多线程之并发容器

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

MAXIMUM_CAPACITY = 1 << 30                      //最大容量 1 073 741 824
DEFAULT_CAPACITY = 16                                //默认容量
MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8   //最大数组大小
DEFAULT_CONCURRENCY_LEVEL = 16           //
LOAD_FACTOR = 0.75f                                  //负载因子
TREEIFY_THRESHOLD = 8                      //树化的阙值
UNTREEIFY_THRESHOLD = 6                //链表化的阙值
MIN_TREEIFY_CAPACITY = 64              //树化，hash表中元素最小容量
MIN_TRANSFER_STRIDE = 16
RESIZE_STAMP_BITS = 16
MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1
RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS
MOVED     = -1                                                 //表示正在转移
TREEBIN   = -2                                                 //已经转移成树
RESERVED  = -3                                               //hash for transient reservations
HASH_BITS = 0x7fffffff                                    //usable bits of normal node hash

transient volatile Node<K,V>[] table;  //默认为null，初始化发生在第一次插入操作，默认大小为16的数组，用来存储Node节点数据，扩容时大小总是2的幂次方
private transient volatile Node<K,V>[] nextTable; // 默认为null，扩容时新生成的数组，其大小为原数组的两倍
private transient volatile long baseCount; //
private transient volatile int sizeCtl;  // -1表示正在初始化，-N标示有N-1个线程进行扩容；table未初始化，表示需要初始化的大小，table初始化过，表示table的容量，默认table大小的0.75

Node：保存key，value及key的hash值的数据结构。 
      其中value和next都用volatile修饰，保证并发的可见性。
ForwardingNode：一个特殊的Node节点，hash值为-1，其中存储nextTable的引用。 
            只有table发生扩容的时候，ForwardingNode才会发挥作用，
            作为一个占位符放在table中表示当前节点为null或则已经被移动

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

final V putVal(K key, V value, boolean onlyIfAbsent) {
         //key 和value 都不能是null
        if (key == null || value == null) throw new NullPointerException();
         //计算hash值 (h ^ (h >>> 16)) & HASH_BITS;//0x7fffffff；保证了hash >= 0.
        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)
                // 初始化table数组操作，若sizeCtl为0则table大小为16，否则为sizeCtl大小
                tab = initTable();
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
                
                //通过hash得到数组下标，若该位置为null，新建节点放在该位置上
                if (casTabAt(tab, i, null,
                             new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            }
            //MOVED为-1，代表该位置的头节点为forwarding nodes，表明该位置正在进行扩容
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            
            //下面分普通链表与树进行插入操作
            else {
                V oldVal = null;
                // 插入操作被锁保护，锁为头节点对象
                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;
    }

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;
    }