hashmap（jdk1.8）扩容机制：
hashmap在实例化时（new HashMap()）时，只初始化了扩张因子
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted 默认值为0.75
}
向hashmap中添加元素时，主要使用方法 put(K, V);
源代码如下：
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0) //如果hashmap刚初始化，不含有任何元素时
n = (tab = resize()).length; //执行resize()方法，来初始化tab
if ((p = tab[i = (n - 1) & hash]) == null) //如果table在该下标下为null，则new一个node并赋值给该值
tab[i] = newNode(hash, key, value, null);
else { //否则就往后继续添加，链表的形式添加
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold) //如果当前size大于阀值，则就进行扩容操作
resize();
afterNodeInsertion(evict);
return null;
}
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table; //获取原来的table
int oldCap = (oldTab == null) ? 0 : oldTab.length; //获取node数组长度
int oldThr = threshold; //获取原有的阀值
int newCap, newThr = 0; //变量：存储新table的大小，新阀值的大小
if (oldCap > 0) { //如果已有node数组存在，不为0说明数组是存在
if (oldCap >= MAXIMUM_CAPACITY) { //如果超过最大允许容量(1<<30)，
threshold = Integer.MAX_VALUE; //设定最大阀值为0x7fffffff
return oldTab; //返回原来的table数组
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && //node数组长度设置为*2
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold 总容量*2
}
else if (oldThr > 0) // initial capacity was placed in threshold 若实例化时，采用的有参的构造函数，会进这一步
newCap = oldThr; //设置原来的阀值作为新的数组长度
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY; //默认的初始化
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); //16*扩展因子
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr; //重设阀值
@SuppressWarnings({"rawtypes","unchecked"})
//初始化新的node数组，大小为扩展后的大小
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null; //注意此处，多线程情况下，若线程同时进行操作，复制数据速度不一致，就会造成数据丢失。
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
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;
}
下面解释resize代码部分
该部分代码，主要将原数组中的数据移入新的数组中
紫色部分代码解释：
注： 在put时，数组下标的算法为 e.hash & (tabSize - 1)，在扩容后，该算法不变，只是将tabSize更新为newTabSize
newTabSize = tabSize << 1，e.hash & oldCap == 0时，则e.hash & (newTabSize-1) = 0 + e.hash & (tabSize - 1)
e.hash & oldCap != 0时，则e.hash & (newTabSize - 1) = oldCap + e.hash & (tabSize - 1)
举例：数组大小由 1 << 6(oldCap) 即 100 0000 扩容至 1 << 7(newCap)即 1000 0000
原数组上点 e.hash = p11 0001(p的值具体未知)
tabSize = 100 0000
newTabSize = 1000 0000
tabSize - 1 = 11 1111
newTabSize - 1 = 111 1111
所在的原数组下标 e.hash & (tabSize - 1) = 11 0001 &
11 1111 = 11 0001
若 p为0
新数组下标： e.hash & (newTabSize - 1) = 011 0001 &
111 1111 = 011 0001 = 0 + 11 0001 = 0 + e.hash & (tabSize - 1)
若p为1
新数组下标： e.hash & (newTabSize - 1) = 111 0001 &
111 1111 = 111 0001 = 1 << 6 + 11 0001 = oldCap + e.hash & (tabSize - 1)
从上述推导可以看出： e.hash & oldCap目的是为了计算e.hash与oldCap最高位（1）“与”运算的值，从而推算出p的值
同时也分为p为0或1的两种处理情况，对map中的所有元素重新遍历计算后，将其填进新node数组，完成扩容
从put操作中，我们可以看出，hashmap是thread-unsafe
特别是在扩容时，我们会发现，若两个线程同时进行扩容，就会造成数据丢失。因为扩容时，都涉及到重新分配至新数组。
如：A、B线程都进行写操作，两者均发现要超阀值，就进行扩容操作，但hashmap的扩容机制是重新分配至新数组，再将该新数组作为hashmap对象的table数组，这样就只会返回最后进行扩容的数组，这样就会导致其中一个线程的数据丢失。（因为在扩容时，会将原有数组下数据进行释放oldTab[j] =null）