0. ConcurrentHashMap是什么

• key和value都不能為null，和HashTable一樣
• 默認(rèn)大小為16，擴(kuò)容時(shí)為2的冪，擴(kuò)容閾值為0.75*cap
• 節(jié)點(diǎn)相同的標(biāo)準(zhǔn)為hash相等并且k1==k2 || k1.equals(k2)

1. 實(shí)現(xiàn)的本質(zhì)

• 數(shù)組+鏈表+紅黑樹
• volatile + CAS

2. 常用api解析

2.0 重要子類解析

Node節(jié)點(diǎn)，數(shù)組的元素類型

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    volatile V val;
    volatile Node<K,V> next;
    Node(int hash, K key, V val, Node<K,V> next) {
        this.hash = hash;
        this.key = key;
        this.val = val;
        this.next = next;
    }
}

TreeBin：放在tab[i]位置的節(jié)點(diǎn)，當(dāng)該節(jié)點(diǎn)的內(nèi)容是紅黑樹時(shí)使用，其中：

• root：紅黑樹的根節(jié)點(diǎn)
• first：鏈表的頭節(jié)點(diǎn)

static final class TreeBin<K,V> extends Node<K,V> {
    TreeNode<K,V> root;
    volatile TreeNode<K,V> first;
    volatile Thread waiter;
    volatile int lockState;
    // values for lockState
    static final int WRITER = 1; // set while holding write lock
    static final int WAITER = 2; // set when waiting for write lock
    static final int READER = 4; // increment value for setting read lock
    ...
}

//紅黑樹的節(jié)點(diǎn)
static final class TreeNode<K,V> extends Node<K,V> {
    TreeNode<K,V> parent;  // red-black tree links
    TreeNode<K,V> left;
    TreeNode<K,V> right;
    TreeNode<K,V> prev;    // needed to unlink next upon deletion
    boolean red;
    TreeNode(int hash, K key, V val, Node<K,V> next,
             TreeNode<K,V> parent) {
        super(hash, key, val, next);
        this.parent = parent;
    }
}

Forwarding節(jié)點(diǎn)，擴(kuò)容時(shí)把舊表的tab[i]位置移動(dòng)到新表后，在舊表的i位置插入該節(jié)點(diǎn)。

static final class ForwardingNode<K,V> extends Node<K,V> {
    final Node<K,V>[] nextTable;
    ForwardingNode(Node<K,V>[] tab) {
        super(MOVED, null, null, null);
        this.nextTable = tab;
    }
}

2.1 構(gòu)造函數(shù)

public ConcurrentHashMap()
public ConcurrentHashMap(int initialCapacity)
public ConcurrentHashMap(Map<? extends K, ? extends V> m)
public ConcurrentHashMap(int initialCapacity, float loadFactor)
public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)

2.2 initTable初始化

初始化只能由一個(gè)線程進(jìn)行，搶占到的線程將sizeCtl設(shè)為-1，未搶占到的線程進(jìn)行yield()操作。初始化完成之后sizeCtl的值為0.75*n。

/**
 * Initializes table, using the size recorded in sizeCtl.
 */
private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    while ((tab = table) == null || tab.length == 0) {
        //sc小于0表示未搶占到，自旋
        if ((sc = sizeCtl) < 0)
            Thread.yield(); // lost initialization race; just spin
        //搶占到的線程把sizeCtl置為-1，防止其他線程進(jìn)入
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
            try {
                if ((tab = table) == null || tab.length == 0) {
                    //默認(rèn)大小為16
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                    @SuppressWarnings("unchecked")
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    //寫入table
                    table = tab = nt;
                    //寫入sc=0.75*n
                    sc = n - (n >>> 2);
                }
            } finally {
                //table初始化完成后，寫入正確的sizeCtl
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}

