Mega Code Archive
A hash table with emweak keysem, full concurrency of retrievals, and adjustable expected concurrency for update
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/*
* 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
*/
//package xbird.util.concurrent.jsr166;
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 <em>weak keys</em>, 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 <tt>size</tt> and
* <tt>isEmpty</tt> 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.ConcurrentHashMap}, this class obeys the same
* functional specification as {@link java.util.Hashtable}, and includes
* versions of methods corresponding to each method of <tt>Hashtable</tt>.
* However, even though all operations are thread-safe, retrieval operations do
* <em>not</em> entail locking, and there is <em>not</em> any support for
* locking the entire table in a way that prevents all access. This class is
* fully interoperable with <tt>Hashtable</tt> in programs that rely on its
* thread safety but not on its synchronization details.
*
* <p>
* Retrieval operations (including <tt>get</tt>) generally do not block, so
* may overlap with update operations (including <tt>put</tt> and
* <tt>remove</tt>). Retrievals reflect the results of the most recently
* <em>completed</em> update operations holding upon their onset. For
* aggregate operations such as <tt>putAll</tt> and <tt>clear</tt>,
* 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 <em>not</em> throw
* {@link ConcurrentModificationException}. However, iterators are designed to
* be used by only one thread at a time.
*
* <p>
* The allowed concurrency among update operations is guided by the optional
* <tt>concurrencyLevel</tt> constructor argument (default <tt>16</tt>),
* 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.
*
* <p>
* This class and its views and iterators implement all of the <em>optional</em>
* methods of the {@link Map} and {@link Iterator} interfaces.
*
* <p>
* Like {@link Hashtable} but unlike {@link HashMap}, this class does
* <em>not</em> allow <tt>null</tt> to be used as a key or value.
*
* <p>
* This class is a member of the <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @author Doug Lea
* @author Jason T. Greene
* @param <K> the type of keys maintained by this map
* @param <V> the type of mapped values
*/
public class ConcurrentWeakHashMap<K, V> extends AbstractMap<K, V>
implements java.util.concurrent.ConcurrentMap<K, V>, 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 initial 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<K, V>[] segments;
transient Set<K> keySet;
transient Set<Map.Entry<K, V>> entrySet;
transient Collection<V> 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<K, V> 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<K> extends WeakReference<K> {
final int hash;
WeakKeyReference(K key, int hash, ReferenceQueue<K> 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 initial 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<K, V> {
final WeakReference<K> keyRef;
final int hash;
volatile V value;
final HashEntry<K, V> next;
HashEntry(K key, int hash, HashEntry<K, V> next, V value, ReferenceQueue<K> refQueue) {
this.keyRef = new WeakKeyReference<K>(key, hash, refQueue);
this.hash = hash;
this.next = next;
this.value = value;
}
@SuppressWarnings("unchecked")
static final <K, V> HashEntry<K, V>[] 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<K, V> 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 <tt>(int)(capacity *
* loadFactor)</tt>.)
*/
transient int threshold;
/**
* The per-segment table.
*/
transient volatile HashEntry<K, V>[] 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<K> refQueue;
Segment(int initialCapacity, float lf) {
loadFactor = lf;
setTable(HashEntry.<K, V> newArray(initialCapacity));
}
@SuppressWarnings("unchecked")
static final <K, V> Segment<K, V>[] newArray(int i) {
return new Segment[i];
}
/**
* Sets table to new HashEntry array.
* Call only while holding lock or in constructor.
*/
void setTable(HashEntry<K, V>[] newTable) {
threshold = (int) (newTable.length * loadFactor);
table = newTable;
refQueue = new ReferenceQueue<K>();
}
/**
* Returns properly casted first entry of bin for given hash.
*/
HashEntry<K, V> getFirst(int hash) {
HashEntry<K, V>[] 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<K, V> 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<K, V> 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<K, V> 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<K, V>[] tab = table;
int len = tab.length;
for(int i = 0; i < len; i++) {
for(HashEntry<K, V> 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<K, V> 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<K, V> 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<K, V>[] tab = table;
int index = hash & (tab.length - 1);
HashEntry<K, V> first = tab[index];
HashEntry<K, V> 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<K, V>(key, hash, first, value, refQueue);
count = c; // write-volatile
}
return oldValue;
} finally {
unlock();
}
}
int rehash() {
HashEntry<K, V>[] 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<K, V>[] 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<K, V> e = oldTable[i];
if(e != null) {
HashEntry<K, V> 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<K, V> lastRun = e;
int lastIdx = idx;
for(HashEntry<K, V> 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<K, V> 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<K, V> n = newTable[k];
newTable[k] = new HashEntry<K, V>(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<K, V>[] tab = table;
int index = hash & (tab.length - 1);
HashEntry<K, V> first = tab[index];
HashEntry<K, V> 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<K, V> newFirst = e.next;
for(HashEntry<K, V> p = first; p != e; p = p.next) {
K pKey = p.keyRef.get();
if(pKey == null) { // Skip GC'd keys
c--;
continue;
}
newFirst = new HashEntry<K, V>(pKey, p.hash, newFirst, p.value, refQueue);
}
tab[index] = newFirst;
count = c; // write-volatile
}
}
return oldValue;
} finally {
unlock();
}
}
@SuppressWarnings("unchecked")
void removeStale() {
WeakKeyReference<K> ref;
while((ref = (WeakKeyReference<K>) refQueue.poll()) != null) {
remove(ref, ref.hash, null, true);
}
}
void clear() {
if(count != 0) {
lock();
try {
HashEntry<K, V>[] 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<K>();
count = 0; // write-volatile
} finally {
unlock();
}
}
}
}
/* ---------------- Public operations -------------- */
/**
* Creates a new, empty map with the specified initial
* capacity, load factor and concurrency level.
