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/*
 * Hibernate, Relational Persistence for Idiomatic Java
 *
 * License: GNU Lesser General Public License (LGPL), version 2.1 or later.
 * See the lgpl.txt file in the root directory or .
 */

/*
 * 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.hibernate.internal.util.collections;

import java.io.IOException;
import java.io.Serializable;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.EnumSet;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.locks.ReentrantLock;

/**
 * An advanced hash table supporting configurable garbage collection semantics
 * of keys and values, optional referential-equality, full concurrency of
 * retrievals, and adjustable expected concurrency for updates.
 * 

 * This table is designed around specific advanced use-cases. If there is any
 * doubt whether this table is for you, you most likely should be using
 * {@link java.util.concurrent.ConcurrentHashMap} instead.
 * 

 * This table supports strong, weak, and soft keys and values. By default keys
 * are weak, and values are strong. Such a configuration offers similar behavior
 * to {@link java.util.WeakHashMap}, entries of this table are periodically
 * removed once their corresponding keys are no longer referenced outside of
 * this table. In other words, this table will not prevent a key from being
 * discarded by the garbage collector. Once a key has been discarded by the
 * collector, the corresponding entry is no longer visible to this table;
 * however, the entry may occupy space until a future table operation decides to
 * reclaim it. For this reason, summary functions such as size and
 * isEmpty might return a value greater than the observed number of
 * entries. In order to support a high level of concurrency, stale entries are
 * only reclaimed during blocking (usually mutating) operations.
 * 

 * Enabling soft keys allows entries in this table to remain until their space
 * is absolutely needed by the garbage collector. This is unlike weak keys which
 * can be reclaimed as soon as they are no longer referenced by a normal strong
 * reference. The primary use case for soft keys is a cache, which ideally
 * occupies memory that is not in use for as long as possible.
 * 

 * By default, values are held using a normal strong reference. This provides
 * the commonly desired 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 this is unavoidable, then it is recommended to use
 * the same reference type in use for the key. However, it should be noted that
 * non-strong values may disappear before their corresponding key.
 * 

 * While this table does allow the use of both strong keys and values, it is
 * recommended to use {@link java.util.concurrent.ConcurrentHashMap} for such a
 * configuration, since it is optimized for that case.
 * 

 * Just like {@link java.util.concurrent.ConcurrentHashMap}, this class obeys
 * the same functional specification as {@link java.util.Hashtable}, and
 * includes versions of methods corresponding to each method of
 * Hashtable. However, even though all operations are thread-safe,
 * retrieval operations do not entail locking, and there is
 * not any support for locking the entire table in a way that
 * prevents all access. This class is fully interoperable with
 * Hashtable in programs that rely on its thread safety but not on
 * its synchronization details.
 * 

 * 

 * Retrieval operations (including get) generally do not block, so
 * may overlap with update operations (including put and
 * remove). Retrievals reflect the results of the most recently
 * completed update operations holding upon their onset. For
 * aggregate operations such as putAll and clear,
 * concurrent retrievals may reflect insertion or removal of only some entries.
 * Similarly, Iterators and Enumerations return elements reflecting the state of
 * the hash table at some point at or since the creation of the
 * iterator/enumeration. They do not throw
 * {@link java.util.ConcurrentModificationException}. However, iterators are designed to
 * be used by only one thread at a time.
 * 

 * 

 * The allowed concurrency among update operations is guided by the optional
 * concurrencyLevel constructor argument (default 16),
 * which is used as a hint for internal sizing. The table is internally
 * partitioned to try to permit the indicated number of concurrent updates
 * without contention. Because placement in hash tables is essentially random,
 * the actual concurrency will vary. Ideally, you should choose a value to
 * accommodate as many threads as will ever concurrently modify the table. Using
 * a significantly higher value than you need can waste space and time, and a
 * significantly lower value can lead to thread contention. But overestimates
 * and underestimates within an order of magnitude do not usually have much
 * noticeable impact. A value of one is appropriate when it is known that only
 * one thread will modify and all others will only read. Also, resizing this or
 * any other kind of hash table is a relatively slow operation, so, when
 * possible, it is a good idea to provide estimates of expected table sizes in
 * constructors.
 * 

 * 

 * This class and its views and iterators implement all of the optional
 * methods of the {@link Map} and {@link Iterator} interfaces.
 * 

 * 

 * Like {@link java.util.Hashtable} but unlike {@link java.util.HashMap}, this class does
 * not allow null to be used as a key or value.
 * 

 * 

 * This class is a member of the 
 * Java Collections Framework.
 *
 * @param  the type of keys maintained by this map
 * @param  the type of mapped values
 *
 * @author Doug Lea
 * @author Jason T. Greene
 */
public class ConcurrentReferenceHashMap extends AbstractMap
		implements java.util.concurrent.ConcurrentMap, Serializable {
	private static final long serialVersionUID = 7249069246763182397L;

