聊聊java中的哪些Map：（八）ConcurrentSkipListMap源码分析

/**
 * A scalable concurrent {@link ConcurrentNavigableMap} implementation.
 * The map is sorted according to the {@linkplain Comparable natural
 * ordering} of its keys, or by a {@link Comparator} provided at map
 * creation time, depending on which constructor is used.
 *
 * <p>This class implements a concurrent variant of <a
 * href="http://en.wikipedia.org/wiki/Skip_list" target="_top">SkipLists</a>
 * providing expected average <i>log(n)</i> time cost for the
 * {@code containsKey}, {@code get}, {@code put} and
 * {@code remove} operations and their variants.  Insertion, removal,
 * update, and access operations safely execute concurrently by
 * multiple threads.
 *
 * <p>Iterators and spliterators are
 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
 *
 * <p>Ascending key ordered views and their iterators are faster than
 * descending ones.
 *
 * <p>All {@code Map.Entry} pairs returned by methods in this class
 * and its views represent snapshots of mappings at the time they were
 * produced. They do <em>not</em> support the {@code Entry.setValue}
 * method. (Note however that it is possible to change mappings in the
 * associated map using {@code put}, {@code putIfAbsent}, or
 * {@code replace}, depending on exactly which effect you need.)
 *
 * <p>Beware that, unlike in most collections, the {@code size}
 * method is <em>not</em> a constant-time operation. Because of the
 * asynchronous nature of these maps, determining the current number
 * of elements requires a traversal of the elements, and so may report
 * inaccurate results if this collection is modified during traversal.
 * Additionally, the bulk operations {@code putAll}, {@code equals},
 * {@code toArray}, {@code containsValue}, and {@code clear} are
 * <em>not</em> guaranteed to be performed atomically. For example, an
 * iterator operating concurrently with a {@code putAll} operation
 * might view only some of the added elements.
 *
 * <p>This class and its views and iterators implement all of the
 * <em>optional</em> methods of the {@link Map} and {@link Iterator}
 * interfaces. Like most other concurrent collections, this class does
 * <em>not</em> permit the use of {@code null} keys or values because some
 * null return values cannot be reliably distinguished from the absence of
 * elements.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author Doug Lea
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 * @since 1.6
 */
public class ConcurrentSkipListMap<K,V> extends AbstractMap<K,V>
    implements ConcurrentNavigableMap<K,V>, Cloneable, Serializable {
}

