JUC回顾之-ConcurrentHashMap源码解读及原理理解

ConcurrentHashMap结构图如下：

ConcurrentHashMap实现类图如下：

segment的结构图如下：

package concurrentMy.juc_collections.hashMap;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
/**
 *
 * (ConcurrentHashMap 原理理解)
 *
 * <p>
 * 修改历史:                                            <br>
 * 修改日期            修改人员       版本             修改内容<br>
 * -------------------------------------------------<br>
 * 2016年8月5日 下午3:37:13   user     1.0        初始化创建<br>
 * </p>
 *
 * @author        Peng.Li
 * @version        1.0
 * @since        JDK1.7
 *
 *
 *
 *
 *
 * 0.HashMap是非线程安全的哈希表，常用于单线程程序中。
 *
 * 1.HashTable容器在激烈的并发环境下面效率低的原因：强一致性
 *     （1）HashTable通过synchronized来保证线程安全的，当一个线程进行put到HashTable添加元素时，线程2不但不能put方法添加元素，也不能通过get获取元素。
 *      （2） 访问HashTable的线程都必须竞争同一把锁
 * 2.ConcurrentHashMap效率高的原因：弱一致性
 *     （1）容器中有多把锁，每一把锁锁住的是容器中的一部分数据，当多个线程访问容器中的不同的数据段的时候，由于获取的是不同的锁，
 *             所以不存在竞争的问题，从而提高并发访问效率。
 *  （2）采用锁分段技术：首先将数据分成一段一段的存储，然后给每一段数据配置一把锁，当一个线程占有锁访问其中一个段数据的时候，其他段的数据也能被其他线程访问。
 *  （3）多线程对于同一个段数据的访问，是互斥的；但是对于不同片段的访问，却是可以同步进行的。
 *  3.结构原理
 *    （1）ConcurrentHashMap是由Segment数组接口和HashEntry数组结构组成。
 *            Segment是一种可重入锁ReentrantLock，在ConcurrentHashMap扮演锁的角色；
 *            HashEntry用于存储键值对数据。
 *    (2) Segment的结构和HashMap类似，是一组数组和链表结构。一个Segment包含一个HashEntry数组，每个HashEntry是一个链表结构的元素，
 *            每个Segment守护者一个HashEntry数组里面的元素，当对HashEntry的数组进行修改的时候，首先需要获得HashEntry数组对应的数据段的Segment锁。
 *
 *    (3) 读不加锁：定义成volatile的变量，能够在线程之间保持可见性，能够被多线程同时读，并且保证不会读到过期的值，但是只能被单线程写（有一种情况可以被多线程写，就是写入的值不依赖于原值），
 *              在get操作里只需要读写 transient volatile HashEntry<K,V>[] table;所以可以不用加锁。之所以不会读到过期的值，是根据java内存模型的happen before原则，
 *              对volatile字段的写入操作先于读操作，即使两个线程同时修改和获取volatile变量，get操作也能拿到最新的值，这是用volatile替换锁的经典应用场景。
 *
 *
       (4) 从图中可以看到，ConcurrentHashMap内部分为很多个Segment，每一个Segment拥有一把锁，然后每个Segment（继承ReentrantLock）下面包含很多个HashEntry列表数组。对于一个key，
           需要经过三次（为什么要hash三次下文会详细讲解）hash操作，才能最终定位这个元素的位置，这三次hash分别为：
           对于一个key，先进行一次hash操作，得到hash值h1，也即h1 = hash1(key)；
           将得到的h1的高几位进行第二次hash，得到hash值h2，也即h2 = hash2(h1高几位)，通过h2能够确定该元素的放在哪个Segment；
           将得到的h1进行第三次hash，得到hash值h3，也即h3 = hash3(h1)，通过h3能够确定该元素放置在哪个HashEntry。

 *
 * 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/
 */
import java.util.concurrent.locks.ReentrantLock;

/**
 * A hash table supporting full concurrency of retrievals and
 * adjustable 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
 * <tt>Hashtable</tt>. However, even though all operations are
 * thread-safe, retrieval operations do <em>not</em> entail locking,
 * and there is <em>not</em> any support for locking the entire table
 * in a way that prevents all access.  This class is fully
 * interoperable with <tt>Hashtable</tt> in programs that rely on its
 * thread safety but not on its synchronization details.
 *
 * <p> Retrieval operations (including <tt>get</tt>) generally do not
 * block, so may overlap with update operations (including
 * <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results
 * of the most recently <em>completed</em> update operations holding
 * upon their onset.  For aggregate operations such as <tt>putAll</tt>
 * and <tt>clear</tt>, concurrent retrievals may reflect insertion or
 * removal of only some entries.  Similarly, Iterators and
 * Enumerations return elements reflecting the state of the hash table
 * at some point at or since the creation of the iterator/enumeration.
 * They do <em>not</em> throw {@link ConcurrentModificationException}.
 * However, iterators are designed to be used by only one thread at a time.
 *
 * <p> The allowed concurrency among update operations is guided by
 * the optional <tt>concurrencyLevel</tt> constructor argument
 * (default <tt>16</tt>), which is used as a hint for internal sizing.  The
 * table is internally partitioned to try to permit the indicated
 * number of concurrent updates without contention. Because placement
 * in hash tables is essentially random, the actual concurrency will
 * vary.  Ideally, you should choose a value to accommodate as many
 * threads as will ever concurrently modify the table. Using a
 * significantly higher value than you need can waste space and time,
 * and a significantly lower value can lead to thread contention. But
 * overestimates and underestimates within an order of magnitude do
 * not usually have much noticeable impact. A value of one is
 * appropriate when it is known that only one thread will modify and
 * all others will only read. Also, resizing this or any other kind of
 * hash table is a relatively slow operation, so, when possible, it is
 * a good idea to provide estimates of expected table sizes in
 * constructors.
 *
 * <p>This class and its views and iterators implement all of the
 * <em>optional</em> methods of the {@link Map} and {@link Iterator}
 * interfaces.
 *
 * <p> Like {@link Hashtable} but unlike {@link HashMap}, this class
 * does <em>not</em> allow <tt>null</tt> to be used as a key or value.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @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 ConcurrentHashMapSourceCode<K, V> extends AbstractMap<K, V>
        implements ConcurrentMap<K, V>, Serializable {
    private static final long serialVersionUID = 7249069246763182397L;

    /*
     * The basic strategy is to subdivide the table among Segments,
     * each of which itself is a concurrently readable hash table.  To
     * reduce footprint, all but one segments are constructed only
     * when first needed (see ensureSegment). To maintain visibility
     * in the presence of lazy construction, accesses to segments as
     * well as elements of segment's table must use volatile access,
     * which is done via Unsafe within methods segmentAt etc
     * below. These provide the functionality of AtomicReferenceArrays
     * but reduce the levels of indirection. Additionally,
     * volatile-writes of table elements and entry "next" fields
     * within locked operations use the cheaper "lazySet" forms of
     * writes (via putOrderedObject) because these writes are always
     * followed by lock releases that maintain sequential consistency
     * of table updates.
     *
     * Historical note: The previous version of this class relied
     * heavily on "final" fields, which avoided some volatile reads at
     * the expense of a large initial footprint.  Some remnants of
     * that design (including forced construction of segment 0) exist
     * to ensure serialization compatibility.
     */

