hashmap 源码
是jdk21的源码 但是整体逻辑没怎么变
先找到key，找到则覆盖，找不到插入 当链表长度>=8 并且table>= 64 则树化 当size》capacity*loadFactor 扩容
/**
* The default initial capacity - MUST be a power of two.
* 计算数组下标
* i = (n - 1) & hash n-1=15&hash = 1111&hash 如果不是2的幂：n=10 n-1=9 1001 分布会很不均衡
* 这里只用了低位
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka(also known as 也就是) 16 0001 1 0000 2^4
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30; //aka 1073741824
/**
* The load factor used when none specified in constructor.
* 如果设为 1.0： 空间利用率极高，但必须等到桶全满才扩容。这会导致在接近 1.0 时发生大量的哈希碰撞，查询性能大幅下降
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2 and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
*/
static final int TREEIFY_THRESHOLD = 8;
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
*/
static final int UNTREEIFY_THRESHOLD = 6;
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds.
*/
static final int MIN_TREEIFY_CAPACITY = 64;
transient Node<K,V>[] table;
/**
* 链表中的一个节点
* Basic hash bin node, used for most entries. (See below for
* TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
*/
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;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
return o instanceof Map.Entry<?, ?> e
&& Objects.equals(key, e.getKey())
&& Objects.equals(value, e.getValue());
}
}
static final int hash(Object key) {
int h;
// 无符号右移 直接补0
// h = 1010 0000 0000 0000 0000 0000 0000 1111
// h >>> 16 = 0000 0000 0000 0000 1010 0000 0000 0000
// 让高位参与低位运算，避免hash冲突
// 对hashcode做扰动 让哈希分布更加均匀
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
// 数组 链表 第一个节点
Node<K,V>[] tab; Node<K,V> p; int n, i;
// 懒加载 第一次put才初始化
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 计算数组下标 没有冲突直接插入
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
// 有冲突
else {
// e 下一个节点 p 当前节点
Node<K,V> e; K k;
// key 已经存在 后面会覆盖旧值 引用或者内容
if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// rbtree
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
// linkedlist
for (int binCount = 0; ; ++binCount) {
// 链表尾部 插入 p = 链表中第一个元素
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st 链表长度>8 则树化 tab 要小于64
treeifyBin(tab, hash);
break;
}
// 判断链表中有没有重复的
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
// 覆盖旧值
if (e != null) { // existing mapping for key
V oldValue = e.value;
// hashmap 允许值为null
if (!onlyIfAbsent || oldValue == null)
e.value = value;
// 给linkedhashmap用的 节点被访问之后用的
afterNodeAccess(e);
return oldValue;
}
}
// 记录 HashMap 被“结构性修改”的次数，用于迭代器检测并发修改（fail-fast）
// scene 一遍遍历 一遍修改结构 HashIterator
++modCount;
if (++size > threshold)
resize();
// 给linkedhashmap用的 节点被插入之后用的 删除最老的节点 实现lru removeEldestEntry
// template method design pattern
afterNodeInsertion(evict);
return null;
}
final Node<K,V>[] resize() {
// 旧数组
Node<K,V>[] oldTab = table;
// 旧容量
int oldCap = (oldTab == null) ? 0 : oldTab.length;
// 旧扩容阈值 capacity*loadFactor
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {// 初始化过
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab; // 到最大容量直接返回旧数组
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
// 容量和阈值都翻倍
}
// 有thresold 但是table还没有建 new HashMap(16)
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;//threshold 代理了 capacity 的职责 tableSizeFor
// new HashMap()
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;//16
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);//16*0.75=12
}
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;
// 迁移数据
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;// 断开引用 帮助gc
if (e.next == null)// 没有链表 单节点
// p = tab[i = (n - 1) & hash]) 重新计算位置
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode) // 是rbtree
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order 链表
Node<K,V> loHead = null, loTail = null;//原位置
Node<K,V> hiHead = null, hiTail = null;//新位置
Node<K,V> next;
do {
next = e.next;// 保存下一个节点 防止断链
/*
oldIndex = hash & (oldCapacity - 1)
newIndex = hash & (oldCapacity*2 - 1)
16 -> 32 01111 -> 11111 只多了一个高位1
*/
if ((e.hash & oldCap) == 0) {// 高位=0 留在原位置
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {// 高位 = 1 去新的位置
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;// 原位置
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;//新位置
}
}
}
}
}
return newTab;
}
// binCount >= TREEIFY_THRESHOLD - 1 = 7
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)//64
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode<K,V> hd = null, tl = null;
do {
TreeNode<K,V> p = replacementTreeNode(e, null);
if (tl == null)
hd = p;
else {
p.prev = tl;
tl.next = p;
}
tl = p;
} while ((e = e.next) != null);
if ((tab[index] = hd) != null)
hd.treeify(tab);
}
}