/*
     * This class implements a tree-like two-dimensionally linked skip
     * list in which the index levels are represented in separate
     * nodes from the base nodes holding data.  There are two reasons
     * for taking this approach instead of the usual array-based
     * structure: 1) Array based implementations seem to encounter
     * more complexity and overhead 2) We can use cheaper algorithms
     * for the heavily-traversed index lists than can be used for the
     * base lists.  Here's a picture of some of the basics for a
     * possible list with 2 levels of index:
     *
     * Head nodes          Index nodes
     * +-+    right        +-+                      +-+
     * |2|---------------->| |--------------------->| |->null
     * +-+                 +-+                      +-+
     *  | down              |                        |
     *  v                   v                        v
     * +-+            +-+  +-+       +-+            +-+       +-+
     * |1|----------->| |->| |------>| |----------->| |------>| |->null
     * +-+            +-+  +-+       +-+            +-+       +-+
     *  v              |    |         |              |         |
     * Nodes  next     v    v         v              v         v
     * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
     * | |->|A|->|B|->|C|->|D|->|E|->|F|->|G|->|H|->|I|->|J|->|K|->null
     * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
     *
     * The base lists use a variant of the HM linked ordered set
     * algorithm. See Tim Harris, "A pragmatic implementation of
     * non-blocking linked lists"
     * http://www.cl.cam.ac.uk/~tlh20/publications.html and Maged
     * Michael "High Performance Dynamic Lock-Free Hash Tables and
     * List-Based Sets"
     * http://www.research.ibm.com/people/m/michael/pubs.htm.  The
     * basic idea in these lists is to mark the "next" pointers of
     * deleted nodes when deleting to avoid conflicts with concurrent
     * insertions, and when traversing to keep track of triples
     * (predecessor, node, successor) in order to detect when and how
     * to unlink these deleted nodes.
     *
     * Rather than using mark-bits to mark list deletions (which can
     * be slow and space-intensive using AtomicMarkedReference), nodes
     * use direct CAS'able next pointers.  On deletion, instead of
     * marking a pointer, they splice in another node that can be
     * thought of as standing for a marked pointer (indicating this by
     * using otherwise impossible field values).  Using plain nodes
     * acts roughly like "boxed" implementations of marked pointers,
     * but uses new nodes only when nodes are deleted, not for every
     * link.  This requires less space and supports faster
     * traversal. Even if marked references were better supported by
     * JVMs, traversal using this technique might still be faster
     * because any search need only read ahead one more node than
     * otherwise required (to check for trailing marker) rather than
     * unmasking mark bits or whatever on each read.
     *
     * This approach maintains the essential property needed in the HM
     * algorithm of changing the next-pointer of a deleted node so
     * that any other CAS of it will fail, but implements the idea by
     * changing the pointer to point to a different node, not by
     * marking it.  While it would be possible to further squeeze
     * space by defining marker nodes not to have key/value fields, it
     * isn't worth the extra type-testing overhead.  The deletion
     * markers are rarely encountered during traversal and are
     * normally quickly garbage collected. (Note that this technique
     * would not work well in systems without garbage collection.)
     *
     * In addition to using deletion markers, the lists also use
     * nullness of value fields to indicate deletion, in a style
     * similar to typical lazy-deletion schemes.  If a node's value is
     * null, then it is considered logically deleted and ignored even
     * though it is still reachable. This maintains proper control of
     * concurrent replace vs delete operations -- an attempted replace
     * must fail if a delete beat it by nulling field, and a delete
     * must return the last non-null value held in the field. (Note:
     * Null, rather than some special marker, is used for value fields
     * here because it just so happens to mesh with the Map API
     * requirement that method get returns null if there is no
     * mapping, which allows nodes to remain concurrently readable
     * even when deleted. Using any other marker value here would be
     * messy at best.)
     *
     * Here's the sequence of events for a deletion of node n with
     * predecessor b and successor f, initially:
     *
     *        +------+       +------+      +------+
     *   ...  |   b  |------>|   n  |----->|   f  | ...
     *        +------+       +------+      +------+
     *
     * 1. CAS n's value field from non-null to null.
     *    From this point on, no public operations encountering
     *    the node consider this mapping to exist. However, other
     *    ongoing insertions and deletions might still modify
     *    n's next pointer.
     *
     * 2. CAS n's next pointer to point to a new marker node.
     *    From this point on, no other nodes can be appended to n.
     *    which avoids deletion errors in CAS-based linked lists.
     *
     *        +------+       +------+      +------+       +------+
     *   ...  |   b  |------>|   n  |----->|marker|------>|   f  | ...
     *        +------+       +------+      +------+       +------+
     *
     * 3. CAS b's next pointer over both n and its marker.
     *    From this point on, no new traversals will encounter n,
     *    and it can eventually be GCed.
     *        +------+                                    +------+
     *   ...  |   b  |----------------------------------->|   f  | ...
     *        +------+                                    +------+
     *
     * A failure at step 1 leads to simple retry due to a lost race
     * with another operation. Steps 2-3 can fail because some other
     * thread noticed during a traversal a node with null value and
     * helped out by marking and/or unlinking.  This helping-out
     * ensures that no thread can become stuck waiting for progress of
     * the deleting thread.  The use of marker nodes slightly
     * complicates helping-out code because traversals must track
     * consistent reads of up to four nodes (b, n, marker, f), not
     * just (b, n, f), although the next field of a marker is
     * immutable, and once a next field is CAS'ed to point to a
     * marker, it never again changes, so this requires less care.
