src/jsr166e/ConcurrentHashMapV8.java

/*
 * 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/publicdomain/zero/1.0/
 */

package jsr166e;
import jsr166e.LongAdder;
import java.util.Arrays;
import java.util.Map;
import java.util.Set;
import java.util.Collection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.AbstractCollection;
import java.util.Hashtable;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Enumeration;
import java.util.ConcurrentModificationException;
import java.util.NoSuchElementException;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.locks.LockSupport;
import java.io.Serializable;

/**
 * A hash table supporting full concurrency of retrievals and
 * high expected concurrency for updates. This class obeys the
 * same functional specification as {@link java.util.Hashtable}, and
 * includes versions of methods corresponding to each method of
 * {@code Hashtable}. 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 {@code Hashtable} in programs that rely on its
 * thread safety but not on its synchronization details.
 *
 * <p> Retrieval operations (including {@code get}) generally do not
 * block, so may overlap with update operations (including {@code put}
 * and {@code remove}). Retrievals reflect the results of the most
 * recently <em>completed</em> update operations holding upon their
 * onset.  For aggregate operations such as {@code putAll} and {@code
 * 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
 * <em>not</em> throw {@link ConcurrentModificationException}.
 * However, iterators are designed to be used by only one thread at a
 * time.  Bear in mind that the results of aggregate status methods
 * including {@code size}, {@code isEmpty}, and {@code containsValue}
 * are typically useful only when a map is not undergoing concurrent
 * updates in other threads.  Otherwise the results of these methods
 * reflect transient states that may be adequate for monitoring
 * or estimation purposes, but not for program control.
 *
 * <p> The table is dynamically expanded when there are too many
 * collisions (i.e., keys that have distinct hash codes but fall into
 * the same slot modulo the table size), with the expected average
 * effect of maintaining roughly two bins per mapping (corresponding
 * to a 0.75 load factor threshold for resizing). There may be much
 * variance around this average as mappings are added and removed, but
 * overall, this maintains a commonly accepted time/space tradeoff for
 * hash tables.  However, resizing this or any other kind of hash
 * table may be a relatively slow operation. When possible, it is a
 * good idea to provide a size estimate as an optional {@code
 * initialCapacity} constructor argument. An additional optional
 * {@code loadFactor} constructor argument provides a further means of
 * customizing initial table capacity by specifying the table density
 * to be used in calculating the amount of space to allocate for the
 * given number of elements.  Also, for compatibility with previous
 * versions of this class, constructors may optionally specify an
 * expected {@code concurrencyLevel} as an additional hint for
 * internal sizing.  Note that using many keys with exactly the same
 * {@code hashCode{}} is a sure way to slow down performance of any
 * hash table.
 *
 * <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 {@code null} 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>.
 *
 * <p><em>jsr166e note: This class is a candidate replacement for
 * java.util.concurrent.ConcurrentHashMap.<em>
 *
 * @since 1.5
 * @author Doug Lea
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */
public class ConcurrentHashMapV8<K, V>
        implements ConcurrentMap<K, V>, Serializable {
    private static final long serialVersionUID = 7249069246763182397L;

    /**
     * A function computing a mapping from the given key to a value,
     * or {@code null} if there is no mapping. This is a place-holder
     * for an upcoming JDK8 interface.
     */
    public static interface MappingFunction<K, V> {
        /**
         * Returns a value for the given key, or null if there is no
         * mapping. If this function throws an (unchecked) exception,
         * the exception is rethrown to its caller, and no mapping is
         * recorded.  Because this function is invoked within
         * atomicity control, the computation should be short and
         * simple. The most common usage is to construct a new object
         * serving as an initial mapped value.
         *
         * @param key the (non-null) key
         * @return a value, or null if none
         */
        V map(K key);
    }

    /*
     * Overview:
     *
     * The primary design goal of this hash table is to maintain
     * concurrent readability (typically method get(), but also
     * iterators and related methods) while minimizing update
     * contention. Secondary goals are to keep space consumption about
     * the same or better than java.util.HashMap, and to support high
     * initial insertion rates on an empty table by many threads.
     *
     * Each key-value mapping is held in a Node.  Because Node fields
     * can contain special values, they are defined using plain Object
     * types. Similarly in turn, all internal methods that use them
     * work off Object types. And similarly, so do the internal
     * methods of auxiliary iterator and view classes.  All public
     * generic typed methods relay in/out of these internal methods,
     * supplying null-checks and casts as needed.
     *
     * The table is lazily initialized to a power-of-two size upon the
     * first insertion.  Each bin in the table contains a list of
     * Nodes (most often, zero or one Node).  Table accesses require
     * volatile/atomic reads, writes, and CASes.  Because there is no
     * other way to arrange this without adding further indirections,
     * we use intrinsics (sun.misc.Unsafe) operations.  The lists of
     * nodes within bins are always accurately traversable under
     * volatile reads, so long as lookups check hash code and
     * non-nullness of value before checking key equality.
     *
     * We use the top two bits of Node hash fields for control
     * purposes -- they are available anyway because of addressing
     * constraints.  As explained further below, these top bits are
     * usd as follows:
     *  00 - Normal
     *  01 - Locked
     *  11 - Locked and may have a thread waiting for lock
     *  10 - Node is a forwarding node
     *
     * The lower 30 bits of each Node's hash field contain a
     * transformation (for better randomization -- method "spread") of
     * the key's hash code, except for forwarding nodes, for which the
     * lower bits are zero (and so always have hash field == "MOVED").
     *
     * Insertion (via put or putIfAbsent) of the first node in an
     * empty bin is performed by just CASing it to the bin.  This is
     * by far the most common case for put operations.  Other update
     * operations (insert, delete, and replace) require locks.  We do
     * not want to waste the space required to associate a distinct
     * lock object with each bin, so instead use the first node of a
     * bin list itself as a lock. Blocking support for these locks
     * relies on the builtin "synchronized" monitors.  However, we
     * also need a tryLock construction, so we overlay these by using
     * bits of the Node hash field for lock control (see above), and
     * so normally use builtin monitors only for blocking and
     * signalling using wait/notifyAll constructions. See
     * Node.tryAwaitLock.
     *
     * Using the first node of a list as a lock does not by itself
     * suffice though: When a node is locked, any update must first
     * validate that it is still the first node after locking it, and
     * retry if not. Because new nodes are always appended to lists,
     * once a node is first in a bin, it remains first until deleted
     * or the bin becomes invalidated.  However, operations that only
     * conditionally update may inspect nodes until the point of
     * update. This is a converse of sorts to the lazy locking
     * technique described by Herlihy & Shavit.
     *
     * The main disadvantage of per-bin locks is that other update
     * operations on other nodes in a bin list protected by the same
     * lock can stall, for example when user equals() or mapping
     * functions take a long time.  However, statistically, this is
     * not a common enough problem to outweigh the time/space overhead
     * of alternatives: Under random hash codes, the frequency of
     * nodes in bins follows a Poisson distribution
     * (http://en.wikipedia.org/wiki/Poisson_distribution) with a
     * parameter of about 0.5 on average, given the resizing threshold
     * of 0.75, although with a large variance because of resizing
     * granularity. Ignoring variance, the expected occurrences of
     * list size k are (exp(-0.5) * pow(0.5, k) / factorial(k)). The
     * first few values are:
     *
     * 0:    0.607
     * 1:    0.303
     * 2:    0.076
     * 3:    0.012
     * more: 0.002
     *
     * Lock contention probability for two threads accessing distinct
     * elements is roughly 1 / (8 * #elements).  Function "spread"
     * performs hashCode randomization that improves the likelihood
     * that these assumptions hold unless users define exactly the
     * same value for too many hashCodes.
     *
     * The table is resized when occupancy exceeds an occupancy
     * threshold (nominally, 0.75, but see below).  Only a single
     * thread performs the resize (using field "sizeCtl", to arrange
     * exclusion), but the table otherwise remains usable for reads
     * and updates. Resizing proceeds by transferring bins, one by
     * one, from the table to the next table.  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.  On
     * average, 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 concurrently traversing table.  Upon
     * transfer, the old table bin contains only a special forwarding
     * node (with hash field "MOVED") that contains the next table as
     * its key. On encountering a forwarding node, access and update
     * operations restart, using the new table.
     *
     * Each bin transfer requires its bin lock. However, unlike other
     * cases, a transfer can skip a bin if it fails to acquire its
     * lock, and revisit it later. Method rebuild maintains a buffer
     * of TRANSFER_BUFFER_SIZE bins that have been skipped because of
     * failure to acquire a lock, and blocks only if none are
     * available (i.e., only very rarely).  The transfer operation
     * must also ensure that all accessible bins in both the old and
     * new table are usable by any traversal.  When there are no lock
     * acquisition failures, this is arranged simply by proceeding
     * from the last bin (table.length - 1) up towards the first.
     * Upon seeing a forwarding node, traversals (see class
     * InternalIterator) arrange to move to the new table without
     * revisiting nodes.  However, when any node is skipped during a
     * transfer, all earlier table bins may have become visible, so
     * are initialized with a reverse-forwarding node back to the old
     * table until the new ones are established. (This sometimes
     * requires transiently locking a forwarding node, which is
     * possible under the above encoding.) These more expensive
     * mechanics trigger only when necessary.
     *
     * The traversal scheme also applies to partial traversals of
     * ranges of bins (via an alternate InternalIterator constructor)
     * to support partitioned aggregate operations (that are not
     * otherwise implemented yet).  Also, read-only operations give up
     * if ever forwarded to a null table, which provides support for
     * shutdown-style clearing, which is also not currently
     * implemented.
     *
     * Lazy table initialization minimizes footprint until first use,
     * and also avoids resizings when the first operation is from a
     * putAll, constructor with map argument, or deserialization.
     * These cases attempt to override the initial capacity settings,
     * but harmlessly fail to take effect in cases of races.
     *
     * The element count is maintained using a LongAdder, which avoids
     * contention on updates but can encounter cache thrashing if read
     * too frequently during concurrent access. To avoid reading so
     * often, resizing is normally attempted only upon adding to a bin
     * already holding two or more nodes. Under uniform hash
     * distributions, the probability of this occurring at threshold
     * is around 13%, meaning that only about 1 in 8 puts check
     * threshold (and after resizing, many fewer do so). But this
     * approximation has high variance for small table sizes, so we
     * check on any collision for sizes <= 64.
     *
     * Maintaining API and serialization compatibility with previous
     * versions of this class introduces several oddities. Mainly: We
     * leave untouched but unused constructor arguments refering to
     * concurrencyLevel. We accept a loadFactor constructor argument,
     * but apply it only to initial table capacity (which is the only
     * time that we can guarantee to honor it.) We also declare an
     * unused "Segment" class that is instantiated in minimal form
     * only when serializing.
     */

