ConcurrentHashMap源码剖析

先贴上源码

使用的JDK版本：1.7.0_13

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 * 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.ConcurrentMap;
import java.util.concurrent.locks.*;
import java.util.*;
import java.io.Serializable;
import java.io.IOException;

public class ConcurrentHashMap<K, V> extends AbstractMap<K, V>
        implements ConcurrentMap<K, V>, Serializable {
    private static final long serialVersionUID = 7249069246763182397L;

    //默认初始容量
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    //默认扩容因子
    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;

    //最大容量
    static final int MAXIMUM_CAPACITY = 1 << 30;

    //最小分分片数
    static final int MIN_SEGMENT_TABLE_CAPACITY = 2;

    //最大的片数
    static final int MAX_SEGMENTS = 1 << 16; // slightly conservative

    /**
     * Number of unsynchronized retries in size and containsValue
     * methods before resorting to locking. This is used to avoid
     * unbounded retries if tables undergo continuous modification
     * which would make it impossible to obtain an accurate result.
     */
    /**
     * 在计算size和containsValue时，重试次数，如果这几次算出来的值发生了变化，
     * 则加锁重算
     */
    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(ConcurrentHashMap 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.
     */
    //Map中所有的片，每一个片相当于一个Hashtable，实现单独锁
    final Segment<K,V>[] segments;

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

    /**
     * 对于volatile修饰的变量，jvm虚拟机只是保证从主内存加载到线程工作内存的值是最新的
     *key和hash是final的
     */
    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 hash function to a given hashCode, which
     * defends against poor quality hash functions.  This is critical
     * because ConcurrentHashMap uses power-of-two length hash tables,
     * that otherwise encounter collisions for hashCodes that do not
     * differ in lower or upper bits.
     */
    private int hash(Object k) {
        int h = hashSeed;

        if ((0 != h) && (k instanceof String)) {
            retur
4000
n sun.misc.Hashing.stringHash32((String) k);
        }

        h ^= k.hashCode();

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

    /**
     * Segments are specialized versions of hash tables.  This
     * subclasses from ReentrantLock opportunistically, just to
     * simplify some locking and avoid separate construction.
     */
    //片结构，该类继承自ReentrantLock，因此可以直接调用lock进行锁定，
    //该对象是ConcurrentHashMap的核心，在该对象内实现了同步机制，因此对该类做了详细的描述
    static final class Segment<K,V> extends ReentrantLock implements Serializable {

        private static final long serialVersionUID = 2249069246763182397L;

        //最大尝试去锁的尝试次数，对于单核CPU，为1，多核为64
        static final int MAX_SCAN_RETRIES =
            Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;

        //当前片中的table数组，这个数组是volatile修饰的
        transient volatile HashEntry<K,V>[] table;

        //当前片中的元素个数
        transient int count;

        //修改计数器
        transient int modCount;

        //扩容点
        transient int threshold;

        //扩容因子
        final float loadFactor;

        Segment(float lf, int threshold, HashEntry<K,V>[] tab) {
            this.loadFactor = lf;
            this.threshold = threshold;
            this.table = tab;
        }
        
        /**
         * 每个片的put操作
         * @param key
         * @param hash
         * @param value
         * @param onlyIfAbsent
         *               尔值onlyIfAbsent可以这样理解：
         *               如果要put一个值当调用putIfAbsent方法时，onlyIfAbsent为true，此时当这个值已经存在了，则忽略，不存在时，则创建
         *               和传统的put（onlyIfAbsent为false,有则替换，没有则创建新的放入）相比，是可放可不放的
         * @return
         */
        final V put(K key, int hash, V value, boolean onlyIfAbsent) {
            /**
             * tryLock()解释：
             *       如果锁可用，则获取锁，并立即返回值 true。
             *    如果锁不可用，则此方法将立即返回值 false。
             *    
             *    该行执行后，一定能能获取到一个锁,没有直接调用lock()也是为了提高并发，使用了尝试锁机制
             *    put操作会锁定整个片（这里是不是可以把精度再小一点，精确到锁某个链呢？不行，因为有可能要扩容，要打破原有链式结构，哈哈）
             */
            HashEntry<K,V> node = tryLock() ? null :scanAndLockForPut(key, hash, value);
            V oldValue;
            try {
                HashEntry<K,V>[] tab = table;
                //找到存放位置，这个存放位置是相对于片的，是片内数组的存放位置
                int index = (tab.length - 1) & hash;
                //该位置的链表首元素
                HashEntry<K,V> first = entryAt(tab, index);
                for (HashEntry<K,V> e = first;;) {
                    if (e != null) {//链表存在，则循环遍历
                        K k;
                        if ((k = e.key) == key || (e.hash == hash && key.equals(k))) {
                            oldValue = e.value;
                            if (!onlyIfAbsent) {//如果找到了，则替换
                                e.value = value;
                                ++modCount;
                            }
                            break;
                        }
                        e = e.next;//如果没找到，则到链表尾部时，e=null,走入else
                    } else {//如果e为null，则表示此时链表为空或者已走到链表尾部，此时可以断定其他线程也没有创建该Entry
                        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)//reshah，重置大小
                            rehash(node);
                        else
                            setEntryAt(tab, index, node);
                        ++modCount;
                        count = c;
                        oldValue = null;
                        break;
                    }
                }
            } finally {
                //释放锁，这里是使用ReentrantLock的规则，释放锁一定要放在finally中。
                unlock();
            }
            return oldValue;
        }

