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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 org.infinispan.commons.util.concurrent;
   
  import java.util.Map;
  import java.util.Set;
An alternative weak-key java.util.concurrent.ConcurrentMap which is similar to java.util.concurrent.ConcurrentHashMap.

Parameters:
<K> the type of keys maintained by this map
<V> the type of mapped values
Author(s):
The Netty Project
Doug Lea
Jason T. Greene
Trustin Lee
Version:
$Rev: 2371 $, $Date: 2010-10-19 15:00:42 +0900 (Tue, 19 Oct 2010) $
  
  public final class ConcurrentWeakKeyHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {
  
     /*
     * The basic strategy is to subdivide the table among Segments,
     * each of which itself is a concurrently readable hash table.
     */

   
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 integers.
  
     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;
  
     Set<K> keySet;
     Set<Map.Entry<K, V>> entrySet;
     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 ConcurrentReferenceHashMap 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.

Parameters:
hash the hash code for the key
Returns:
the segment
 
    final Segment<K, V> segmentFor(int hash) {
       return [hash >>>  & ];
    }
 
    private int hashOf(Object key) {
       return hash(key.hashCode());
    }
 
    /* ---------------- 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 keyint hashReferenceQueue<ObjectrefQueue) {
          super(keyrefQueue);
          this. = hash;
       }
 
       public final int keyHash() {
          return ;
       }
 
       public final Object keyRef() {
          return this;
       }
    }

   
ConcurrentReferenceHashMap 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 Object keyRef;
       final int hash;
       volatile Object valueRef;
       final HashEntry<K, V> next;
 
       HashEntry(
             K keyint hashHashEntry<K, V> next, V value,
             ReferenceQueue<ObjectrefQueue) {
          this. = hash;
          this. = next;
          this. = new WeakKeyReference<K>(keyhashrefQueue);
          this. = value;
       }
 
       @SuppressWarnings("unchecked")
       final K key() {
          return ((WeakReference<K>) ).get();
       }
 
       final V value() {
          return dereferenceValue();
       }
 
       @SuppressWarnings("unchecked")
       final V dereferenceValue(Object value) {
          if (value instanceof WeakKeyReference) {
             return ((Reference<V>) value).get();
          }
 
          return (V) value;
       }
 
       final void setValue(V value) {
          this. = value;
       }
 
       @SuppressWarnings("unchecked")
       static <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 {
       /*
       * 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 = -8328104880676891126L;

      
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.
 
       int modCount;

      
The table is rehashed when its size exceeds this threshold. (The value of this field is always (capacity * loadFactor).)
 
       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.
 
       final float loadFactor;

      
The collected weak-key reference queue for this segment. This should be (re)initialized whenever table is assigned,
 
       transient volatile ReferenceQueue<ObjectrefQueue;
 
       Segment(int initialCapacityfloat lf) {
           = lf;
          setTable(HashEntry.<K, V>newArray(initialCapacity));
       }
 
       @SuppressWarnings("unchecked")
       static <K, V> Segment<K, V>[] newArray(int i) {
          return new Segment[i];
       }
 
       private boolean keyEq(Object srcObject dest) {
          return src.equals(dest);
       }

      
Sets table to new HashEntry array. Call only while holding lock or in constructor.
 
       void setTable(HashEntry<K, V>[] newTable) {
           = (int) (newTable.length * );
           = newTable;
           = new ReferenceQueue<Object>();
       }

      
Returns properly casted first entry of bin for given hash.
 
       HashEntry<K, V> getFirst(int hash) {
          HashEntry<K, V>[] tab = ;
          return tab[hash & tab.length - 1];
       }
 
       HashEntry<K, V> newHashEntry(
             K keyint hashHashEntry<K, V> next, V value) {
          return new HashEntry<K, V>(
                keyhashnextvalue);
       }

