首先要知道java8中HashMap由数组(bucket)、链表、红黑树这3种数据结构组成
下面就从源码分析理解HashMap~GO
-
1
2
3
4
5
6
7
8
9
10//初始数组容量（桶的数量），默认为16，并且总是2的幂方
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大数组容量，如果new HashMap时指定CAPACITY超过此限制则默认为最大值
static final int MAXIMUM_CAPACITY = 1 << 30;
//负载因子默认为0.75
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//当一个桶上的链表长度>=8时，链表转化为红黑树
static final int TREEIFY_THRESHOLD = 8;
//当链表长度<6时红黑树转化为链表
static final int UNTREEIFY_THRESHOLD = 6; -
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15static 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;
}
}Java8HashMap不同Java7,8中节点是由Node对象实现，而7中则是Entry。上述是在HashMap定义了静态Node类
1
2
3
4
5
6
7
8
9
10
11public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}这是Node重写的equals方法，说明判断2个节点相等要Key和Value都相等。
-
1
2
3
4static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}HashMap结合元素自身的hashcode以及再hash的方式来获取元素的哈希值，在通过取模的方式映射到数组中，其中取模运算是通过优化后的位运算来实现的。
-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31public 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);
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}HashMap重载的3个构造器 -
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29public 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) {
//变量说明：tab用于存储table引用，first用于存储数组中第一个节点，e为目标节点
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) {
//比较key的hash值和key的值
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
//判断是否有子节点
if ((e = first.next) != null) {
//检测节点为链表还是红黑树，然后分别调用不同的方法进行匹配
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;
}上述是常用的get(Object key)方法，核心是getNode()方法，从源码可得知方法先用hash()算出key的值在找到数组中对应的位置，在判断节点的类型，调用不同的方法进行处理。在匹配key时不仅要hash值相等，key的值也要相等，这也验证了hashCode()和equals()返回的结果要相等。
-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54public 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;
//检测数组是否为空，如果为空则调用resize()进行扩容初始化
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//通过hash找到桶的位置判断是否有节点存在，若无则直接newNode
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
//若当前首节点匹配，则直接返回此节点
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//对节点类型进行检测，若为红黑树，则调用红黑树的插入逻辑
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//对链表进行遍历匹配
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;
}
}
//对匹配成功的节点进行值的更新
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;
}
上述就是整个put操作的逻辑。
-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
//更新新table的容器大小，
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;
({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
//原来的table指向新的table
table = newTab;
if (oldTab != null) {
//把oldtab的节点移到newtab上
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
//只有一个节点，不用遍历
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
//如果是红黑树结构，转向split
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
//如果是链表
Node<K,V> loHead = null, loTail = null;//记录在newtab下位置也是j的节点
Node<K,V> hiHead = null, hiTail = null;///记录在newtab下位置是j + oldtab的节点
Node<K,V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
//如果该节点的(e.hash & oldCap) == 0，说明(e.hash & newCap - 1) == j
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
//否则就是(e.hash & newCap - 1) =j + oldtab
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;
}上述代码中最核心的部分即为对扩容后的数组的重建，不同于Java7中数组的构建（头插法），Java8采用了尾插法，保证了新构建的数组与原数组在碰撞后的元素组织次序的一致。同时还需要注意的是，HashMap不通过线程安全的支持，因此在数组扩容的过程中会造成死循环。多线程下最好还是使用ConcurrentHashMap
抽空更新┗|｀O′|┛ 嗷~~