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

    /**
     * The default initial capacity for this table,
     * used when not otherwise specified in a constructor.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**
     * The default load factor for this table, used when not
     * otherwise specified in a constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The default concurrency level for this table, used when not
     * otherwise specified in a constructor.
     */
    static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    /**
     * The maximum capacity, used if a higher value is implicitly
     * specified by either of the constructors with arguments.  MUST
     * be a power of two <= 1<<30 to ensure that entries are indexable
     * using ints.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The minimum capacity for per-segment tables.  Must be a power
     * of two, at least two to avoid immediate resizing on next use
     * after lazy construction.
     */
    static final int MIN_SEGMENT_TABLE_CAPACITY = 2;

    /**
     * The maximum number of segments to allow; used to bound
     * constructor arguments. Must be power of two less than 1 << 24.
     */
    static final int MAX_SEGMENTS = 1 << 16; // slightly conservative

    /**
     * Number of unsynchronized retries in size and containsValue
     * methods before resorting to locking. This is used to avoid
     * unbounded（不受限制的） retries if tables undergo （经受）continuous（连续的） modification
     * which would make it impossible to obtain an accurate result.
     */
    static final int RETRIES_BEFORE_LOCK = 2;

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

    /**
     * holds values which can't be initialized until after VM is booted.
     */
    private static class Holder {

        /**
        * Enable alternative hashing of String keys?
        *
        * <p>Unlike the other hash map implementations we do not implement a
        * threshold for regulating whether alternative hashing is used for
        * String keys. Alternative hashing is either enabled for all instances
        * or disabled for all instances.
        */
        static final boolean ALTERNATIVE_HASHING;

        static {
            // Use the "threshold" system property even though our threshold
            // behaviour is "ON" or "OFF".
            String altThreshold = java.security.AccessController.doPrivileged(
                new sun.security.action.GetPropertyAction(
                    "jdk.map.althashing.threshold"));

            int threshold;
            try {
                threshold = (null != altThreshold)
                        ? Integer.parseInt(altThreshold)
                        : Integer.MAX_VALUE;

                // disable alternative hashing if -1
                if (threshold == -1) {
                    threshold = Integer.MAX_VALUE;
                }

                if (threshold < 0) {
                    throw new IllegalArgumentException("value must be positive integer.");
                }
            } catch(IllegalArgumentException failed) {
                throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
            }
            ALTERNATIVE_HASHING = threshold <= MAXIMUM_CAPACITY;
        }
    }

    /**
     * A randomizing value associated with this instance that is applied to
     * hash code of keys to make hash collisions harder to find.
     */
    private transient final int hashSeed = randomHashSeed(this);

    private static int randomHashSeed(ConcurrentHashMapSourceCode instance) {
        if (sun.misc.VM.isBooted() && Holder.ALTERNATIVE_HASHING) {
            return sun.misc.Hashing.randomHashSeed(instance);
        }

        return 0;
    }

    /**
     * Mask value for indexing into segments. The upper bits of a
     * key's hash code are used to choose the segment.
     */
    final int segmentMask;

    /**
     * Shift value for indexing within segments.
     */
    final int segmentShift;

    /**
     * The segments, each of which is a specialized hash table.
     */
    final Segment<K,V>[] segments;

    transient Set<K> keySet;
    transient Set<Map.Entry<K,V>> entrySet;
    transient Collection<V> values;

    /**
     * ConcurrentHashMap list entry. Note that this is never exported
     * out as a user-visible Map.Entry.
     */
    static final class HashEntry<K,V> {
        final int hash;
        final K key;
        volatile V value;
        volatile HashEntry<K,V> next;

        HashEntry(int hash, K key, V value, HashEntry<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

        /**
         * Sets next field with volatile write semantics.  (See above
         * about use of putOrderedObject.)
         */
        final void setNext(HashEntry<K,V> n) {
            UNSAFE.putOrderedObject(this, nextOffset, n);
        }

        // Unsafe mechanics
        static final sun.misc.Unsafe UNSAFE;
        static final long nextOffset;
        static {
            try {
                UNSAFE = sun.misc.Unsafe.getUnsafe();
                Class k = HashEntry.class;
                nextOffset = UNSAFE.objectFieldOffset
                    (k.getDeclaredField("next"));
            } catch (Exception e) {
                throw new Error(e);
            }
        }
    }

    /**
     * Gets the ith element of given table (if nonnull) with volatile
     * read semantics. Note: This is manually integrated into a few
     * performance-sensitive methods to reduce call overhead.
     */
    @SuppressWarnings("unchecked")
    static final <K,V> HashEntry<K,V> entryAt(HashEntry<K,V>[] tab, int i) {
        return (tab == null) ? null :
            (HashEntry<K,V>) UNSAFE.getObjectVolatile
            (tab, ((long)i << TSHIFT) + TBASE);
    }

    /**
     * Sets the ith element of given table, with volatile write
     * semantics. (See above about use of putOrderedObject.)
     */
    static final <K,V> void setEntryAt(HashEntry<K,V>[] tab, int i,
                                       HashEntry<K,V> e) {
        UNSAFE.putOrderedObject(tab, ((long)i << TSHIFT) + TBASE, e);
    }

    /**
     * Applies a supplemental(补充的，追加的) hash function to a given hashCode, which
     * defends against (对抗) poor quality （质量差的） hash functions.  This is critical(爱挑剔的)
     * because ConcurrentHashMap uses power-of-two length hash tables,
     * that otherwise encounter（遭遇） collisions（hash碰撞） for hashCodes that do not
     * differ in lower or upper bits.
     *
     * 用到了Wang/Jenkins算法变种，主要的目的为了减少hash冲突，使元素能够均匀的分布到不同的Segment上，从而提高容器的存取的效率。
     * 假如哈希的质量差到极点，所有的元素都在同一个Segment中，不仅存取缓慢，分段锁也会失去意义。
     *
     *      System.out.println(Integer.parseInt("0001111", 2) & 15);
         System.out.println(Integer.parseInt("0011111", 2) & 15);
         System.out.println(Integer.parseInt("0111111", 2) & 15);
         System.out.println(Integer.parseInt("1111111", 2) & 15);

         上面的结果全都是15，通过这个例子发现如果不进行再hash，hash冲突非常严重，因为只要低位一样，无论高位是什么，与15做&操作都为15。
         发生冲突的几率还是很大的，但是如果我们先把上例中的二进制数字使用hash()函数先进行一次预hash，得到的结果是这样的：

