jdk 源码分析（5）java ConcurrentSkipListMap结构

理解ConcurrentSkipListMap 只需要理解存储方式就行了

跳表结构之下往上分层，比如需要找117，首先会判断最高层，先比较21，在比较37 ，再比较37左边的，因为左边结束了，所以只能往下走，一直找到目标为止：

寻找规则，从最高层开始，先右，再下，直到最后。

1）单个节点定义：链式。

static final class Node<K,V> {    final K key;    volatile Object value;    volatile Node<K,V> next;    /**     * Creates a new regular node.     */    Node(K key, Object value, Node<K,V> next) {        this.key = key;        this.value = value;        this.next = next;    }

2）单个节点，主要增加对right和down的指引

static class Index<K,V> {    final Node<K,V> node;    final Index<K,V> down;    volatile Index<K,V> right;    /**     * Creates index node with given values.     */    Index(Node<K,V> node, Index<K,V> down, Index<K,V> right) {        this.node = node;        this.down = down;        this.right = right;    }

3）再次包装，产生对leve的定义。

static final class HeadIndex<K,V> extends Index<K,V> {    final int level;    HeadIndex(Node<K,V> node, Index<K,V> down, Index<K,V> right, int level) {        super(node, down, right);        this.level = level;    }}

如果对数据结构了解，上面headIndex 就可以构成跳表。

put的时候查找位置：

private Node<K,V> findPredecessor(Object key, Comparator<? super K> cmp) {
        if (key == null)
            throw new NullPointerException(); // don't postpone errors
        for (;;) {//先找right
            for (Index<K,V> q = head, r = q.right, d;;) {
                if (r != null) {
                    Node<K,V> n = r.node;
                    K k = n.key;
                    if (n.value == null) {
                        if (!q.unlink(r))
                            break;           // restart
                        r = q.right;         // reread r
                        continue;
                    }
                    if (cpr(cmp, key, k) > 0) {
                        q = r;
                        r = r.right;
                        continue;
                    }
                }
             //再找down
                if ((d = q.down) == null)
                    return q.node;
                q = d;
                r = d.right;
            }
        }

可以看到都是先右边再下边。

不过这里的跳表有点不同，每个节点不是一个单点，而是一个链表。通过next 连接。

 for (Node<K,V> b = findPredecessor(key, cmp), n = b.next;;) {
                if (n != null) {
                    Object v; int c;
                    Node<K,V> f = n.next;
                    if (n != b.next)               // inconsistent read
                        break;
                    if ((v = n.value) == null) {   // n is deleted
                        n.helpDelete(b, f);
                        break;
                    }
                    if (b.value == null || v == n) // b is deleted
                        break;
                    if ((c = cpr(cmp, key, n.key)) > 0) {
                        b = n;
                        n = f;
                        continue;
                    }
                    if (c == 0) {
                        if (onlyIfAbsent || n.casValue(v, value)) {
                            @SuppressWarnings("unchecked") V vv = (V)v;
                            return vv;
                        }
                        break; // restart if lost race to replace value
                    }
                    // else c < 0; fall through
                }
                z = new Node<K,V>(key, value, n);
                if (!b.casNext(n, z))
                    break;         // restart if lost race to append to b
                break outer;
            }
        }

里面还有// find insertion points and splice in

数据的level配置，已经插入新的节点。