1 put操作
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
//tab即bucket数组 n为数组长度
Node<K,V>[] tab; Node<K,V> p; int n, i;
//如果bucket数组为空
if ((tab = table) == null || (n = tab.length) == 0)
//扩容 n为扩容后bucket数组的长度 即原来的2倍
n = (tab = resize()).length;
//将待插入node的hash&(n-1)后得到索引，如果该索引处为空
if ((p = tab[i = (n - 1) & hash]) == null)
//将待插入node放入的该索引处
tab[i] = newNode(hash, key, value, null);
else {
//p为上一步计算得索引下的头结点,k是p的key值
Node<K,V> e; K k;
//hash相等，并且key也相等，则该key存在，将p赋给e
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//为红黑树
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//遍历链表
for (int binCount = 0; ; ++binCount) {
//如果p的后继为空
if ((e = p.next) == null) {
//将插入到p的后继
p.next = newNode(hash, key, value, null);
//当node数大于等于8时，转为红黑树
//插入一个结点后，node已经是8个了，然而此时binCount还
//未更新仍是7；所以这里是binCount>=8-1.
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//key值存在，终止循环
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
//p指针沿链表下移
p = e;
}
}
//已存在的关于key的键值对
if (e != null) { // existing mapping for key
V oldValue = e.value;
//onlyIfAbsent表示是否仅在oldValue为空时更新键值对
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}2、扩容
final Node<K,V>[] resize() {
//原bucket数组
Node<K,V>[] oldTab = table;
//原bucket数组的容量
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
//容量已经最大了，返回原bucket数组,随你怎么碰撞也没办法了
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//扩容为原来的两倍
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; //threshold也扩大为原来的两倍
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
//将原本的node重新分到新的hashmap中
if (oldTab != null) {
//遍历bucket数组
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
//释放原本的空间
oldTab[j] = null;
//如果next节点为空
if (e.next == null)
//重新hash，并放入相应的位置
newTab[e.hash & (newCap - 1)] = e;
//如果是红黑树的根节点
else if (e instanceof TreeNode)
//拆树
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
//next节点不为空
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
//链表中的后继节点
next = e.next;
//为0表明(e.hash & (newCap - 1))还会和 e.hash & (oldCap - 1)一
//样,即该节点所处索引不变，即在新map的未扩容之前的部分 low链表中
//因为扩容一次，原本的链表最多会成为两个链表，一个在原来的索引下
//(low链表)一个在扩容后大一点的索引中(high链表)。这样扩容时构造链
//表的效率很高，而且不会产生循环链表
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
1.8下，resize操作维护了两个链表，每次在尾部插入节点;在多线程下也不会造成环形链表