      
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 keyint hash) {
          if ( != 0) { // read-volatile
             HashEntry<K, V> e = getFirst(hash);
             while (e != null) {
                if (e.hash == hash && keyEq(keye.key())) {
                   Object opaque = e.valueRef;
                   if (opaque != null) {
                      return e.dereferenceValue(opaque);
                   }
 
                   return readValueUnderLock(e); // recheck
                }
                e = e.next;
             }
          }
          return null;
       }
 
       boolean containsKey(Object keyint hash) {
          if ( != 0) { // read-volatile
             HashEntry<K, V> e = getFirst(hash);
             while (e != null) {
                if (e.hash == hash && keyEq(keye.key())) {
                   return true;
                }
                e = e.next;
             }
          }
          return false;
       }
 
       boolean containsValue(Object value) {
          if ( != 0) { // read-volatile
             HashEntry<K, V>[] tab = ;
             int len = tab.length;
             for (int i = 0; i < leni++) {
                for (HashEntry<K, V> e = tab[i]; e != nulle = e.next) {
                   Object opaque = e.valueRef;
                   V v;
 
                   if (opaque == null) {
                      v = readValueUnderLock(e); // recheck
                   } else {
                      v = e.dereferenceValue(opaque);
                   }
 
                   if (value.equals(v)) {
                      return true;
                   }
                }
             }
          }
          return false;
       }
 
       boolean replace(K keyint hash, V oldValue, V newValue) {
          lock();
          try {
             removeStale();
             HashEntry<K, V> e = getFirst(hash);
             while (e != null && (e.hash != hash || !keyEq(keye.key()))) {
                e = e.next;
             }
 
             boolean replaced = false;
             if (e != null && oldValue.equals(e.value())) {
                replaced = true;
                e.setValue(newValue);
             }
             return replaced;
          } finally {
             unlock();
          }
       }
 
       V replace(K keyint hash, V newValue) {
          lock();
          try {
             removeStale();
             HashEntry<K, V> e = getFirst(hash);
             while (e != null && (e.hash != hash || !keyEq(keye.key()))) {
                e = e.next;
             }
 
             V oldValue = null;
             if (e != null) {
                oldValue = e.value();
                e.setValue(newValue);
             }
             return oldValue;
          } finally {
             unlock();
          }
       }
 
       V put(K keyint hash, V valueboolean onlyIfAbsent) {
          lock();
          try {
             removeStale();
             int c = ;
             if (c++ > ) { // ensure capacity
                int reduced = rehash();
                if (reduced > 0) {
                    = (c -= reduced) - 1; // write-volatile
                }
             }
 
             HashEntry<K, V>[] tab = ;
             int index = hash & tab.length - 1;
             HashEntry<K, V> first = tab[index];
             HashEntry<K, V> e = first;
             while (e != null && (e.hash != hash || !keyEq(keye.key()))) {
                e = e.next;
             }
 
             V oldValue;
             if (e != null) {
                oldValue = e.value();
                if (!onlyIfAbsent) {
                   e.setValue(value);
                }
             } else {
                oldValue = null;
                ++;
                tab[index] = newHashEntry(keyhashfirstvalue);
                 = c// write-volatile
             }
             return oldValue;
          } finally {
             unlock();
          }
       }
 
       int rehash() {
          HashEntry<K, V>[] oldTable = ;
          int oldCapacity = oldTable.length;
          if (oldCapacity >= ) {
             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);
           = (int) (newTable.length * );
          int sizeMask = newTable.length - 1;
          int reduce = 0;
          for (int i = 0; i < oldCapacityi++) {
             // 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 = nextlast != nulllast = 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 = ep != lastRunp = p.next) {
                      // Skip GC'd weak references
                      K key = p.key();
                      if (key == null) {
                         reduce++;
                         continue;
                      }
                      int k = p.hash & sizeMask;
                      HashEntry<K, V> n = newTable[k];
                      newTable[k] = newHashEntry(keyp.hashnp.value());
                   }
                }
             }
          }
           = newTable;
          return reduce;
       }

      
Remove; match on key only if value null, else match both.
 
       V remove(Object keyint hashObject valueboolean refRemove) {
          lock();
          try {
             if (!refRemove) {
                removeStale();
             }
             int c =  - 1;
             HashEntry<K, V>[] tab = ;
             int index = hash & tab.length - 1;
             HashEntry<K, V> first = tab[index];
             HashEntry<K, V> e = first;
             // a reference remove operation compares the Reference instance
             while (e != null && key != e.keyRef &&
                   (refRemove || hash != e.hash || !keyEq(keye.key()))) {
                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.
                   ++;
                   HashEntry<K, V> newFirst = e.next;
                   for (HashEntry<K, V> p = firstp != ep = p.next) {
                      K pKey = p.key();
                      if (pKey == null) { // Skip GC'd keys
                         c--;
                         continue;
                      }
 
                      newFirst = newHashEntry(
                            pKeyp.hashnewFirstp.value());
                   }
                   tab[index] = newFirst;
                    = c// write-volatile
                }
             }
             return oldValue;
          } finally {
             unlock();
          }
       }
 
