ConcurrentHashMap源码阅读

public class ConcurrentHashMap<K,V> extends AbstractMap<K,V>
    implements ConcurrentMap<K,V>, Serializable {
    private static final long serialVersionUID = 7249069246763182397L;


    //table初始化
    private final Node<K,V>[] initTable() {
        Node<K,V>[] tab; 
        int sc;
        while ((tab = table) == null || tab.length == 0) {
            if ((sc = sizeCtl) < 0)// sizeCtl小于0，则进行线程让步等待
                Thread.yield(); // lost initialization race; just spin
            else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {// 比较sizeCtl的值与sc是否相等，相等则用-1替换
                try {
                    if ((tab = table) == null || tab.length == 0) {
                        //根据sizeCtl的值进行设置，如果没有设置szieCtl的值，那么默认生成的table大小为16
                        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);// sc为n * 3/4
                    }
                } finally {
                    sizeCtl = sc;
                }
                break;
            }
        }
        return tab;
    }

    //返回table数组中下标为i的结点
    static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
    //ASHIFT是指tab[i]中第i个元素相对于数组第一个元素的偏移量，而ABASE是数组在内存中第一个元素的偏移地址
        return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
    }

    //比较table数组下标为i的结点是否为c，若为c，则用v置换
    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);
    }

    //在扩容时将table表中的结点转移到nextTable中
    final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
        Node<K,V>[] nextTab; 
        int sc;
        //table不为null & 结点类型是ForwardingNode & 结点的nextTable域不为空
        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.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
                    transfer(tab, nextTab);// 将table的结点转移到nextTab中
                    break;
                }
            }
            return nextTab;
        }
        return table;
    }

     //添加key-value
    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;
            if (tab == null || (n = tab.length) == 0)//表为null或者长度为0
                tab = initTable();//初始化表
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
                //table的第i项为空则用新生成的node置换
                if (casTabAt(tab, i, null,new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)// 该结点的hash值为MOVED
                tab = helpTransfer(tab, f);// 进行结点的转移（在扩容的过程中）
            else {
                V oldVal = null;
                synchronized (f) { //锁定了f,f是链表的第一个元素，则锁住了整个链表
                    if (tabAt(tab, i) == f) {// 找到table表下标为i的节点并且等于f
                        if (fh >= 0) { // table表中该结点的hash值>=0
                            binCount = 1;
                            for (Node<K,V> e = f;; ++binCount) {
                                K ek;
                                //结点hash值相等且key相等，则为同一个key
                                if (e.hash == hash &&
                                    ((ek = e.key) == key ||
                                     (ek != null && key.equals(ek)))) {
                                    oldVal = e.val;//保存结点旧值
                                    if (!onlyIfAbsent)
                                        e.val = value;//更新结点value值
                                    break;
                                }
                                Node<K,V> pred = e;//保存当前结点
                                //当前结点的后继结点为空，当前结点为最后一个结点
                                if ((e = e.next) == null) {
                                    //生成新结点，最后一个结点pre的next指向它
                                    pred.next = new Node<K,V>(hash, key,
                                                              value, null);
                                    break;//key-value添加成功退出循环
                                }
                            }
                        }
                        else if (f instanceof TreeBin) {// 结点为红黑树结点类型
                            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)// 如果binCount大于等于转化为红黑树的阈值
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);
        return null;
    }

}