ConcurrentHashMap核心操作

继承/实现

//和HashMap一样 AbstractMap子类，但接口实现自Map的子接口ConcurrentMap
ConcurrentHashMap<K,V> extends AbstractMap<K,V>
    implements ConcurrentMap<K,V>, Serializable

构造函数

//无参方法 默认大小16 只有在第一次存入数据才会进行真正的初始化（所有构造函数里唯一一个没有设置 sizeCtl的 ）
    public ConcurrentHashMap() {
    }

  //指定初始化大小，实际上以默认参数调用复杂初始化
    public ConcurrentHashMap(int initialCapacity) {
        this(initialCapacity, LOAD_FACTOR, 1);
    }
//将现有map数据载入一个新的实例
    public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
        this.sizeCtl = DEFAULT_CAPACITY;
//putAll方法内会重新初始化一次大小
        putAll(m);
    }
    public ConcurrentHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, 1);
    }
//基础构造函数，从说明和作用范围表示loadFactor和concurrencyLevel仅仅负责初始化（向下兼容），不会影响实际的扩容因子
    public ConcurrentHashMap(int initialCapacity,
                             float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        if (initialCapacity < concurrencyLevel)   // Use at least as many bins
            initialCapacity = concurrencyLevel;   // as estimated threads
        long size = (long)(1.0 + (long)initialCapacity / loadFactor);
//这里根据initialCapacity确定初始化大小，实际会取用大于initialCapacity且最小的2的幂次
        int cap = (size >= (long)MAXIMUM_CAPACITY) ?
            MAXIMUM_CAPACITY : tableSizeFor((int)size);
        this.sizeCtl = cap;
    }

put方法实现

//调用putVal方法
public V put(K key, V value) {
        return putVal(key, value, false);
    }
//putAll，初始化size后再一个个执行putVal
 public void putAll(Map<? extends K, ? extends V> m) {
        tryPresize(m.size());
        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
            putVal(e.getKey(), e.getValue(), false);
    }
//最终实现方法
final V putVal(K key, V value, boolean onlyIfAbsent) {
        if (key == null || value == null) throw new NullPointerException();
//hash计算
        int hash = spread(key.hashCode());
        int binCount = 0;
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh; K fk; V fv;
//并发下懒初始化行为
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
//查看桶位头节点，为空则进行cas竞争，成功获取cas则对此桶赋予新头节点。cas成功后退出自旋
            else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
//竞争，并赋予头节点
                if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value)))
                    break;                   // no lock when adding to empty bin
            }
//头节点为FWD节点则处理扩容
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
//判断是不是putIfAbsent，直接检查节点（有冲突不会发生写操作所以，能写则表示没冲突）
            else if (onlyIfAbsent // check first node without acquiring lock
                     && fh == hash
                     && ((fk = f.key) == key || (fk != null && key.equals(fk)))
                     && (fv = f.val) != null)
                return fv;
            else {
//旧值赋予
                V oldVal = null;
//使用了sync
                synchronized (f) {
//对头节点的双重检查
                    if (tabAt(tab, i) == f) {
//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;
                                    if (!onlyIfAbsent)
                                        e.val = value;
                                    break;
                                }
                                Node<K,V> pred = e;
                                if ((e = e.next) == null) {
                                    pred.next = new Node<K,V>(hash, key, value);
                                    break;
                                }
                            }
                        }
//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;
                            }
                        }
                        else if (f instanceof ReservationNode)
                            throw new IllegalStateException("Recursive update");
                    }
                }
//判断是不是需要节点转换，链表模式下bincount会大于8
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
//累加
        addCount(1L, binCount);
        return null;
    }

扩容判断

节点转换

private final void treeifyBin(Node<K,V>[] tab, int index) {
        Node<K,V> b; int n;
        if (tab != null) {
//只有在桶位置超过64的情况下才会考虑转换成红黑树，否则扩容
            if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
                tryPresize(n << 1);
            else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
//否则sync锁对应桶下标元素
                synchronized (b) {
                    if (tabAt(tab, index) == b) {
                        TreeNode<K,V> hd = null, tl = null;
                        for (Node<K,V> e = b; e != null; e = e.next) {
                            TreeNode<K,V> p =
                                new TreeNode<K,V>(e.hash, e.key, e.val,
                                                  null, null);
                            if ((p.prev = tl) == null)
                                hd = p;
                            else
                                tl.next = p;
                            tl = p;
                        }
                        setTabAt(tab, index, new TreeBin<K,V>(hd));
                    }
                }
            }
        }
    }