Android ConcurrentHashMap

以下内容基于Android Api 28 。

ConcurrentHashMap中，数据实际存储在其内部的Node数组table之中。保存数据时，实际调用putVal(K key, V value, boolean onlyIfAbsent)方法。

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) {

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)

tab = helpTransfer(tab, f);

else {

V oldVal = null;

synchronized (f) {

if (tabAt(tab, i) == f) {

if (fh >= 0) {

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;

if (!onlyIfAbsent)

e.val = value;

break;

}

Node pred = e;

if ((e = e.next) == null) {

pred.next = new Node(hash, key,

value, null);

break;

}

}

}

else if (f instanceof TreeBin) {

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) {

if (binCount >= TREEIFY_THRESHOLD)

treeifyBin(tab, i);

if (oldVal != null)

return oldVal;

break;

}

}

}

addCount(1L, binCount);

return null;

}

其中分为几种状态：

1.table为null；需要执行Node[] initTable()方法初始化Node数组，并将sizeCtl赋值为当前数组大小的0.75；

2.根据hash值计算的数组下标对应位置的Node为null；执行boolean casTabAt(Node[] tab, int i,Node c, Node v)方法初始化当前Node，操作为原子化操作；

3.需要扩容；执行helpTransfer(Node[] tab, Node f)，将table指向nextTable，nextTable大小为当前table的2倍；

4.查找对应Node中的保存位置；对当前下标位置对Node加锁，做二次检查，遍历链表，查找存放对位置。