	/*
		 * The basic strategy is to subdivide the table among Segments,
		 * each of which itself is a concurrently readable hash table.
		 */

	/**
	 * An option specifying which Java reference type should be used to refer
	 * to a key and/or value.
	 */
	public static enum ReferenceType {
		/**
		 * Indicates a normal Java strong reference should be used
		 */
		STRONG,
		/**
		 * Indicates a {@link WeakReference} should be used
		 */
		WEAK,
		/**
		 * Indicates a {@link SoftReference} should be used
		 */
		SOFT
	}

	;


	public static enum Option {
		/**
		 * Indicates that referential-equality (== instead of .equals()) should
		 * be used when locating keys. This offers similar behavior to {@link java.util.IdentityHashMap}
		 */
		IDENTITY_COMPARISONS
	}

	;

	/* ---------------- Constants -------------- */

	static final ReferenceType DEFAULT_KEY_TYPE = ReferenceType.WEAK;

	static final ReferenceType DEFAULT_VALUE_TYPE = ReferenceType.STRONG;


	/**
	 * 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[] segments;

	boolean identityComparisons;

	transient Set keySet;
	transient Set> entrySet;
	transient Collection values;

	/* ---------------- Small Utilities -------------- */

	/**
	 * Applies a supplemental hash function to a given hashCode, which
	 * defends against poor quality hash functions.  This is critical
	 * because 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
	 *
	 * @param hash the hash code for the key
	 *
	 * @return the segment
	 */
	final Segment segmentFor(int hash) {
		return segments[( hash >>> segmentShift ) & segmentMask];
	}

	private int hashOf(Object key) {
		return hash(
				identityComparisons ?
						System.identityHashCode( key ) : key.hashCode()
		);
	}

	/* ---------------- Inner Classes -------------- */

	static interface KeyReference {
		int keyHash();

		Object keyRef();
	}

	/**
	 * A weak-key reference which stores the key hash needed for reclamation.
	 */
	static final class WeakKeyReference extends WeakReference implements KeyReference {
		final int hash;

		WeakKeyReference(K key, int hash, ReferenceQueue refQueue) {
			super( key, refQueue );
			this.hash = hash;
		}

		@Override
		public final int keyHash() {
			return hash;
		}

		@Override
		public final Object keyRef() {
			return this;
		}
	}

	/**
	 * A soft-key reference which stores the key hash needed for reclamation.
	 */
	static final class SoftKeyReference extends SoftReference implements KeyReference {
		final int hash;

		SoftKeyReference(K key, int hash, ReferenceQueue refQueue) {
			super( key, refQueue );
			this.hash = hash;
		}

		@Override
		public final int keyHash() {
			return hash;
		}

		@Override
		public final Object keyRef() {
			return this;
		}
	}

	static final class WeakValueReference extends WeakReference implements KeyReference {
		final Object keyRef;
		final int hash;

		WeakValueReference(V value, Object keyRef, int hash, ReferenceQueue refQueue) {
			super( value, refQueue );
			this.keyRef = keyRef;
			this.hash = hash;
		}

		@Override
		public final int keyHash() {
			return hash;
		}

		@Override
		public final Object keyRef() {
			return keyRef;
		}
	}

	static final class SoftValueReference extends SoftReference implements KeyReference {
		final Object keyRef;
		final int hash;

		SoftValueReference(V value, Object keyRef, int hash, ReferenceQueue refQueue) {
			super( value, refQueue );
			this.keyRef = keyRef;
			this.hash = hash;
		}

		@Override
		public final int keyHash() {
			return hash;
		}

		@Override
		public final Object keyRef() {
			return keyRef;
		}
	}

	/**
	 * 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 {
		final Object keyRef;
		final int hash;
		volatile Object valueRef;
		final HashEntry next;

		HashEntry(
				K key, int hash, HashEntry next, V value,
				ReferenceType keyType, ReferenceType valueType,
				ReferenceQueue refQueue) {
			this.hash = hash;
			this.next = next;
			this.keyRef = newKeyReference( key, keyType, refQueue );
			this.valueRef = newValueReference( value, valueType, refQueue );
		}

		final Object newKeyReference(
				K key, ReferenceType keyType,
				ReferenceQueue refQueue) {
			if ( keyType == ReferenceType.WEAK ) {
				return new WeakKeyReference( key, hash, refQueue );
			}
			if ( keyType == ReferenceType.SOFT ) {
				return new SoftKeyReference( key, hash, refQueue );
			}

			return key;
		}

		final Object newValueReference(
				V value, ReferenceType valueType,
				ReferenceQueue refQueue) {
			if ( valueType == ReferenceType.WEAK ) {
				return new WeakValueReference( value, keyRef, hash, refQueue );
			}
			if ( valueType == ReferenceType.SOFT ) {
				return new SoftValueReference( value, keyRef, hash, refQueue );
			}