2.3 數(shù)組元素原子操作

arrayBaseOffset獲取數(shù)組中第一個(gè)元素的偏移地址。即數(shù)組對(duì)象頭的偏移距離。
arrayIndexScale獲取數(shù)組中每一個(gè)元素的大小。
若2^n = scale，則ASHIFT = n。
i<<ASHIFT = i * 2^ASHIFT = i * scale
所以((long)i << ASHIFT) + ABASE=i * scale + ABASE即為內(nèi)存中元素的真實(shí)位置。
使用getObjectVolatile putObjectVolatile是為了保證讀寫原子性，同時(shí)直接讀寫到內(nèi)存而不是線程緩存。

...
private static final long ABASE;
private static final int ASHIFT;
...
ABASE = U.arrayBaseOffset(ak);
int scale = U.arrayIndexScale(ak);
if ((scale & (scale - 1)) != 0)
    throw new Error("data type scale not a power of two");
// 2^ASHIFT = scale
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);

@SuppressWarnings("unchecked")
static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
    return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
}
//參數(shù)分別為 table數(shù)組、index、expect、update
static final <K,V> boolean casTabAt(Node<K,V>[] tab, int i,
                                    Node<K,V> c, Node<K,V> v) {
    return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
}
static final <K,V> void setTabAt(Node<K,V>[] tab, int i, Node<K,V> v) {
    U.putObjectVolatile(tab, ((long)i << ASHIFT) + ABASE, v);
}

2.4 put方法

/**
 * Maps the specified key to the specified value in this table.
 * Neither the key nor the value can be null.
 *
 * <p>The value can be retrieved by calling the {@code get} method
 * with a key that is equal to the original key.
 *
 * @param key key with which the specified value is to be associated
 * @param value value to be associated with the specified key
 * @return the previous value associated with {@code key}, or
 *         {@code null} if there was no mapping for {@code key}
 * @throws NullPointerException if the specified key or value is null
 */
public V put(K key, V value) {
    return putVal(key, value, false);
}
/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
    //key和value都不能為null
    if (key == null || value == null) throw new NullPointerException();
    //將key的hash無符號(hào)右移16位，然后與其本身異或，再將符號(hào)位置0
    int hash = spread(key.hashCode());
    int binCount = 0;
    //原子操作失敗時(shí)自旋使用
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;
        //如果表是空的，初始化
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();
        //如果tab[i]是空，使用原子操作更新，不加鎖
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            if (casTabAt(tab, i, null,
                         new Node<K,V>(hash, key, value, null)))
                //操作成功，退出循環(huán)，停止自旋
                break;                   // no lock when adding to empty bin
        }
        //如果當(dāng)前節(jié)點(diǎn)是一個(gè)fwd節(jié)點(diǎn)，則本線程幫助完成擴(kuò)容
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            //鎖住tab[i]位置的第一個(gè)元素，相當(dāng)于鎖住tab[i]整個(gè)位置
            synchronized (f) {
                //檢查tab[i]位置元素有無變化
                if (tabAt(tab, i) == f) {
                    if (fh >= 0) {
                        binCount = 1;
                        for (Node<K,V> e = f;; ++binCount) {
                            K ek;
                            //如果節(jié)點(diǎn)已經(jīng)存儲(chǔ)過(key和hash分別相等)
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                oldVal = e.val;
                                //如果onlyIfAbsent為false，更新
                                if (!onlyIfAbsent)
                                    e.val = value;
                                break;
                            }
                            Node<K,V> pred = e;
                            //tab[i]位置的鏈表上沒有元素，則插入到鏈表最后
                            if ((e = e.next) == null) {
                                pred.next = new Node<K,V>(hash, key,
                                                          value, null);
                                break;
                            }
                        }
                    }
                    //如果f是紅黑樹的根節(jié)點(diǎn)
                    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) {
                //如果鏈表上的節(jié)點(diǎn)大于等于8個(gè)，轉(zhuǎn)成紅黑樹
                if (binCount >= TREEIFY_THRESHOLD)
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    addCount(1L, binCount);
    return null;
}

//將key的hash無符號(hào)右移16位，然后與其本身異或，再將符號(hào)位置0
//目的是為了減少hash沖突，使分布更均勻
static final int spread(int h) {
    return (h ^ (h >>> 16)) & HASH_BITS;
}

helpTransfer方法，如果正在擴(kuò)容，則幫助進(jìn)行擴(kuò)容：

/**
 * Helps transfer if a resize is in progress.
 */
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;
            //多一個(gè)線程幫助擴(kuò)容時(shí)，sc+1
            if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
                transfer(tab, nextTab);
                break;
            }
        }
        return nextTab;
    }
    return table;
}