         0100｜0111｜0110｜0111｜1101｜1010｜0100｜1110
         1111｜0111｜0100｜0011｜0000｜0001｜1011｜1000
         0111｜0111｜0110｜1001｜0100｜0110｜0011｜1110
         1000｜0011｜0000｜0000｜1100｜1000｜0001｜1010

         可以看到每一位的数据都散开了，并且ConcurrentHashMap中是使用预hash值的高位参与运算的。比如之前说的先将hash值向右按位移动28位，
         再与15做&运算，得到的结果都别为：4，15，7，8，没有冲突！

     */
    public int hash(Object k) {
        int h = hashSeed;

        if ((0 != h) && (k instanceof String)) {
            return sun.misc.Hashing.stringHash32((String) k);
        }

        h ^= k.hashCode();

        // Spread bits to regularize(调整；使合法化) both segment and index locations（位置，地点）,
        // using variant(不同的) of single-word Wang/Jenkins hash.
        h += (h <<  15) ^ 0xffffcd7d;
        h ^= (h >>> 10);
        h += (h <<   3);
        h ^= (h >>>  6);
        h += (h <<   2) + (h << 14);
        return h ^ (h >>> 16);
    }

    /**
     * Segments are specialized versions of hash tables.  This
     * subclasses from ReentrantLock opportunistically, just to
     * simplify some locking and avoid separate construction.
     */
    static final class Segment<K,V> extends ReentrantLock implements Serializable {
        /*
         * Segments maintain a table of entry lists that are always
         * kept in a consistent state, so can be read (via volatile
         * reads of segments and tables) without locking.  This
         * requires replicating nodes when necessary during table
         * resizing, so the old lists can be traversed by readers
         * still using old version of table.
         *
         * This class defines only mutative methods requiring locking.
         * Except as noted, the methods of this class perform the
         * per-segment versions of ConcurrentHashMap methods.  (Other
         * methods are integrated directly into ConcurrentHashMap
         * methods.) These mutative methods use a form of controlled
         * spinning on contention via methods scanAndLock and
         * scanAndLockForPut. These intersperse tryLocks with
         * traversals to locate nodes.  The main benefit is to absorb
         * cache misses (which are very common for hash tables) while
         * obtaining locks so that traversal is faster once
         * acquired. We do not actually use the found nodes since they
         * must be re-acquired under lock anyway to ensure sequential
         * consistency of updates (and in any case may be undetectably
         * stale), but they will normally be much faster to re-locate.
         * Also, scanAndLockForPut speculatively creates a fresh node
         * to use in put if no node is found.
         */

        private static final long serialVersionUID = 2249069246763182397L;

        /**
         * The maximum number of times to tryLock in a prescan before
         * possibly blocking on acquire in preparation for a locked
         * segment operation. On multiprocessors, using a bounded
         * number of retries maintains cache acquired while locating
         * nodes.
         */
        static final int MAX_SCAN_RETRIES =
            Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;

        /**
         * The per-segment table. Elements are accessed via
         * entryAt/setEntryAt providing volatile semantics.
         */
        transient volatile HashEntry<K,V>[] table;

        /**
         * The number of elements. Accessed only either within locks
         * or among other volatile reads that maintain visibility.
         */
        transient int count;

        /**
         * The total number of mutative operations in this segment.
         * Even though this may overflows 32 bits, it provides
         * sufficient accuracy for stability checks in CHM isEmpty()
         * and size() methods.  Accessed only either within locks or
         * among other volatile reads that maintain visibility.
         */
        transient int modCount;

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

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

        Segment(float lf, int threshold, HashEntry<K,V>[] tab) {
            this.loadFactor = lf;
            this.threshold = threshold;
            this.table = tab;
        }

        final V put(K key, int hash, V value, boolean onlyIfAbsent) {
            HashEntry<K,V> node = tryLock() ? null :
                scanAndLockForPut(key, hash, value);
            V oldValue;
            try {
                HashEntry<K,V>[] tab = table;
                int index = (tab.length - 1) & hash;
                HashEntry<K,V> first = entryAt(tab, index);
                for (HashEntry<K,V> e = first;;) {
                    if (e != null) {
                        K k;
                        if ((k = e.key) == key ||
                            (e.hash == hash && key.equals(k))) {
                            oldValue = e.value;
                            if (!onlyIfAbsent) {
                                e.value = value;
                                ++modCount;
                            }
                            break;
                        }
                        e = e.next;
                    }
                    else {
                        if (node != null)
                            node.setNext(first);
                        else
                            node = new HashEntry<K,V>(hash, key, value, first);
                        int c = count + 1;
                        if (c > threshold && tab.length < MAXIMUM_CAPACITY)
                            rehash(node);
                        else
                            setEntryAt(tab, index, node);
                        ++modCount;
                        count = c;
                        oldValue = null;
                        break;
                    }
                }
            } finally {
                unlock();
            }
            return oldValue;
        }

        /**
         * Doubles size of table and repacks entries, also adding the
         * given node to new table
         */
        @SuppressWarnings("unchecked")
        private void rehash(HashEntry<K,V> node) {
            /*
             * Reclassify nodes in each list to new 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.
             * Statistically, at the default threshold, only about
             * one-sixth of them need cloning when a table
             * doubles. The nodes they replace will be garbage
             * collectable as soon as they are no longer referenced by
             * any reader thread that may be in the midst of
             * concurrently traversing table. Entry accesses use plain
             * array indexing because they are followed by volatile
             * table write.
             */
            HashEntry<K,V>[] oldTable = table;
            int oldCapacity = oldTable.length;
            int newCapacity = oldCapacity << 1;
            threshold = (int)(newCapacity * loadFactor);
            HashEntry<K,V>[] newTable =
                (HashEntry<K,V>[]) new HashEntry[newCapacity];
            int sizeMask = newCapacity - 1;
            for (int i = 0; i < oldCapacity ; i++) {
                HashEntry<K,V> e = oldTable[i];
                if (e != null) {
                    HashEntry<K,V> next = e.next;
                    int idx = e.hash & sizeMask;
                    if (next == null)   //  Single node on list
                        newTable[idx] = e;
                    else { // Reuse consecutive sequence at same slot
                        HashEntry<K,V> lastRun = e;
                        int lastIdx = idx;
                        for (HashEntry<K,V> last = next;
                             last != null;
                             last = last.next) {
                            int k = last.hash & sizeMask;
                            if (k != lastIdx) {
                                lastIdx = k;
                                lastRun = last;
                            }
                        }
                        newTable[lastIdx] = lastRun;
                        // Clone remaining nodes
                        for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {
                            V v = p.value;
                            int h = p.hash;
                            int k = h & sizeMask;
                            HashEntry<K,V> n = newTable[k];
                            newTable[k] = new HashEntry<K,V>(h, p.key, v, n);
                        }
                    }
                }
            }
            int nodeIndex = node.hash & sizeMask; // add the new node
            node.setNext(newTable[nodeIndex]);
            newTable[nodeIndex] = node;
            table = newTable;
        }