			return value;
		}

		@SuppressWarnings("unchecked")
		final K key() {
			if ( keyRef instanceof KeyReference ) {
				return ( (Reference) keyRef ).get();
			}

			return (K) keyRef;
		}

		final V value() {
			return dereferenceValue( valueRef );
		}

		@SuppressWarnings("unchecked")
		final V dereferenceValue(Object value) {
			if ( value instanceof KeyReference ) {
				return ( (Reference) value ).get();
			}

			return (V) value;
		}

		final void setValue(V value, ReferenceType valueType, ReferenceQueue refQueue) {
			this.valueRef = newValueReference( value, valueType, refQueue );
		}

		@SuppressWarnings("unchecked")
		static final  HashEntry[] newArray(int i) {
			return new HashEntry[i];
		}
	}

	/**
	 * Segments are specialized versions of hash tables.  This
	 * subclasses from ReentrantLock opportunistically, just to
	 * simplify some locking and avoid separate construction.
	 */
	static final class Segment extends ReentrantLock implements Serializable {
		/*
				 * Segments maintain a table of entry lists that are ALWAYS
				 * kept in a consistent state, so can be read without locking.
				 * Next fields of nodes are immutable (final).  All list
				 * additions are performed at the front of each bin. This
				 * makes it easy to check changes, and also fast to traverse.
				 * When nodes would otherwise be changed, new nodes are
				 * created to replace them. This works well for hash tables
				 * since the bin lists tend to be short. (The average length
				 * is less than two for the default load factor threshold.)
				 *
				 * Read operations can thus proceed without locking, but rely
				 * on selected uses of volatiles to ensure that completed
				 * write operations performed by other threads are
				 * noticed. For most purposes, the "count" field, tracking the
				 * number of elements, serves as that volatile variable
				 * ensuring visibility.  This is convenient because this field
				 * needs to be read in many read operations anyway:
				 *
				 *   - All (unsynchronized) read operations must first read the
				 *     "count" field, and should not look at table entries if
				 *     it is 0.
				 *
				 *   - All (synchronized) write operations should write to
				 *     the "count" field after structurally changing any bin.
				 *     The operations must not take any action that could even
				 *     momentarily cause a concurrent read operation to see
				 *     inconsistent data. This is made easier by the nature of
				 *     the read operations in Map. For example, no operation
				 *     can reveal that the table has grown but the threshold
				 *     has not yet been updated, so there are no atomicity
				 *     requirements for this with respect to reads.
				 *
				 * As a guide, all critical volatile reads and writes to the
				 * count field are marked in code comments.
				 */

		private static final long serialVersionUID = 2249069246763182397L;

		/**
		 * The number of elements in this segment's region.
		 */
		transient volatile int count;

		/**
		 * Number of updates that alter the size of the table. This is
		 * used during bulk-read methods to make sure they see a
		 * consistent snapshot: If modCounts change during a traversal
		 * of segments computing size or checking containsValue, then
		 * we might have an inconsistent view of state so (usually)
		 * must retry.
		 */
		transient int modCount;

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

		/**
		 * The per-segment table.
		 */
		transient volatile HashEntry[] table;

		/**
		 * The load factor for the hash table.  Even though this value
		 * is same for all segments, it is replicated to avoid needing
		 * links to outer object.
		 *
		 * @serial
		 */
		final float loadFactor;

		/**
		 * The collected weak-key reference queue for this segment.
		 * This should be (re)initialized whenever table is assigned,
		 */
		transient volatile ReferenceQueue refQueue;

		final ReferenceType keyType;

		final ReferenceType valueType;

		final boolean identityComparisons;

		Segment(
				int initialCapacity, float lf, ReferenceType keyType,
				ReferenceType valueType, boolean identityComparisons) {
			loadFactor = lf;
			this.keyType = keyType;
			this.valueType = valueType;
			this.identityComparisons = identityComparisons;
			setTable( HashEntry.newArray( initialCapacity ) );
		}

		@SuppressWarnings("unchecked")
		static final  Segment[] newArray(int i) {
			return new Segment[i];
		}

		private boolean keyEq(Object src, Object dest) {
			return identityComparisons ? src == dest : src.equals( dest );
		}

		/**
		 * Sets table to new HashEntry array.
		 * Call only while holding lock or in constructor.
		 */
		void setTable(HashEntry[] newTable) {
			threshold = (int) ( newTable.length * loadFactor );
			table = newTable;
			refQueue = new ReferenceQueue();
		}

		/**
		 * Returns properly casted first entry of bin for given hash.
		 */
		HashEntry getFirst(int hash) {
			HashEntry[] tab = table;
			return tab[hash & ( tab.length - 1 )];
		}