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

    /**
     * The largest possible table capacity.  This value must be
     * exactly 1<<30 to stay within Java array allocation and indexing
     * bounds for power of two table sizes, and is further required
     * because the top two bits of 32bit hash fields are used for
     * control purposes.
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The default initial table capacity.  Must be a power of 2
     * (i.e., at least 1) and at most MAXIMUM_CAPACITY.
     */
    private static final int DEFAULT_CAPACITY = 16;

    /**
     * The largest possible (non-power of two) array size.
     * Needed by toArray and related methods.
     */
    static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * The default concurrency level for this table. Unused but
     * defined for compatibility with previous versions of this class.
     */
    private static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    /**
     * The load factor for this table. Overrides of this value in
     * constructors affect only the initial table capacity.  The
     * actual floating point value isn't normally used -- it is
     * simpler to use expressions such as {@code n - (n >>> 2)} for
     * the associated resizing threshold.
     */
    private static final float LOAD_FACTOR = 0.75f;

    /**
     * The buffer size for skipped bins during transfers. The
     * value is arbitrary but should be large enough to avoid
     * most locking stalls during resizes.
     */
    private static final int TRANSFER_BUFFER_SIZE = 32;

    /*
     * Encodings for special uses of Node hash fields. See above for
     * explanation.
     */
    static final int MOVED     = 0x80000000; // hash field for fowarding nodes
    static final int LOCKED    = 0x40000000; // set/tested only as a bit
    static final int WAITING   = 0xc0000000; // both bits set/tested together
    static final int HASH_BITS = 0x3fffffff; // usable bits of normal node hash

    /* ---------------- Fields -------------- */

    /**
     * The array of bins. Lazily initialized upon first insertion.
     * Size is always a power of two. Accessed directly by iterators.
     */
    transient volatile Node[] table;

    /**
     * The counter maintaining number of elements.
     */
    private transient final LongAdder counter;

    /**
     * Table initialization and resizing control.  When negative, the
     * table is being initialized or resized. Otherwise, when table is
     * null, holds the initial table size to use upon creation, or 0
     * for default. After initialization, holds the next element count
     * value upon which to resize the table.
     */
    private transient volatile int sizeCtl;

    // views
    private transient KeySet<K,V> keySet;
    private transient Values<K,V> values;
    private transient EntrySet<K,V> entrySet;

    /** For serialization compatibility. Null unless serialized; see below */
    private Segment<K,V>[] segments;

    /* ---------------- Nodes -------------- */

    /**
     * Key-value entry. Note that this is never exported out as a
     * user-visible Map.Entry (see WriteThroughEntry and SnapshotEntry
     * below). Nodes with a negative hash field are special, and do
     * not contain user keys or values.  Otherwise, keys are never
     * null, and null val fields indicate that a node is in the
     * process of being deleted or created. For purposes of read-only
     * access, a key may be read before a val, but can only be used
     * after checking val to be non-null.
     */
    static final class Node {
        volatile int hash;
        final Object key;
        volatile Object val;
        volatile Node next;

        Node(int hash, Object key, Object val, Node next) {
            this.hash = hash;
            this.key = key;
            this.val = val;
            this.next = next;
        }

        /** CompareAndSet the hash field */
        final boolean casHash(int cmp, int val) {
            return UNSAFE.compareAndSwapInt(this, hashOffset, cmp, val);
        }

        /** The number of spins before blocking for a lock */
        static final int MAX_SPINS =
            Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;

        /**
         * Spins a while if LOCKED bit set and this node is the first
         * of its bin, and then sets WAITING bits on hash field and
         * blocks (once) if they are still set.  It is OK for this
         * method to return even if lock is not available upon exit,
         * which enables these simple single-wait mechanics.
         *
         * The corresponding signalling operation is performed within
         * callers: Upon detecting that WAITING has been set when
         * unlocking lock (via a failed CAS from non-waiting LOCKED
         * state), unlockers acquire the sync lock and perform a
         * notifyAll.
         */
        final void tryAwaitLock(Node[] tab, int i) {
            if (tab != null && i >= 0 && i < tab.length) { // bounds check
                int spins = MAX_SPINS, h;
                while (tabAt(tab, i) == this && ((h = hash) & LOCKED) != 0) {
                    if (spins >= 0) {
                        if (--spins == MAX_SPINS >>> 1)
                            Thread.yield();  // heuristically yield mid-way
                    }
                    else if (casHash(h, h | WAITING)) {
                        synchronized(this) {
                            if (tabAt(tab, i) == this &&
                                (hash & WAITING) == WAITING) {
                                try {
                                    wait();
                                } catch (InterruptedException ie) {
                                    Thread.currentThread().interrupt();
                                }
                            }
                            else
                                notifyAll(); // possibly won race vs signaller
                        }
                        break;
                    }
                }
            }
        }

        // Unsafe mechanics for casHash
        private static final sun.misc.Unsafe UNSAFE;
        private static final long hashOffset;

        static {
            try {
                UNSAFE = getUnsafe();
                Class<?> k = Node.class;
                hashOffset = UNSAFE.objectFieldOffset
                    (k.getDeclaredField("hash"));
            } catch (Exception e) {
                throw new Error(e);
            }
        }
    }