*
* @param initialCapacity the initial 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 initial 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<K, V>(cap, loadFactor);
}
/**
* Creates a new, empty map with the specified initial 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 initial 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 initial capacity,
* and with default load factor (0.75) and concurrencyLevel (16).
*
* @param initialCapacity the initial capacity. The implementation
* performs internal sizing to accommodate this many elements.
* @throws IllegalArgumentException if the initial 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 initial 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<? extends K, ? extends V> m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
putAll(m);
}
/**
* 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() {
final Segment<K, V>[] 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 <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* @return the number of key-value mappings in this map
*/
public int size() {
final Segment<K, V>[] 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.
*
* <p>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 <tt>true</tt> if and only if the specified object
* is a key in this table, as determined by the
* <tt>equals</tt> method; <tt>false</tt> 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 <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
* @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<K, V>[] 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 java.util.Hashtable},
* which supported this method prior to introduction of the
* Java Collections framework.
* @param value a value to search for
* @return <tt>true</tt> if and only if some key maps to the
* <tt>value</tt> argument in this table as
* determined by the <tt>equals</tt> method;
* <tt>false</tt> 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.
*
* <p> The value can be retrieved by calling the <tt>get</tt> 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 <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>
* @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 <tt>null</tt> 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<? extends K, ? extends V> m) {
for(Map.Entry<? extends K, ? extends V> 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 <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>
* @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 <tt>null</tt> 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 <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.
*
* <p>The view's <tt>iterator</tt> 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<K> keySet() {
Set<K> 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 <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.
*
* <p>The view's <tt>iterator</tt> 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<V> values() {
Collection<V> 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 <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.
*
* <p>The view's <tt>iterator</tt> 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<Map.Entry<K, V>> entrySet() {
Set<Map.Entry<K, V>> 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<K> 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<V> elements() {
return new ValueIterator();
}
/* ---------------- Iterator Support -------------- */
abstract class HashIterator {
int nextSegmentIndex;
int nextTableIndex;
HashEntry<K, V>[] currentTable;
HashEntry<K, V> nextEntry;
HashEntry<K, V> 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<K, V> 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<K, V> 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<K>, Enumeration<K> {
public K next() {
return super.nextEntry().keyRef.get();
}
public K nextElement() {
return super.nextEntry().keyRef.get();
}
}
final class ValueIterator extends HashIterator implements Iterator<V>, Enumeration<V> {
public V next() {
return super.nextEntry().value;
}
public V nextElement() {
return super.nextEntry().value;
}
}
/*
* This class is needed for JDK5 compatibility.
*/
static class SimpleEntry<K, V> implements Entry<K, V>, java.io.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<? extends K, ? extends V> 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")
Map.Entry e = (Map.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<K, V> {
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<Entry<K, V>> {
public Map.Entry<K, V> next() {
HashEntry<K, V> e = super.nextEntry();
return new WriteThroughEntry(e.keyRef.get(), e.value);
}
}
final class KeySet extends AbstractSet<K> {
public Iterator<K> 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<V> {
public Iterator<V> 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<Map.Entry<K, V>> {
public Iterator<Map.Entry<K, V>> iterator() {
return new EntryIterator();
}
public boolean contains(Object o) {
if(!(o instanceof Map.Entry))
return false;
Map.Entry<?, ?> e = (Map.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;
Map.Entry<?, ?> e = (Map.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 <tt>ConcurrentWeakHashMap</tt> 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<K, V> seg = segments[k];
seg.lock();
try {
HashEntry<K, V>[] tab = seg.table;
for(int i = 0; i < tab.length; ++i) {
for(HashEntry<K, V> 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 <tt>ConcurrentWeakHashMap</tt> 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);
}
}
}
]]>