        /**
         * 重置hash表，这个方法感觉不如HashMap中的重置Hash表，
         * 执行此方法时，已经加了锁，可以和HashMap的处理方式一样
         * 也不用重新new对象，只改变next指针和位置即可！！
         * 这里的代码没有完全领悟^_^^_^
         */
        @SuppressWarnings("unchecked")
        private void rehash(HashEntry<K,V> node) {
            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;//lastIdx是新的index
                        for (HashEntry<K,V> last = next;last != null; last = last.next) {
                            /**
                             * 这里是为了找到当前链表的最后一个元素，由于该元素的next指针指向了null，不需要重构，可以重用，
                             * 这里假设一种情况，如果oldTable[0]的最后一个元素和oldTable[1]的最后一个元素重构后
                             * 都在newTable[2]的位置，不是会出错？oldTable[1]会覆盖掉oldTable[0]的值吧
                             */
                            int k = last.hash & sizeMask;
                            if (k != lastIdx) {
                                lastIdx = k;
                                lastRun = last;
                            }
                        }
                        newTable[lastIdx] = lastRun;//可重用的直接拿来使用
                        // Clone remaining nodes
                        for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {//从链表头部开始，到刚找到的那个元素为止
                            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;
        }

        /**
         * 该方法最终一定会返回一个锁，加上该方法后，先尝试去获取锁（tryLock），如果获取不到锁，
         * 则在第一次循环时，如果是新对象，则依据key，value创建一个HashEntry，并每隔一次再遍历该片，
         * 看看其他线程是否创建了该HashEntry，如果其他已经线程创建了，则取得其他线程创建的对象，
         * 继续循环等待锁。
         *
         * 此处用到了自旋锁机制，自旋锁的概念参考百度百科
         */
        private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
            //通过给出的hash值，找到当前Entry，第一次放置应该找不到，第二次能找到旧的。
            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()) {//如果没有获取到锁
                HashEntry<K,V> f; // to recheck first below
                if (retries < 0) {//只有第一次循环会进入该部分，后面的调用了++retries
                    if (e == null) {//e==null表示是第一次放入
                        if (node == null) // speculatively create node
                            node = new HashEntry<K,V>(hash, key, value, null);
                        retries = 0;
                    }else if (key.equals(e.key))//如果找到了Entry对象
                        retries = 0;
                    else
                        e = e.next;
                } else if (++retries > MAX_SCAN_RETRIES) {
                    //如果已经达到最大尝试次数，则强制锁定，使用阻塞同步方法
                    lock();
                    break;
                } else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) {
                    //当retries为偶数时，再一次遍历当前片区，如果找到要创建的对象但是却不等于first，
                    //说明此时发生了并发，其他线程已经创建了该对象，则将e和first都置为其他线程创建的Entry
                    //此时重试次数重置，重新开始请求锁，
                    //此时的node好像还是指向新创建的那个对象，为什么不重新指向找到的这个Entry呢？?????
                    //这里看懂了，此方法的关键是获取锁，如果其他线程已经创建了该对象，则不管他，继续循环获取锁
                    //返回去的node是创建的新对象，可以理解为预创建，就是在等待释放锁的时间里顺带做了这件事，
                    //后面用不用看情况（若已有，则抛弃，若无则用这个对象加入链表）。
                    e = first = f; // re-traverse if entry changed
                    retries = -1;
                }
            }
            return node;
        }