     *
     * Skip lists add indexing to this scheme, so that the base-level
     * traversals start close to the locations being found, inserted
     * or deleted -- usually base level traversals only traverse a few
     * nodes. This doesn't change the basic algorithm except for the
     * need to make sure base traversals start at predecessors (here,
     * b) that are not (structurally) deleted, otherwise retrying
     * after processing the deletion.
     *
     * Index levels are maintained as lists with volatile next fields,
     * using CAS to link and unlink.  Races are allowed in index-list
     * operations that can (rarely) fail to link in a new index node
     * or delete one. (We can't do this of course for data nodes.)
     * However, even when this happens, the index lists remain sorted,
     * so correctly serve as indices.  This can impact performance,
     * but since skip lists are probabilistic anyway, the net result
     * is that under contention, the effective "p" value may be lower
     * than its nominal value. And race windows are kept small enough
     * that in practice these failures are rare, even under a lot of
     * contention.
     *
     * The fact that retries (for both base and index lists) are
     * relatively cheap due to indexing allows some minor
     * simplifications of retry logic. Traversal restarts are
     * performed after most "helping-out" CASes. This isn't always
     * strictly necessary, but the implicit backoffs tend to help
     * reduce other downstream failed CAS's enough to outweigh restart
     * cost.  This worsens the worst case, but seems to improve even
     * highly contended cases.
     *
     * Unlike most skip-list implementations, index insertion and
     * deletion here require a separate traversal pass occurring after
     * the base-level action, to add or remove index nodes.  This adds
     * to single-threaded overhead, but improves contended
     * multithreaded performance by narrowing interference windows,
     * and allows deletion to ensure that all index nodes will be made
     * unreachable upon return from a public remove operation, thus
     * avoiding unwanted garbage retention. This is more important
     * here than in some other data structures because we cannot null
     * out node fields referencing user keys since they might still be
     * read by other ongoing traversals.
     *
     * Indexing uses skip list parameters that maintain good search
     * performance while using sparser-than-usual indices: The
     * hardwired parameters k=1, p=0.5 (see method doPut) mean
     * that about one-quarter of the nodes have indices. Of those that
     * do, half have one level, a quarter have two, and so on (see
     * Pugh's Skip List Cookbook, sec 3.4).  The expected total space
     * requirement for a map is slightly less than for the current
     * implementation of java.util.TreeMap.
     *
     * Changing the level of the index (i.e, the height of the
     * tree-like structure) also uses CAS. The head index has initial
     * level/height of one. Creation of an index with height greater
     * than the current level adds a level to the head index by
     * CAS'ing on a new top-most head. To maintain good performance
     * after a lot of removals, deletion methods heuristically try to
     * reduce the height if the topmost levels appear to be empty.
     * This may encounter races in which it possible (but rare) to
     * reduce and "lose" a level just as it is about to contain an
     * index (that will then never be encountered). This does no
     * structural harm, and in practice appears to be a better option
     * than allowing unrestrained growth of levels.
     *
     * The code for all this is more verbose than you'd like. Most
     * operations entail locating an element (or position to insert an
     * element). The code to do this can't be nicely factored out
     * because subsequent uses require a snapshot of predecessor
     * and/or successor and/or value fields which can't be returned
     * all at once, at least not without creating yet another object
     * to hold them -- creating such little objects is an especially
     * bad idea for basic internal search operations because it adds
     * to GC overhead.  (This is one of the few times I've wished Java
     * had macros.) Instead, some traversal code is interleaved within
     * insertion and removal operations.  The control logic to handle
     * all the retry conditions is sometimes twisty. Most search is
     * broken into 2 parts. findPredecessor() searches index nodes
     * only, returning a base-level predecessor of the key. findNode()
     * finishes out the base-level search. Even with this factoring,
     * there is a fair amount of near-duplication of code to handle
     * variants.
     *
     * To produce random values without interference across threads,
     * we use within-JDK thread local random support (via the
     * "secondary seed", to avoid interference with user-level
     * ThreadLocalRandom.)
     *
     * A previous version of this class wrapped non-comparable keys
     * with their comparators to emulate Comparables when using
     * comparators vs Comparables.  However, JVMs now appear to better
     * handle infusing comparator-vs-comparable choice into search
     * loops. Static method cpr(comparator, x, y) is used for all
     * comparisons, which works well as long as the comparator
     * argument is set up outside of loops (thus sometimes passed as
     * an argument to internal methods) to avoid field re-reads.
     *
     * For explanation of algorithms sharing at least a couple of
     * features with this one, see Mikhail Fomitchev's thesis
     * (http://www.cs.yorku.ca/~mikhail/), Keir Fraser's thesis
     * (http://www.cl.cam.ac.uk/users/kaf24/), and Hakan Sundell's
     * thesis (http://www.cs.chalmers.se/~phs/).
     *
     * Given the use of tree-like index nodes, you might wonder why
     * this doesn't use some kind of search tree instead, which would
     * support somewhat faster search operations. The reason is that
     * there are no known efficient lock-free insertion and deletion
     * algorithms for search trees. The immutability of the "down"
     * links of index nodes (as opposed to mutable "left" fields in
     * true trees) makes this tractable using only CAS operations.
     *
     * Notation guide for local variables
     * Node:         b, n, f    for  predecessor, node, successor
     * Index:        q, r, d    for index node, right, down.
     *               t          for another index node
     * Head:         h
     * Levels:       j
     * Keys:         k, key
     * Values:       v, value
     * Comparisons:  c
     */