紅黑樹節(jié)點(diǎn)的put方法：

/**
 * Finds or adds a node.
 * @return null if added
 */
//紅黑樹中添加節(jié)點(diǎn)，如果是添加加點(diǎn)返回null，如果是更新節(jié)點(diǎn)返回舊節(jié)點(diǎn)
final TreeNode<K,V> putTreeVal(int h, K k, V v) {
    Class<?> kc = null;
    boolean searched = false;
    for (TreeNode<K,V> p = root;;) {
        int dir, ph; K pk;
        //如果根節(jié)點(diǎn)是空，新建根節(jié)點(diǎn)
        if (p == null) {
            first = root = new TreeNode<K,V>(h, k, v, null, null);
            break;
        }
        //先比較hash
        else if ((ph = p.hash) > h)
            dir = -1;
        else if (ph < h)
            dir = 1;
        //hash相等時(shí)比較key
        else if ((pk = p.key) == k || (pk != null && k.equals(pk)))
            return p;
        //hash相等，但是key不等
        else if ((kc == null &&
                  (kc = comparableClassFor(k)) == null) ||
                 (dir = compareComparables(kc, k, pk)) == 0) {
            //如果key沒實(shí)現(xiàn)Comparable<K>接口，或者compareTo()方法返回0
            if (!searched) {
                TreeNode<K,V> q, ch;
                searched = true;
                if (((ch = p.left) != null &&
                     (q = ch.findTreeNode(h, k, kc)) != null) ||
                    ((ch = p.right) != null &&
                     (q = ch.findTreeNode(h, k, kc)) != null))
                    return q;
            }
            //決勝局，使用系統(tǒng)的hash值比較，若還有相等，dir=-1
            dir = tieBreakOrder(k, pk);
        }
        TreeNode<K,V> xp = p;
        if ((p = (dir <= 0) ? p.left : p.right) == null) {
            //在樹中找到插入的位置，鏈表中放到鏈表頭部
            TreeNode<K,V> x, f = first;
            first = x = new TreeNode<K,V>(h, k, v, f, xp);
            if (f != null)
                f.prev = x;
            if (dir <= 0)
                xp.left = x;
            else
                xp.right = x;
            //如果父節(jié)點(diǎn)是黑色，x設(shè)為紅色
            if (!xp.red)
                x.red = true;
            //如果父節(jié)點(diǎn)是紅色，進(jìn)行調(diào)整
            else {
                lockRoot();
                try {
                    //按照紅黑樹的規(guī)則進(jìn)行調(diào)整
                    root = balanceInsertion(root, x);
                } finally {
                    unlockRoot();
                }
            }
            break;
        }
    }
    assert checkInvariants(root);
    return null;
}

檢查key是否實(shí)現(xiàn)了Comparable<Key>接口：

/**
 * Returns x's Class if it is of the form "class C implements
 * Comparable<C>", else null.
 */
static Class<?> comparableClassFor(Object x) {
    if (x instanceof Comparable) {
        Class<?> c; Type[] ts, as; Type t; ParameterizedType p;
        //如果是string，直接返回
        if ((c = x.getClass()) == String.class) // bypass checks
            return c;
        //獲取c實(shí)現(xiàn)的全部接口
        if ((ts = c.getGenericInterfaces()) != null) {
            for (int i = 0; i < ts.length; ++i) {
                //如果是參數(shù)化類型，并且原始類型是Comparable，參數(shù)只有一個(gè)，且為c
                if (((t = ts[i]) instanceof ParameterizedType) &&
                    ((p = (ParameterizedType)t).getRawType() ==
                     Comparable.class) &&
                    (as = p.getActualTypeArguments()) != null &&
                    as.length == 1 && as[0] == c) // type arg is c
                    return c;
            }
        }
    }
    return null;
}