        /**
         * Scans for a node containing given key while trying to
         * acquire lock, creating and returning one if not found. Upon
         * return, guarantees that lock is held. UNlike in most
         * methods, calls to method equals are not screened: Since
         * traversal speed doesn't matter, we might as well help warm
         * up the associated code and accesses as well.
         *
         * @return a new node if key not found, else null
         */
        private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
            HashEntry<K,V> first = entryForHash(this, hash);
            HashEntry<K,V> e = first;
            HashEntry<K,V> node = null;
            int retries = -1; // negative while locating node
            while (!tryLock()) {
                HashEntry<K,V> f; // to recheck first below
                if (retries < 0) {
                    if (e == null) {
                        if (node == null) // speculatively create node
                            node = new HashEntry<K,V>(hash, key, value, null);
                        retries = 0;
                    }
                    else if (key.equals(e.key))
                        retries = 0;
                    else
                        e = e.next;
                }
                else if (++retries > MAX_SCAN_RETRIES) {
                    lock();
                    break;
                }
                else if ((retries & 1) == 0 &&
                         (f = entryForHash(this, hash)) != first) {
                    e = first = f; // re-traverse if entry changed
                    retries = -1;
                }
            }
            return node;
        }

        /**
         * Scans for a node containing the given key while trying to
         * acquire lock for a remove or replace operation. Upon
         * return, guarantees that lock is held.  Note that we must
         * lock even if the key is not found, to ensure sequential
         * consistency of updates.
         */
        private void scanAndLock(Object key, int hash) {
            // similar to but simpler than scanAndLockForPut
            HashEntry<K,V> first = entryForHash(this, hash);
            HashEntry<K,V> e = first;
            int retries = -1;
            while (!tryLock()) {
                HashEntry<K,V> f;
                if (retries < 0) {
                    if (e == null || key.equals(e.key))
                        retries = 0;
                    else
                        e = e.next;
                }
                else if (++retries > MAX_SCAN_RETRIES) {
                    lock();
                    break;
                }
                else if ((retries & 1) == 0 &&
                         (f = entryForHash(this, hash)) != first) {
                    e = first = f;
                    retries = -1;
                }
            }
        }

        /**
         * Remove; match on key only if value null, else match both.
         */
        final V remove(Object key, int hash, Object value) {
            if (!tryLock())
                scanAndLock(key, hash);
            V oldValue = null;
            try {
                HashEntry<K,V>[] tab = table;
                int index = (tab.length - 1) & hash;
                HashEntry<K,V> e = entryAt(tab, index);
                HashEntry<K,V> pred = null;
                while (e != null) {
                    K k;
                    HashEntry<K,V> next = e.next;
                    if ((k = e.key) == key ||
                        (e.hash == hash && key.equals(k))) {
                        V v = e.value;
                        if (value == null || value == v || value.equals(v)) {
                            if (pred == null)
                                setEntryAt(tab, index, next);
                            else
                                pred.setNext(next);
                            ++modCount;
                            --count;
                            oldValue = v;
                        }
                        break;
                    }
                    pred = e;
                    e = next;
                }
            } finally {
                unlock();
            }
            return oldValue;
        }

        final boolean replace(K key, int hash, V oldValue, V newValue) {
            if (!tryLock())
                scanAndLock(key, hash);
            boolean replaced = false;
            try {
                HashEntry<K,V> e;
                for (e = entryForHash(this, hash); e != null; e = e.next) {
                    K k;
                    if ((k = e.key) == key ||
                        (e.hash == hash && key.equals(k))) {
                        if (oldValue.equals(e.value)) {
                            e.value = newValue;
                            ++modCount;
                            replaced = true;
                        }
                        break;
                    }
                }
            } finally {
                unlock();
            }
            return replaced;
        }

        final V replace(K key, int hash, V value) {
            if (!tryLock())
                scanAndLock(key, hash);
            V oldValue = null;
            try {
                HashEntry<K,V> e;
                for (e = entryForHash(this, hash); e != null; e = e.next) {
                    K k;
                    if ((k = e.key) == key ||
                        (e.hash == hash && key.equals(k))) {
                        oldValue = e.value;
                        e.value = value;
                        ++modCount;
                        break;
                    }
                }
            } finally {
                unlock();
            }
            return oldValue;
        }

        final void clear() {
            lock();
            try {
                HashEntry<K,V>[] tab = table;
                for (int i = 0; i < tab.length ; i++)
                    setEntryAt(tab, i, null);
                ++modCount;
                count = 0;
            } finally {
                unlock();
            }
        }
    }

    // Accessing segments

    /**
     * Gets the jth element of given segment array (if nonnull) with
     * volatile element access semantics via Unsafe. (The null check
     * can trigger harmlessly only during deserialization.) Note:
     * because each element of segments array is set only once (using
     * fully ordered writes), some performance-sensitive methods rely
     * on this method only as a recheck upon null reads.
     */
    @SuppressWarnings("unchecked")
    static final <K,V> Segment<K,V> segmentAt(Segment<K,V>[] ss, int j) {
        long u = (j << SSHIFT) + SBASE;
        return ss == null ? null :
            (Segment<K,V>) UNSAFE.getObjectVolatile(ss, u);
    }

    /**
     * Returns the segment for the given index, creating it and
     * recording in segment table (via CAS) if not already present.
     *
     * @param k the index
     * @return the segment
     */
    @SuppressWarnings("unchecked")
    private Segment<K,V> ensureSegment(int k) {
        final Segment<K,V>[] ss = this.segments;
        long u = (k << SSHIFT) + SBASE; // raw offset
        Segment<K,V> seg;
        if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
            Segment<K,V> proto = ss[0]; // use segment 0 as prototype
            int cap = proto.table.length;
            float lf = proto.loadFactor;
            int threshold = (int)(cap * lf);
            HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
            if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
                == null) { // recheck
                Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
                while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
                       == null) {
                    if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
                        break;
                }
            }
        }
        return seg;
    }

    // Hash-based segment and entry accesses

    /**
     * Get the segment for the given hash
     */
    @SuppressWarnings("unchecked")
    private Segment<K,V> segmentForHash(int h) {
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        return (Segment<K,V>) UNSAFE.getObjectVolatile(segments, u);
    }

    /**
     * Gets the table entry for the given segment and hash
     */
    @SuppressWarnings("unchecked")
    static final <K,V> HashEntry<K,V> entryForHash(Segment<K,V> seg, int h) {
        HashEntry<K,V>[] tab;
        return (seg == null || (tab = seg.table) == null) ? null :
            (HashEntry<K,V>) UNSAFE.getObjectVolatile
            (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE);
    }