    /* ---------------- Table element access -------------- */

    /*
     * Volatile access methods are used for table elements as well as
     * elements of in-progress next table while resizing.  Uses are
     * null checked by callers, and implicitly bounds-checked, relying
     * on the invariants that tab arrays have non-zero size, and all
     * indices are masked with (tab.length - 1) which is never
     * negative and always less than length. Note that, to be correct
     * wrt arbitrary concurrency errors by users, bounds checks must
     * operate on local variables, which accounts for some odd-looking
     * inline assignments below.
     */

    static final Node tabAt(Node[] tab, int i) { // used by InternalIterator
        return (Node)UNSAFE.getObjectVolatile(tab, ((long)i<<ASHIFT)+ABASE);
    }

    private static final boolean casTabAt(Node[] tab, int i, Node c, Node v) {
        return UNSAFE.compareAndSwapObject(tab, ((long)i<<ASHIFT)+ABASE, c, v);
    }

    private static final void setTabAt(Node[] tab, int i, Node v) {
        UNSAFE.putObjectVolatile(tab, ((long)i<<ASHIFT)+ABASE, v);
    }

    /* ---------------- Internal access and update methods -------------- */

    /**
     * Applies a supplemental hash function to a given hashCode, which
     * defends against poor quality hash functions.  The result must
     * be have the top 2 bits clear. For reasonable performance, this
     * function must have good avalanche properties; i.e., that each
     * bit of the argument affects each bit of the result. (Although
     * we don't care about the unused top 2 bits.)
     */
    private static final int spread(int h) {
        // Apply base step of MurmurHash; see http://code.google.com/p/smhasher/
        h ^= h >>> 16;
        h *= 0x85ebca6b;
        h ^= h >>> 13;
        h *= 0xc2b2ae35;
        return ((h >>> 16) ^ h) & HASH_BITS; // mask out top bits
    }

    /** Implementation for get and containsKey */
    private final Object internalGet(Object k) {
        int h = spread(k.hashCode());
        retry: for (Node[] tab = table; tab != null;) {
            Node e; Object ek, ev; int eh;    // locals to read fields once
            for (e = tabAt(tab, (tab.length - 1) & h); e != null; e = e.next) {
                if ((eh = e.hash) == MOVED) {
                    tab = (Node[])e.key;      // restart with new table
                    continue retry;
                }
                if ((eh & HASH_BITS) == h && (ev = e.val) != null &&
                    ((ek = e.key) == k || k.equals(ek)))
                    return ev;
            }
            break;
        }
        return null;
    }

    /** Implementation for put and putIfAbsent */
    private final Object internalPut(Object k, Object v, boolean replace) {
        int h = spread(k.hashCode());
        Object oldVal = null;               // previous value or null if none
        for (Node[] tab = table;;) {
            int i; Node f; int fh; Object fk, fv;
            if (tab == null)
                tab = initTable();
            else if ((f = tabAt(tab, i = (tab.length - 1) & h)) == null) {
                if (casTabAt(tab, i, null, new Node(h, k, v, null)))
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)
                tab = (Node[])f.key;
            else if (!replace && (fh & HASH_BITS) == h && (fv = f.val) != null &&
                     ((fk = f.key) == k || k.equals(fk))) {
                oldVal = fv;                // precheck 1st node for putIfAbsent
                break;
            }
            else if ((fh & LOCKED) != 0)
                f.tryAwaitLock(tab, i);
            else if (f.casHash(fh, fh | LOCKED)) {
                boolean validated = false;
                boolean checkSize = false;
                try {
                    if (tabAt(tab, i) == f) {
                        validated = true;    // retry if 1st already deleted
                        for (Node e = f;;) {
                            Object ek, ev;
                            if ((e.hash & HASH_BITS) == h &&
                                (ev = e.val) != null &&
                                ((ek = e.key) == k || k.equals(ek))) {
                                oldVal = ev;
                                if (replace)
                                    e.val = v;
                                break;
                            }
                            Node last = e;
                            if ((e = e.next) == null) {
                                last.next = new Node(h, k, v, null);
                                if (last != f || tab.length <= 64)
                                    checkSize = true;
                                break;
                            }
                        }
                    }
                } finally {                  // unlock and signal if needed
                    if (!f.casHash(fh | LOCKED, fh)) {
                        f.hash = fh;
                        synchronized(f) { f.notifyAll(); };
                    }
                }
                if (validated) {
                    int sc;
                    if (checkSize && tab.length < MAXIMUM_CAPACITY &&
                        (sc = sizeCtl) >= 0 && counter.sum() >= (long)sc)
                        growTable();
                    break;
                }
            }
        }
        if (oldVal == null)
            counter.increment();             // update counter outside of locks
        return oldVal;
    }

    /**
     * Implementation for the four public remove/replace methods:
     * Replaces node value with v, conditional upon match of cv if
     * non-null.  If resulting value is null, delete.
     */
    private final Object internalReplace(Object k, Object v, Object cv) {
        int h = spread(k.hashCode());
        Object oldVal = null;
        for (Node[] tab = table;;) {
            Node f; int i, fh;
            if (tab == null ||
                (f = tabAt(tab, i = (tab.length - 1) & h)) == null)
                break;
            else if ((fh = f.hash) == MOVED)
                tab = (Node[])f.key;
            else if ((fh & HASH_BITS) != h && f.next == null) // precheck
                break;                          // rules out possible existence
            else if ((fh & LOCKED) != 0)
                f.tryAwaitLock(tab, i);
            else if (f.casHash(fh, fh | LOCKED)) {
                boolean validated = false;
                boolean deleted = false;
                try {
                    if (tabAt(tab, i) == f) {
                        validated = true;
                        for (Node e = f, pred = null;;) {
                            Object ek, ev;
                            if ((e.hash & HASH_BITS) == h &&
                                ((ev = e.val) != null) &&
                                ((ek = e.key) == k || k.equals(ek))) {
                                if (cv == null || cv == ev || cv.equals(ev)) {
                                    oldVal = ev;
                                    if ((e.val = v) == null) {
                                        deleted = true;
                                        Node en = e.next;
                                        if (pred != null)
                                            pred.next = en;
                                        else
                                            setTabAt(tab, i, en);
                                    }
                                }
                                break;
                            }
                            pred = e;
                            if ((e = e.next) == null)
                                break;
                        }
                    }
                } finally {
                    if (!f.casHash(fh | LOCKED, fh)) {
                        f.hash = fh;
                        synchronized(f) { f.notifyAll(); };
                    }
                }
                if (validated) {
                    if (deleted)
                        counter.decrement();
                    break;
                }
            }
        }
        return oldVal;
    }

    /** Implementation for computeIfAbsent and compute. Like put, but messier. */
    // Todo: Somehow reinstate non-termination check
    @SuppressWarnings("unchecked")
    private final V internalCompute(K k,
                                    MappingFunction<? super K, ? extends V> fn,
                                    boolean replace) {
        int h = spread(k.hashCode());
        V val = null;
        boolean added = false;
        Node[] tab = table;
        outer:for (;;) {
            Node f; int i, fh; Object fk, fv;
            if (tab == null)
                tab = initTable();
            else if ((f = tabAt(tab, i = (tab.length - 1) & h)) == null) {
                Node node = new Node(fh = h | LOCKED, k, null, null);
                boolean validated = false;
                if (casTabAt(tab, i, null, node)) {
                    validated = true;
                    try {
                        val = fn.map(k);
                        if (val != null) {
                            node.val = val;
                            added = true;
                        }
                    } finally {
                        if (!added)
                            setTabAt(tab, i, null);
                        if (!node.casHash(fh, h)) {
                            node.hash = fh;
                            synchronized(node) { node.notifyAll(); };
                        }
                    }
                }
                if (validated)
                    break;
            }
            else if ((fh = f.hash) == MOVED)
                tab = (Node[])f.key;
            else if (!replace && (fh & HASH_BITS) == h && (fv = f.val) != null &&
                     ((fk = f.key) == k || k.equals(fk))) {
                if (tabAt(tab, i) == f) {
                    val = (V)fv;
                    break;
                }
            }
            else if ((fh & LOCKED) != 0)
                f.tryAwaitLock(tab, i);
            else if (f.casHash(fh, fh | LOCKED)) {
                boolean validated = false;
                boolean checkSize = false;
                try {
                    if (tabAt(tab, i) == f) {
                        validated = true;
                        for (Node e = f;;) {
                            Object ek, ev, v;
                            if ((e.hash & HASH_BITS) == h &&
                                (ev = e.val) != null &&
                                ((ek = e.key) == k || k.equals(ek))) {
                                if (replace && (v = fn.map(k)) != null)
                                    ev = e.val = v;
                                val = (V)ev;
                                break;
                            }
                            Node last = e;
                            if ((e = e.next) == null) {
                                if ((val = fn.map(k)) != null) {
                                    last.next = new Node(h, k, val, null);
                                    added = true;
                                    if (last != f || tab.length <= 64)
                                        checkSize = true;
                                }
                                break;
                            }
                        }
                    }
                } finally {
                    if (!f.casHash(fh | LOCKED, fh)) {
                        f.hash = fh;
                        synchronized(f) { f.notifyAll(); };
                    }
                }
                if (validated) {
                    int sc;
                    if (checkSize && tab.length < MAXIMUM_CAPACITY &&
                        (sc = sizeCtl) >= 0 && counter.sum() >= (long)sc)
                        growTable();
                    break;
                }
            }
        }
        if (added)
            counter.increment();
        return val;
    }