     * Head nodes          Index nodes
     * +-+    right        +-+                      +-+
     * |2|---------------->| |--------------------->| |->null
     * +-+                 +-+                      +-+
     *  | down              |                        |
     *  v                   v                        v
     * +-+            +-+  +-+       +-+            +-+       +-+
     * |1|----------->| |->| |------>| |----------->| |------>| |->null
     * +-+            +-+  +-+       +-+            +-+       +-+
     *  v              |    |         |              |         |
     * Nodes  next     v    v         v              v         v
     * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
     * | |->|A|->|B|->|C|->|D|->|E|->|F|->|G|->|H|->|I|->|J|->|K|->null
     * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
     *

 *        +------+       +------+      +------+
 *   ...  |   b  |------>|   n  |----->|   f  | ...
 *        +------+       +------+      +------+

*        +------+       +------+      +------+       +------+
*   ...  |   b  |------>|   n  |----->|marker|------>|   f  | ...
*        +------+       +------+      +------+       +------+
*

*        +------+                                    +------+
*   ...  |   b  |----------------------------------->|   f  | ...
*        +------+                                    +------+
*

private static final long serialVersionUID = -8627078645895051609L;

/**
 * Special value used to identify base-level header
 */
private static final Object BASE_HEADER = new Object();

/**
 * The topmost head index of the skiplist.
 */
private transient volatile HeadIndex<K,V> head;

/**
 * The comparator used to maintain order in this map, or null if
 * using natural ordering.  (Non-private to simplify access in
 * nested classes.)
 * @serial
 */
final Comparator<? super K> comparator;

/** Lazily initialized key set */
private transient KeySet<K> keySet;
/** Lazily initialized entry set */
private transient EntrySet<K,V> entrySet;
/** Lazily initialized values collection */
private transient Values<V> values;
/** Lazily initialized descending key set */
private transient ConcurrentNavigableMap<K,V> descendingMap;

/**
 * Constructs a new, empty map, sorted according to the
 * {@linkplain Comparable natural ordering} of the keys.
 */
public ConcurrentSkipListMap() {
    this.comparator = null;
    initialize();
}

/**
 * Initializes or resets state. Needed by constructors, clone,
 * clear, readObject. and ConcurrentSkipListSet.clone.
 * (Note that comparator must be separately initialized.)
 */
private void initialize() {
    keySet = null;
    entrySet = null;
    values = null;
    descendingMap = null;
    head = new HeadIndex<K,V>(new Node<K,V>(null, BASE_HEADER, null),
                              null, null, 1);
}

/**
 * Constructs a new, empty map, sorted according to the specified
 * comparator.
 *
 * @param comparator the comparator that will be used to order this map.
 *        If {@code null}, the {@linkplain Comparable natural
 *        ordering} of the keys will be used.
 */
public ConcurrentSkipListMap(Comparator<? super K> comparator) {
    this.comparator = comparator;
    initialize();
}