       @SuppressWarnings("rawtypes")
       final void removeStale() {
          WeakKeyReference ref;
          while ((ref = (WeakKeyReference.poll()) != null) {
             remove(ref.keyRef(), ref.keyHash(), nulltrue);
          }
       }
 
       void clear() {
          if ( != 0) {
             lock();
             try {
                HashEntry<K, V>[] tab = ;
                for (int i = 0; i < tab.lengthi++) {
                   tab[i] = null;
                }
                ++;
                // replace the reference queue to avoid unnecessary stale
                // cleanups
                 = new ReferenceQueue<Object>();
                 = 0; // write-volatile
             } finally {
                unlock();
             }
          }
       }
    }
 
    /* ---------------- Public operations -------------- */

   
Creates a new, empty map with the specified initial capacity, load factor and concurrency level.

Parameters:
initialCapacity the initial capacity. The implementation performs internal sizing to accommodate this many elements.
loadFactor the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.
concurrencyLevel the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
Throws:
java.lang.IllegalArgumentException if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.
 
          int initialCapacityfloat loadFactorint concurrencyLevel) {
       if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) {
          throw new IllegalArgumentException();
       }
 
       if (concurrencyLevel > ) {
          concurrencyLevel = ;
       }
 
       // Find power-of-two sizes best matching arguments
       int sshift = 0;
       int ssize = 1;
       while (ssize < concurrencyLevel) {
          ++sshift;
          ssize <<= 1;
       }
        = 32 - sshift;
        = ssize - 1;
       this. = Segment.newArray(ssize);
 
       if (initialCapacity > ) {
          initialCapacity = ;
       }
       int c = initialCapacity / ssize;
       if (c * ssize < initialCapacity) {
          ++c;
       }
       int cap = 1;
       while (cap < c) {
          cap <<= 1;
       }
 
       for (int i = 0; i < this..length; ++i) {
          this.[i] = new Segment<K, V>(caploadFactor);
       }
    }

   
Creates a new, empty map with the specified initial capacity and load factor and with the default reference types (weak keys, strong values), and concurrencyLevel (16).

Parameters:
initialCapacity The implementation performs internal sizing to accommodate this many elements.
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:
java.lang.IllegalArgumentException if the initial capacity of elements is negative or the load factor is nonpositive
 
    public ConcurrentWeakKeyHashMap(int initialCapacityfloat loadFactor) {
       this(initialCapacityloadFactor);
    }

   
Creates a new, empty map with the specified initial capacity, and with default reference types (weak keys, strong values), load factor (0.75) and concurrencyLevel (16).

Parameters:
initialCapacity the initial capacity. The implementation performs internal sizing to accommodate this many elements.
Throws:
java.lang.IllegalArgumentException if the initial capacity of elements is negative.
 
    public ConcurrentWeakKeyHashMap(int initialCapacity) {
       this(initialCapacity);
    }

   
Creates a new, empty map with a default initial capacity (16), reference types (weak keys, strong values), default load factor (0.75) and concurrencyLevel (16).
 
    public ConcurrentWeakKeyHashMap() {
    }

   
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).

Parameters:
m the map
 
    public ConcurrentWeakKeyHashMap(Map<? extends K, ? extends V> m) {
       this(Math.max((int) (m.size() / ) + 1,
                     ), ,
            );
       putAll(m);
    }

   
Returns true if this map contains no key-value mappings.

Returns:
true if this map contains no key-value mappings
 
    @Override
    public boolean isEmpty() {
       final Segment<K, V>[] segments = this.;
       /*
       * 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]. != 0) {
             return false;
          } else {
             mcsum += mc[i] = segments[i].;
          }
       }
       // 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]. != 0 || mc[i] != segments[i].) {
                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.

