一、简介
- HashMap的源码我们之前解读过，数组加链表，链表过长时裂变为红黑树。自动扩容机制没细说，今天详细看一下
往期回顾：
- Java1.7的HashMap源码分析-面试必备技能
- Java1.8的HashMap源码分析-面试必备技能
二、扩容机制
先说结论：
- hashmap的容量都是2的倍数，比如2，4，8，16，32，64 ...
- 每次扩容都是扩一倍，2到4 ，4到8，8到16， 16到32 等等
- 扩容因子：默认是0.75，也可以指定一个小数
- 扩容时间点：当容器内的元素数量到达：容量*扩容因子 开始扩容
三、源码分析
（1）先看构造函数
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
static final float DEFAULT_LOAD_FACTOR = 0.75f;
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}默认的构造函数指定了扩容因子：0.75, 默认容量是16
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}指定初始容量，默认扩容因子：0.75
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}同时指定初始容量和扩容因子
/**
* The next size value at which to resize (capacity * load factor).
*
* @serial
*/
int threshold;- 注意这个变量：下一个要扩容的值，扩容容量，容量*扩容因子
- 看这一句：this.threshold = tableSizeFor(initialCapacity);
/**
* Returns a power of two size for the given target capacity.
*/
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}- 这个方法是取给定值四舍五入之后的2的倍数，比如3—->4 ,15->16, 27->32
- 至此准备工作就做好了，下面看put方法
（2）put方法
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
// ① 最开始table为null, 调用resize（）方法
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
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;
// ② 结束的时候判断容量是不是大于扩容容量，大于则调用resize方法
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}- ① 最开始table为null, 调用resize（）方法
- ② 结束的时候判断容量是不是大于扩容容量，大于则调用resize（）方法
- 看resize（）方法
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
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;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
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;
}- 先分析第一种情况：Map map = new HashMap();
- 走最后一个分支 , 容量为16，扩容容量为12
else {
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}- 分析第二种情况：Map map = new HashMap(20);
- 走第二个分支，前面分析过，threshold = tableSizeFor(20) 为 32
- 新容量newcap = oldThr 为32
// 容量
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;- 新扩容容量newThr = newCap * loadFactor 为 24
// 扩容容量
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;- 分析第三种情况：上面的map已经插入24个元素，新插入一个要扩容
- 走第一个分支，oldCap=32，oldThr=24
- 扩容：newCap = oldCap << 1 为64
- 扩扩容容量newThr = oldThr << 1 为48
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}- 最后是复制元素到新的table
- 单个元素直接复制
- 如果是树，调用树的复制方法
- 如果是链表，循环链表复制
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