    /**
     * Implementation for clear. Steps through each bin, removing all nodes.
     */
    private final void internalClear() {
        long delta = 0L; // negative number of deletions
        int i = 0;
        Node[] tab = table;
        while (tab != null && i < tab.length) {
            int fh;
            Node f = tabAt(tab, i);
            if (f == null)
                ++i;
            else if ((fh = f.hash) == MOVED)
                tab = (Node[])f.key;
            else if ((fh & LOCKED) != 0)
                f.tryAwaitLock(tab, i);
            else if (f.casHash(fh, fh | LOCKED)) {
                boolean validated = false;
                try {
                    if (tabAt(tab, i) == f) {
                        validated = true;
                        for (Node e = f; e != null; e = e.next) {
                            if (e.val != null) { // currently always true
                                e.val = null;
                                --delta;
                            }
                        }
                        setTabAt(tab, i, null);
                    }
                } finally {
                    if (!f.casHash(fh | LOCKED, fh)) {
                        f.hash = fh;
                        synchronized(f) { f.notifyAll(); };
                    }
                }
                if (validated)
                    ++i;
            }
        }
        counter.add(delta);
    }

    /* ----------------Table Initialization and Resizing -------------- */

    /**
     * Returns a power of two table size for the given desired capacity.
     * See Hackers Delight, sec 3.2
     */
    private static final int tableSizeFor(int c) {
        int n = c - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }

    /**
     * Initializes table, using the size recorded in sizeCtl.
     */
    private final Node[] initTable() {
        Node[] tab; int sc;
        while ((tab = table) == null) {
            if ((sc = sizeCtl) < 0)
                Thread.yield(); // lost initialization race; just spin
            else if (UNSAFE.compareAndSwapInt(this, sizeCtlOffset, sc, -1)) {
                try {
                    if ((tab = table) == null) {
                        int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                        tab = table = new Node[n];
                        sc = n - (n >>> 2) - 1;
                    }
                } finally {
                    sizeCtl = sc;
                }
                break;
            }
        }
        return tab;
    }

    /**
     * If not already resizing, creates next table and transfers bins.
     * Rechecks occupancy after a transfer to see if another resize is
     * already needed because resizings are lagging additions.
     */
    private final void growTable() {
        int sc = sizeCtl;
        if (sc >= 0 && UNSAFE.compareAndSwapInt(this, sizeCtlOffset, sc, -1)) {
            try {
                Node[] tab; int n;
                while ((tab = table) != null &&
                       (n = tab.length) > 0 && n < MAXIMUM_CAPACITY &&
                       counter.sum() >= (long)sc) {
                    table = rebuild(tab);
                    sc = (n << 1) - (n >>> 1) - 1;
                }
            } finally {
                sizeCtl = sc;
            }
        }
    }

    /*
     * Moves and/or copies the nodes in each bin to new table. See
     * above for explanation.
     *
     * @return the new table
     */
    private static final Node[] rebuild(Node[] tab) {
        int n = tab.length;
        Node[] nextTab = new Node[n << 1];
        Node fwd = new Node(MOVED, nextTab, null, null);
        int[] buffer = null;       // holds bins to revisit; null until needed
        Node rev = null;           // reverse forwarder; null until needed
        int nbuffered = 0;         // the number of bins in buffer list
        int bufferIndex = 0;       // buffer index of current buffered bin
        int bin = n - 1;           // current non-buffered bin or -1 if none

        for (int i = bin;;) {      // start upwards sweep
            int fh; Node f;
            if ((f = tabAt(tab, i)) == null) {
                if (bin >= 0) {    // no lock needed (or available)
                    if (!casTabAt(tab, i, f, fwd))
                        continue;
                }
                else {             // transiently use a locked forwarding node
                    Node g =  new Node(MOVED|LOCKED, nextTab, null, null);
                    if (!casTabAt(tab, i, f, g))
                        continue;
                    setTabAt(nextTab, i, null);
                    setTabAt(nextTab, i + n, null);
                    setTabAt(tab, i, fwd);
                    if (!g.casHash(MOVED|LOCKED, MOVED)) {
                        g.hash = MOVED;
                        synchronized(g) { g.notifyAll(); }
                    }
                }
            }
            else if (((fh = f.hash) & LOCKED) == 0 && f.casHash(fh, fh|LOCKED)) {
                boolean validated = false;
                try {              // split to lo and hi lists; copying as needed
                    if (tabAt(tab, i) == f) {
                        validated = true;
                        Node e = f, lastRun = f;
                        Node lo = null, hi = null;
                        int runBit = e.hash & n;
                        for (Node p = e.next; p != null; p = p.next) {
                            int b = p.hash & n;
                            if (b != runBit) {
                                runBit = b;
                                lastRun = p;
                            }
                        }
                        if (runBit == 0)
                            lo = lastRun;
                        else
                            hi = lastRun;
                        for (Node p = e; p != lastRun; p = p.next) {
                            int ph = p.hash & HASH_BITS;
                            Object pk = p.key, pv = p.val;
                            if ((ph & n) == 0)
                                lo = new Node(ph, pk, pv, lo);
                            else
                                hi = new Node(ph, pk, pv, hi);
                        }
                        setTabAt(nextTab, i, lo);
                        setTabAt(nextTab, i + n, hi);
                        setTabAt(tab, i, fwd);
                    }
                } finally {
                    if (!f.casHash(fh | LOCKED, fh)) {
                        f.hash = fh;
                        synchronized(f) { f.notifyAll(); };
                    }
                }
                if (!validated)
                    continue;
            }
            else {
                if (buffer == null) // initialize buffer for revisits
                    buffer = new int[TRANSFER_BUFFER_SIZE];
                if (bin < 0 && bufferIndex > 0) {
                    int j = buffer[--bufferIndex];
                    buffer[bufferIndex] = i;
                    i = j;         // swap with another bin
                    continue;
                }
                if (bin < 0 || nbuffered >= TRANSFER_BUFFER_SIZE) {
                    f.tryAwaitLock(tab, i);
                    continue;      // no other options -- block
                }
                if (rev == null)   // initialize reverse-forwarder
                    rev = new Node(MOVED, tab, null, null);
                if (tabAt(tab, i) != f || (f.hash & LOCKED) == 0)
                    continue;      // recheck before adding to list
                buffer[nbuffered++] = i;
                setTabAt(nextTab, i, rev);     // install place-holders
                setTabAt(nextTab, i + n, rev);
            }

            if (bin > 0)
                i = --bin;
            else if (buffer != null && nbuffered > 0) {
                bin = -1;
                i = buffer[bufferIndex = --nbuffered];
            }
            else
                return nextTab;
        }
    }