/**
 * Constructs a new map containing the same mappings as the given map,
 * sorted according to the {@linkplain Comparable natural ordering} of
 * the keys.
 *
 * @param  m the map whose mappings are to be placed in this map
 * @throws ClassCastException if the keys in {@code m} are not
 *         {@link Comparable}, or are not mutually comparable
 * @throws NullPointerException if the specified map or any of its keys
 *         or values are null
 */
public ConcurrentSkipListMap(Map<? extends K, ? extends V> m) {
    this.comparator = null;
    initialize();
    putAll(m);
}

public void putAll(Map<? extends K, ? extends V> m) {
    for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
        put(e.getKey(), e.getValue());
}

/**
 * Constructs a new map containing the same mappings and using the
 * same ordering as the specified sorted map.
 *
 * @param m the sorted map whose mappings are to be placed in this
 *        map, and whose comparator is to be used to sort this map
 * @throws NullPointerException if the specified sorted map or any of
 *         its keys or values are null
 */
public ConcurrentSkipListMap(SortedMap<K, ? extends V> m) {
    this.comparator = m.comparator();
    initialize();
    buildFromSorted(m);
}

private void buildFromSorted(SortedMap<K, ? extends V> map) {
    //map不能为null
    if (map == null)
        throw new NullPointerException();
    //定义head索引和basepred节点。
    HeadIndex<K,V> h = head;
    Node<K,V> basepred = h.node;

    // Track the current rightmost node at each level. Uses an
    // ArrayList to avoid committing to initial or maximum level.
    //在list中定义index
    ArrayList<Index<K,V>> preds = new ArrayList<Index<K,V>>();
    //初始化
    // initialize
    for (int i = 0; i <= h.level; ++i)
        preds.add(null);
    Index<K,V> q = h;
    //初始化最右边的head节点
    for (int i = h.level; i > 0; --i) {
        preds.set(i, q);
        q = q.down;
    }

    //定义entry的迭代器
    Iterator<? extends Map.Entry<? extends K, ? extends V>> it =
        map.entrySet().iterator();
    //迭代遍历
    while (it.hasNext()) {
        //取出next
        Map.Entry<? extends K, ? extends V> e = it.next();
        //随机数
        int rnd = ThreadLocalRandom.current().nextInt();
        int j = 0;
        //用随机数 将随机数截取之后右移动再和1取模
        if ((rnd & 0x80000001) == 0) {
            do {
                ++j;
            } while (((rnd >>>= 1) & 1) != 0);
            if (j > h.level) j = h.level + 1;
        }
        //上述部分根据随机数计算level判断节点index的层数
        K k = e.getKey();
        V v = e.getValue();
        //k v不能为null
        if (k == null || v == null)
            throw new NullPointerException();
        //new一个node 并将basepred指向这个节点
        Node<K,V> z = new Node<K,V>(k, v, null);
        basepred.next = z;
        basepred = z;
        //根据层数 进行构造
        if (j > 0) {
            Index<K,V> idx = null;
            for (int i = 1; i <= j; ++i) {
                idx = new Index<K,V>(z, idx, null);
                if (i > h.level)
                    h = new HeadIndex<K,V>(h.node, h, idx, i);

                if (i < preds.size()) {
                    preds.get(i).right = idx;
                    preds.set(i, idx);
                } else
                    preds.add(idx);
            }
        }
    }
    head = h;
}

private static final sun.misc.Unsafe UNSAFE;
private static final long valueOffset;
private static final long nextOffset;

static {
    try {
        UNSAFE = sun.misc.Unsafe.getUnsafe();
        Class<?> k = Node.class;
        valueOffset = UNSAFE.objectFieldOffset
            (k.getDeclaredField("value"));
        nextOffset = UNSAFE.objectFieldOffset
            (k.getDeclaredField("next"));
    } catch (Exception e) {
        throw new Error(e);
    }
}
}

/**
 * Nodes hold keys and values, and are singly linked in sorted
 * order, possibly with some intervening marker nodes. The list is
 * headed by a dummy node accessible as head.node. The value field
 * is declared only as Object because it takes special non-V
 * values for marker and header nodes.
 */
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;
    }