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

    /**
     * Creates a new, empty map with the specified initial
     * capacity, load factor and concurrency level.
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements.
     * @param loadFactor  the load factor threshold, used to control resizing.
     * Resizing may be performed when the average number of elements per
     * bin exceeds this threshold.
     * @param concurrencyLevel the estimated number of concurrently
     * updating threads. The implementation performs internal sizing
     * to try to accommodate this many threads.
     * @throws IllegalArgumentException if the initial capacity is
     * negative or the load factor or concurrencyLevel are
     * nonpositive.
     *
     * 实现原理：
     *     ConcurrentHashMap使用分段锁技术，将数据分成一段一段的存储，然后给每一段数据配一把锁，当一个线程占用锁访问其中一个段数据的时候，
     *      其他段的数据也能被其他线程访问，能够实现真正的并发访问。如下图是ConcurrentHashMap的内部结构图：
     *
     * 1.initialCapacity 表示新建的这个ConcurrentHashMap的初始容量，也就是上线结构图中的Entry数量。
     * 默认值为static final int DEFAULT_INITIAL_CAPACITY = 16;
     *
     * 2.loadFactor表示负载因子，就是当ConcurrentHashMap中的元素个数大于loadFactor * 最大容量时候就需要rehash和扩容。
     * 默认值为static final float DEFAULT_LOAD_FACTOR = 0.75f;
     *
     * 3.concurrencyLevel表示并发级别，这个值用来确定segment的个数，segment的个数大于等于concurrencyLevel的第一个2的n次方的数。
     * 比如，如果concurrencyLevel为12，13，14，15，16，则Segment的数目为16(2的4次方)。
     *
     * 4.理想情况下ConcurrentHashMap真正的访问量能够达到concurrencyLevel，因为有concurrencyLevel个Segment，
     * 假如有concurrencyLevel个线程要访问Map，并且需要访问的数据都恰好分别落在不同的segment中，则这些线程能够无竞
     * 争的自由访问（因为不需要竞争同一把锁）达到同时访问的效果。这也是这个concurrencyLevel参数为什么起名为“并发级别”的原因。
     *
     *
     */
    @SuppressWarnings("unchecked")
    public ConcurrentHashMapSourceCode(int initialCapacity,
                             float loadFactor, int concurrencyLevel) {
        //1.验证参数的合法性，如果不合法，直接抛出异常
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();

        //2.concurrencyLevel也就是Segment的个数不能超过最大Segment的个数，最大个数MAX_SEGMENTS默认值为 2 << 16,如果超过这个值，设置这个值。
        if (concurrencyLevel > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;

        // Find power-of-two sizes best matching arguments
        //比如concurrencyLevel=16默认值，则ssize也会等于16（2的4次方，sshift=4），如果concurrencyLevel=18，则ssize=32（也就是2的5次方，sshift=5），
        //3.这段代码的使用循环找到>=concurrencyLevel的第一个2的n次方的数ssize，这个数ssize就是Segment数组的大小;并记录一共向左按位移动的次数sshift。

        int sshift = 0;
        int ssize = 1;
        while (ssize < concurrencyLevel) {
            //sshift记录ssize向左移动的次数
            ++sshift;
            //ssize就是Segment数组的大小
            ssize <<= 1;
        }
        //segmentShift 默认的情况下为28
        this.segmentShift = 32 - sshift;
        //segmentMask 默认情况下为15，segmentMask的各个二进制位都为1，目的是之后可以通过key的hash值与这个值做&运算确定Segment的索引。
        this.segmentMask = ssize - 1;

        //4 检查给的容量值是否大于允许的最大容量，如果大于MAXIMUM_CAPACITY，就设置为该值。initialCapacity默认值也为16。static final int MAXIMUM_CAPACITY = 1 << 30;
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        //5 计算每个Segment平均应该放置多少元素，这个值c是向上取整的值。比如初始容量initialCapacity=15，Segment数组的大小为16，Segment的个数为4，则每个Segment平均需要放置4个元素。
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        while (cap < c)
            cap <<= 1;
        //6 创建一个Segment的实例，将其当做Segment数组的第一个元素。
        // create segments and segments[0]，cap * loadFactor = 1.5，cap=2
        Segment<K,V> s0 =
            new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
                             (HashEntry<K,V>[])new HashEntry[cap]);

        // ssize默认=16，表示Segment数组的大小
        Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
        UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
        this.segments = ss;
    }

    /**
     * Creates a new, empty map with the specified initial capacity
     * and load factor and with the default concurrencyLevel (16).
     *
     * @param initialCapacity The implementation performs internal
     * sizing to accommodate this many elements.
     * @param loadFactor  the load factor threshold, used to control resizing.
     * Resizing may be performed when the average number of elements per
     * bin exceeds this threshold.
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative or the load factor is nonpositive
     *
     * @since 1.6
     */
    public ConcurrentHashMapSourceCode(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with the specified initial capacity,
     * and with default load factor (0.75) and concurrencyLevel (16).
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements.
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative.
     */
    public ConcurrentHashMapSourceCode(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new, empty map with a default initial capacity (16),
     * load factor (0.75) and concurrencyLevel (16).
     */
    public ConcurrentHashMapSourceCode() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    /**
     * Creates a new map with the same mappings as the given map.
     * The map is created with a capacity of 1.5 times the number
     * of mappings in the given map or 16 (whichever is greater),
     * and a default load factor (0.75) and concurrencyLevel (16).
     *
     * @param m the map
     */
    public ConcurrentHashMapSourceCode(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                      DEFAULT_INITIAL_CAPACITY),
             DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
        putAll(m);
    }

    /**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     *
     * @return <tt>true</tt> if this map contains no key-value mappings
     */
    public boolean isEmpty() {
        /*
         * Sum per-segment modCounts to avoid mis-reporting when
         * elements are concurrently added and removed in one segment
         * while checking another, in which case the table was never
         * actually empty at any point. (The sum ensures accuracy up
         * through at least 1<<31 per-segment modifications before
         * recheck.)  Methods size() and containsValue() use similar
         * constructions for stability checks.
         */
        long sum = 0L;
        final Segment<K,V>[] segments = this.segments;
        for (int j = 0; j < segments.length; ++j) {
            Segment<K,V> seg = segmentAt(segments, j);
            if (seg != null) {
                if (seg.count != 0)
                    return false;
                sum += seg.modCount;
            }
        }
        if (sum != 0L) { // recheck unless no modifications
            for (int j = 0; j < segments.length; ++j) {
                Segment<K,V> seg = segmentAt(segments, j);
                if (seg != null) {
                    if (seg.count != 0)
                        return false;
                    sum -= seg.modCount;
                }
            }
            if (sum != 0L)
                return false;
        }
        return true;
    }