以指定的節(jié)點(diǎn)為根，在樹種查找key節(jié)點(diǎn)，未找到返回null：

/**
 * Returns the TreeNode (or null if not found) for the given key
 * starting at given root.
 */
final TreeNode<K,V> findTreeNode(int h, Object k, Class<?> kc) {
    if (k != null) {
        TreeNode<K,V> p = this;
        do  {
            int ph, dir; K pk; TreeNode<K,V> q;
            TreeNode<K,V> pl = p.left, pr = p.right;
            if ((ph = p.hash) > h)
                p = pl;
            else if (ph < h)
                p = pr;
            else if ((pk = p.key) == k || (pk != null && k.equals(pk)))
                return p;
            else if (pl == null)
                p = pr;
            else if (pr == null)
                p = pl;
            //如果key實(shí)現(xiàn)了Comparable<K>接口，并且compareTo()方法返回值不為0
            else if ((kc != null ||
                      (kc = comparableClassFor(k)) != null) &&
                     (dir = compareComparables(kc, k, pk)) != 0)
                p = (dir < 0) ? pl : pr;
            //遞歸查找右子樹
            else if ((q = pr.findTreeNode(h, k, kc)) != null)
                return q;
            else
                p = pl;
        } while (p != null);
    }
    return null;
}

進(jìn)行紅黑樹平衡調(diào)整時(shí)，先鎖住樹根：

/**
 * Acquires write lock for tree restructuring.
 */
private final void lockRoot() {
    if (!U.compareAndSwapInt(this, LOCKSTATE, 0, WRITER))
        contendedLock(); // offload to separate method
}
/**
 * Releases write lock for tree restructuring.
 */
private final void unlockRoot() {
    lockState = 0;
}
/**
 * Possibly blocks awaiting root lock.
 */
private final void contendedLock() {
    boolean waiting = false;
    for (int s;;) {
        //lockState為0或WAITER時(shí)，搶占
        if (((s = lockState) & ~WAITER) == 0) {
            if (U.compareAndSwapInt(this, LOCKSTATE, s, WRITER)) {
                if (waiting)
                    waiter = null;
                return;
            }
        }
        //如果WAITER位為0，將WAITER位置1
        else if ((s & WAITER) == 0) {
            if (U.compareAndSwapInt(this, LOCKSTATE, s, s | WAITER)) {
                waiting = true;
                waiter = Thread.currentThread();
            }
        }
        else if (waiting)
            LockSupport.park(this);
    }
}

2.5 get方法

/**
 * 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) {
    Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
    int h = spread(key.hashCode());
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (e = tabAt(tab, (n - 1) & h)) != null) {
        //如果tab[i]就是要查找的節(jié)點(diǎn)
        if ((eh = e.hash) == h) {
            if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                return e.val;
        }
        //如果是特殊節(jié)點(diǎn)，用find方法
        else if (eh < 0)
            return (p = e.find(h, key)) != null ? p.val : null;
        //如果是正常的單鏈表，往后找
        while ((e = e.next) != null) {
            if (e.hash == h &&
                ((ek = e.key) == key || (ek != null && key.equals(ek))))
                return e.val;
        }
    }
    return null;
}

Node節(jié)點(diǎn)調(diào)用find方法有3種情況：

• MOVED: ForwardingNode
• TREEBIN: TreeBin
• RESERVED: ReservationNode
  都繼承了Node類，在子類中分別重寫了find方法

