JAVA集合类深入的HashMap
如何放进元素
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
hash算法如下，就是调用对象自己的hashcode。
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
然后核心代码如下
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//如果第一次添加元素，会在这一步初始化存储数组
//存储为Node[] table
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//查看是否发生hash碰撞，方法是数组长度n-1和hash码的按位与。如果否则直接创建。假设你重写了key对象中的equals方法而忘记重写hashcode，此时你就要接受equals的key存在存进去多次的可能性。当然也要接受相等的key取出来的内容不同。
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
//这里的NODE是一个链表
Node<K,V> e; K k;
//检查是否已经存在这个key。此时使用的方法是equals。
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//假设这个位置已经发生过很多此哈希碰撞，就把他放在一个树形结构里（红黑树，一种自平衡的二叉查找树）。TreeNode继承自LinkedHashMap里的Entry静态内部类，而该类又继承HashMap里的Node。LinkedHashMap则是继承自HashMap。笔者完全不可理解这么设计的理由。
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//寻找链表尾部。如果链表太长，则改用树结构存储。
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;//修改计数，用来在并发时的快速失败
if (++size > threshold)//超过当前的阈值，则进行扩容。
resize();
afterNodeInsertion(evict);
return null;
}
resize的过程如下
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 //扩大阈值到一倍
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;//新数组的大小为上次所扩大到的阈值
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;//负载因子*当前的容量得出当前的阈值，一般而言负载因子小于1
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];//新建数组
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;
//按顺序依次把旧数据拷贝到新的数组
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;
}
如何取出
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
//首先检查该hashmap是否存在
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
//检查存储位置对应链表的第一个元素
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
//检查存储位置是否是树结构
if (first instanceof TreeNode)
//内部是递归查找树的的内容
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
//如果是普通股链表，则遍历寻找
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
序列化
hashmap里面的数组是被transient修饰的。所以，序列化的时候需要借助额外的方法。
//这个方法虽然是private的，但是实际运行时会调用他进行序列化
private void writeObject(java.io.ObjectOutputStream s)
throws IOException {
int buckets = capacity();
// Write out the threshold, loadfactor, and any hidden stuff
s.defaultWriteObject();
s.writeInt(buckets);
s.writeInt(size);
internalWriteEntries(s);
}
void internalWriteEntries(java.io.ObjectOutputStream s) throws IOException {
Node<K,V>[] tab;
if (size > 0 && (tab = table) != null) {
for (int i = 0; i < tab.length; ++i) {
for (Node<K,V> e = tab[i]; e != null; e = e.next) {
s.writeObject(e.key);
s.writeObject(e.value);
}
}
}
}
复制
hashmap有clone标记接口，表示可以调用clone。clone并没有什么特别的，无非是调用父类clone，然后把自己的每一个元素放在新产生的HashMap里。@SuppressWarnings(“unchecked”)
@SuppressWarnings("unchecked")
@Override
public Object clone() {
HashMap<K,V> result;
try {
result = (HashMap<K,V>)super.clone();
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError(e);
}
result.reinitialize();
result.putMapEntries(this, false);
return result;
}
并发
在对象里有modCount字段，用来记录hashmap被修改次数。如果使用迭代器遍历的时候发现该值出现变化，则发生并发访问，抛出异常，快速失败。
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
transient int modCount;
实际使用的方法如下
//初始化时把expectedModCount置为和外部类对象的modCount一致
HashIterator() {
expectedModCount = modCount;
Node<K,V>[] t = table;
current = next = null;
index = 0;
if (t != null && size > 0) { // advance to first entry
do {} while (index < t.length && (next = t[index++]) == null);
}
}
public final boolean hasNext() {
return next != null;
}
final Node<K,V> nextNode() {
Node<K,V>[] t;
Node<K,V> e = next;
//如果实际使用时发现二者不一致，则抛出异常
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (e == null)
throw new NoSuchElementException();
if ((next = (current = e).next) == null && (t = table) != null) {
do {} while (index < t.length && (next = t[index++]) == null);
}
return e;
}
public final void remove() {
Node<K,V> p = current;
if (p == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
current = null;
K key = p.key;
removeNode(hash(key), key, null, false, false);
expectedModCount = modCount;
}
}
你以为这样抛出个异常就完了！！！其实这个异常只是提醒作用。免的真有小白程序员不知道这个事。事实上，多线程的put（具体来说是在put引起resize的时候）会导致死循环。JDK开发者们是尽最大努力抛出这个异常，但是他们不能保证不会发生最惨烈的后果。另外，即使代码不陷入死循环，并发的put显然可能导致相同keyhash的元素在并发存储时在链表中相互覆盖。所以，千万不要在多线程中使用hashmap。