    /* ----------------Table Traversal -------------- */

    /**
     * Encapsulates traversal for methods such as containsValue; also
     * serves as a base class for other iterators.
     *
     * At each step, the iterator snapshots the key ("nextKey") and
     * value ("nextVal") of a valid node (i.e., one that, at point of
     * snapshot, has a nonnull user value). Because val fields can
     * change (including to null, indicating deletion), field nextVal
     * might not be accurate at point of use, but still maintains the
     * weak consistency property of holding a value that was once
     * valid.
     *
     * Internal traversals directly access these fields, as in:
     * {@code while (it.next != null) { process(it.nextKey); it.advance(); }}
     *
     * Exported iterators (subclasses of ViewIterator) extract key,
     * value, or key-value pairs as return values of Iterator.next(),
     * and encapsulate the it.next check as hasNext();
     *
     * The iterator visits each valid node that was reachable upon
     * iterator construction once. It might miss some that were added
     * to a bin after the bin was visited, which is OK wrt consistency
     * guarantees. Maintaining this property in the face of possible
     * ongoing resizes requires a fair amount of bookkeeping state
     * that is difficult to optimize away amidst volatile accesses.
     * Even so, traversal maintains reasonable throughput.
     *
     * Normally, iteration proceeds bin-by-bin traversing lists.
     * However, if the table has been resized, then all future steps
     * must traverse both the bin at the current index as well as at
     * (index + baseSize); and so on for further resizings. To
     * paranoically cope with potential sharing by users of iterators
     * across threads, iteration terminates if a bounds checks fails
     * for a table read.
     *
     * The range-based constructor enables creation of parallel
     * range-splitting traversals. (Not yet implemented.)
     */
    static class InternalIterator {
        Node next;           // the next entry to use
        Node last;           // the last entry used
        Object nextKey;      // cached key field of next
        Object nextVal;      // cached val field of next
        Node[] tab;          // current table; updated if resized
        int index;           // index of bin to use next
        int baseIndex;       // current index of initial table
        final int baseLimit; // index bound for initial table
        final int baseSize;  // initial table size

        /** Creates iterator for all entries in the table. */
        InternalIterator(Node[] tab) {
            this.tab = tab;
            baseLimit = baseSize = (tab == null) ? 0 : tab.length;
            index = baseIndex = 0;
            next = null;
            advance();
        }

        /** Creates iterator for the given range of the table */
        InternalIterator(Node[] tab, int lo, int hi) {
            this.tab = tab;
            baseSize = (tab == null) ? 0 : tab.length;
            baseLimit = (hi <= baseSize) ? hi : baseSize;
            index = baseIndex = lo;
            next = null;
            advance();
        }

        /** Advances next. See above for explanation. */
        final void advance() {
            Node e = last = next;
            outer: do {
                if (e != null)                  // advance past used/skipped node
                    e = e.next;
                while (e == null) {             // get to next non-null bin
                    Node[] t; int b, i, n;      // checks must use locals
                    if ((b = baseIndex) >= baseLimit || (i = index) < 0 ||
                        (t = tab) == null || i >= (n = t.length))
                        break outer;
                    else if ((e = tabAt(t, i)) != null && e.hash == MOVED)
                        tab = (Node[])e.key;    // restarts due to null val
                    else                        // visit upper slots if present
                        index = (i += baseSize) < n ? i : (baseIndex = b + 1);
                }
                nextKey = e.key;
            } while ((nextVal = e.val) == null);// skip deleted or special nodes
            next = e;
        }
    }

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

    /**
     * Creates a new, empty map with the default initial table size (16),
     */
    public ConcurrentHashMapV8() {
        this.counter = new LongAdder();
    }

    /**
     * Creates a new, empty map with an initial table size
     * accommodating the specified number of elements without the need
     * to dynamically resize.
     *
     * @param initialCapacity The implementation performs internal
     * sizing to accommodate this many elements.
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative
     */
    public ConcurrentHashMapV8(int initialCapacity) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException();
        int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
                   MAXIMUM_CAPACITY :
                   tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
        this.counter = new LongAdder();
        this.sizeCtl = cap;
    }

    /**
     * Creates a new map with the same mappings as the given map.
     *
     * @param m the map
     */
    public ConcurrentHashMapV8(Map<? extends K, ? extends V> m) {
        this.counter = new LongAdder();
        this.sizeCtl = DEFAULT_CAPACITY;
        putAll(m);
    }

    /**
     * Creates a new, empty map with an initial table size based on
     * the given number of elements ({@code initialCapacity}) and
     * initial table density ({@code loadFactor}).
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements,
     * given the specified load factor.
     * @param loadFactor the load factor (table density) for
     * establishing the initial table size
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative or the load factor is nonpositive
     *
     * @since 1.6
     */
    public ConcurrentHashMapV8(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, 1);
    }

    /**
     * Creates a new, empty map with an initial table size based on
     * the given number of elements ({@code initialCapacity}), table
     * density ({@code loadFactor}), and number of concurrently
     * updating threads ({@code concurrencyLevel}).
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements,
     * given the specified load factor.
     * @param loadFactor the load factor (table density) for
     * establishing the initial table size
     * @param concurrencyLevel the estimated number of concurrently
     * updating threads. The implementation may use this value as
     * a sizing hint.
     * @throws IllegalArgumentException if the initial capacity is
     * negative or the load factor or concurrencyLevel are
     * nonpositive
     */
    public ConcurrentHashMapV8(int initialCapacity,
                               float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        if (initialCapacity < concurrencyLevel)   // Use at least as many bins
            initialCapacity = concurrencyLevel;   // as estimated threads
        long size = (long)(1.0 + (long)initialCapacity / loadFactor);
        int cap =  ((size >= (long)MAXIMUM_CAPACITY) ?
                    MAXIMUM_CAPACITY: tableSizeFor((int)size));
        this.counter = new LongAdder();
        this.sizeCtl = cap;
    }

    /**
     * {@inheritDoc}
     */
    public boolean isEmpty() {
        return counter.sum() <= 0L; // ignore transient negative values
    }

    /**
     * {@inheritDoc}
     */
    public int size() {
        long n = counter.sum();
        return ((n < 0L) ? 0 :
                (n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
                (int)n);
    }

    final long longSize() { // accurate version of size needed for views
        long n = counter.sum();
        return (n < 0L) ? 0L : n;
    }

    /**
     * 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
     */
    @SuppressWarnings("unchecked")
    public V get(Object key) {
        if (key == null)
            throw new NullPointerException();
        return (V)internalGet(key);
    }

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

    /**
     * Returns {@code true} if this map maps one or more keys to the
     * specified value. Note: This method may require a full traversal
     * of the map, and is much slower than method {@code containsKey}.
     *
     * @param value value whose presence in this map is to be tested
     * @return {@code true} if this map maps one or more keys to the
     *         specified value
     * @throws NullPointerException if the specified value is null
     */
    public boolean containsValue(Object value) {
        if (value == null)
            throw new NullPointerException();
        Object v;
        InternalIterator it = new InternalIterator(table);
        while (it.next != null) {
            if ((v = it.nextVal) == value || value.equals(v))
                return true;
            it.advance();
        }
        return false;
    }

    /**
     * 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 {@code true} if and only if some key maps to the
     *         {@code value} argument in this table as
     *         determined by the {@code equals} method;
     *         {@code 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.
     *
     * <p> The value can be retrieved by calling the {@code 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 {@code key}, or
     *         {@code null} if there was no mapping for {@code key}
     * @throws NullPointerException if the specified key or value is null
     */
    @SuppressWarnings("unchecked")
    public V put(K key, V value) {
        if (key == null || value == null)
            throw new NullPointerException();
        return (V)internalPut(key, value, true);
    }

    /**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     *         or {@code null} if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    @SuppressWarnings("unchecked")
    public V putIfAbsent(K key, V value) {
        if (key == null || value == null)
            throw new NullPointerException();
        return (V)internalPut(key, value, false);
    }