Returns:
the number of key-value mappings in this map
 
    @Override
    public int size() {
       final Segment<K, V>[] segments = this.;
       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 < ; ++k) {
          check = 0;
          sum = 0;
          int mcsum = 0;
          for (int i = 0; i < segments.length; ++i) {
             sum += segments[i].;
             mcsum += mc[i] = segments[i].;
          }
          if (mcsum != 0) {
             for (int i = 0; i < segments.length; ++i) {
                check += segments[i].;
                if (mc[i] != segments[i].) {
                   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();
          }
          try {
             for (int i = 0; i < segments.length; ++i) {
                sum += segments[i].;
             }
          } finally {
             for (int i = 0; i < segments.length; ++i) {
                segments[i].unlock();
             }
          }
       }
       if (sum > .) {
          return .;
       } else {
          return (intsum;
       }
    }

   
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key k to a value v such that key.equals(k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

Throws:
java.lang.NullPointerException if the specified key is null
 
    @Override
    public V get(Object key) {
       int hash = hashOf(key);
       return segmentFor(hash).get(keyhash);
    }

   
Tests if the specified object is a key in this table.

Parameters:
key possible key
Returns:
true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise.
Throws:
java.lang.NullPointerException if the specified key is null
 
    @Override
    public boolean containsKey(Object key) {
       int hash = hashOf(key);
       return segmentFor(hash).containsKey(keyhash);
    }

   
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.

Parameters:
value value whose presence in this map is to be tested
Returns:
true if this map maps one or more keys to the specified value
Throws:
java.lang.NullPointerException if the specified value is null
 
 
    @Override
    public boolean containsValue(Object value) {
       if (value == null) {
          throw new NullPointerException();
       }
 
       // See explanation of modCount use above
 
       final Segment<K, V>[] segments = this.;
       int[] mc = new int[segments.length];
 
       // Try a few times without locking
       for (int k = 0; k < ; ++k) {
          int mcsum = 0;
          for (int i = 0; i < segments.length; ++i) {
             mcsum += mc[i] = segments[i].;
             if (segments[i].containsValue(value)) {
                return true;
             }
          }
          boolean cleanSweep = true;
          if (mcsum != 0) {
             for (int i = 0; i < segments.length; ++i) {
                if (mc[i] != segments[i].) {
                   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 containsValue(java.lang.Object), and exists solely to ensure full compatibility with class java.util.Hashtable, which supported this method prior to introduction of the Java Collections framework.

Parameters:
value a value to search for
Returns:
true if and only if some key maps to the value argument in this table as determined by the equals method; false otherwise
Throws:
java.lang.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.

Parameters:
key key with which the specified value is to be associated
value value to be associated with the specified key
Returns:
the previous value associated with key, or null if there was no mapping for key
Throws:
java.lang.NullPointerException if the specified key or value is null
 
    @Override
    public V put(K key, V value) {
       if (value == null) {
          throw new NullPointerException();
       }
       int hash = hashOf(key);
       return segmentFor(hash).put(keyhashvaluefalse);
    }

   

Returns:
the previous value associated with the specified key, or null if there was no mapping for the key
Throws:
java.lang.NullPointerException if the specified key or value is null
 
    @Override
    public V putIfAbsent(K key, V value) {
       if (value == null) {
          throw new NullPointerException();
       }
       int hash = hashOf(key);
       return segmentFor(hash).put(keyhashvaluetrue);
    }

   
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.

Parameters:
m mappings to be stored in this map
 
    @Override
    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.

Parameters:
key the key that needs to be removed
Returns:
the previous value associated with key, or null if there was no mapping for key
Throws:
java.lang.NullPointerException if the specified key is null
 
    @Override
    public V remove(Object key) {
       int hash = hashOf(key);
       return segmentFor(hash).remove(keyhashnullfalse);
    }

   

Throws:
java.lang.NullPointerException if the specified key is null
 
    @Override
    public boolean remove(Object keyObject value) {
       int hash = hashOf(key);
       if (value == null) {
          return false;
       }
       return segmentFor(hash).remove(keyhashvaluefalse) != null;
    }

   

Throws:
java.lang.NullPointerException if any of the arguments are null
 
    @Override
    public boolean replace(K key, V oldValue, V newValue) {
       if (oldValue == null || newValue == null) {
          throw new NullPointerException();
      }
      int hash = hashOf(key);
      return segmentFor(hash).replace(keyhasholdValuenewValue);
   }

   

Returns:
the previous value associated with the specified key, or null if there was no mapping for the key
Throws:
java.lang.NullPointerException if the specified key or value is null
   public V replace(K key, V value) {
      if (value == null) {
         throw new NullPointerException();
      }
      int hash = hashOf(key);
      return segmentFor(hash).replace(keyhashvalue);
   }

   
Removes all of the mappings from this map.
   public void clear() {
      for (int i = 0; i < .; ++i) {
         [i].clear();
      }
   }

   
Removes any stale entries whose keys have been finalized. Use of this method is normally not necessary since stale entries are automatically removed lazily, when blocking operations are required. However, there are some cases where this operation should be performed eagerly, such as cleaning up old references to a ClassLoader in a multi-classloader environment.

