核心成员变量
JDK7
- Entry[] table。Entry存储了HashMap的真正数据
- size大小，代表HashMap内存储了多少个键值对
- capacity容量。实际上HashMap中没有一个成员叫capacity，它是作为table这个数组的大小而隐式存在
- threshold阈值和loadFactor（默认0.75f）装载因子。threshold是通过capacity*loadFactor得到的。当size超过threshold时（刚好相等时不会扩容），HashMap扩容会再次计算每个元素的hash位置
- entrySet、keySet和values这三个都是一种试图，真正地数据都来自table
JDK8
-
table。这个Node类型的数组存储了HashMap的真正数据 Node<K,V> implements Map.Entry<K,V>
- size大小，代表HashMap内存储了多少个键值对
- capacity容量。实际上HashMap中没有一个成员叫capacity，它是作为table这个数组的大小而隐式存在
- threshold阈值和loadFactor（默认0.75f）装载因子。threshold是通过capacity*loadFactor得到的。当size超过threshold时（刚好相等时不会扩容），HashMap会扩容（
）
- entrySet、keySet和values这三个都是一种试图，真正地数据都来自table
JDK8阈值变化原因
到8转为红黑树
- TreeNode（红黑树中）占用空间是普通Node（链表中）的两倍，为了时间和空间的权衡
- 节点的分布频率会遵循泊松分布，链表长度达到8个元素的概率为0.00000006
到6转为链表
- 若是7，则当极端情况下（频繁插入和删除的都是同一个hash桶）对一个链表长度为8的hash桶进行频繁的删除和插入，同样也会导致频繁的树化<=>非树化
源码分析
put过程
JDK7
JDK8
put操作
// JDK7
public V put(K key, V value) {
if (table == EMPTY_TABLE) { // 表空进行初始化
inflateTable(threshold);
}
if (key == null) // 键空
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length); // 计算bucket的位置
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
// hash相同 equals 比较是真则替换返回旧值
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i); // 添加数据 这里包含hash冲突的处理
return null;
}
void addEntry(int hash, K key, V value, int bucketIndex) {
if ((size >= threshold) && (null != table[bucketIndex])) {
// 扩容操作，将数据元素重新计算位置后放入newTable中，Hash位置会重新计算
resize(2 * table.length);
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}
createEntry(hash, key, value, bucketIndex);
}
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
// 根据头结点构建
table[bucketIndex] = new Entry<>(hash, key, value, e);
size++;
}
// JDK8
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;
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;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
hash算法
// JDK7
final int hash(Object k) {
int h = hashSeed;
if (0 != h && k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}
h ^= k.hashCode();
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
// JDK8
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
实体结构
// JDK7
static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
int hash;
/**
* Creates new entry.
*/
Entry(int h, K k, V v, Entry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}
}
// JDK8
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
}
static class LinkedHashMap.Entry<K,V> extends HashMap.Node<K,V> {
Entry<K,V> before, after;
Entry(int hash, K key, V value, Node<K,V> next) {
super(hash, key, value, next);
}
}
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
TreeNode<K, V> parent; // red-black tree links
TreeNode<K, V> left;
TreeNode<K, V> right;
TreeNode<K, V> prev; // needed to unlink next upon deletion
boolean red;
TreeNode(int hash, K key, V val, Node<K, V> next) {
super(hash, key, val, next);
}
}
get过程
JDK7
// JDK7
public V get(Object key) {
if (key == null)
return getForNullKey();
Entry<K,V> entry = getEntry(key);
return null == entry ? null : entry.getValue();
}
final Entry<K,V> getEntry(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
}
// JDK8
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
JDK7&JDK8区别
-
底层结构
JDK7是数组+链表 存储节点数据Entry
JDK8是数组+链表+红黑树 存储节点数据Node。JDK8当链表长度大于8同时HashMap元素个数大于64则转换成红黑树 -
哈希表为空
JDK7会先调用inflateTable初始化一个数组
JDK8则直接调用热size扩容 -
hash冲突时插入数据
JDK7采用头插
JDK8会将节点插入到链表尾部 -
hash函数
JDK7中的hash函数直接使用key的hashcode值进行5次异或和4次位移，扰动复杂
JDK8中的hash函数采用key的hashcode值异或上key的hashcode无符号右移16位，扰动简单 -
扩容策略
JDK7中会重新计算hash
JDK8中不需要像JDK7那样重新计算hash，只需要看看原来的hash值新增的那个bit是1还是0就好了，是0的话索引没变，是1的话索引变成“原索引+oldCap”
JDK8扩容
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