    /**
     * 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) {
        if (m == null)
            throw new NullPointerException();
        /*
         * If uninitialized, try to preallocate big enough table
         */
        if (table == null) {
            int size = m.size();
            int n = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :
                tableSizeFor(size + (size >>> 1) + 1);
            int sc = sizeCtl;
            if (n < sc)
                n = sc;
            if (sc >= 0 &&
                UNSAFE.compareAndSwapInt(this, sizeCtlOffset, sc, -1)) {
                try {
                    if (table == null) {
                        table = new Node[n];
                        sc = n - (n >>> 2) - 1;
                    }
                } finally {
                    sizeCtl = sc;
                }
            }
        }
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
            Object ek = e.getKey(), ev = e.getValue();
            if (ek == null || ev == null)
                throw new NullPointerException();
            internalPut(ek, ev, true);
        }
    }

    /**
     * If the specified key is not already associated with a value,
     * computes its value using the given mappingFunction, and if
     * non-null, enters it into the map.  This is equivalent to
     *  <pre> {@code
     * if (map.containsKey(key))
     *   return map.get(key);
     * value = mappingFunction.map(key);
     * if (value != null)
     *   map.put(key, value);
     * return value;}</pre>
     *
     * except that the action is performed atomically.  Some attempted
     * update operations on this map by other threads may be blocked
     * while computation is in progress, so the computation should be
     * short and simple, and must not attempt to update any other
     * mappings of this Map. The most appropriate usage is to
     * construct a new object serving as an initial mapped value, or
     * memoized result, as in:
     *  <pre> {@code
     * map.computeIfAbsent(key, new MappingFunction<K, V>() {
     *   public V map(K k) { return new Value(f(k)); }});}</pre>
     *
     * @param key key with which the specified value is to be associated
     * @param mappingFunction the function to compute a value
     * @return the current (existing or computed) value associated with
     *         the specified key, or {@code null} if the computation
     *         returned {@code null}
     * @throws NullPointerException if the specified key or mappingFunction
     *         is null
     * @throws IllegalStateException if the computation detectably
     *         attempts a recursive update to this map that would
     *         otherwise never complete
     * @throws RuntimeException or Error if the mappingFunction does so,
     *         in which case the mapping is left unestablished
     */
    public V computeIfAbsent(K key, MappingFunction<? super K, ? extends V> mappingFunction) {
        if (key == null || mappingFunction == null)
            throw new NullPointerException();
        return internalCompute(key, mappingFunction, false);
    }

    /**
     * Computes the value associated with the given key using the given
     * mappingFunction, and if non-null, enters it into the map.  This
     * is equivalent to
     *  <pre> {@code
     * value = mappingFunction.map(key);
     * if (value != null)
     *   map.put(key, value);
     * else
     *   value = map.get(key);
     * return value;}</pre>
     *
     * except that the action is performed atomically.  Some attempted
     * update operations on this map by other threads may be blocked
     * while computation is in progress, so the computation should be
     * short and simple, and must not attempt to update any other
     * mappings of this Map.
     *
     * @param key key with which the specified value is to be associated
     * @param mappingFunction the function to compute a value
     * @return the current value associated with
     *         the specified key, or {@code null} if the computation
     *         returned {@code null} and the value was not otherwise present
     * @throws NullPointerException if the specified key or mappingFunction
     *         is null
     * @throws IllegalStateException if the computation detectably
     *         attempts a recursive update to this map that would
     *         otherwise never complete
     * @throws RuntimeException or Error if the mappingFunction does so,
     *         in which case the mapping is unchanged
     */
    public V compute(K key, MappingFunction<? super K, ? extends V> mappingFunction) {
        if (key == null || mappingFunction == null)
            throw new NullPointerException();
        return internalCompute(key, mappingFunction, true);
    }

    /**
     * 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 {@code key}, or
     *         {@code null} if there was no mapping for {@code key}
     * @throws NullPointerException if the specified key is null
     */
    @SuppressWarnings("unchecked")
    public V remove(Object key) {
        if (key == null)
            throw new NullPointerException();
        return (V)internalReplace(key, null, null);
    }

    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if the specified key is null
     */
    public boolean remove(Object key, Object value) {
        if (key == null)
            throw new NullPointerException();
        if (value == null)
            return false;
        return internalReplace(key, null, value) != null;
    }

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

    /**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     *         or {@code null} if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    @SuppressWarnings("unchecked")
    public V replace(K key, V value) {
        if (key == null || value == null)
            throw new NullPointerException();
        return (V)internalReplace(key, value, null);
    }

    /**
     * Removes all of the mappings from this map.
     */
    public void clear() {
        internalClear();
    }

    /**
     * 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 {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.  It does not support the {@code add} or
     * {@code addAll} operations.
     *
     * <p>The view's {@code iterator} is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Set<K> keySet() {
        KeySet<K,V> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet<K,V>(this));
    }

    /**
     * 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 {@code Iterator.remove},
     * {@code Collection.remove}, {@code removeAll},
     * {@code retainAll}, and {@code clear} operations.  It does not
     * support the {@code add} or {@code addAll} operations.
     *
     * <p>The view's {@code iterator} is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Collection<V> values() {
        Values<K,V> vs = values;
        return (vs != null) ? vs : (values = new Values<K,V>(this));
    }

    /**
     * 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 {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.  It does not support the {@code add} or
     * {@code addAll} operations.
     *
     * <p>The view's {@code iterator} is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Set<Map.Entry<K,V>> entrySet() {
        EntrySet<K,V> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet<K,V>(this));
    }

    /**
     * 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<K,V>(this);
    }

    /**
     * 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<K,V>(this);
    }

    /**
     * Returns the hash code value for this {@link Map}, i.e.,
     * the sum of, for each key-value pair in the map,
     * {@code key.hashCode() ^ value.hashCode()}.
     *
     * @return the hash code value for this map
     */
    public int hashCode() {
        int h = 0;
        InternalIterator it = new InternalIterator(table);
        while (it.next != null) {
            h += it.nextKey.hashCode() ^ it.nextVal.hashCode();
            it.advance();
        }
        return h;
    }

    /**
     * Returns a string representation of this map.  The string
     * representation consists of a list of key-value mappings (in no
     * particular order) enclosed in braces ("{@code {}}").  Adjacent
     * mappings are separated by the characters {@code ", "} (comma
     * and space).  Each key-value mapping is rendered as the key
     * followed by an equals sign ("{@code =}") followed by the
     * associated value.
     *
     * @return a string representation of this map
     */
    public String toString() {
        InternalIterator it = new InternalIterator(table);
        StringBuilder sb = new StringBuilder();
        sb.append('{');
        if (it.next != null) {
            for (;;) {
                Object k = it.nextKey, v = it.nextVal;
                sb.append(k == this ? "(this Map)" : k);
                sb.append('=');
                sb.append(v == this ? "(this Map)" : v);
                it.advance();
                if (it.next == null)
                    break;
                sb.append(',').append(' ');
            }
        }
        return sb.append('}').toString();
    }

    /**
     * Compares the specified object with this map for equality.
     * Returns {@code true} if the given object is a map with the same
     * mappings as this map.  This operation may return misleading
     * results if either map is concurrently modified during execution
     * of this method.
     *
     * @param o object to be compared for equality with this map
     * @return {@code true} if the specified object is equal to this map
     */
    public boolean equals(Object o) {
        if (o != this) {
            if (!(o instanceof Map))
                return false;
            Map<?,?> m = (Map<?,?>) o;
            InternalIterator it = new InternalIterator(table);
            while (it.next != null) {
                Object val = it.nextVal;
                Object v = m.get(it.nextKey);
                if (v == null || (v != val && !v.equals(val)))
                    return false;
                it.advance();
            }
            for (Map.Entry<?,?> e : m.entrySet()) {
                Object mk, mv, v;
                if ((mk = e.getKey()) == null ||
                    (mv = e.getValue()) == null ||
                    (v = internalGet(mk)) == null ||
                    (mv != v && !mv.equals(v)))
                    return false;
            }
        }
        return true;
    }