Note: this method will acquire locks, one at a time, across all segments of this table, so if it is to be used, it should be used sparingly.

   public void purgeStaleEntries() {
      for (int i = 0; i < .; ++i) {
         [i].removeStale();
      }
   }

   
Returns a java.util.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 java.util.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 = ;
      return ks != null ? ks : ( = new KeySet());
   }

   
Returns a java.util.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 java.util.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 = ;
      return vs != null ? vs : ( = new Values());
   }

   
Returns a java.util.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 java.util.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 = ;
      return es != null ? es : ( = new EntrySet());
   }

   
Returns an enumeration of the keys in this table.

Returns:
an enumeration of the keys in this table
See also:
keySet()
   public Enumeration<K> keys() {
      return new KeyIterator();
   }

   
Returns an enumeration of the values in this table.

Returns:
an enumeration of the values in this table
See also:
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() {
          = . - 1;
          = -1;
         advance();
      }
      public void rewind() {
          = . - 1;
          = -1;
          = null;
          = null;
          = null;
          = null;
         advance();
      }
      public boolean hasMoreElements() {
         return hasNext();
      }
      final void advance() {
         if ( != null && ( = .) != null) {
            return;
         }
         while ( >= 0) {
            if (( = [--]) != null) {
               return;
            }
         }
         while ( >= 0) {
            Segment<K, V> seg = [--];
            if (seg.count != 0) {
                = seg.table;
               for (int j = . - 1; j >= 0; --j) {
                  if (( = [j]) != null) {
                      = j - 1;
                     return;
                  }
               }
            }
         }
      }
      public boolean hasNext() {
         while ( != null) {
            if (.key() != null) {
               return true;
            }
            advance();
         }
         return false;
      }
      HashEntry<K, V> nextEntry() {
         do {
            if ( == null) {
               throw new NoSuchElementException();
            }
             = ;
             = .key();
            advance();
         } while ( == null); // Skip GC'd keys
         return ;
      }
      public void remove() {
         if ( == null) {
            throw new IllegalStateException();
         }
          = null;
      }
   }
   final class KeyIterator
         extends HashIterator implements ReusableIterator<K>, Enumeration<K> {
      @Override
      public K next() {
         return super.nextEntry().key();
      }
      @Override
      public K nextElement() {
         return super.nextEntry().key();
      }
   }
   final class ValueIterator
         extends HashIterator implements ReusableIterator<V>, Enumeration<V> {
      @Override
      public V next() {
         return super.nextEntry().value();
      }
      @Override
      public V nextElement() {
         return super.nextEntry().value();
      }
   }
   /*
   * This class is needed for JDK5 compatibility.
   */
   static class SimpleEntry<K, V> implements Entry<K, V> {
      private final K key;
      private V value;
      public SimpleEntry(K key, V value) {
         this. = key;
         this. = value;
      }
      public SimpleEntry(Entry<? extends K, ? extends V> entry) {
         this. = entry.getKey();
         this. = entry.getValue();
      }
      @Override
      public K getKey() {
         return ;
      }
      @Override
      public V getValue() {
         return ;
      }
      @Override
      public V setValue(V value) {
         V oldValue = this.;
         this. = value;
         return oldValue;
      }
      @Override
      public boolean equals(Object o) {
         if (!(o instanceof Map.Entry<?, ?>)) {
            return false;
         }
         @SuppressWarnings("rawtypes")
         Map.Entry e = (Map.Entryo;
         return eq(e.getKey()) && eq(e.getValue());
      }
      @Override
      public int hashCode() {
         return ( == null ? 0 : .hashCode()) ^ ( == null ? 0 : .hashCode());
      }
      @Override
      public String toString() {
         return  + "=" + ;
      }
      private static boolean eq(Object o1Object 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> {
      WriteThroughEntry(K k, V v) {
         super(kv);
      }

      
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 can not guarantee more.
      @Override
      public V setValue(V value) {
         if (value == null) {
            throw new NullPointerException();
         }
         V v = super.setValue(value);
         ConcurrentWeakKeyHashMap.this.put(getKey(), value);
         return v;
      }
   }
   final class EntryIterator extends