/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * 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 java.util.concurrent;

import java.io.ObjectStreamField;

import java.io.Serializable;

import java.lang.reflect.ParameterizedType;

import java.lang.reflect.Type;

import java.util.AbstractMap;

import java.util.Arrays;

import java.util.Collection;

import java.util.Enumeration;

import java.util.HashMap;

import java.util.Hashtable;

import java.util.Iterator;

import java.util.Map;

import java.util.NoSuchElementException;

import java.util.Set;

import java.util.Spliterator;

import java.util.concurrent.atomic.AtomicReference;

import java.util.concurrent.locks.LockSupport;

import java.util.concurrent.locks.ReentrantLock;

import java.util.function.BiConsumer;

import java.util.function.BiFunction;

import java.util.function.Consumer;

import java.util.function.DoubleBinaryOperator;

import java.util.function.Function;

import java.util.function.IntBinaryOperator;

import java.util.function.LongBinaryOperator;

import java.util.function.Predicate;

import java.util.function.ToDoubleBiFunction;

import java.util.function.ToDoubleFunction;

import java.util.function.ToIntBiFunction;

import java.util.function.ToIntFunction;

import java.util.function.ToLongBiFunction;

import java.util.function.ToLongFunction;

import java.util.stream.Stream;

import jdk.internal.misc.Unsafe;

/**

 * 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. (More formally, an update operation for a given key bears a

 * <em>happens-before</em> relation with any (non-null) retrieval for

 * that key reporting the updated value.)  For aggregate operations

 * such as {@code putAll} and {@code clear}, concurrent retrievals may

 * reflect insertion or removal of only some entries.  Similarly,

 * Iterators, Spliterators 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

 * java.util.ConcurrentModificationException 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. To ameliorate impact, when keys are {@link Comparable},

 * this class may use comparison order among keys to help break ties.

 *

 * <p>A {@link Set} projection of a ConcurrentHashMap may be created

 * (using {@link #newKeySet()} or {@link #newKeySet(int)}), or viewed

 * (using {@link #keySet(Object)} when only keys are of interest, and the

 * mapped values are (perhaps transiently) not used or all take the

 * same mapping value.

 *

 * <p>A ConcurrentHashMap can be used as a scalable frequency map (a

 * form of histogram or multiset) by using {@link

 * java.util.concurrent.atomic.LongAdder} values and initializing via

 * {@link #computeIfAbsent computeIfAbsent}. For example, to add a count

 * to a {@code ConcurrentHashMap<String,LongAdder> freqs}, you can use

 * {@code freqs.computeIfAbsent(key, k -> new LongAdder()).increment();}

 *

 * <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>ConcurrentHashMaps support a set of sequential and parallel bulk

 * operations that, unlike most {@link Stream} methods, are designed

 * to be safely, and often sensibly, applied even with maps that are

 * being concurrently updated by other threads; for example, when

 * computing a snapshot summary of the values in a shared registry.

 * There are three kinds of operation, each with four forms, accepting

 * functions with keys, values, entries, and (key, value) pairs as

 * arguments and/or return values. Because the elements of a

 * ConcurrentHashMap are not ordered in any particular way, and may be

 * processed in different orders in different parallel executions, the

 * correctness of supplied functions should not depend on any

 * ordering, or on any other objects or values that may transiently

 * change while computation is in progress; and except for forEach

 * actions, should ideally be side-effect-free. Bulk operations on

 * {@link java.util.Map.Entry} objects do not support method {@code

 * setValue}.

 *

 * <ul>

 * <li>forEach: Performs a given action on each element.

 * A variant form applies a given transformation on each element

 * before performing the action.

 *

 * <li>search: Returns the first available non-null result of

 * applying a given function on each element; skipping further

 * search when a result is found.

 *

 * <li>reduce: Accumulates each element.  The supplied reduction

 * function cannot rely on ordering (more formally, it should be

 * both associative and commutative).  There are five variants:

 *

 * <ul>

 *

 * <li>Plain reductions. (There is not a form of this method for

 * (key, value) function arguments since there is no corresponding

 * return type.)

 *

 * <li>Mapped reductions that accumulate the results of a given

 * function applied to each element.

 *

 * <li>Reductions to scalar doubles, longs, and ints, using a

 * given basis value.

 *

 * </ul>

 * </ul>

 *

 * <p>These bulk operations accept a {@code parallelismThreshold}

 * argument. Methods proceed sequentially if the current map size is

 * estimated to be less than the given threshold. Using a value of

 * {@code Long.MAX_VALUE} suppresses all parallelism.  Using a value

 * of {@code 1} results in maximal parallelism by partitioning into

 * enough subtasks to fully utilize the {@link

 * ForkJoinPool#commonPool()} that is used for all parallel

 * computations. Normally, you would initially choose one of these

 * extreme values, and then measure performance of using in-between

 * values that trade off overhead versus throughput.

