ConcurrentHashMap

Posted 2020-12-29 renqiqiang

ConcurrentHashMap探究

• ConcurrentHashMap的key和value不能为空

Node

    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        volatile V val;
        volatile Node<K,V> next;
    }

put操作

hash操作

    // (h = key.hashCode()) ^ (h >>> 16)
    static final int spread(int h) {
        //比HashMap多并上了一个HASH_BITS
        //HASH_BITS
        //static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash
        //HASH_BITS是0x7fffffff，该步是为了消除最高位上的负符号 hash的负在ConcurrentHashMap中有特殊意义表示在扩容或者是树节点
        return (h ^ (h >>> 16)) & HASH_BITS;  
    }

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

  //casTabAt主要用于CAS比较该索引处是否为null防止其它线程已改变该值，null则插入，break结束死循环  
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);
    }

    //获取数组i索引的Node CAS操作 保持其它线程对table的改变在这里可见
static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
        return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
    }

    /** Implementation for put and putIfAbsent */
    final V putVal(K key, V value, boolean onlyIfAbsent) {
        //key或者value为空，抛出空指针异常
        if (key == null || value == null) throw new NullPointerException();
        //获取hash值
        int hash = spread(key.hashCode());
        int binCount = 0;
        //???
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh;
            //table为null或空map时进行初始化
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            // 判断当前索引位置的数组上是否有值  无值那就是首元素插入
            //获取数组i索引的Node CAS操作 保持其它线程对table的改变在这里可见
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
                //casTabAt主要用于CAS比较该索引处是否为null防止其它线程已改变该值，null则插入，break结束死循环
                if (casTabAt(tab, i, null,
                             new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            }
            //首地址处不null 并且Node的hash是-1  表示是ForwardingNode节点正在rehash扩容
            else if ((fh = f.hash) == MOVED)
                //帮助扩容
                tab = helpTransfer(tab, f);
            else {
                V oldVal = null;
                //对桶的首元素上锁独占
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                          // 桶中键值对组织形式是链表
                        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;
                                     // 查找到对应键值对，更新值
                                    if (!onlyIfAbsent)
                                        e.val = value;
                                    break;
                                }
                                Node<K,V> pred = e;
                                // 桶中没有对应键值对，插入到链表尾部
                                if ((e = e.next) == null) {
                                    pred.next = new Node<K,V>(hash, key,
                                                              value, null);
                                    break;
                                }
                            }
                        }
                          // 桶中键值对组织形式是红黑树
                        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;
                            }
                        }
                    }
                }
                 // 检查桶中键值对总数
                if (binCount != 0) {
                    //大于固定长度，转换为红黑树
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        // 更新baseCount
        addCount(1L, binCount);
        return null;
    }