Deep Dive into Java 8 ConcurrentHashMap: Initialization, Treeification, Resizing, and Transfer Mechanics

The article provides a detailed analysis of Java 8's ConcurrentHashMap implementation, focusing on its initialization, treeification, resizing, and data migration mechanisms.

Table Initialization (initTable)

The initTable method creates an appropriately sized array and sets sizeCtl. Concurrency is handled by a CAS operation on sizeCtl to ensure only one thread performs the initialization.

private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    while ((tab = table) == null || tab.length == 0) {
        if ((sc = sizeCtl) < 0)
            Thread.yield(); // lost initialization race; just spin
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
            try {
                if ((tab = table) == null || tab.length == 0) {
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    table = tab = nt;
                    sc = n - (n >>> 2); // 0.75 * n
                }
            } finally {
                sizeCtl = sc; // set sizeCtl to 12 (example)
            }
            break;
        }
    }
    return tab;
}

Bin Treeification (treeifyBin)

The treeifyBin method may either resize the array or convert a linked list bin into a red‑black tree, depending on the current capacity.

private final void treeifyBin(Node<K,V>[] tab, int index) {
    Node<K,V> b;
    int n, sc;
    if (tab != null) {
        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;
                    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;
                    }
                    setTabAt(tab, index, new TreeBin<K,V>(hd));
                }
            }
        }
    }
}

Resizing Logic (tryPresize)

The tryPresize method prepares the map for a capacity increase, calculating a new size (approximately 1.5× the requested size) and handling the CAS updates to sizeCtl. It may trigger the transfer method to move entries.

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;
        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;
                }
                break;
            }
        } 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;
                if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
                    transfer(tab, nt);
            } else if (U.compareAndSwapInt(this, SIZECTL, sc,
                    (rs << RESIZE_STAMP_SHIFT) + 2))
                transfer(tab, null);
        }
    }
}

Data Migration (transfer)

The transfer method moves entries from the old table to a newly allocated one. It works in parallel using a global transferIndex and a calculated stride to assign chunks of work to threads. The method handles plain nodes, forwarding nodes, linked‑list bins, and tree bins, converting them appropriately in the new table.

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;
    if (nextTab == null) {
        try {
            Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1];
            nextTab = nt;
        } catch (Throwable ex) {
            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, finishing = false;
    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;
            }
        }
        if ((f = tabAt(tab, i)) == null)
            advance = casTabAt(tab, i, null, fwd);
        else if ((fh = f.hash) == MOVED)
            advance = true;
        else {
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    if (fh >= 0) {
                        // split linked list into two halves
                        // ... (omitted for brevity) ...
                    } else if (f instanceof TreeBin) {
                        // split tree bin
                        // ... (omitted for brevity) ...
                    }
                }
            }
        }
    }
}

Lookup Operation (get)

The get method computes the hash, locates the bucket, and then handles three cases: direct match, forwarding/ tree node (during resize), or linear traversal of a linked list.

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) {
        if ((eh = e.hash) == h) {
            if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                return e.val;
        } else if (eh < 0)
            return (p = e.find(h, key)) != null ? p.val : null;
        while ((e = e.next) != null) {
            if (e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek))))
                return e.val;
        }
    }
    return null;
}

In summary, the article walks through the most complex parts of ConcurrentHashMap —initialization, bin treeification, resizing, and multi‑threaded transfer—showing how Java 8 achieves high‑performance concurrent access.