    /**
     * Creates a new marker node. A marker is distinguished by
     * having its value field point to itself.  Marker nodes also
     * have null keys, a fact that is exploited in a few places,
     * but this doesn't distinguish markers from the base-level
     * header node (head.node), which also has a null key.
     */
    Node(Node<K,V> next) {
        this.key = null;
        this.value = this;
        this.next = next;
    }
}

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;
    }

    /**
     * compareAndSet right field
     */
    final boolean casRight(Index<K,V> cmp, Index<K,V> val) {
        return UNSAFE.compareAndSwapObject(this, rightOffset, cmp, val);
    }

    /**
     * Returns true if the node this indexes has been deleted.
     * @return true if indexed node is known to be deleted
     */
    final boolean indexesDeletedNode() {
        return node.value == null;
    }

    /**
     * Tries to CAS newSucc as successor.  To minimize races with
     * unlink that may lose this index node, if the node being
     * indexed is known to be deleted, it doesn't try to link in.
     * @param succ the expected current successor
     * @param newSucc the new successor
     * @return true if successful
     */
    final boolean link(Index<K,V> succ, Index<K,V> newSucc) {
        Node<K,V> n = node;
        newSucc.right = succ;
        return n.value != null && casRight(succ, newSucc);
    }

    /**
     * Tries to CAS right field to skip over apparent successor
     * succ.  Fails (forcing a retraversal by caller) if this node
     * is known to be deleted.
     * @param succ the expected current successor
     * @return true if successful
     */
    final boolean unlink(Index<K,V> succ) {
        return node.value != null && casRight(succ, succ.right);
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long rightOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = Index.class;
            rightOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("right"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}

/**
 * Nodes heading each level keep track of their level.
 */
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;
    }
}

/**
 * Returns the value to which the specified key is mapped,
 * or {@code null} if this map contains no mapping for the key.
 *
 * <p>More formally, if this map contains a mapping from a key
 * {@code k} to a value {@code v} such that {@code key} compares
 * equal to {@code k} according to the map's ordering, then this
 * method returns {@code v}; otherwise it returns {@code null}.
 * (There can be at most one such mapping.)
 *
 * @throws ClassCastException if the specified key cannot be compared
 *         with the keys currently in the map
 * @throws NullPointerException if the specified key is null
 */
public V get(Object key) {
    return doGet(key);
}

/**
 * Gets value for key. Almost the same as findNode, but returns
 * the found value (to avoid retries during re-reads)
 *
 * @param key the key
 * @return the value, or null if absent
 */
private V doGet(Object key) {
    //key不能为空
    if (key == null)
        throw new NullPointerException();
    //比较器
    Comparator<? super K> cmp = comparator;
    //死循环
    outer: for (;;) {
        //内层循环 next
        for (Node<K,V> b = findPredecessor(key, cmp), n = b.next;;) {
            Object v; int c;
            //首先查找next如果为空则重新开始
            if (n == null)
                break outer;
            Node<K,V> f = n.next;
            //如果next与n不等 则break
            if (n != b.next)                // inconsistent read
                break;
            //如果n的value为null 则是已删除的节点调用delete
            if ((v = n.value) == null) {    // n is deleted
                n.helpDelete(b, f);
                break;
            }
            //如果b的value为null 那么同理删除b
            if (b.value == null || v == n)  // b is deleted
                break;
                //cpr方法返回v
            if ((c = cpr(cmp, key, n.key)) == 0) {
                @SuppressWarnings("unchecked") V vv = (V)v;
                return vv;
            }
            //重新开始循环
            if (c < 0)
                break outer;
            b = n;
            n = f;
        }
    }
    return null;
}

/**
 * Compares using comparator or natural ordering if null.
 * Called only by methods that have performed required type checks.
 */
@SuppressWarnings({"unchecked", "rawtypes"})
static final int cpr(Comparator c, Object x, Object y) {
    return (c != null) ? c.compare(x, y) : ((Comparable)x).compareTo(y);
}