2.6 remove方法

/**
 * Removes the key (and its corresponding value) from this map.
 * This method does nothing if the key is not in the map.
 *
 * @param  key the key that needs to be removed
 * @return the previous value associated with {@code key}, or
 *         {@code null} if there was no mapping for {@code key}
 * @throws NullPointerException if the specified key is null
 */
public V remove(Object key) {
    return replaceNode(key, null, null);
}
/**
 * Implementation for the four public remove/replace methods:
 * Replaces node value with v, conditional upon match of cv if
 * non-null.  If resulting value is null, delete.
 */
final V replaceNode(Object key, V value, Object cv) {
    int hash = spread(key.hashCode());
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;
        //如果tab是空，或者tab[i]位置為空，結(jié)束
        if (tab == null || (n = tab.length) == 0 ||
            (f = tabAt(tab, i = (n - 1) & hash)) == null)
            break;
        //如果tab[i]位置是fwd節(jié)點(diǎn)，參與擴(kuò)容
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        else {
            V oldVal = null;
            boolean validated = false;
            //鎖住tab[i]位置
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    //普通節(jié)點(diǎn)
                    if (fh >= 0) {
                        validated = true;
                        for (Node<K,V> e = f, pred = null;;) {
                            K ek;
                            //鏈表循環(huán)往后查找，如果找到
                            if (e.hash == hash &&
                                ((ek = e.key) == key ||
                                 (ek != null && key.equals(ek)))) {
                                V ev = e.val;
                                if (cv == null || cv == ev ||
                                    (ev != null && cv.equals(ev))) {
                                    oldVal = ev;
                                    //如果新值不為空，替換
                                    if (value != null)
                                        e.val = value;
                                    //如果不是tab[i]位置的第一個(gè)節(jié)點(diǎn)，刪除節(jié)點(diǎn)
                                    else if (pred != null)
                                        pred.next = e.next;
                                    //如果是tab[i]位置的第一個(gè)節(jié)點(diǎn)，原子替換
                                    else
                                        setTabAt(tab, i, e.next);
                                }
                                break;
                            }
                            //往鏈表后查找
                            pred = e;
                            if ((e = e.next) == null)
                                break;
                        }
                    }
                    //如果是紅黑樹
                    else if (f instanceof TreeBin) {
                        validated = true;
                        TreeBin<K,V> t = (TreeBin<K,V>)f;
                        TreeNode<K,V> r, p;
                        //如果找到節(jié)點(diǎn)
                        if ((r = t.root) != null &&
                            (p = r.findTreeNode(hash, key, null)) != null) {
                            V pv = p.val;
                            if (cv == null || cv == pv ||
                                (pv != null && cv.equals(pv))) {
                                oldVal = pv;
                                //如果新值不為空，替換
                                if (value != null)
                                    p.val = value;
                                //否則刪除節(jié)點(diǎn)
                                else if (t.removeTreeNode(p))
                                    setTabAt(tab, i, untreeify(t.first));
                            }
                        }
                    }
                }
            }
            if (validated) {
                if (oldVal != null) {
                    if (value == null)
                        addCount(-1L, -1);
                    return oldVal;
                }
                break;
            }
        }
    }
    return null;
}

2.7 擴(kuò)容方法

如果tab[i]位置的節(jié)點(diǎn)超過8個(gè)，轉(zhuǎn)換成紅黑樹

/**
 * Replaces all linked nodes in bin at given index unless table is
 * too small, in which case resizes instead.
 */
private final void treeifyBin(Node<K,V>[] tab, int index) {
    Node<K,V> b; int n, sc;
    if (tab != null) {
        //如果長度小于64，擴(kuò)容
        if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
            tryPresize(n << 1);
        else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
            synchronized (b) {
                if (tabAt(tab, index) == b) {
                    TreeNode<K,V> hd = null, tl = null;
                    //轉(zhuǎn)為雙向鏈表
                    for (Node<K,V> e = b; e != null; e = e.next) {
                        TreeNode<K,V> p =
                            new TreeNode<K,V>(e.hash, e.key, e.val,
                                              null, null);
                        if ((p.prev = tl) == null)
                            hd = p;
                        else
                            tl.next = p;
                        tl = p;
                    }
                    //把紅黑樹放到tab[i]位置
                    setTabAt(tab, index, new TreeBin<K,V>(hd));
                }
            }
        }
    }
}