        //这个也是个自旋锁，和上面一样，不过少了预创建的功能
        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;
                }
            }
        }
        
        //删除
        final V remove(Object key, int hash, Object value) {
            //加锁
            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 ||
                  
13ee9
      (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;
        }
        
        //replace和put相比，不可能造成rehash
        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;
        }
        
        //返回替换的对象，和上面那个方法一样
        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;
        }
        
        //清空
        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

    /**
     * 根据hash值计算元素在那个分片
     */
    @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
     */
    /**
     * 根据哈希值和片，找到该Entry，使用UNSAFE.getObjectVolatile()保证读到的是最新的对象。
     */
    @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 -------------- */

    /**
     * 由构造函数可知，ConcurrentHashMap维护了一个数组大小固定的Segment，可以理解为片或者段
     * Segment数组的大小由构造函数的concurrencyLevel决定，默认为16
     *由于Segment数组大小固定，rehash发生在Segment内部。
     *有Segment的构造可知，其内部类似于一个高效的Hashtable，不过Segment的实效比Hashtable要更好。
     *ConcurrentHashMap的锁的粒度是Segment
     */
    @SuppressWarnings("unchecked")
    public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        if (concurrencyLevel > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;
        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
        while (ssize < concurrencyLevel) {
            ++sshift;
            ssize <<= 1;
        }
        this.segmentShift = 32 - sshift;
        this.segmentMask = ssize - 1;
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        while (cap < c)
            cap <<= 1;
        // create segments and segments[0]
        Segment<K,V> s0 =
            new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
                             (HashEntry<K,V>[])new HashEntry[cap]);
        Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
        UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
        this.segments = ss;
    }
    
    public ConcurrentHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
    }
    
    public ConcurrentHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    public ConcurrentHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    public ConcurrentHashMap(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);
    }

    /**
     * 此方法没有加锁，为了相对安全，校验了三次^_^，也是拼了
     */
    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;
    }

    /**
     * 这个方法更有意思，先查两次，如果结果相同就返回了，万一不相同，加锁，让你乱动我的数据！fuck！
     */
    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 (;;) {
                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) {
                    Segment<K,V> seg = segmentAt(segments, j);
                    if (seg != null) {
                        sum += seg.modCount;
                        int c = seg.count;
                        if (c < 0 || (size += c) < 0)
                            overflow = true;
                    }
                }
                if (sum == last)
                    break;
                last = sum;
            }
        } finally {
            if (retries > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j < segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return overflow ? Integer.MAX_VALUE : size;
    }

    /**
     * get操作也没有加锁，调用了UNSAFE.getObjectVolatile方法来强制从内存中取得最新值
     */
    public V get(Object key) {
        Segment<K,V> s; // manually integrate access methods to reduce overhead
        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 e.value;
            }
        }
        return null;
    }

    //和get一样
    @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;
    }
    
    //这个思路和size一样，老外也说了Same idea as size()
    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 (;;) {
                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;
                    Segment<K,V> seg = segmentAt(segments, j);
                    if (seg != null && (tab = seg.table) != null) {
                        for (int i = 0 ; i < tab.length; i++) {
                            HashEntry<K,V> e;
                            for (e = entryAt(tab, i); e != null; e = e.next) {
                                V v = e.value;
                                if (v != null && value.equals(v)) {
                                    found = true;
                                    break outer;
                                }
                            }
                        }
                        sum += seg.modCount;
                    }
                }
                if (retries > 0 && sum == last)
                    break;
                last = sum;
            }
        } finally {
            if (retries > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j < segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return found;
    }

    public boolean contains(Object value) {
        return containsValue(value);
    }

    //put的时候，先根据hash值找到Segment放置位置，再在Segment内部查找
    @SuppressWarnings("unchecked")
    public V put(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          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            s = ensureSegment(j);
        return s.put(key, hash, value, false);
    }

    //这个前面讲过
    @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);
    }
    
    //这里每次put都需要加锁释放锁，感觉可以加个标志位，标示是否要加锁，这样就可以在外层加锁
    public void putAll(Map<? extends K, ? extends V> m) {
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

    //和put一样，先找到位置，加锁，删除
    public V remove(Object key) {
        int hash = hash(key);
        Segment<K,V> s = segmentForHash(hash);
        return s == null ? null : s.remove(key, hash, null);
    }