		HashEntry newHashEntry(K key, int hash, HashEntry next, V value) {
			return new HashEntry( key, hash, next, value, keyType, valueType, refQueue );
		}

		/**
		 * Reads value field of an entry under lock. Called if value
		 * field ever appears to be null. This is possible only if a
		 * compiler happens to reorder a HashEntry initialization with
		 * its table assignment, which is legal under memory model
		 * but is not known to ever occur.
		 */
		V readValueUnderLock(HashEntry e) {
			lock();
			try {
				removeStale();
				return e.value();
			}
			finally {
				unlock();
			}
		}

		/* Specialized implementations of map methods */

		V get(Object key, int hash) {
			if ( count != 0 ) { // read-volatile
				HashEntry e = getFirst( hash );
				while ( e != null ) {
					if ( e.hash == hash && keyEq( key, e.key() ) ) {
						Object opaque = e.valueRef;
						if ( opaque != null ) {
							return e.dereferenceValue( opaque );
						}

						return readValueUnderLock( e );  // recheck
					}
					e = e.next;
				}
			}
			return null;
		}

		boolean containsKey(Object key, int hash) {
			if ( count != 0 ) { // read-volatile
				HashEntry e = getFirst( hash );
				while ( e != null ) {
					if ( e.hash == hash && keyEq( key, e.key() ) ) {
						return true;
					}
					e = e.next;
				}
			}
			return false;
		}

		boolean containsValue(Object value) {
			if ( count != 0 ) { // read-volatile
				HashEntry[] tab = table;
				int len = tab.length;
				for ( int i = 0; i < len; i++ ) {
					for ( HashEntry e = tab[i]; e != null; e = e.next ) {
						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 key, int hash, V oldValue, V newValue) {
			lock();
			try {
				removeStale();
				HashEntry e = getFirst( hash );
				while ( e != null && ( e.hash != hash || !keyEq( key, e.key() ) ) ) {
					e = e.next;
				}

				boolean replaced = false;
				if ( e != null && oldValue.equals( e.value() ) ) {
					replaced = true;
					e.setValue( newValue, valueType, refQueue );
				}
				return replaced;
			}
			finally {
				unlock();
			}
		}

		V replace(K key, int hash, V newValue) {
			lock();
			try {
				removeStale();
				HashEntry e = getFirst( hash );
				while ( e != null && ( e.hash != hash || !keyEq( key, e.key() ) ) ) {
					e = e.next;
				}

				V oldValue = null;
				if ( e != null ) {
					oldValue = e.value();
					e.setValue( newValue, valueType, refQueue );
				}
				return oldValue;
			}
			finally {
				unlock();
			}
		}


		V put(K key, int hash, V value, boolean onlyIfAbsent) {
			lock();
			try {
				removeStale();
				int c = count;
				if ( c++ > threshold ) {// ensure capacity
					int reduced = rehash();
					if ( reduced > 0 ) {
						// adjust from possible weak cleanups
						count = ( c -= reduced ) - 1; // write-volatile
					}
				}

				HashEntry[] tab = table;
				int index = hash & ( tab.length - 1 );
				HashEntry first = tab[index];
				HashEntry e = first;
				while ( e != null && ( e.hash != hash || !keyEq( key, e.key() ) ) ) {
					e = e.next;
				}

				V oldValue;
				if ( e != null ) {
					oldValue = e.value();
					if ( !onlyIfAbsent ) {
						e.setValue( value, valueType, refQueue );
					}
				}
				else {
					oldValue = null;
					++modCount;
					tab[index] = newHashEntry( key, hash, first, value );
					count = c; // write-volatile
				}
				return oldValue;
			}
			finally {
				unlock();
			}
		}

		int rehash() {
			HashEntry[] oldTable = table;
			int oldCapacity = oldTable.length;
			if ( oldCapacity >= MAXIMUM_CAPACITY ) {
				return 0;
			}

			/*
						 * Reclassify nodes in each list to new Map.  Because we are
						 * using power-of-two expansion, the elements from each bin
						 * must either stay at same index, or move with a power of two
						 * offset. We eliminate unnecessary node creation by catching
						 * cases where old nodes can be reused because their next
						 * fields won't change. Statistically, at the default
						 * threshold, only about one-sixth of them need cloning when
						 * a table doubles. The nodes they replace will be garbage
						 * collectable as soon as they are no longer referenced by any
						 * reader thread that may be in the midst of traversing table
						 * right now.
						 */

			HashEntry[] newTable = HashEntry.newArray( oldCapacity << 1 );
			threshold = (int) ( newTable.length * loadFactor );
			int sizeMask = newTable.length - 1;
			int reduce = 0;
			for ( int i = 0; i < oldCapacity; i++ ) {
				// We need to guarantee that any existing reads of old Map can
				//  proceed. So we cannot yet null out each bin.
				HashEntry e = oldTable[i];

				if ( e != null ) {
					HashEntry next = e.next;
					int idx = e.hash & sizeMask;

