jdk1.8 ConCurrentHashMap CAS乐观锁 + Synchronized

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面试中经常问到的 ConCurrentHashMap jdk1.7 与 1.8 的区别

简单描述：

jdk1.7分段锁segment 桶分段

jdk1.8 使用了基于UnSafe类的compareAndSet 原子乐观锁+synchronized 高效实现

说明：UnSafe类不能用于业务编程，只能用于jdk jvm 原子乐观锁操作实现，及脱离JVM

内存使用及释放

大概图解：

代码分析：

（1）put代码主体逻辑：cas乐观锁 + synchronized + 自旋

transient volatile Node<K,V>[] table;
static final int MOVED     = -1; // hash for forwarding nodes

 final V putVal(K key, V value, boolean onlyIfAbsent) {
        //1.key或 value 为null则抛出异常
        if (key == null || value == null) throw new NullPointerException();
        //2.计算key的 hashCode
        int hash = spread(key.hashCode());
        int binCount = 0;
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
                if (casTabAt(tab, i, null,
                             new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                V oldVal = null;
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {
                            binCount = 1;
                            for (Node<K,V> e = f;; ++binCount) {
                                K ek;
                                if (e.hash == hash &&
                                    ((ek = e.key) == key ||
                                     (ek != null && key.equals(ek)))) {
                                    oldVal = e.val;
                                    if (!onlyIfAbsent)
                                        e.val = value;
                                    break;
                                }
                                Node<K,V> pred = e;
                                if ((e = e.next) == null) {
                                    pred.next = new Node<K,V>(hash, key,
                                                              value, null);
                                    break;
                                }
                            }
                        }
                        else if (f instanceof TreeBin) {
                            Node<K,V> p;
                            binCount = 2;
                            if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                           value)) != null) {
                                oldVal = p.val;
                                if (!onlyIfAbsent)
                                    p.val = value;
                            }
                        }
                    }
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);
        return null;
    }

（2）初始化桶：cas + 线程自旋等待

    /*表（指桶数组）的初始化与扩容控制器。
        当该值为负数时，表正在进行初始化或扩容：值为 -1 表示正在初始化，
        值为其他负数时，其绝对值减 1 即为当前活跃的扩容线程数（即 -(1 + 活跃扩容线程数)）。
        若该值非负：当表（桶数组）为 null 时，该值表示表创建时要使用的初始容量（值为 0 时表示使用默认初始容量）；
        表初始化完成后，该值表示触发下一次扩容的元素数量阈值。 
    */
    private transient volatile int sizeCtl;
    // Unsafe mechanics
    private static final sun.misc.Unsafe U;
    //默认初始表容量。必须是 2 的幂（即至少为 1），且最大不得超过 MAXIMUM_CAPACITY（最大容量常量）。
    private static final int DEFAULT_CAPACITY = 16;
    /*初始化桶数组 */
    private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    while ((tab = table) == null || tab.length == 0) {
        //如果sizeCtl < 0 则表明有线程正在桶扩容，执行等待
        if ((sc = sizeCtl) < 0)
            Thread.yield(); //初始化竞争失败；只需自旋等待即可。
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {//原子cas操作，参数：当前对象、偏移量、期待原值、更新后的值（SIZECTL, sc：SIZECTL有没有变更过 ）
            //如果没有并发，则直接加乐观锁进行桶扩容
            try {
                if ((tab = table) == null || tab.length == 0) {
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                    @SuppressWarnings("unchecked")
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    table = tab = nt;
                    sc = n - (n >>> 2);
                }
            } finally {
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}

（3）tabAt 索引定位：

static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
    //提前获取  Object getObjectVolatile(Object obj, long offset);
    //obj：目标对象（要读取字段所属的对象）； 
    //offset：字段在对象内存布局中的 “偏移量”（需通过 Unsafe.objectFieldOffset(Field) 提前获取）。
    //主内存同步 强制读主内存 禁止重排 无锁并发场景，需确保读取最新值
    return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
}

（4）casTabAt 更新扩展node链表：cas原子乐观锁

static final <K,V> boolean casTabAt(Node<K,V>[] tab, int i,
                                    Node<K,V> c, Node<K,V> v) {
//CAS（Compare-And-Swap，比较并交换）方式原子性地更新对象的字段值，是 Java 并发编程中实现无锁线程安全的底层基石。
//Object obj  目标对象（要修改的字段所属对象）
//long offset  字段在对象内存中的偏移量
//Object expect  预期的当前值
//Object update  要更新的新值
    return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
}

（5）helpTransfer 并发高效迁移数据：

//当哈希表处于扩容过程时，某些辅助方法会参与到 “桶数组之间的数据迁移” 
//工作中 —— 将旧数组中的元素重新计算哈希并转移到新数组的对应位置，以配合扩容多线程并发扩容，提升扩容效率。
final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
    Node<K,V>[] nextTab; int sc;
    if (tab != null && (f instanceof ForwardingNode) &&
        (nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {
        int rs = resizeStamp(tab.length);
        while (nextTab == nextTable && table == tab &&
                (sc = sizeCtl) < 0) {
            if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
                sc == rs + MAX_RESIZERS || transferIndex <= 0)
                break;
            if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
                transfer(tab, nextTab);
                break;
            }
        }
        return nextTab;
    }
    return table;
}

