ConcurrentHashMap

public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Seriablizable {
private static final long serialVersionUID = 7249069246763182397L;
private static final int MAXIMUM_CAPACITY = 1 << 30;
private static final int DEFAULT_CAPACITY = 16;
static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
private static final float LOAD_FACTOR = 0.75f;
static final int TREEIFY_THRESHOLD = 8;
static final int MIN_TREEIFY_CAPACITY = 64;
private static final int MIN_TRANSFER_STRIDE = 16;
private static final int RESIZE_STAMP_BITS = 16;
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;

static final int MOVED = -1;
static final int TREEBIN = -2;
static final int RESERVED = -3;
static final int HASH_BITS = 0x7fffffff;

static final int NCPU = Runtime.getRuntime().availableProcessors();
}

//内部类
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) {  //构造函数，hash值、key值、val值三参
            this.hash = hash;
            this.key = key;
            this.val = val;
        }

        Node(int hash, K key, V val, Node<K,V> next) {  //构造函数，指针，链表
            this(hash, key, val);
            this.next = next;
        }

        public final K getKey()     { return key; }
        public final V getValue()   { return val; }
        public final int hashCode() { return key.hashCode() ^ val.hashCode(); }  ??
        public final String toString() {  //toString返回格式：类名@16进制无符号整数
            return Helpers.mapEntryToString(key, val);
        }
        public final V setValue(V value) {
            throw new UnsupportedOperationException();
        }

        public final boolean equals(Object o) {
            Object k, v, u; Map.Entry<?,?> e;
            return ((o instanceof Map.Entry) &&
                    (k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
                    (v = e.getValue()) != null &&
                    (k == key || k.equals(key)) &&
                    (v == (u = val) || v.equals(u)));
        }

        /**
         * Virtualized support for map.get(); overridden in subclasses.
         */
        Node<K,V> find(int h, Object k) {
            Node<K,V> e = this;
            if (k != null) {
                do {
                    K ek;
                    if (e.hash == h &&
                        ((ek = e.key) == k || (ek != null && k.equals(ek))))
                        return e;
                } while ((e = e.next) != null);
            }
            return null;
        }
    }

public ConcurrentHashMap(){}

public ConcurrentHashMap(int initialCapacity) {
    this(initialCapacity, LOAD_FACTOR, 1);
}

public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
    this.sizeCtl = DEFAULT_CAPACITY;
    putAll(m);
}

//@param concurrencyLevel the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint.

public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
    if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)  throw new IllegalArgumentException();
    if (initialCapacity < concurrencyLevel) 
        initialCapacity = concurrencyLevel;
    long size = (long)(1.0 + (long)initialCapacity / loadFactor);
    int cap = (size >= (long)MAXIMUM_CAPACITY) ? 
        MAXIMUM_CAPACITY : tableSizeFor((int)size);
    this.sizeCtl = cap;
}

public int size() {
        long n = sumCount();
        return ((n < 0L) ? 0 :
                (n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
                (int)n);
    }

// private static final Unsafe U = Unsafe.getUnsafe();
public V get(Object key) {
        Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
        int h = spread(key.hashCode());  //计算哈希值
// static final int spread(int h) {return (h ^ (h >>>16)) & HASH_BITS;}-->增加高位影响，减小哈希冲突（2的幂次方）
// 扰动函数，高16位与低16位进行异或，混合高位低位信息
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (e = tabAt(tab, (n - 1) & h)) != null) {
// static final <K, V> Node<K, V> tabAt(Node<K, V>[] tab, int i){
//return (Node<K, V>)U.getObjectAcquire(tab, ((long)i << ASHIFT) + ABASE);}
            if ((eh = e.hash) == h) {  //先根据哈希值查找
                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;  //遍历链表？ find的逻辑与下面的while一致
            while ((e = e.next) != null) {  //遍历链表
                if (e.hash == h &&
                    ((ek = e.key) == key || (ek != null && key.equals(ek))))
                    return e.val;
            }
        }
        return null;
    }

public boolean containsKey(Object key) {
    return get(key) != null;
}

public boolean containsValue(Object value) {
    if (value == null)  throw new NullPointerException();
    Node<K, V>[] t;
    if ((t = table) != null) {
        Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
        for (Node<K, V> p; (p = it.advance()) != null;) {
            V v;
            if ((v = p.val) == value || (v != null && value.equals(v)))
                return true;
        }
    }
    return false;
}

public V put(K key, V value) { return putVal(key, value, false)}

final V putVal(K key, V value, boolean onlyIfAbsent) {
        if (key == null || value == null) throw new NullPointerException();
        int hash = spread(key.hashCode());
        int binCount = 0;
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh; K fk; V fv;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
//static final <K, V> Node<K, V> tabAt(Node<K, V>[] tab, int i) {
//return (Node<K, V>)U.getObjectAcquire(tab, ((long)i << ASHIFT) + ABASE);}
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
//static final <K, V> boolean casTabAt(Node<K, V>[] tab, int i, Node<K, V> c, Node<K, V> v) {
//return U.compareAndSetObject(tab, ((long)i << ASHIFT) + ABASE, c, v);}
                if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value)))
break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)
//static final int MOVED = -1; hash for forwarding nodes
                tab = helpTransfer(tab, f);  //helpTransfer-->如果正在调整大小则帮助转移
            else if (onlyIfAbsent // 检查第一个节点但不获取锁
                     && fh == hash
                     && ((fk = f.key) == key || (fk != null && key.equals(fk)))
                     && (fv = f.val) != null)
                return fv;
            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);
                                    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;
                            }
                        }
                        else if (f instanceof ReservationNode)
                            throw new IllegalStateException("Recursive update");
                    }
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);
        return null;
    }

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.compareAndSetInt(this, SIZECTL, sc, -1)) {
                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;
    }

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.compareAndSetInt(this, SIZECTL, sc, sc + 1)) {
                    transfer(tab, nextTab);
                    break;
                }
            }
            return nextTab;
        }
        return table;
    }