Java系列-ConcurrentHashMap源码-putVal

1.putVal

cas+自旋保证线程安全

处理某个槽位时使用synchronized

public class ConcurrentHashMap extends AbstractMap
    implements ConcurrentMap, Serializable {

    static final  Node tabAt(Node[] tab, int i) {
        return (Node)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
    }

    static final  boolean casTabAt(Node[] tab, int i,
                                        Node c, Node v) {
        return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
    }  
  
    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[] tab = table;;) {
            Node f; 
            int n, i, fh;
            if (tab == null || (n = tab.length) == 0){
                tab = initTable();
            } else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {//tab[i]
                //创建Node，放到tab[i]
                if (casTabAt(tab, i, null,
                             new Node(hash, key, value, null))){
                    break;                   // no lock when adding to empty bin
                }
            } else if ((fh = f.hash) == MOVED){//表示正在扩容
                tab = helpTransfer(tab, f); //协助扩容
            } else {
                V oldVal = null;
                synchronized (f) { //对tab[i]加锁
                    if (tabAt(tab, i) == f) {
                        if (fh >= 0) {//hash>=0 表示是普通的链表结构
                            binCount = 1;
                            for (Node e = f;; ++binCount) {
                                K ek;
                                if (e.hash == hash &&
                                    ((ek = e.key) == key ||
                                     (ek != null && key.equals(ek)))) {
                                    oldVal = e.val;//找到key一样的节点，更新val
                                    if (!onlyIfAbsent){
                                        e.val = value;
                                    }
                                    break;
                                }
                                Node pred = e;
                                if ((e = e.next) == null) {//遍历到链表结尾也没有key相等的
                                    //放到链表尾
                                    pred.next = new Node(hash, key, value, null);
                                    break;
                                }
                            }
                        } else if (f instanceof TreeBin) {//如果是tree
                            Node p;
                            binCount = 2;
                            if ((p = ((TreeBin)f).putTreeVal(hash, key,
                                                           value)) != null) {
                                oldVal = p.val;
                                if (!onlyIfAbsent){
                                    p.val = value;
                                }
                            }
                        } else if (f instanceof ReservationNode){
                            throw new IllegalStateException("Recursive update");
                        }
                    }
                }
                if (binCount != 0) {//看是否需要扩容
                    if (binCount >= TREEIFY_THRESHOLD){
                        treeifyBin(tab, i);
                    }
                    if (oldVal != null){
                        return oldVal;
                    }
                    break;
                }
            }
        }
        addCount(1L, binCount);//维护size和看是否需要扩容
        return null;
    }
}