擴(kuò)大table的長度：

/**
 * Tries to presize table to accommodate the given number of elements.
 *
 * @param size number of elements (doesn't need to be perfectly accurate)
 */
private final void tryPresize(int size) {
    int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :
        tableSizeFor(size + (size >>> 1) + 1);
    int sc;
    while ((sc = sizeCtl) >= 0) {
        Node<K,V>[] tab = table; int n;
        //未初始化，進(jìn)行初始化
        if (tab == null || (n = tab.length) == 0) {
            n = (sc > c) ? sc : c;
            if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
                try {
                    if (table == tab) {
                        @SuppressWarnings("unchecked")
                        Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                        table = nt;
                        sc = n - (n >>> 2);
                    }
                } finally {
                    sizeCtl = sc;
                }
            }
        }
        else if (c <= sc || n >= MAXIMUM_CAPACITY)
            break;
        else if (tab == table) {
            int rs = resizeStamp(n);
            if (sc < 0) {
                Node<K,V>[] nt;
                if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
                    sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
                    transferIndex <= 0)
                    break;
                //多一個(gè)線程進(jìn)行擴(kuò)容操作，sc+1
                if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
                    transfer(tab, nt);
            }
            //實(shí)際進(jìn)行擴(kuò)容的路徑
            else if (U.compareAndSwapInt(this, SIZECTL, sc,
                                         (rs << RESIZE_STAMP_SHIFT) + 2))
                transfer(tab, null);
        }
    }
}

把雙向鏈表轉(zhuǎn)為紅黑樹：

/**
 * Creates bin with initial set of nodes headed by b.
 */
TreeBin(TreeNode<K,V> b) {
    super(TREEBIN, null, null, null);
    this.first = b;
    TreeNode<K,V> r = null;
    for (TreeNode<K,V> x = b, next; x != null; x = next) {
        next = (TreeNode<K,V>)x.next;
        x.left = x.right = null;
        //如果根為空
        if (r == null) {
            x.parent = null;
            x.red = false;
            r = x;
        }
        else {
            K k = x.key;
            int h = x.hash;
            Class<?> kc = null;
            for (TreeNode<K,V> p = r;;) {
                int dir, ph;
                K pk = p.key;
                //確定節(jié)點(diǎn)x的插入方向
                if ((ph = p.hash) > h)
                    dir = -1;
                else if (ph < h)
                    dir = 1;
                else if ((kc == null &&
                          (kc = comparableClassFor(k)) == null) ||
                         (dir = compareComparables(kc, k, pk)) == 0)
                    dir = tieBreakOrder(k, pk);
                    TreeNode<K,V> xp = p;
                //插入節(jié)點(diǎn)x
                if ((p = (dir <= 0) ? p.left : p.right) == null) {
                    x.parent = xp;
                    if (dir <= 0)
                        xp.left = x;
                    else
                        xp.right = x;
                    //調(diào)整紅黑樹
                    r = balanceInsertion(r, x);
                    break;
                }
            }
        }
    }
    this.root = r;
    assert checkInvariants(root);
}

并行擴(kuò)容transfer方法：

• 每個(gè)線程每次處理stride個(gè)元素。
• 索引i到bound時(shí)，本線程本次處理完成，i到0時(shí)，整體數(shù)組處理完成，更新table和sizeCtl的值。
• 多一個(gè)線程進(jìn)行擴(kuò)容操作時(shí)，sc+1，完成后每個(gè)線程sc-1