    /* ----------------Iterators -------------- */

    /**
     * Base class for key, value, and entry iterators.  Adds a map
     * reference to InternalIterator to support Iterator.remove.
     */
    static abstract class ViewIterator<K,V> extends InternalIterator {
        final ConcurrentHashMapV8<K, V> map;
        ViewIterator(ConcurrentHashMapV8<K, V> map) {
            super(map.table);
            this.map = map;
        }

        public final void remove() {
            if (last == null)
                throw new IllegalStateException();
            map.remove(last.key);
            last = null;
        }

        public final boolean hasNext()         { return next != null; }
        public final boolean hasMoreElements() { return next != null; }
    }

    static final class KeyIterator<K,V> extends ViewIterator<K,V>
        implements Iterator<K>, Enumeration<K> {
        KeyIterator(ConcurrentHashMapV8<K, V> map) { super(map); }

        @SuppressWarnings("unchecked")
        public final K next() {
            if (next == null)
                throw new NoSuchElementException();
            Object k = nextKey;
            advance();
            return (K)k;
        }

        public final K nextElement() { return next(); }
    }

    static final class ValueIterator<K,V> extends ViewIterator<K,V>
        implements Iterator<V>, Enumeration<V> {
        ValueIterator(ConcurrentHashMapV8<K, V> map) { super(map); }

        @SuppressWarnings("unchecked")
        public final V next() {
            if (next == null)
                throw new NoSuchElementException();
            Object v = nextVal;
            advance();
            return (V)v;
        }

        public final V nextElement() { return next(); }
    }

    static final class EntryIterator<K,V> extends ViewIterator<K,V>
        implements Iterator<Map.Entry<K,V>> {
        EntryIterator(ConcurrentHashMapV8<K, V> map) { super(map); }

        @SuppressWarnings("unchecked")
        public final Map.Entry<K,V> next() {
            if (next == null)
                throw new NoSuchElementException();
            Object k = nextKey;
            Object v = nextVal;
            advance();
            return new WriteThroughEntry<K,V>((K)k, (V)v, map);
        }
    }

    static final class SnapshotEntryIterator<K,V> extends ViewIterator<K,V>
        implements Iterator<Map.Entry<K,V>> {
        SnapshotEntryIterator(ConcurrentHashMapV8<K, V> map) { super(map); }

        @SuppressWarnings("unchecked")
        public final Map.Entry<K,V> next() {
            if (next == null)
                throw new NoSuchElementException();
            Object k = nextKey;
            Object v = nextVal;
            advance();
            return new SnapshotEntry<K,V>((K)k, (V)v);
        }
    }

    /**
     * Base of writeThrough and Snapshot entry classes
     */
    static abstract class MapEntry<K,V> implements Map.Entry<K, V> {
        final K key; // non-null
        V val;       // non-null
        MapEntry(K key, V val)        { this.key = key; this.val = val; }
        public final K getKey()       { return key; }
        public final V getValue()     { return val; }
        public final int hashCode()   { return key.hashCode() ^ val.hashCode(); }
        public final String toString(){ return key + "=" + val; }

        public final boolean equals(Object o) {
            Object k, v; Map.Entry<?,?> e;
            return ((o instanceof Map.Entry) &&
                    (k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
                    (v = e.getValue()) != null &&
                    (k == key || k.equals(key)) &&
                    (v == val || v.equals(val)));
        }

        public abstract V setValue(V value);
    }

    /**
     * Entry used by EntryIterator.next(), that relays setValue
     * changes to the underlying map.
     */
    static final class WriteThroughEntry<K,V> extends MapEntry<K,V>
        implements Map.Entry<K, V> {
        final ConcurrentHashMapV8<K, V> map;
        WriteThroughEntry(K key, V val, ConcurrentHashMapV8<K, V> map) {
            super(key, val);
            this.map = map;
        }

        /**
         * Sets our entry's value and writes 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 final V setValue(V value) {
            if (value == null) throw new NullPointerException();
            V v = val;
            val = value;
            map.put(key, value);
            return v;
        }
    }

    /**
     * Internal version of entry, that doesn't write though changes
     */
    static final class SnapshotEntry<K,V> extends MapEntry<K,V>
        implements Map.Entry<K, V> {
        SnapshotEntry(K key, V val) { super(key, val); }
        public final V setValue(V value) { // only locally update
            if (value == null) throw new NullPointerException();
            V v = val;
            val = value;
            return v;
        }
    }

    /* ----------------Views -------------- */

    /**
     * Base class for views. This is done mainly to allow adding
     * customized parallel traversals (not yet implemented.)
     */
    static abstract class MapView<K, V> {
        final ConcurrentHashMapV8<K, V> map;
        MapView(ConcurrentHashMapV8<K, V> map)  { this.map = map; }
        public final int size()                 { return map.size(); }
        public final boolean isEmpty()          { return map.isEmpty(); }
        public final void clear()               { map.clear(); }

        // implementations below rely on concrete classes supplying these
        abstract Iterator<?> iter();
        abstract public boolean contains(Object o);
        abstract public boolean remove(Object o);

        private static final String oomeMsg = "Required array size too large";

        public final Object[] toArray() {
            long sz = map.longSize();
            if (sz > (long)(MAX_ARRAY_SIZE))
                throw new OutOfMemoryError(oomeMsg);
            int n = (int)sz;
            Object[] r = new Object[n];
            int i = 0;
            Iterator<?> it = iter();
            while (it.hasNext()) {
                if (i == n) {
                    if (n >= MAX_ARRAY_SIZE)
                        throw new OutOfMemoryError(oomeMsg);
                    if (n >= MAX_ARRAY_SIZE - (MAX_ARRAY_SIZE >>> 1) - 1)
                        n = MAX_ARRAY_SIZE;
                    else
                        n += (n >>> 1) + 1;
                    r = Arrays.copyOf(r, n);
                }
                r[i++] = it.next();
            }
            return (i == n) ? r : Arrays.copyOf(r, i);
        }

        @SuppressWarnings("unchecked")
        public final <T> T[] toArray(T[] a) {
            long sz = map.longSize();
            if (sz > (long)(MAX_ARRAY_SIZE))
                throw new OutOfMemoryError(oomeMsg);
            int m = (int)sz;
            T[] r = (a.length >= m) ? a :
                (T[])java.lang.reflect.Array
                .newInstance(a.getClass().getComponentType(), m);
            int n = r.length;
            int i = 0;
            Iterator<?> it = iter();
            while (it.hasNext()) {
                if (i == n) {
                    if (n >= MAX_ARRAY_SIZE)
                        throw new OutOfMemoryError(oomeMsg);
                    if (n >= MAX_ARRAY_SIZE - (MAX_ARRAY_SIZE >>> 1) - 1)
                        n = MAX_ARRAY_SIZE;
                    else
                        n += (n >>> 1) + 1;
                    r = Arrays.copyOf(r, n);
                }
                r[i++] = (T)it.next();
            }
            if (a == r && i < n) {
                r[i] = null; // null-terminate
                return r;
            }
            return (i == n) ? r : Arrays.copyOf(r, i);
        }

        public final int hashCode() {
            int h = 0;
            for (Iterator<?> it = iter(); it.hasNext();)
                h += it.next().hashCode();
            return h;
        }

        public final String toString() {
            StringBuilder sb = new StringBuilder();
            sb.append('[');
            Iterator<?> it = iter();
            if (it.hasNext()) {
                for (;;) {
                    Object e = it.next();
                    sb.append(e == this ? "(this Collection)" : e);
                    if (!it.hasNext())
                        break;
                    sb.append(',').append(' ');
                }
            }
            return sb.append(']').toString();
        }

        public final boolean containsAll(Collection<?> c) {
            if (c != this) {
                for (Iterator<?> it = c.iterator(); it.hasNext();) {
                    Object e = it.next();
                    if (e == null || !contains(e))
                        return false;
                }
            }
            return true;
        }

        public final boolean removeAll(Collection c) {
            boolean modified = false;
            for (Iterator<?> it = iter(); it.hasNext();) {
                if (c.contains(it.next())) {
                    it.remove();
                    modified = true;
                }
            }
            return modified;
        }

        public final boolean retainAll(Collection<?> c) {
            boolean modified = false;
            for (Iterator<?> it = iter(); it.hasNext();) {
                if (!c.contains(it.next())) {
                    it.remove();
                    modified = true;
                }
            }
            return modified;
        }

