HashMap原理
Posted 哈特谢普苏特
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1.构造器
hashmap有四个构造器
(1) HashMap() 如下所示代码 DEFAULT_LOAD_FACTOR = 0.75f
/**
* 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
默认的初始大小为16
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
（2）HashMap(int initialCapacity)
public HashMap(int initialCapacity)
this(initialCapacity, DEFAULT_LOAD_FACTOR);
在（2）的情况下，最终会调用（3），如果我们initialCapacity = 10，在（3）中经过tableSizeFor（），HashMap的threshold最终为16.
(3) HashMap(int initialCapacity, float loadFactor)
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);
tableSizeFor（） 函数将输入的cap通过位运算（或运算）变成一个大于等于cap且是2的幂的一个数并 返回。
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;
（4）HashMap(Map<? extends K, ? extends V> m)
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m)
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
在这里会调用putMapEntries（）方法
/**
* Implements Map.putAll and Map constructor.
*
* @param m the map
* @param evict false when initially constructing this map, else
* true (relayed to method afterNodeInsertion).
*/
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict)
int s = m.size();
if (s > 0)
if (table == null) // pre-size
//构造器调用 因此table == null
float ft = ((float)s / loadFactor) + 1.0F;
//浮点数ft的值为传入的map的大小除以loadFactor + 1.0f,加1.0f是为了向上取整，这可能会导致一定的问题
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
//如果t大于threshold 则进行tablseSizeFor()
//实际上如果是构造器调用的putMapEntries()threshold默认为0，因此肯定会调用tableSizeFor()方法
threshold = tableSizeFor(t);
else if (s > threshold)
resize();
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
/**
* The next size value at which to resize (capacity * load factor).
*
* @serial
*/
// (The javadoc description is true upon serialization.
// Additionally, if the table array has not been allocated, this
// field holds the initial array capacity, or zero signifying
// DEFAULT_INITIAL_CAPACITY.)
int threshold;这是一个用（4）构造的HashMap， 在这种构造器中就会导致一个问题，如果传入的hashmap中有12个key-value键值对，那么在putMapEntries（）中就会导致计算出来的浮点数ft 为17.0，从而导致新的hashmap在进行tableSizeFor时大小变为32而不是旧的hashmap的16。如下图debug结果。
import java.util.*;
public class HashMapDemo
public static void main(String[] args)
HashMap<String,Integer> oldMap = new HashMap<String,Integer>();
for(int i=0;i<12;i++)
oldMap.put(""+i,i);
HashMap<String,Integer> newMap = new HashMap<String,Integer>(oldMap);
System.out.println();//此处打断点
2.put(),get()方法
（1）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;
if ((tab = table) == null || (n = tab.length) == 0)
//如果当前tab为null，则进行resize()
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
//如果当前位置为null,直接插入一个结点 位置为n-1 & hash
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))))
//如果当前有值且key相等 则进行替换
e = p;
else if (p instanceof TreeNode)
//如果是树结点 按照树结点的方式进行插入
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else
//如果不是树结点 当前位置的key和参数也不相等 则需要进行遍历
for (int binCount = 0; ; ++binCount)
if ((e = p.next) == null)
//如果遍历到最后都没有发现，则创建新结点插在最后，并且需要判断是否需要树化
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
//如果在遍历的时候发现key相等 同上 需要替换
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;
//操作次数++
++modCount;
if (++size > threshold)
//如果当前个数大于threshold 进行resize()
resize();
afterNodeInsertion(evict);
return null;
这里用到了resize() 扩容为原来的2倍并通过e.hash() & oldCap 判断是否需要挪位
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;
（2）get() 获取hashmap中特定key的值很简单，通过hash获取结点即可
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)
//如果第一个就是 直接返回
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null)
//否则进行遍历 如果是树结点 则通过树的方式
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);
//如果没有 返回null
return null;
hashmap中的hash()函数，实际上是key进行hashcode后高16位和低16位进行异或操作
static final int hash(Object key)
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
3.线程不安全
（1）在进行put（）操作的时候，假设线程A,B同时进行put操作且需要存入的值的key的hashcode()是相同的，并且此时当前槽位没有元素，当A,B都进行if ((p = tab[i = (n - 1) & hash]) == null)操作后，A被阻塞，而B继续执行并顺利将值存入。此时A开始继续运行（如果是线程安全的话，A本应该进行不为null的逻辑），但由于是线程不安全的，因此A会将B写的值进行覆盖。导致B的值丢失
（2）resize()操作
同样两个线程A和B，进行put()操作且刚好在同一槽位插入成功，需要进行扩容，由于jdk1.8是头插法，因此两个线程都需要进行扩容操作，假设此时A被阻塞，B先开始扩容，当B扩容结束后A此时开始继续执行，按照原来的逻辑A又进行了头插，最终导致当get()方法计算出来的位置刚好是发生多线程resize()的位置，最终导致死循环。
（3）put()中的++modCount
++本来就不是原子操作，多个线程同时put()很容易导致++modCount最终数值的不准确。
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