValue() == null : v.equals(e.getValue());
}
public int hashCode() {
Object v = value;
return hash ^ ((v == null) ? 0 : v.hashCode());
}
public String toString() {
return key + "=" + value;
}
}
protected class HashIterator implements Iterator, Enumeration {
protected final Entry[] tab; // snapshot of table
protected int index; // current slot
protected Entry entry = null; // current node of slot
protected Object currentKey; // key for current node
protected Object currentValue; // value for current node
protected Entry lastReturned = null; // last node returned by next
protected HashIterator() {
// force all segments to synch
synchronized(segments[0]) { tab = table; }
for (int i = 1; i < segments.length; ++i) segments[i].synch();
index = tab.length - 1;
}
public boolean hasMoreElements() { return hasNext(); }
public Object nextElement() { return next(); }
public boolean hasNext() {
/*
currentkey and currentValue are set here to ensure that next()
returns normally if hasNext() returns true. This avoids
surprises especially when final element is removed during
traversal -- instead, we just ignore the removal during
current traversal.
*/
for (;;) {
if (entry != null) {
Object v = entry.value;
if (v != null) {
currentKey = entry.key;
currentValue = v;
return true;
}
else
entry = entry.next;
}
while (entry == null && index >= 0)
entry = tab[index--];
if (entry == null) {
currentKey = currentValue = null;
return false;
}
}
}
protected Object returnValueOfNext() { return entry; }
public Object next() {
if (currentKey == null && !hasNext())
throw new NoSuchElementException();
Object result = returnValueOfNext();
lastReturned = entry;
currentKey = currentValue = null;
entry = entry.next;
return result;
}
public void remove() {
if (lastReturned == null)
throw new IllegalStateException();
ConcurrentHashMap.this.remove(lastReturned.key);
}
}
protected class KeyIterator extends HashIterator {
protected Object returnValueOfNext() { return currentKey; }
}
protected class ValueIterator extends HashIterator {
protected Object returnValueOfNext() { return currentValue; }
}
/**
* Save the state of the <tt>ConcurrentHashMap</tt>
* instance to a stream (i.e.,
* serialize it).
*
* @serialData
* An estimate of the table size, followed by
* 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.
*/
protected void writeObject(java.io.ObjectOutputStream s)
throws IOException {
// Write out the loadfactor, and any hidden stuff
s.defaultWriteObject();
// Write out capacity estimate. It is OK if this
// changes during the write, since it is only used by
// readObject to set initial capacity, to avoid needless resizings.
int cap;
synchronized(segments[0]) { cap = table.length; }
s.writeInt(cap);
// Write out keys and values (alternating)
for (int k = 0; k < segments.length; ++k) {
Segment seg = segments[k];
Entry[] tab;
synchronized(seg) { tab = table; }
for (int i = k; i < tab.length; i+= segments.length) {
for (Entry e = tab[i]; e != null; e = e.next) {
s.writeObject(e.key);
s.writeObject(e.value);
}
}
}
s.writeObject(null);
s.writeObject(null);
}
/**
* Reconstitute the <tt>ConcurrentHashMap</tt>
* instance from a stream (i.e.,
* deserialize it).
*/
protected void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException {
// Read in the threshold, loadfactor, and any hidden stuff
s.defaultReadObject();
int cap = s.readInt();
table = new Entry[cap];
threshold = (int)(cap * loadFactor / segments.length) + 1;
for (int i = 0; i < segments.length; ++i)
segments[i] = new Segment();
// Read the keys and values, and put the mappings in the table
for (;;) {
Object key = s.readObject();
Object value = s.readObject();
if (key == null)
break;
put(key, value);
}
}
}