    }

    static final class KeySet<K,V> extends MapView<K,V> implements Set<K> {
        KeySet(ConcurrentHashMapV8<K, V> map)   { super(map); }
        public final boolean contains(Object o) { return map.containsKey(o); }
        public final boolean remove(Object o)   { return map.remove(o) != null; }

        public final Iterator<K> iterator() {
            return new KeyIterator<K,V>(map);
        }
        final Iterator<?> iter() {
            return new KeyIterator<K,V>(map);
        }
        public final boolean add(K e) {
            throw new UnsupportedOperationException();
        }
        public final boolean addAll(Collection<? extends K> c) {
            throw new UnsupportedOperationException();
        }
        public boolean equals(Object o) {
            Set<?> c;
            return ((o instanceof Set) &&
                    ((c = (Set<?>)o) == this ||
                     (containsAll(c) && c.containsAll(this))));
        }
    }

    static final class Values<K,V> extends MapView<K,V>
        implements Collection<V>  {
        Values(ConcurrentHashMapV8<K, V> map)   { super(map); }
        public final boolean contains(Object o) { return map.containsValue(o); }

        public final boolean remove(Object o) {
            if (o != null) {
                Iterator<V> it = new ValueIterator<K,V>(map);
                while (it.hasNext()) {
                    if (o.equals(it.next())) {
                        it.remove();
                        return true;
                    }
                }
            }
            return false;
        }
        public final Iterator<V> iterator() {
            return new ValueIterator<K,V>(map);
        }
        final Iterator<?> iter() {
            return new ValueIterator<K,V>(map);
        }
        public final boolean add(V e) {
            throw new UnsupportedOperationException();
        }
        public final boolean addAll(Collection<? extends V> c) {
            throw new UnsupportedOperationException();
        }
    }

    static final class EntrySet<K,V>  extends MapView<K,V>
        implements Set<Map.Entry<K,V>> {
        EntrySet(ConcurrentHashMapV8<K, V> map) { super(map); }

        public final boolean contains(Object o) {
            Object k, v, r; Map.Entry<?,?> e;
            return ((o instanceof Map.Entry) &&
                    (k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
                    (r = map.get(k)) != null &&
                    (v = e.getValue()) != null &&
                    (v == r || v.equals(r)));
        }

        public final boolean remove(Object o) {
            Object k, v; Map.Entry<?,?> e;
            return ((o instanceof Map.Entry) &&
                    (k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
                    (v = e.getValue()) != null &&
                    map.remove(k, v));
        }

        public final Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator<K,V>(map);
        }
        final Iterator<?> iter() {
            return new SnapshotEntryIterator<K,V>(map);
        }
        public final boolean add(Entry<K,V> e) {
            throw new UnsupportedOperationException();
        }
        public final boolean addAll(Collection<? extends Entry<K,V>> c) {
            throw new UnsupportedOperationException();
        }
        public boolean equals(Object o) {
            Set<?> c;
            return ((o instanceof Set) &&
                    ((c = (Set<?>)o) == this ||
                     (containsAll(c) && c.containsAll(this))));
        }
    }

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

    /**
     * Stripped-down version of helper class used in previous version,
     * declared for the sake of serialization compatibility
     */
    static class Segment<K,V> implements Serializable {
        private static final long serialVersionUID = 2249069246763182397L;
        final float loadFactor;
        Segment(float lf) { this.loadFactor = lf; }
    }

    /**
     * Saves the state of the {@code ConcurrentHashMapV8} instance to a
     * stream (i.e., serializes 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.
     */
    @SuppressWarnings("unchecked")
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        if (segments == null) { // for serialization compatibility
            segments = (Segment<K,V>[])
                new Segment<?,?>[DEFAULT_CONCURRENCY_LEVEL];
            for (int i = 0; i < segments.length; ++i)
                segments[i] = new Segment<K,V>(LOAD_FACTOR);
        }
        s.defaultWriteObject();
        InternalIterator it = new InternalIterator(table);
        while (it.next != null) {
            s.writeObject(it.nextKey);
            s.writeObject(it.nextVal);
            it.advance();
        }
        s.writeObject(null);
        s.writeObject(null);
        segments = null; // throw away
    }

    /**
     * Reconstitutes the instance from a stream (that is, deserializes it).
     * @param s the stream
     */
    @SuppressWarnings("unchecked")
    private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        this.segments = null; // unneeded
        // initialize transient final field
        UNSAFE.putObjectVolatile(this, counterOffset, new LongAdder());

        // Create all nodes, then place in table once size is known
        long size = 0L;
        Node p = null;
        for (;;) {
            K k = (K) s.readObject();
            V v = (V) s.readObject();
            if (k != null && v != null) {
                p = new Node(spread(k.hashCode()), k, v, p);
                ++size;
            }
            else
                break;
        }
        if (p != null) {
            boolean init = false;
            int n;
            if (size >= (long)(MAXIMUM_CAPACITY >>> 1))
                n = MAXIMUM_CAPACITY;
            else {
                int sz = (int)size;
                n = tableSizeFor(sz + (sz >>> 1) + 1);
            }
            int sc = sizeCtl;
            if (n > sc &&
                UNSAFE.compareAndSwapInt(this, sizeCtlOffset, sc, -1)) {
                try {
                    if (table == null) {
                        init = true;
                        Node[] tab = new Node[n];
                        int mask = n - 1;
                        while (p != null) {
                            int j = p.hash & mask;
                            Node next = p.next;
                            p.next = tabAt(tab, j);
                            setTabAt(tab, j, p);
                            p = next;
                        }
                        table = tab;
                        counter.add(size);
                        sc = n - (n >>> 2) - 1;
                    }
                } finally {
                    sizeCtl = sc;
                }
            }
            if (!init) { // Can only happen if unsafely published.
                while (p != null) {
                    internalPut(p.key, p.val, true);
                    p = p.next;
                }
            }
        }
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long counterOffset;
    private static final long sizeCtlOffset;
    private static final long ABASE;
    private static final int ASHIFT;

    static {
        int ss;
        try {
            UNSAFE = getUnsafe();
            Class<?> k = ConcurrentHashMapV8.class;
            counterOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("counter"));
            sizeCtlOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("sizeCtl"));
            Class<?> sc = Node[].class;
            ABASE = UNSAFE.arrayBaseOffset(sc);
            ss = UNSAFE.arrayIndexScale(sc);
        } catch (Exception e) {
            throw new Error(e);
        }
        if ((ss & (ss-1)) != 0)
            throw new Error("data type scale not a power of two");
        ASHIFT = 31 - Integer.numberOfLeadingZeros(ss);
    }

    /**
     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
     * Replace with a simple call to Unsafe.getUnsafe when integrating
     * into a jdk.
     *
     * @return a sun.misc.Unsafe
     */
    private static sun.misc.Unsafe getUnsafe() {
        try {
            return sun.misc.Unsafe.getUnsafe();
        } catch (SecurityException se) {
            try {
                return java.security.AccessController.doPrivileged
                    (new java.security
                     .PrivilegedExceptionAction<sun.misc.Unsafe>() {
                        public sun.misc.Unsafe run() throws Exception {
                            java.lang.reflect.Field f = sun.misc
                                .Unsafe.class.getDeclaredField("theUnsafe");
                            f.setAccessible(true);
                            return (sun.misc.Unsafe) f.get(null);
                        }});
            } catch (java.security.PrivilegedActionException e) {
                throw new RuntimeException("Could not initialize intrinsics",
                                           e.getCause());
            }
        }
    }

}
Revision:	1.24
Committed:	Thu Sep 15 14:25:46 2011 UTC (12 years, 7 months ago) by dl
Branch:	MAIN
Changes since 1.23:	+813 -384 lines
Log Message:	Overlay trylocks; atomic size control; more traversal scaffolding