 *

 * <p>The concurrency properties of bulk operations follow

 * from those of ConcurrentHashMap: Any non-null result returned

 * from {@code get(key)} and related access methods bears a

 * happens-before relation with the associated insertion or

 * update.  The result of any bulk operation reflects the

 * composition of these per-element relations (but is not

 * necessarily atomic with respect to the map as a whole unless it

 * is somehow known to be quiescent).  Conversely, because keys

 * and values in the map are never null, null serves as a reliable

 * atomic indicator of the current lack of any result.  To

 * maintain this property, null serves as an implicit basis for

 * all non-scalar reduction operations. For the double, long, and

 * int versions, the basis should be one that, when combined with

 * any other value, returns that other value (more formally, it

 * should be the identity element for the reduction). Most common

 * reductions have these properties; for example, computing a sum

 * with basis 0 or a minimum with basis MAX_VALUE.

 *

 * <p>Search and transformation functions provided as arguments

 * should similarly return null to indicate the lack of any result

 * (in which case it is not used). In the case of mapped

 * reductions, this also enables transformations to serve as

 * filters, returning null (or, in the case of primitive

 * specializations, the identity basis) if the element should not

 * be combined. You can create compound transformations and

 * filterings by composing them yourself under this "null means

 * there is nothing there now" rule before using them in search or

 * reduce operations.

 *

 * <p>Methods accepting and/or returning Entry arguments maintain

 * key-value associations. They may be useful for example when

 * finding the key for the greatest value. Note that "plain" Entry

 * arguments can be supplied using {@code new

 * AbstractMap.SimpleEntry(k,v)}.

 *

 * <p>Bulk operations may complete abruptly, throwing an

 * exception encountered in the application of a supplied

 * function. Bear in mind when handling such exceptions that other

 * concurrently executing functions could also have thrown

 * exceptions, or would have done so if the first exception had

 * not occurred.

 *

 * <p>Speedups for parallel compared to sequential forms are common

 * but not guaranteed.  Parallel operations involving brief functions

 * on small maps may execute more slowly than sequential forms if the

 * underlying work to parallelize the computation is more expensive

 * than the computation itself.  Similarly, parallelization may not

 * lead to much actual parallelism if all processors are busy

 * performing unrelated tasks.

 *

 * <p>All arguments to all task methods must be non-null.

 *

 * <p>This class is a member of the

 * <a href="{@docRoot}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * @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 ConcurrentHashMap<K,V> extends AbstractMap<K,V>

    implements ConcurrentMap<K,V>, Serializable {

    private static final long serialVersionUID = 7249069246763182397L;

    /*

     * 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.

     *

     * This map usually acts as a binned (bucketed) hash table.  Each

     * key-value mapping is held in a Node.  Most nodes are instances

     * of the basic Node class with hash, key, value, and next

     * fields. However, various subclasses exist: TreeNodes are

     * arranged in balanced trees, not lists.  TreeBins hold the roots

     * of sets of TreeNodes. ForwardingNodes are placed at the heads

     * of bins during resizing. ReservationNodes are used as

     * placeholders while establishing values in computeIfAbsent and

     * related methods.  The types TreeBin, ForwardingNode, and

     * ReservationNode do not hold normal user keys, values, or

     * hashes, and are readily distinguishable during search etc

     * because they have negative hash fields and null key and value

     * fields. (These special nodes are either uncommon or transient,

     * so the impact of carrying around some unused fields is

     * insignificant.)

     *

     * The table is lazily initialized to a power-of-two size upon the

     * first insertion.  Each bin in the table normally contains a

     * list of Nodes (most often, the list has only 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

     * (jdk.internal.misc.Unsafe) operations.

     *

     * We use the top (sign) bit of Node hash fields for control

     * purposes -- it is available anyway because of addressing

     * constraints.  Nodes with negative hash fields are specially

     * handled or ignored in map methods.

     *

     * Insertion (via put or its variants) 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 under most

     * key/hash distributions.  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. Locking support for these locks relies on builtin

     * "synchronized" monitors.

     *

     * 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 (upon resizing).

     *

     * 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, under

     * random hash codes, this is not a common problem.  Ideally, 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 values are:

     *

     * 0:    0.60653066

     * 1:    0.30326533

     * 2:    0.07581633

     * 3:    0.01263606

     * 4:    0.00157952

     * 5:    0.00015795

     * 6:    0.00001316

     * 7:    0.00000094

     * 8:    0.00000006

     * more: less than 1 in ten million

     *

     * Lock contention probability for two threads accessing distinct

     * elements is roughly 1 / (8 * #elements) under random hashes.

     *

     * Actual hash code distributions encountered in practice

     * sometimes deviate significantly from uniform randomness.  This

     * includes the case when N > (1<<30), so some keys MUST collide.

     * Similarly for dumb or hostile usages in which multiple keys are

     * designed to have identical hash codes or ones that differs only

     * in masked-out high bits. So we use a secondary strategy that

     * applies when the number of nodes in a bin exceeds a

     * threshold. These TreeBins use a balanced tree to hold nodes (a

     * specialized form of red-black trees), bounding search time to

     * O(log N).  Each search step in a TreeBin is at least twice as

     * slow as in a regular list, but given that N cannot exceed

     * (1<<64) (before running out of addresses) this bounds search

     * steps, lock hold times, etc, to reasonable constants (roughly

     * 100 nodes inspected per operation worst case) so long as keys

     * are Comparable (which is very common -- String, Long, etc).

