ConcurrentHashMap

Base

• Node Definition:val and next are volatile.

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

• sizeCtl
  Table initialization and resizing control. When negative, the
  table is being initialized or resized: -1 for initialization,
  else -(1 + the number of active resizing threads). Otherwise,
  when table is null, holds the initial table size to use upon
  creation, or 0 for default. After initialization, holds the
  next element count value upon which to resize the table.
• concurrentLevel
  as estimated threads,also the number of bins.
• table:volatile
  the array of bins. Lazily initialized upon first insertion.
  Size is always a power of two. Accessed directly by iterators.
• nextTable
  The next table to use; non-null only while resizing.
• transferIndex
  The next table index (plus one) to split while resizing.init to n while resizing.

Spread method

static final int spread(int h){
  return  h^(h >>>16) &0x7fffffff;// the high 16 bits XOR the low 16 bits.
}

Thread.yield()

the current Thread is willing to yield its current use of a processor, but the scheduler is free to ignore the hint.

putVal()

key points.

• null key or null value is not accessible.
• method of compute hash(spread)
• if table is empty,initialize tab(lazy init).No global lock,use
  CAS and sizeCtl(volatile) to ensure single thread do initTable.
• if tab[i] == null,try CAS put.
• if tab[i].hash == -1,help resize.
• else ,synchronized(tab[i]). if(tab[i].hash >= 0), add to the end of the list and record binCount.else (tab[i].hash == 0), put to a red_black tree.then, if(binCount >= 8),treeifyBin(may be do not treeify, resize can be possible.)

final V putVal(K key, V value, boolean onlyIfAbsent) {
        if (key == null || value == null) throw new NullPointerException();//is different from HashMap
        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();//when in init, no lock. here use volatile  parameter:"sizeCtl" and CAS and spin to ensure init.  
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {//when this bin is null,use CAS Operation to do put.
                if (casTabAt(tab, i, null,
                             new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin,just CAS.
            }
            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;
    }

initTable()

• lazy init.
• sizeCtl to -1 when initializing.
• only one thread do this.

 private final Node<K,V>[] initTable() {
        Node<K,V>[] tab; int sc;
        while ((tab = table) == null || tab.length == 0) {//note,use while here.
            if ((sc = sizeCtl) < 0)
                Thread.yield(); // lost initialization race; just spin
            else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {//CAS Operation, use sizeCtl param.
                try {
                    if ((tab = table) == null || tab.length == 0) {//it`s also means.
                        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);//why ?
                    }
                } finally {
                    sizeCtl = sc;//when to resize.
                }
                break;
            }
        }
        return tab;
    }

transfer

resize table
key points.

• nextTable.length = tab.length <<1;(double)
• transferIndex is volatile(shared in multi threads.),to allow more than one threads handle tab bins.
• ForwardingNode (hash = -1) in tab[i] means the tab is resizing and the ith bin has already been processed.
• the Nodes in tab[i] will store to nextTab[i] (node.hash & n == 0),otherwise nextTable[i + n].
  -for ith bin process: CAS when bin is empty, ignore when bin`s first node is an instance of ForwardingNode(hash == -1),otherwise,synchronized(first node the bin).
• question.when finished, why sizeCtl - 1 ?

 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];//double size.
                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;
/** find the next bin to handle, if exist.**/
            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))) {//this #{stride} bins will be handled by this thread,while others may be handled by other threads.
                    bound = nextBound;
                    i = nextIndex - 1;
                    advance = false;
                }
            }
/** i reprsents the bin index, so it should be between 0 and n - 1. **/
            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)//why ?
                        return;
                    finishing = advance = true;
                    i = n; // recheck before commit,why?
                }
            }
            else if ((f = tabAt(tab, i)) == null)// CAS operation
                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;//must be 0 or 1. because n is power of 2.
                            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);//one bin in old table depart to two bin in new table.
                            setTabAt(tab, i, fwd); //marked as processed.
                            advance = true;
                        }
                    }
                }
            }
        }
    }

get()

• use tab(volatile) and tabAt(i) and node.next(volatile) and node.val (volatile) to get element(up-to-date).
• volatile :write happens before read.
• when resizing, read from the new table(hash & (nextTab.length - 1))
• when a tree, read from the tree.

 public V get(Object key) {
        Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
        int h = spread(key.hashCode());
        //tab = table （volatile：write happens before read)  
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (e = tabAt(tab, (n - 1) & h)) != null) {
            if ((eh = e.hash) == h) {
                if ((ek = e.key) == key || (ek != null && key.equals(ek)))//for no collision situation
                    return e.val;
            }
            else if (eh < 0) // find in the nextTable(-1) or Tree(-2)
                return (p = e.find(h, key)) != null ? p.val : null;
//at last,find in the list
            while ((e = e.next) != null) {
                if (e.hash == h &&
                    ((ek = e.key) == key || (ek != null && key.equals(ek))))
                    return e.val;
            }
        }
        return null;
    }

tabAt()

• tabAt: use Unsafe.getObjectVolatile to get tab[i](up-to-date)

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);
    }