    /**
     * Returns the number of key-value mappings in this map.  If the
     * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * @return the number of key-value mappings in this map
     *
     *1. size 操作和put与get的区别在于，size操作需要遍历所有的segment才能算出整个map的大小，而put和get操作只需要关心一个segment；
     *2. 假设我们当前遍历的Segment为SA，那么在遍历SA过程中，其他的Segment比如SB可能会被修改，那么这一次计算出来的size值并不是Map的当前真正大小。
     *     所以一个比较简单的办法是就是计算Map大小的时候所有的segment都lock住，不能更新数据（put 和 remove，计算完之后unlock；
     *
     *3. 作者Doug Lea 想出一个更好的idea：先给3次机会（retries初始化为-1，一直重试到RETRIES_BEFORE_LOCK值为2 ，不锁定lock所有Segment；
     *   遍历所有的segment，累加各个segment的大小得到整个Map的大小。
     *
     *4.如果某相邻的2次计算获取的所有Segment的所有更新次数（每个Segment都有一个变量modCount变量，这个变量在Segment的Entry被修改的时候会加1
     * 通过这个值可以得到每个Segment的更新操作的次数）是一样的，说明在计算的过程中没有更新操作，直接结束循环，返回当前的size；
     *
     * 5. 如果重试3次计算的结果中，Map的更新次数和前一次不一致，则之后的计算先对所有的Segment加锁，遍历所有segment计算map的大小，最后当重试计算>3次后再解锁所有的
     * segment。
     *
     * 6.例子：
     *
     * 假如一个Map有4个segment，标记S1,S2,S3,S4,现在我们要获取Map的Size；
     * 计算过程是这样的：
     *                 第一次计算不对segment S1,S2,S3,S4加锁，遍历所有的segment，假设这次每个segment的大小变成了1，2，3，4；更新次数分别为2，2，3，1；则这次计算可以得到Map的总大小为1+2+3+4=10，总更新次数modCount=2+2+3+1=8；
     *                 第二次计算，不对S1,S2,S3,S4加锁，遍历所有的Segment，假设这次每个segment的大小变成了2，2，3，4;更新次数变为了3，2，3，1； 则Map的size=2+2+3+4=11；modCount=9
     *                 那么第一次和第二次计算得到的更新次数不一致，第一次是8，第二次是9；则可以判定这段时间Map的数据被更新；因此必须进行第3次重试计算；
     *                 第三次计算，不对S1,S2,S3,S4加锁，遍历所有的Segment，假设每个Segment的更新次数还是为3，2，3，1；则因为第2次计算和第3次计算的得到的Map的modCount次数是一致的，则说明这段时间内第2次和第3次这段时间内Map的数据没有被更新
     *                 此时可以返回第3次计算的Map大小；最坏的情况：第3次计算得到的计算结果和第2次不一致，则只能先对所有的Segment加锁再计算，最后解锁。
     */
    public int size() {
        // Try a few times to get accurate count. On failure due to
        // continuous async changes in table, resort to locking.
        final Segment<K,V>[] segments = this.segments;
        int size;
        boolean overflow; // true if size overflows 32 bits
        long sum;         // sum of modCounts
        long last = 0L;   // previous sum
        int retries = -1; // first iteration isn't retry
        try {
            for (;;) {
                // 如果重试次数为3次，锁定segment
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    for (int j = 0; j < segments.length; ++j)
                        ensureSegment(j).lock(); // force creation
                }

                sum = 0L;
                size = 0;
                overflow = false;
                for (int j = 0; j < segments.length; ++j) {
                    //遍历所有的Segment
                    Segment<K,V> seg = segmentAt(segments, j);
                    if (seg != null) {
                        //累加修改的次数
                        sum += seg.modCount;
                        //c代表segment的
                        int c = seg.count;
                        if (c < 0 || (size += c) < 0)
                            overflow = true;
                    }
                }
                //如果和前一次计算的Map的size一致，结束循环，返回最终的size值
                if (sum == last)
                    break;
                last = sum;
            }
        } finally {
            // 如果重试次数>3次则，释放segment锁
            if (retries > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j < segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return overflow ? Integer.MAX_VALUE : size;
    }

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code key.equals(k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * @throws NullPointerException if the specified key is null
     */
    public V get(Object key) {
        Segment<K,V> s; // manually integrate access methods to reduce overhead
        HashEntry<K,V>[] tab;
        //1 和put操作一样，先通过key进行两次hash确定取哪个segment中的数据
        int h = hash(key);
        //2 使用UNSAFE方法获取对应的Segment，然后再进行一次&运算得到HashEntry链表的位置，然后从链表头开始遍历整个链表。
        //（由于hash会碰撞，所以用一个链表保存），如果找到对应的key，则返回对应的value值，如果链表遍历完都没有找到对应的key，
        // 则说明map中不包含该key，返回null
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null &&
            (tab = s.table) != null) {
            for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile
                     (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE);
                 e != null; e = e.next) {
                K k;
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    return e.value;
            }
        }
        return null;
    }

    /**
     * Tests if the specified object is a key in this table.
     *
     * @param  key   possible key
     * @return <tt>true</tt> if and only if the specified object
     *         is a key in this table, as determined by the
     *         <tt>equals</tt> method; <tt>false</tt> otherwise.
     * @throws NullPointerException if the specified key is null
     */
    @SuppressWarnings("unchecked")
    public boolean containsKey(Object key) {
        Segment<K,V> s; // same as get() except no need for volatile value read
        HashEntry<K,V>[] tab;
        int h = hash(key);
        long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
        if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null &&
            (tab = s.table) != null) {
            for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile
                     (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE);
                 e != null; e = e.next) {
                K k;
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    return true;
            }
        }
        return false;
    }

    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the
     * specified value. Note: This method requires a full internal
     * traversal of the hash table, and so is much slower than
     * method <tt>containsKey</tt>.
     *
     * @param value value whose presence in this map is to be tested
     * @return <tt>true</tt> if this map maps one or more keys to the
     *         specified value
     * @throws NullPointerException if the specified value is null
     */
    public boolean containsValue(Object value) {
        // Same idea as size()
        if (value == null)
            throw new NullPointerException();
        final Segment<K,V>[] segments = this.segments;
        boolean found = false;
        long last = 0;
        int retries = -1;
        try {
            outer: for (;;) {

                //重试3次，计算size后才给所有segment加锁，计算Map的size
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    for (int j = 0; j < segments.length; ++j)
                        ensureSegment(j).lock(); // force creation
                }
                long hashSum = 0L;
                int sum = 0;
                for (int j = 0; j < segments.length; ++j) {
                    HashEntry<K,V>[] tab;
                    //遍历所有的Segment
                    Segment<K,V> seg = segmentAt(segments, j);
                    if (seg != null && (tab = seg.table) != null) {
                        //遍历每个Segment里面的HashEntry
                        for (int i = 0 ; i < tab.length; i++) {
                            HashEntry<K,V> e;
                            for (e = entryAt(tab, i); e != null; e = e.next) {
                                //获取value值，并且与入参value进行比较
                                V v = e.value;
                                //相同返回，found=true，退出循环
                                if (v != null && value.equals(v)) {
                                    found = true;
                                    break outer;
                                }
                            }
                        }
                        //累加各个segment的更新次数
                        sum += seg.modCount;
                    }
                }
                //前一次计算的更新次数modCount和当前计算的segment的更新次数进行比较，相同，退出循环，返回found = true
                if (retries > 0 && sum == last)
                    break;
                last = sum;
            }
        } finally {
            //重试计算次数>3次后，释放segment锁
            if (retries > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j < segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return found;
    }

    /**
     * Legacy method testing if some key maps into the specified value
     * in this table.  This method is identical in functionality to
     * {@link #containsValue}, and exists solely to ensure
     * full compatibility with class {@link java.util.Hashtable},
     * which supported this method prior to introduction of the
     * Java Collections framework.