（6）resizeStamp 扩容时的标记位

//返回大小为 n 的表进行扩容时的标记位。当向左移动 RESIZE_STAMP_SHIFT 位时，该值必须为负数。
static final int resizeStamp(int n) {
    return Integer.numberOfLeadingZeros(n) | (1 << (RESIZE_STAMP_BITS - 1));
}

（7）transfer 移动和 / 或复制每个桶（bin）中的节点到新表

//移动和 / 或复制每个桶（bin）中的节点到新表（数组）中
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
    int n = tab.length, stride;
    if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
        stride = MIN_TRANSFER_STRIDE; // subdivide range
    if (nextTab == null) {            // initiating
        try {
            @SuppressWarnings("unchecked")
            Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1];
            nextTab = nt;
        } catch (Throwable ex) {      // try to cope with OOME
            sizeCtl = Integer.MAX_VALUE;
            return;
        }
        nextTable = nextTab;
        transferIndex = n;
    }
    int nextn = nextTab.length;
    ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);
    boolean advance = true;
    boolean finishing = false; // to ensure sweep before committing nextTab
    for (int i = 0, bound = 0;;) {
        Node<K,V> f; int fh;
        while (advance) {
            int nextIndex, nextBound;
            if (--i >= bound || finishing)
                advance = false;
            else if ((nextIndex = transferIndex) <= 0) {
                i = -1;
                advance = false;
            }
            else if (U.compareAndSwapInt
                        (this, TRANSFERINDEX, nextIndex,
                        nextBound = (nextIndex > stride ?
                                    nextIndex - stride : 0))) {
                bound = nextBound;
                i = nextIndex - 1;
                advance = false;
            }
        }
        if (i < 0 || i >= n || i + n >= nextn) {
            int sc;
            if (finishing) {
                nextTable = null;
                table = nextTab;
                sizeCtl = (n << 1) - (n >>> 1);
                return;
            }
            if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
                if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)
                    return;
                finishing = advance = true;
                i = n; // recheck before commit
            }
        }
        else if ((f = tabAt(tab, i)) == null)
            advance = casTabAt(tab, i, null, fwd);
        else if ((fh = f.hash) == MOVED)
            advance = true; // already processed
        else {
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    Node<K,V> ln, hn;
                    if (fh >= 0) {
                        int runBit = fh & n;
                        Node<K,V> lastRun = f;
                        for (Node<K,V> p = f.next; p != null; p = p.next) {
                            int b = p.hash & n;
                            if (b != runBit) {
                                runBit = b;
                                lastRun = p;
                            }
                        }
                        if (runBit == 0) {
                            ln = lastRun;
                            hn = null;
                        }
                        else {
                            hn = lastRun;
                            ln = null;
                        }
                        for (Node<K,V> p = f; p != lastRun; p = p.next) {
                            int ph = p.hash; K pk = p.key; V pv = p.val;
                            if ((ph & n) == 0)
                                ln = new Node<K,V>(ph, pk, pv, ln);
                            else
                                hn = new Node<K,V>(ph, pk, pv, hn);
                        }
                        setTabAt(nextTab, i, ln);
                        setTabAt(nextTab, i + n, hn);
                        setTabAt(tab, i, fwd);
                        advance = true;
                    }
                    else if (f instanceof TreeBin) {
                        TreeBin<K,V> t = (TreeBin<K,V>)f;
                        TreeNode<K,V> lo = null, loTail = null;
                        TreeNode<K,V> hi = null, hiTail = null;
                        int lc = 0, hc = 0;
                        for (Node<K,V> e = t.first; e != null; e = e.next) {
                            int h = e.hash;
                            TreeNode<K,V> p = new TreeNode<K,V>
                                (h, e.key, e.val, null, null);
                            if ((h & n) == 0) {
                                if ((p.prev = loTail) == null)
                                    lo = p;
                                else
                                    loTail.next = p;
                                loTail = p;
                                ++lc;
                            }
                            else {
                                if ((p.prev = hiTail) == null)
                                    hi = p;
                                else
                                    hiTail.next = p;
                                hiTail = p;
                                ++hc;
                            }
                        }
                        ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :
                            (hc != 0) ? new TreeBin<K,V>(lo) : t;
                        hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
                            (lc != 0) ? new TreeBin<K,V>(hi) : t;
                        setTabAt(nextTab, i, ln);
                        setTabAt(nextTab, i + n, hn);
                        setTabAt(tab, i, fwd);
                        advance = true;
                    }
                }
            }
        }
    }
}

（8）涉及实现内部类，可参考源码