					//  Single node on list
					if ( next == null ) {
						newTable[idx] = e;
					}

					else {
						// Reuse trailing consecutive sequence at same slot
						HashEntry lastRun = e;
						int lastIdx = idx;
						for ( HashEntry last = next; last != null; last = last.next ) {
							int k = last.hash & sizeMask;
							if ( k != lastIdx ) {
								lastIdx = k;
								lastRun = last;
							}
						}
						newTable[lastIdx] = lastRun;
						// Clone all remaining nodes
						for ( HashEntry p = e; p != lastRun; p = p.next ) {
							// Skip GC'd weak refs
							K key = p.key();
							if ( key == null ) {
								reduce++;
								continue;
							}
							int k = p.hash & sizeMask;
							HashEntry n = newTable[k];
							newTable[k] = newHashEntry( key, p.hash, n, p.value() );
						}
					}
				}
			}
			table = newTable;
			return reduce;
		}

		/**
		 * Remove; match on key only if value null, else match both.
		 */
		V remove(Object key, int hash, Object value, boolean refRemove) {
			lock();
			try {
				if ( !refRemove ) {
					removeStale();
				}
				int c = count - 1;
				HashEntry[] tab = table;
				int index = hash & ( tab.length - 1 );
				HashEntry first = tab[index];
				HashEntry e = first;
				// a ref remove operation compares the Reference instance
				while ( e != null && key != e.keyRef
						&& ( refRemove || hash != e.hash || !keyEq( key, e.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.
						++modCount;
						HashEntry newFirst = e.next;
						for ( HashEntry p = first; p != e; p = p.next ) {
							K pKey = p.key();
							if ( pKey == null ) { // Skip GC'd keys
								c--;
								continue;
							}

							newFirst = newHashEntry( pKey, p.hash, newFirst, p.value() );
						}
						tab[index] = newFirst;
						count = c; // write-volatile
					}
				}
				return oldValue;
			}
			finally {
				unlock();
			}
		}

		final void removeStale() {
			KeyReference ref;
			while ( ( ref = (KeyReference) refQueue.poll() ) != null ) {
				remove( ref.keyRef(), ref.keyHash(), null, true );
			}
		}

		void clear() {
			if ( count != 0 ) {
				lock();
				try {
					HashEntry[] tab = table;
					for ( int i = 0; i < tab.length; i++ ) {
						tab[i] = null;
					}
					++modCount;
					// replace the reference queue to avoid unnecessary stale cleanups
					refQueue = new ReferenceQueue();
					count = 0; // write-volatile
				}
				finally {
					unlock();
				}
			}
		}
	}


	/* ---------------- Public operations -------------- */

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

	 * Behavioral changing options such as {@link Option#IDENTITY_COMPARISONS}
	 * can also be specified.
	 *
	 * @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.
	 * @param keyType the reference type to use for keys
	 * @param valueType the reference type to use for values
	 * @param options the behavioral options
	 *
	 * @throws IllegalArgumentException if the initial capacity is
	 * negative or the load factor or concurrencyLevel are
	 * nonpositive.
	 */
	public ConcurrentReferenceHashMap(
			int initialCapacity,
			float loadFactor, int concurrencyLevel,
			ReferenceType keyType, ReferenceType valueType,
			EnumSet

	 * 
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
	 */
	@Override
	public V get(Object key) {
		if ( key == null ) {
			return null;
		}
		int hash = hashOf( key );
		return segmentFor( hash ).get( key, hash );
	}

	/**
	 * Tests if the specified object is a key in this table.
	 *
	 * @param key possible key
	 *
	 * @return true if and only if the specified object
	 * is a key in this table, as determined by the
	 * equals method; false otherwise.
	 *
	 * @throws NullPointerException if the specified key is null
	 */
	@Override
	public boolean containsKey(Object key) {
		if ( key == null ) {
			return false;
		}
		int hash = hashOf( key );
		return segmentFor( hash ).containsKey( key, hash );
	}

	/**
	 * Returns true if this map maps one or more keys to the
	 * specified value. Note: This method requires a full internal
	 * traversal of the hash table, and so is much slower than
	 * method containsKey.
	 *
	 * @param value value whose presence in this map is to be tested
	 *
	 * @return true if this map maps one or more keys to the
	 * specified value
	 */
	@Override
	public boolean containsValue(Object value) {
		if ( value == null ) {
			return false;
		}

		// See explanation of modCount use above

		final Segment[] segments = this.segments;
		int[] mc = new int[segments.length];