     * @param  value a value to search for
     * @return <tt>true</tt> if and only if some key maps to the
     *         <tt>value</tt> argument in this table as
     *         determined by the <tt>equals</tt> method;
     *         <tt>false</tt> otherwise
     * @throws NullPointerException if the specified value is null
     */
    public boolean contains(Object value) {
        return containsValue(value);
    }

    /**
     * Maps the specified key to the specified value in this table.
     * Neither the key nor the value can be null.
     *
     * <p> The value can be retrieved by calling the <tt>get</tt> method
     * with a key that is equal to the original key.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key or value is null
     */
    @SuppressWarnings("unchecked")
    public V put(K key, V value) {
        Segment<K,V> s;
        //1.value值不能为空
        if (value == null)
            throw new NullPointerException();
        //2.key通过一次hash运算得到一个hash值。(这个hash运算下文详说)
        int hash = hash(key);
        //3.将得到的hash值向右按位移动segmentShift位，然后再与segmentMask做&运算得到Segment的索引
        //在初始化的时候，segmentShift的值等于32-sshift，例如concurrencyLevel等于16，则sshift等于4，那么segmentShift为28。
        //hash值是一个32位的整数，将其向右移动28就变成这个样子：0000 0000 0000 0000 0000 0000 0000 XXXX,然后再用这个值与segmentMask
        //做&运算，也就是说取最后四位的值。这个值确定Segment的索引。
        int j = (hash >>> segmentShift) & segmentMask;
        //4.使用UNSAFE的方式从Segment数组中获取该索引对应的Segment对象
        if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            s = ensureSegment(j);
        //5.向这个Segment对象中put值，这个put操作也是一样的步骤
        return s.put(key, hash, value, false);
    }

    /**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     *         or <tt>null</tt> if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    @SuppressWarnings("unchecked")
    public V putIfAbsent(K key, V value) {
        Segment<K,V> s;
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key);
        int j = (hash >>> segmentShift) & segmentMask;
        if ((s = (Segment<K,V>)UNSAFE.getObject
             (segments, (j << SSHIFT) + SBASE)) == null)
            s = ensureSegment(j);
        return s.put(key, hash, value, true);
    }

    /**
     * Copies all of the mappings from the specified map to this one.
     * These mappings replace any mappings that this map had for any of the
     * keys currently in the specified map.
     *
     * @param m mappings to be stored in this map
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

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

    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if the specified key is null
     */
    public boolean remove(Object key, Object value) {
        int hash = hash(key);
        Segment<K,V> s;
        return value != null && (s = segmentForHash(hash)) != null &&
            s.remove(key, hash, value) != null;
    }

    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if any of the arguments are null
     */
    public boolean replace(K key, V oldValue, V newValue) {
        int hash = hash(key);
        if (oldValue == null || newValue == null)
            throw new NullPointerException();
        Segment<K,V> s = segmentForHash(hash);
        return s != null && s.replace(key, hash, oldValue, newValue);
    }

    /**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     *         or <tt>null</tt> if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    public V replace(K key, V value) {
        int hash = hash(key);
        if (value == null)
            throw new NullPointerException();
        Segment<K,V> s = segmentForHash(hash);
        return s == null ? null : s.replace(key, hash, value);
    }

    /**
     * Removes all of the mappings from this map.
     */
    public void clear() {
        final Segment<K,V>[] segments = this.segments;
        for (int j = 0; j < segments.length; ++j) {
            Segment<K,V> s = segmentAt(segments, j);
            if (s != null)
                s.clear();
        }
    }

    /**
     * Returns a {@link Set} view of the keys contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  The set supports element
     * removal, which removes the corresponding mapping from this map,
     * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
     * operations.  It does not support the <tt>add</tt> or
     * <tt>addAll</tt> operations.
     *
     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

    /**
     * Returns a {@link Collection} view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are
     * reflected in the collection, and vice-versa.  The collection
     * supports element removal, which removes the corresponding
     * mapping from this map, via the <tt>Iterator.remove</tt>,
     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
     * <tt>retainAll</tt>, and <tt>clear</tt> operations.  It does not
     * support the <tt>add</tt> or <tt>addAll</tt> operations.
     *
     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

    /**
     * Returns a {@link Set} view of the mappings contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  The set supports element
     * removal, which removes the corresponding mapping from the map,
     * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
     * operations.  It does not support the <tt>add</tt> or
     * <tt>addAll</tt> operations.
     *
     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Set<Map.Entry<K,V>> entrySet() {
        Set<Map.Entry<K,V>> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

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

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

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

    abstract class HashIterator {
        int nextSegmentIndex;
        int nextTableIndex;
        HashEntry<K,V>[] currentTable;
        HashEntry<K, V> nextEntry;
        HashEntry<K, V> lastReturned;

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

        /**
         * Set nextEntry to first node of next non-empty table
         * (in backwards order, to simplify checks).
         */
        final void advance() {
            for (;;) {
                if (nextTableIndex >= 0) {
                    if ((nextEntry = entryAt(currentTable,
                                             nextTableIndex--)) != null)
                        break;
                }
                else if (nextSegmentIndex >= 0) {
                    Segment<K,V> seg = segmentAt(segments, nextSegmentIndex--);
                    if (seg != null && (currentTable = seg.table) != null)
                        nextTableIndex = currentTable.length - 1;
                }
                else
                    break;
            }
        }

        final HashEntry<K,V> nextEntry() {
            HashEntry<K,V> e = nextEntry;
            if (e == null)
                throw new NoSuchElementException();
            lastReturned = e; // cannot assign until after null check
            if ((nextEntry = e.next) == null)
                advance();
            return e;
        }

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

        public final void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            ConcurrentHashMapSourceCode.this.remove(lastReturned.key);
            lastReturned = null;
        }
    }

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

    final class ValueIterator
        extends HashIterator
        implements Iterator<V>, Enumeration<V>
    {
        public final V next()        { return super.nextEntry().value; }
        public final V nextElement() { return super.nextEntry().value; }
    }