     * TreeBin nodes (TreeNodes) also maintain the same "next"

     * traversal pointers as regular nodes, so can be traversed in

     * iterators in the same way.

     *

     * The table is resized when occupancy exceeds a percentage

     * threshold (nominally, 0.75, but see below).  Any thread

     * noticing an overfull bin may assist in resizing after the

     * initiating thread allocates and sets up the replacement array.

     * However, rather than stalling, these other threads may proceed

     * with insertions etc.  The use of TreeBins shields us from the

     * worst case effects of overfilling while resizes are in

     * progress.  Resizing proceeds by transferring bins, one by one,

     * from the table to the next table. However, threads claim small

     * blocks of indices to transfer (via field transferIndex) before

     * doing so, reducing contention.  A generation stamp in field

     * sizeCtl ensures that resizings do not overlap. 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, which can stall

     * waiting for locks while resizing. However, because other

     * threads can join in and help resize rather than contend for

     * locks, average aggregate waits become shorter as resizing

     * progresses.  The transfer operation must also ensure that all

     * accessible bins in both the old and new table are usable by any

     * traversal.  This is arranged in part by proceeding from the

     * last bin (table.length - 1) up towards the first.  Upon seeing

     * a forwarding node, traversals (see class Traverser) arrange to

     * move to the new table without revisiting nodes.  To ensure that

     * no intervening nodes are skipped even when moved out of order,

     * a stack (see class TableStack) is created on first encounter of

     * a forwarding node during a traversal, to maintain its place if

     * later processing the current table. The need for these

     * save/restore mechanics is relatively rare, but when one

     * forwarding node is encountered, typically many more will be.

     * So Traversers use a simple caching scheme to avoid creating so

     * many new TableStack nodes. (Thanks to Peter Levart for

     * suggesting use of a stack here.)

     *

     * The traversal scheme also applies to partial traversals of

     * ranges of bins (via an alternate Traverser constructor)

     * to support partitioned aggregate operations.  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 specialization of

     * LongAdder. We need to incorporate a specialization rather than

     * just use a LongAdder in order to access implicit

     * contention-sensing that leads to creation of multiple

     * CounterCells.  The counter mechanics avoid contention on

     * updates but can encounter cache thrashing if read too

     * frequently during concurrent access. To avoid reading so often,

     * resizing under contention is 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).

     *

     * TreeBins use a special form of comparison for search and

     * related operations (which is the main reason we cannot use

     * existing collections such as TreeMaps). TreeBins contain

     * Comparable elements, but may contain others, as well as

     * elements that are Comparable but not necessarily Comparable for

     * the same T, so we cannot invoke compareTo among them. To handle

     * this, the tree is ordered primarily by hash value, then by

     * Comparable.compareTo order if applicable.  On lookup at a node,

     * if elements are not comparable or compare as 0 then both left

     * and right children may need to be searched in the case of tied

     * hash values. (This corresponds to the full list search that

     * would be necessary if all elements were non-Comparable and had

     * tied hashes.) On insertion, to keep a total ordering (or as

     * close as is required here) across rebalancings, we compare

     * classes and identityHashCodes as tie-breakers. The red-black

     * balancing code is updated from pre-jdk-collections

     * (http://gee.cs.oswego.edu/dl/classes/collections/RBCell.java)

     * based in turn on Cormen, Leiserson, and Rivest "Introduction to

     * Algorithms" (CLR).

     *

     * TreeBins also require an additional locking mechanism.  While

     * list traversal is always possible by readers even during

     * updates, tree traversal is not, mainly because of tree-rotations

     * that may change the root node and/or its linkages.  TreeBins

     * include a simple read-write lock mechanism parasitic on the

     * main bin-synchronization strategy: Structural adjustments

     * associated with an insertion or removal are already bin-locked

     * (and so cannot conflict with other writers) but must wait for

     * ongoing readers to finish. Since there can be only one such

     * waiter, we use a simple scheme using a single "waiter" field to

     * block writers.  However, readers need never block.  If the root

     * lock is held, they proceed along the slow traversal path (via

     * next-pointers) until the lock becomes available or the list is

     * exhausted, whichever comes first. These cases are not fast, but

     * maximize aggregate expected throughput.

     *

     * Maintaining API and serialization compatibility with previous

     * versions of this class introduces several oddities. Mainly: We

     * leave untouched but unused constructor arguments referring 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.

     *

     * Also, solely for compatibility with previous versions of this

     * class, it extends AbstractMap, even though all of its methods

     * are overridden, so it is just useless baggage.

     *

     * This file is organized to make things a little easier to follow

     * while reading than they might otherwise: First the main static

     * declarations and utilities, then fields, then main public