		// Try a few times without locking
		for ( int k = 0; k < RETRIES_BEFORE_LOCK; ++k ) {
			int sum = 0;
			int mcsum = 0;
			for ( int i = 0; i < segments.length; ++i ) {
				int c = segments[i].count;
				mcsum += mc[i] = segments[i].modCount;
				if ( segments[i].containsValue( value ) ) {
					return true;
				}
			}
			boolean cleanSweep = true;
			if ( mcsum != 0 ) {
				for ( int i = 0; i < segments.length; ++i ) {
					int c = segments[i].count;
					if ( mc[i] != segments[i].modCount ) {
						cleanSweep = false;
						break;
					}
				}
			}
			if ( cleanSweep ) {
				return false;
			}
		}
		// Resort to locking all segments
		for ( int i = 0; i < segments.length; ++i ) {
			segments[i].lock();
		}
		boolean found = false;
		try {
			for ( int i = 0; i < segments.length; ++i ) {
				if ( segments[i].containsValue( value ) ) {
					found = true;
					break;
				}
			}
		}
		finally {
			for ( int i = 0; i < segments.length; ++i ) {
				segments[i].unlock();
			}
		}
		return found;
	}

	/**
	 * Legacy method testing if some key maps into the specified value
	 * in this table.  This method is identical in functionality to
	 * {@link #containsValue}, and exists solely to ensure
	 * full compatibility with class {@link java.util.Hashtable},
	 * which supported this method prior to introduction of the
	 * Java Collections framework.
	 *
	 * @param value a value to search for
	 *
	 * @return true if and only if some key maps to the
	 * value argument in this table as
	 * determined by the equals method;
	 * false otherwise
	 *
	 * @throws NullPointerException if the specified value is null
	 */
	public boolean contains(Object value) {
		return containsValue( value );
	}

	/**
	 * Maps the specified key to the specified value in this table.
	 * Neither the key nor the value can be null.
	 * 

	 * 
 The value can be retrieved by calling the get method
	 * with a key that is equal to the original key.
	 *
	 * @param key key with which the specified value is to be associated
	 * @param value value to be associated with the specified key
	 *
	 * @return the previous value associated with key, or
	 * null if there was no mapping for key
	 *
	 * @throws NullPointerException if the specified key or value is null
	 */
	@Override
	public V put(K key, V value) {
		if ( key == null || value == null ) {
			return null;
		}
		int hash = hashOf( key );
		return segmentFor( hash ).put( key, hash, value, false );
	}

	/**
	 * {@inheritDoc}
	 *
	 * @return the previous value associated with the specified key,
	 * or null if there was no mapping for the key
	 *
	 * @throws NullPointerException if the specified key or value is null
	 */
	@Override
	public V putIfAbsent(K key, V value) {
		if ( key == null || value == null ) {
			return null;
		}
		int hash = hashOf( key );
		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
	 */
	@Override
	public void putAll(Map m) {
		for ( Map.Entry e : m.entrySet() ) {
			put( e.getKey(), e.getValue() );
		}
	}

	/**
	 * Removes the key (and its corresponding value) from this map.
	 * This method does nothing if the key is not in the map.
	 *
	 * @param key the key that needs to be removed
	 *
	 * @return the previous value associated with key, or
	 * null if there was no mapping for key
	 *
	 * @throws NullPointerException if the specified key is null
	 */
	@Override
	public V remove(Object key) {
		if ( key == null ) {
			return null;
		}
		int hash = hashOf( key );
		return segmentFor( hash ).remove( key, hash, null, false );
	}

	/**
	 * {@inheritDoc}
	 *
	 * @throws NullPointerException if the specified key is null
	 */
	@Override
	public boolean remove(Object key, Object value) {
		if ( key == null || value == null ) {
			return false;
		}
		int hash = hashOf( key );
		return segmentFor( hash ).remove( key, hash, value, false ) != null;
	}

	/**
	 * {@inheritDoc}
	 *
	 * @throws NullPointerException if any of the arguments are null
	 */
	@Override
	public boolean replace(K key, V oldValue, V newValue) {
		if ( key == null || oldValue == null || newValue == null ) {
			throw new NullPointerException();
		}
		int hash = hashOf( key );
		return segmentFor( hash ).replace( key, hash, oldValue, newValue );
	}

	/**
	 * {@inheritDoc}
	 *
	 * @return the previous value associated with the specified key,
	 * or null if there was no mapping for the key
	 *
	 * @throws NullPointerException if the specified key or value is null
	 */
	@Override
	public V replace(K key, V value) {
		if ( key == null || value == null ) {
			return null;
		}
		int hash = hashOf( key );
		return segmentFor( hash ).replace( key, hash, value );
	}

	/**
	 * Removes all of the mappings from this map.
	 */
	@Override
	public void clear() {
		for ( int i = 0; i < segments.length; ++i ) {
			segments[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 < segments.length; ++i ) {
			segments[i].removeStale();
		}
	}