/**
 * Moves and/or copies the nodes in each bin to new table. See
 * above for explanation.
 */
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
    int n = tab.length, stride;
    //stride最小為16
    if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
        stride = MIN_TRANSFER_STRIDE; // subdivide range
    if (nextTab == null) {            // initiating
        try {
            //新表擴(kuò)容的大小為2*n
            @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;
            //判斷是否到本線程處理的邊界
            if (--i >= bound || finishing)
                advance = false;
            //判斷是否到table的邊界
            else if ((nextIndex = transferIndex) <= 0) {
                i = -1;
                advance = false;
            }
            //讀取本線程本次需要處理的部分，即i到bound部分
            else if (U.compareAndSwapInt
                     (this, TRANSFERINDEX, nextIndex,
                      nextBound = (nextIndex > stride ?
                                   nextIndex - stride : 0))) {
                bound = nextBound;
                i = nextIndex - 1;
                advance = false;
            }
        }
        //i<0說明table數(shù)組整體處理完了
        if (i < 0 || i >= n || i + n >= nextn) {
            int sc;
            //結(jié)束處理，賦值table和sizeCtl
            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 {
            //鎖住tab[i]位置的節(jié)點(diǎn)f
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    Node<K,V> ln, hn;
                    //如果是鏈表節(jié)點(diǎn)
                    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;
                            }
                        }
                        //循環(huán)完成后lastRun后面的節(jié)點(diǎn)與lastRun在新table中在一個(gè)格子
                        //感覺這一趟循環(huán)沒有什么意義??？反正后面還要循環(huán)一趟
                        if (runBit == 0) {
                            ln = lastRun;
                            hn = null;
                        }
                        else {
                            hn = lastRun;
                            ln = null;
                        }
                        //重新遍歷tab[i]位置的鏈表，第n位為0的放到ln頭部，為1的放到hn頭部
                        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);
                        }
                        //把ln和hn分別放到新表的i和i+n位置
                        setTabAt(nextTab, i, ln);
                        setTabAt(nextTab, i + n, hn);
                        //舊表的i位置標(biāo)為MOVED
                        setTabAt(tab, i, fwd);
                        advance = true;
                    }
                    //如果是紅黑樹節(jié)點(diǎn)
                    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;
                        //紅黑樹的所有節(jié)點(diǎn)同時(shí)也在一個(gè)雙向鏈表上
                        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);
                            //第n位為0時(shí)放在lo后
                            if ((h & n) == 0) {
                                if ((p.prev = loTail) == null)
                                    lo = p;
                                else
                                    loTail.next = p;
                                loTail = p;
                                ++lc;
                            }
                            //第n位為1時(shí)放在hi后
                            else {
                                if ((p.prev = hiTail) == null)
                                    hi = p;
                                else
                                    hiTail.next = p;
                                hiTail = p;
                                ++hc;
                            }
                        }
                        //新的鏈表如果少于6個(gè)，轉(zhuǎn)為單鏈表，否則轉(zhuǎn)為紅黑樹，如果另外一個(gè)鏈?zhǔn)强盏?，直接把原來的t放過去
                        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;
                        //把ln和hn分別放入新表的i和i+n位置
                        setTabAt(nextTab, i, ln);
                        setTabAt(nextTab, i + n, hn);
                        //舊表的i位置設(shè)為MOVED
                        setTabAt(tab, i, fwd);
                        advance = true;
                    }
                }
            }
        }
    }
}

3. 總結(jié)

• 默認(rèn)大小16，每次擴(kuò)容時(shí)為原來的2倍，擴(kuò)容閾值為0.75*n
• 指定初始化大小時(shí)，容量為大于指定數(shù)的最小的2的冪，由tableSizeFor()方法實(shí)現(xiàn)
• table長度大于64，并且單個(gè)位置的節(jié)點(diǎn)數(shù)大于8個(gè)，會(huì)將該位置轉(zhuǎn)為紅黑樹，否則只是擴(kuò)大table的長度
• 在進(jìn)行寫操作時(shí)，每次只鎖tab[i]位置，不是整表上鎖
• sizeCtl比較難理解
  • 為-1是表示在初始化，初始化只能由一個(gè)線程進(jìn)行，其他線程yield()操作，在initTable()方法中
  • 為0x80xx00xx時(shí)(即resizeStamp(n)<<RESIZE_STAMP_SHIFT+2)，表示在擴(kuò)容操作，在tryPresize()方法中
  • 為0x80xx00xx時(shí)(上種情況下+n)，表示并發(fā)擴(kuò)容操作，每多一個(gè)線程，進(jìn)行sizeCtl+1操作，在putVal() remove()方法中，節(jié)點(diǎn)為REMOVED情況下，完成擴(kuò)容操作后，每退出一個(gè)線程，進(jìn)行sizeCtl-1操作，直到(sc - 2) == resizeStamp(n) << RESIZE_STAMP_SHIFT，表示最后一個(gè)線程完成，在transfer()方法中

4. 參考

1. ConcurrentHashMap源碼build 1.8.0_121-b13版本
2. Java7/8 中的 HashMap 和 ConcurrentHashMap 全解析
3. ConcurrentHashMap總結(jié)
4. 二叉樹 - 紅黑樹