    /**
     * Custom Entry class used by EntryIterator.next(), that relays
     * setValue changes to the underlying map.
     */
    final class WriteThroughEntry
        extends AbstractMap.SimpleEntry<K,V>
    {
        WriteThroughEntry(K k, V v) {
            super(k,v);
        }

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

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

    final class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return new KeyIterator();
        }
        public int size() {
            return ConcurrentHashMapSourceCode.this.size();
        }
        public boolean isEmpty() {
            return ConcurrentHashMapSourceCode.this.isEmpty();
        }
        public boolean contains(Object o) {
            return ConcurrentHashMapSourceCode.this.containsKey(o);
        }
        public boolean remove(Object o) {
            return ConcurrentHashMapSourceCode.this.remove(o) != null;
        }
        public void clear() {
            ConcurrentHashMapSourceCode.this.clear();
        }
    }

    final class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator();
        }
        public int size() {
            return ConcurrentHashMapSourceCode.this.size();
        }
        public boolean isEmpty() {
            return ConcurrentHashMapSourceCode.this.isEmpty();
        }
        public boolean contains(Object o) {
            return ConcurrentHashMapSourceCode.this.containsValue(o);
        }
        public void clear() {
            ConcurrentHashMapSourceCode.this.clear();
        }
    }

    final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        public Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator();
        }
        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
            V v = ConcurrentHashMapSourceCode.this.get(e.getKey());
            return v != null && v.equals(e.getValue());
        }
        public boolean remove(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
            return ConcurrentHashMapSourceCode.this.remove(e.getKey(), e.getValue());
        }
        public int size() {
            return ConcurrentHashMapSourceCode.this.size();
        }
        public boolean isEmpty() {
            return ConcurrentHashMapSourceCode.this.isEmpty();
        }
        public void clear() {
            ConcurrentHashMapSourceCode.this.clear();
        }
    }

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

    /**
     * Save the state of the <tt>ConcurrentHashMap</tt> instance to a
     * stream (i.e., serialize it).
     * @param s the stream
     * @serialData
     * the key (Object) and value (Object)
     * for each key-value mapping, followed by a null pair.
     * The key-value mappings are emitted in no particular order.
     */
    private void writeObject(java.io.ObjectOutputStream s) throws IOException {
        // force all segments for serialization compatibility
        for (int k = 0; k < segments.length; ++k)
            ensureSegment(k);
        s.defaultWriteObject();

        final Segment<K,V>[] segments = this.segments;
        for (int k = 0; k < segments.length; ++k) {
            Segment<K,V> seg = segmentAt(segments, k);
            seg.lock();
            try {
                HashEntry<K,V>[] tab = seg.table;
                for (int i = 0; i < tab.length; ++i) {
                    HashEntry<K,V> e;
                    for (e = entryAt(tab, i); e != null; e = e.next) {
                        s.writeObject(e.key);
                        s.writeObject(e.value);
                    }
                }
            } finally {
                seg.unlock();
            }
        }
        s.writeObject(null);
        s.writeObject(null);
    }

    /**
     * Reconstitute the <tt>ConcurrentHashMap</tt> instance from a
     * stream (i.e., deserialize it).
     * @param s the stream
     */
    @SuppressWarnings("unchecked")
    private void readObject(java.io.ObjectInputStream s)
        throws IOException, ClassNotFoundException {
        // Don't call defaultReadObject()
        ObjectInputStream.GetField oisFields = s.readFields();
        final Segment<K,V>[] oisSegments = (Segment<K,V>[])oisFields.get("segments", null);

        final int ssize = oisSegments.length;
        if (ssize < 1 || ssize > MAX_SEGMENTS
            || (ssize & (ssize-1)) != 0 )  // ssize not power of two
            throw new java.io.InvalidObjectException("Bad number of segments:"
                                                     + ssize);
        int sshift = 0, ssizeTmp = ssize;
        while (ssizeTmp > 1) {
            ++sshift;
            ssizeTmp >>>= 1;
        }
        UNSAFE.putIntVolatile(this, SEGSHIFT_OFFSET, 32 - sshift);
        UNSAFE.putIntVolatile(this, SEGMASK_OFFSET, ssize - 1);
        UNSAFE.putObjectVolatile(this, SEGMENTS_OFFSET, oisSegments);

        // set hashMask
        UNSAFE.putIntVolatile(this, HASHSEED_OFFSET, randomHashSeed(this));

        // Re-initialize segments to be minimally sized, and let grow.
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        final Segment<K,V>[] segments = this.segments;
        for (int k = 0; k < segments.length; ++k) {
            Segment<K,V> seg = segments[k];
            if (seg != null) {
                seg.threshold = (int)(cap * seg.loadFactor);
                seg.table = (HashEntry<K,V>[]) new HashEntry[cap];
            }
        }

        // Read the keys and values, and put the mappings in the table
        for (;;) {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            if (key == null)
                break;
            put(key, value);
        }
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long SBASE;
    private static final int SSHIFT;
    private static final long TBASE;
    private static final int TSHIFT;
    private static final long HASHSEED_OFFSET;
    private static final long SEGSHIFT_OFFSET;
    private static final long SEGMASK_OFFSET;
    private static final long SEGMENTS_OFFSET;

    static {
        int ss, ts;
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class tc = HashEntry[].class;
            Class sc = Segment[].class;
            TBASE = UNSAFE.arrayBaseOffset(tc);
            SBASE = UNSAFE.arrayBaseOffset(sc);
            ts = UNSAFE.arrayIndexScale(tc);
            ss = UNSAFE.arrayIndexScale(sc);
            HASHSEED_OFFSET = UNSAFE.objectFieldOffset(
                ConcurrentHashMap.class.getDeclaredField("hashSeed"));
            SEGSHIFT_OFFSET = UNSAFE.objectFieldOffset(
                ConcurrentHashMap.class.getDeclaredField("segmentShift"));
            SEGMASK_OFFSET = UNSAFE.objectFieldOffset(
                ConcurrentHashMap.class.getDeclaredField("segmentMask"));
            SEGMENTS_OFFSET = UNSAFE.objectFieldOffset(
                ConcurrentHashMap.class.getDeclaredField("segments"));
        } catch (Exception e) {
            throw new Error(e);
        }
        if ((ss & (ss-1)) != 0 || (ts & (ts-1)) != 0)
            throw new Error("data type scale not a power of two");
        SSHIFT = 31 - Integer.numberOfLeadingZeros(ss);
        TSHIFT = 31 - Integer.numberOfLeadingZeros(ts);
    }

}

参考文章：

1.https://www.ibm.com/developerworks/cn/java/java-lo-concurrenthashmap/

2.http://www.infoq.com/cn/articles/ConcurrentHashMap

3.http://qifuguang.me/

4.阿里牛人：https://blog.csdn.net/justloveyou_/article/details/72783008

5.掘金面试总结：https://juejin.im/post/5ba591386fb9a05cd31eb85f