	/**
	 * Returns a {@link Set} view of the keys contained in this map.
	 * The set is backed by the map, so changes to the map are
	 * reflected in the set, and vice-versa.  The set supports element
	 * removal, which removes the corresponding mapping from this map,
	 * via the Iterator.remove, Set.remove,
	 * removeAll, retainAll, and clear
	 * operations.  It does not support the add or
	 * addAll operations.
	 * 

	 * 
The view's iterator is a "weakly consistent" iterator
	 * that will never throw {@link 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.
	 */
	@Override
	public Set keySet() {
		Set ks = keySet;
		return ( ks != null ) ? ks : ( keySet = new KeySet() );
	}

	/**
	 * Returns a {@link Collection} view of the values contained in this map.
	 * The collection is backed by the map, so changes to the map are
	 * reflected in the collection, and vice-versa.  The collection
	 * supports element removal, which removes the corresponding
	 * mapping from this map, via the Iterator.remove,
	 * Collection.remove, removeAll,
	 * retainAll, and clear operations.  It does not
	 * support the add or addAll operations.
	 * 

	 * 
The view's iterator is a "weakly consistent" iterator
	 * that will never throw {@link 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.
	 */
	@Override
	public Collection values() {
		Collection vs = values;
		return ( vs != null ) ? vs : ( values = new Values() );
	}

	/**
	 * Returns a {@link Set} view of the mappings contained in this map.
	 * The set is backed by the map, so changes to the map are
	 * reflected in the set, and vice-versa.  The set supports element
	 * removal, which removes the corresponding mapping from the map,
	 * via the Iterator.remove, Set.remove,
	 * removeAll, retainAll, and clear
	 * operations.  It does not support the add or
	 * addAll operations.
	 * 

	 * 
The view's iterator is a "weakly consistent" iterator
	 * that will never throw {@link 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.
	 */
	@Override
	public Set> entrySet() {
		Set> es = entrySet;
		return ( es != null ) ? es : ( entrySet = new EntrySet() );
	}

	/**
	 * Returns an enumeration of the keys in this table.
	 *
	 * @return an enumeration of the keys in this table
	 *
	 * @see #keySet()
	 */
	public Enumeration keys() {
		return new KeyIterator();
	}

	/**
	 * Returns an enumeration of the values in this table.
	 *
	 * @return an enumeration of the values in this table
	 *
	 * @see #values()
	 */
	public Enumeration elements() {
		return new ValueIterator();
	}

	/* ---------------- Iterator Support -------------- */

	abstract class HashIterator {
		int nextSegmentIndex;
		int nextTableIndex;
		HashEntry[] currentTable;
		HashEntry nextEntry;
		HashEntry lastReturned;
		K currentKey; // Strong reference to weak key (prevents gc)

		HashIterator() {
			nextSegmentIndex = segments.length - 1;
			nextTableIndex = -1;
			advance();
		}

		public boolean hasMoreElements() {
			return hasNext();
		}

		final void advance() {
			if ( nextEntry != null && ( nextEntry = nextEntry.next ) != null ) {
				return;
			}

			while ( nextTableIndex >= 0 ) {
				if ( ( nextEntry = currentTable[nextTableIndex--] ) != null ) {
					return;
				}
			}

			while ( nextSegmentIndex >= 0 ) {
				Segment seg = segments[nextSegmentIndex--];
				if ( seg.count != 0 ) {
					currentTable = seg.table;
					for ( int j = currentTable.length - 1; j >= 0; --j ) {
						if ( ( nextEntry = currentTable[j] ) != null ) {
							nextTableIndex = j - 1;
							return;
						}
					}
				}
			}
		}

		public boolean hasNext() {
			while ( nextEntry != null ) {
				if ( nextEntry.key() != null ) {
					return true;
				}
				advance();
			}

			return false;
		}

		HashEntry nextEntry() {
			do {
				if ( nextEntry == null ) {
					throw new NoSuchElementException();
				}

				lastReturned = nextEntry;
				currentKey = lastReturned.key();
				advance();
			} while ( currentKey == null ); // Skip GC'd keys

			return lastReturned;
		}

		public void remove() {
			if ( lastReturned == null ) {
				throw new IllegalStateException();
			}
			ConcurrentReferenceHashMap.this.remove( currentKey );
			lastReturned = null;
		}
	}

	final class KeyIterator
			extends HashIterator
			implements Iterator, Enumeration {
		@Override
		public K next() {
			return super.nextEntry().key();
		}

		@Override
		public K nextElement() {
			return super.nextEntry().key();
		}
	}

	final class ValueIterator
			extends HashIterator
			implements Iterator, Enumeration {
		@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 implements Entry, 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 entry) {
			this.key = entry.getKey();
			this.value = entry.getValue();
		}

		@Override
		public K getKey() {
			return key;
		}

		@Override
		public V getValue() {
			return value;
		}

		@Override
		public V setValue(V value) {
			V oldValue = this.value;
			this.value = value;
			return oldValue;
		}