/**
 * Associates the specified value with the specified key in this map.
 * If the map previously contained a mapping for the key, the old
 * value is replaced.
 *
 * @param key key with which the specified value is to be associated
 * @param value value to be associated with the specified key
 * @return the previous value associated with the specified key, or
 *         {@code null} if there was no mapping for the key
 * @throws ClassCastException if the specified key cannot be compared
 *         with the keys currently in the map
 * @throws NullPointerException if the specified key or value is null
 */
public V put(K key, V value) {
    if (value == null)
        throw new NullPointerException();
    return doPut(key, value, false);
}

/**
 * Main insertion method.  Adds element if not present, or
 * replaces value if present and onlyIfAbsent is false.
 * @param key the key
 * @param value the value that must be associated with key
 * @param onlyIfAbsent if should not insert if already present
 * @return the old value, or null if newly inserted
 */
private V doPut(K key, V value, boolean onlyIfAbsent) {
    Node<K,V> z;             // added node
    //key不能为null
    if (key == null)
        throw new NullPointerException();
    //定义比较器
    Comparator<? super K> cmp = comparator;
    //死循环
    outer: for (;;) {
        //循环查找
        for (Node<K,V> b = findPredecessor(key, cmp), n = b.next;;) {
            if (n != null) {
                Object v; int c;
                Node<K,V> f = n.next;
                //n不等于next 则返回
                if (n != b.next)               // inconsistent read
                    break;
                //如果v为null触发删除
                if ((v = n.value) == null) {   // n is deleted
                    n.helpDelete(b, f);
                    break;
                }
                //如果b为null触发删除
                if (b.value == null || v == n) // b is deleted
                    break;
                //cpr查找
                if ((c = cpr(cmp, key, n.key)) > 0) {
                    b = n;
                    n = f;
                    continue;
                }
                //c==0  设置值
                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
            }
            //new一个新节点
            z = new Node<K,V>(key, value, n);
            if (!b.casNext(n, z))
                break;         // restart if lost race to append to b
            break outer;
        }
    }
    //计算这个新节点的随机数 
    int rnd = ThreadLocalRandom.nextSecondarySeed();
    //根据随机数计算level
    if ((rnd & 0x80000001) == 0) { // test highest and lowest bits
        int level = 1, max;
        while (((rnd >>>= 1) & 1) != 0)
            ++level;
        Index<K,V> idx = null;
        HeadIndex<K,V> h = head;
        if (level <= (max = h.level)) {
            for (int i = 1; i <= level; ++i)
                idx = new Index<K,V>(z, idx, null);
        }
        //循环遍历插入，每层都需要
        else { // try to grow by one level
            level = max + 1; // hold in array and later pick the one to use
            @SuppressWarnings("unchecked")Index<K,V>[] idxs =
                (Index<K,V>[])new Index<?,?>[level+1];
            for (int i = 1; i <= level; ++i)
                idxs[i] = idx = new Index<K,V>(z, idx, null);
            for (;;) {
                h = head;
                int oldLevel = h.level;
                if (level <= oldLevel) // lost race to add level
                    break;
                HeadIndex<K,V> newh = h;
                Node<K,V> oldbase = h.node;
                for (int j = oldLevel+1; j <= level; ++j)
                    newh = new HeadIndex<K,V>(oldbase, newh, idxs[j], j);
                if (casHead(h, newh)) {
                    h = newh;
                    idx = idxs[level = oldLevel];
                    break;
                }
            }
        }
        // find insertion points and splice in
        splice: for (int insertionLevel = level;;) {
            int j = h.level;
            for (Index<K,V> q = h, r = q.right, t = idx;;) {
                if (q == null || t == null)
                    break splice;
                if (r != null) {
                    Node<K,V> n = r.node;
                    // compare before deletion check avoids needing recheck
                    int c = cpr(cmp, key, n.key);
                    if (n.value == null) {
                        if (!q.unlink(r))
                            break;
                        r = q.right;
                        continue;
                    }
                    if (c > 0) {
                        q = r;
                        r = r.right;
                        continue;
                    }
                }

                if (j == insertionLevel) {
                    if (!q.link(r, t))
                        break; // restart
                    if (t.node.value == null) {
                        findNode(key);
                        break splice;
                    }
                    if (--insertionLevel == 0)
                        break splice;
                }

                if (--j >= insertionLevel && j < level)
                    t = t.down;
                q = q.down;
                r = q.right;
            }
        }
    }
    return null;
}