    //同上
    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;
    }

    //找到位置，加锁，替换
    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);
    }

    //同上
    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);
    }

    //同上
    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();
        }
    }
    
    //返回一个KeySet对象，这个对象其实不是一个真正的set
    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 -------------- */
    /**
     * 构造Iterator,用于遍历，从它的构造可以看出来，ConcurrentHashMap的遍历没有加锁判断，
     * 也没有进行modeCount检查，因此可以一边进行remove、put操作，一边遍历，
     * 这一点和HashMap不同，HashMap只支持通过Iterator的删除操作。
     * 如果要保证遍历的是当前状态的绝对值，需要手动加锁，或者复制一份进行遍历（这种模式Iterator的remove功能失效）
     * 由于Segment中的table数组是volatile类型的，可以保证遍历的是内存中的最新数据。
     *
     */
    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).
         */
        //把nextEntry设置为最后一个非空的Segment的最后一个非空链表的表头
        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;
            }
        }

        //如果链表只有一个元素，设置nextEntry为当前元素的next，返回当前元素，设置lastReturned为当前元素
        //否则，接着倒序向前找非空链表
        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; }
        
        //这个操作要加锁
        public final void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            ConcurrentHashMap.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);
            ConcurrentHashMap.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 ConcurrentHashMap.this.size();
        }
        public boolean isEmpty() {
            return ConcurrentHashMap.this.isEmpty();
        }
        public boolean contains(Object o) {
            return ConcurrentHashMap.this.containsKey(o);
        }
        public boolean remove(Object o) {
            return ConcurrentHashMap.this.remove(o) != null;
        }
        public void clear() {
            ConcurrentHashMap.this.clear();
        }
    }

    final class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator();
        }
        public int size() {
            return ConcurrentHashMap.this.size();
        }
        public boolean isEmpty() {
            return ConcurrentHashMap.this.isEmpty();
        }
        public boolean contains(Object o) {
            return ConcurrentHashMap.this.containsValue(o);
        }
        public void clear() {
            ConcurrentHashMap.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 = ConcurrentHashMap.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 ConcurrentHashMap.this.remove(e.getKey(), e.getValue());
        }
        public int size() {
            return ConcurrentHashMap.this.size();
        }
        public boolean isEmpty() {
            return ConcurrentHashMap.this.isEmpty();
        }
        public void clear() {
            ConcurrentHashMap.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 {
        s.defaultReadObject();

        // 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;

    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"));
        } 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);
    }

}

ConcurrentHashMap是一个非常重要的数据结构，通过分析，可以看出：

使用了分段锁技术，ConcurrentHashMap内部有一个Segment数组，该数组大小固定，默认为16，可以成为片或段，每个片内，有一个table数组，table数组内保存的是HashEntry链表结构。
ConcurrentHashMap的put操作，先通过key的hash值算出放在哪个Segment，该操作用的是UNSAFE.getObject（）方法，不用加锁，再在Segment内部和Hashtable类似，计算出table的index，再在链表中加入该元素。其中，在Segment的put操作时，调用了lock操作，加锁（用的自旋锁技术），当容量达到阈值时，自动扩容，扩容发生在片内部。
get操作没有加锁，调用了UNSAFE.getObjectVolatile方法来强制从内存中取得最新值
size()操作和containsValue()操作的设计有巧妙，在整个ConcurrentHashMap内设置了一个自动重试次数（RETRIES_BEFORE_LOCK），会循环执行size()操作，在循环的过程中，如果得到的结果一直没有发生变化，则就返回该值，如果返回结果不一致，则加锁进行统计。
ConcurrentHashMap提供了HashIterator，在HashIterator内部实现可以看出，它的遍历没有没有加锁判断，也没有进行modeCount检查，因此可以一边进行remove、put操作，一边遍历不会抛出ConcurrentModificationException异常，这一点和HashMap不同，HashMap只支持通过Iterator的删除操作。因此， 如果要保证遍历的是当前状态的绝对值，需要手动加锁，或者复制一份进行遍历（这种模式Iterator的remove功能失效）。另外由于Segment中的table数组是volatile类型的，可以保证遍历的是内存中的最新数据。得出结论:ConcurrentHashMap迭代器弱一致。
ConcurrentHashMap不是绝对的线程安全。