		@Override
		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() );
		}

		@Override
		public int hashCode() {
			return ( key == null ? 0 : key.hashCode() )
					^ ( value == null ? 0 : value.hashCode() );
		}

		@Override
		public String toString() {
			return key + "=" + value;
		}

		private static boolean eq(Object o1, Object o2) {
			return o1 == null ? o2 == null : o1.equals( o2 );
		}
	}


	/**
	 * Custom Entry class used by EntryIterator.next(), that relays setValue
	 * changes to the underlying map.
	 */
	final class WriteThroughEntry extends SimpleEntry {
		private static final long serialVersionUID = -7900634345345313646L;

		WriteThroughEntry(K k, V v) {
			super( k, v );
		}

		/**
		 * Set our entry's value and write through to the map. The
		 * value to return is somewhat arbitrary here. Since a
		 * WriteThroughEntry does not necessarily track asynchronous
		 * changes, the most recent "previous" value could be
		 * different from what we return (or could even have been
		 * removed in which case the put will re-establish). We do not
		 * and cannot guarantee more.
		 */
		@Override
		public V setValue(V value) {
			if ( value == null ) {
				throw new NullPointerException();
			}
			V v = super.setValue( value );
			ConcurrentReferenceHashMap.this.put( getKey(), value );
			return v;
		}
	}

	final class EntryIterator
			extends HashIterator
			implements Iterator> {
		@Override
		public Map.Entry next() {
			HashEntry e = super.nextEntry();
			return new WriteThroughEntry( e.key(), e.value() );
		}
	}

	final class KeySet extends AbstractSet {
		@Override
		public Iterator iterator() {
			return new KeyIterator();
		}

		@Override
		public int size() {
			return ConcurrentReferenceHashMap.this.size();
		}

		@Override
		public boolean isEmpty() {
			return ConcurrentReferenceHashMap.this.isEmpty();
		}

		@Override
		public boolean contains(Object o) {
			return ConcurrentReferenceHashMap.this.containsKey( o );
		}

		@Override
		public boolean remove(Object o) {
			return ConcurrentReferenceHashMap.this.remove( o ) != null;
		}

		@Override
		public void clear() {
			ConcurrentReferenceHashMap.this.clear();
		}
	}

	final class Values extends AbstractCollection {
		@Override
		public Iterator iterator() {
			return new ValueIterator();
		}

		@Override
		public int size() {
			return ConcurrentReferenceHashMap.this.size();
		}

		@Override
		public boolean isEmpty() {
			return ConcurrentReferenceHashMap.this.isEmpty();
		}

		@Override
		public boolean contains(Object o) {
			return ConcurrentReferenceHashMap.this.containsValue( o );
		}

		@Override
		public void clear() {
			ConcurrentReferenceHashMap.this.clear();
		}
	}

	final class EntrySet extends AbstractSet> {
		@Override
		public Iterator> iterator() {
			return new EntryIterator();
		}

		@Override
		public boolean contains(Object o) {
			if ( !( o instanceof Map.Entry ) ) {
				return false;
			}
			Map.Entry e = (Map.Entry) o;
			V v = ConcurrentReferenceHashMap.this.get( e.getKey() );
			return v != null && v.equals( e.getValue() );
		}

		@Override
		public boolean remove(Object o) {
			if ( !( o instanceof Map.Entry ) ) {
				return false;
			}
			Map.Entry e = (Map.Entry) o;
			return ConcurrentReferenceHashMap.this.remove( e.getKey(), e.getValue() );
		}

		@Override
		public int size() {
			return ConcurrentReferenceHashMap.this.size();
		}

		@Override
		public boolean isEmpty() {
			return ConcurrentReferenceHashMap.this.isEmpty();
		}

		@Override
		public void clear() {
			ConcurrentReferenceHashMap.this.clear();
		}
	}

	/* ---------------- Serialization Support -------------- */

	/**
	 * Save the state of the ConcurrentReferenceHashMap instance to a
	 * stream (i.e., serialize it).
	 *
	 * @param s the stream
	 *
	 * @serialData the key (Object) and value (Object)
	 * for each key-value mapping, followed by a null pair.
	 * The key-value mappings are emitted in no particular order.
	 */
	private void writeObject(java.io.ObjectOutputStream s) throws IOException {
		s.defaultWriteObject();

		for ( int k = 0; k < segments.length; ++k ) {
			Segment seg = segments[k];
			seg.lock();
			try {
				HashEntry[] tab = seg.table;
				for ( int i = 0; i < tab.length; ++i ) {
					for ( HashEntry e = tab[i]; e != null; e = e.next ) {
						K key = e.key();
						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 ConcurrentReferenceHashMap 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 );
		}
	}
}
 















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