/**
 * Removes the mapping for the specified key from this map if present.
 *
 * @param  key key for which mapping should be removed
 * @return the previous value associated with the specified key, or
 *         {@code null} if there was no mapping for the key
 * @throws ClassCastException if the specified key cannot be compared
 *         with the keys currently in the map
 * @throws NullPointerException if the specified key is null
 */
public V remove(Object key) {
    return doRemove(key, null);
}

/**
 * Main deletion method. Locates node, nulls value, appends a
 * deletion marker, unlinks predecessor, removes associated index
 * nodes, and possibly reduces head index level.
 *
 * Index nodes are cleared out simply by calling findPredecessor.
 * which unlinks indexes to deleted nodes found along path to key,
 * which will include the indexes to this node.  This is done
 * unconditionally. We can't check beforehand whether there are
 * index nodes because it might be the case that some or all
 * indexes hadn't been inserted yet for this node during initial
 * search for it, and we'd like to ensure lack of garbage
 * retention, so must call to be sure.
 *
 * @param key the key
 * @param value if non-null, the value that must be
 * associated with key
 * @return the node, or null if not found
 */
final V doRemove(Object key, Object value) {
    //不能为空
    if (key == null)
        throw new NullPointerException();
        //定义比较器
    Comparator<? super K> cmp = comparator;
    //死循环
    outer: for (;;) {
        for (Node<K,V> b = findPredecessor(key, cmp), n = b.next;;) {
            Object v; int c;
            //n为null则返回
            if (n == null)
                break outer;
            Node<K,V> f = n.next;
            //如果不等则返回
            if (n != b.next)                    // inconsistent read
                break;
            //如果v为null则触发删除
            if ((v = n.value) == null) {        // n is deleted
                n.helpDelete(b, f);
                break;
            }
            //如果b为null则触发删除b
            if (b.value == null || v == n)      // b is deleted
                break;
            if ((c = cpr(cmp, key, n.key)) < 0)
                break outer;
            if (c > 0) {
                b = n;
                n = f;
                continue;
            }
            if (value != null && !value.equals(v))
                break outer;
            if (!n.casValue(v, null))
                break;
            if (!n.appendMarker(f) || !b.casNext(n, f))
                findNode(key);                  // retry via findNode
            else {
                findPredecessor(key, cmp);      // clean index
                if (head.right == null)
                    tryReduceLevel();
            }
            @SuppressWarnings("unchecked") V vv = (V)v;
            return vv;
        }
    }
    return null;
}

/**
 * Returns a base-level node with key strictly less than given key,
 * or the base-level header if there is no such node.  Also
 * unlinks indexes to deleted nodes found along the way.  Callers
 * rely on this side-effect of clearing indices to deleted nodes.
 * @param key the key
 * @return a predecessor of key
 */
private Node<K,V> findPredecessor(Object key, Comparator<? super K> cmp) {
    //key不能为空 
    if (key == null)
        throw new NullPointerException(); // don't postpone errors
    //死循环
    for (;;) {
        //二级死循环
        for (Index<K,V> q = head, r = q.right, d;;) {
            //right如果不为空
            if (r != null) {
                Node<K,V> n = r.node;
                K k = n.key;
                //如果value为null 则执行unlink
                if (n.value == null) {
                    if (!q.unlink(r))
                        break;           // restart
                    r = q.right;         // reread r
                    continue;
                }
                //通过cmp方法比较
                if (cpr(cmp, key, k) > 0) {
                    q = r;
                    r = r.right;
                    continue;
                }
            }
            if ((d = q.down) == null)
                return q.node;
            q = d;
            r = d.right;
        }
    }
}

public int size() {
    long count = 0;
    for (Node<K,V> n = findFirst(); n != null; n = n.next) {
        if (n.getValidValue() != null)
            ++count;
    }
    return (count >= Integer.MAX_VALUE) ? Integer.MAX_VALUE : (int) count;
}