java HashTable源码解析
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本文基于JDK1.7,HashTable是用同步来实现线程安全的Map,使用Hash算法定位
与HashMap类似,HashMap是线程不安全的,单线程下效率更高,多线程下ConcurrentHashMap可保证线程安全且效率优于HashTable
Hashtable 概要
- 与HashMap主要区别是Hashtable的put,get方法都是同步的,线程安全,但是性能较差
- key和value都不能为null,HashMap中key与value都可以为 null
- 与HashMap类似,key必须实现hashCode()和equals方法,由于equals判断前都会先判断hashCode方法是否相等,两个equals的对象的hashCode()必须相同,否则在put等方法中不会覆盖
- 与HashMap类似,capacity和loadFactor是影响其性能的两个关键参数。capacity代表桶的个数,初始化initialcapacity为较大值可以减少扩容(rehash,transfer)开销,但是初始消耗更多空间,且增大了遍历时间(与capacity和size成正比,没有元素的数组点也需要遍历)开销。loadFactor代表其空间时间性能交换权衡系数,loadFactor默认为0.75,调大该系数使得空间利用率提高,但是get和put方法的时间性能降低。
- 与HashMap类似,其实现基于数组,用开放定址法解决Hash冲突,每个数组点存储一个链表,当元素个数
size>capacity*loadFactor
时进行扩容 - Hashtable迭代器以及其集合视图(keySet,values)的迭代器都具有fail-fast机制,迭代器被创建后,所有除了迭代器外对集合结构性(插入,删除,更新不是结构修改)的修改都会抛出异常。迭代器通过检查modCount来判断是否在迭代过程中出现了结构性的修改。
- Hashtable是线程安全的,其线程安全是基于同步的,如果不需要线程安全建议使用HashMap,如果需要高并发,建议使用ConcurrentHashMap
Hashtable 类头部
- Hashtable继承Dictionary,而HashMap继承AbstractMap。Dictionary只是提供了虚函数,没有实现任何方法,AbstractMap实现了丰富的方法,如:equals,toString等。
- HashMap与Hashtable实现的其他接口都是一样的
public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable
public class HashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
主要成员变量
- table数组用来存储元素链表
- count计数元素个数
- threshold 扩容的阈值
- loadFactor 扩容因子,控制扩容时机(capacity*loadFactor
private transient Entry<K,V>[] table;
private transient int count;
private int threshold;
private float loadFactor;
private transient int modCount = 0;
transient int hashSeed;
构造方法
- 根据initialCapacity,loadFactor,创建table数组,计算threshold
- 根据Map初始化,首先创建二倍于原Map size的table数组,将原有元素transfer到新table中,该过程是同步的
- 与HashMap不同,其容量capacity并不是2的幂次
public Hashtable(int initialCapacity, float loadFactor)
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load: "+loadFactor);
if (initialCapacity==0)
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry[initialCapacity];
threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
initHashSeedAsNeeded(initialCapacity);
public Hashtable(int initialCapacity)
this(initialCapacity, 0.75f);
public Hashtable()
this(11, 0.75f);
public Hashtable(Map<? extends K, ? extends V> t)
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
public synchronized void putAll(Map<? extends K, ? extends V> t)
for (Map.Entry<? extends K, ? extends V> e : t.entrySet())
put(e.getKey(), e.getValue());
基本节点 Entry
- clone为浅拷贝,没有创建key和value
- 单链表节点除了保存key和value外,还保存了指向下一节点的指针next
- 有hash值域
private static class Entry<K,V> implements Map.Entry<K,V>
int hash;
final K key;
V value;
Entry<K,V> next;
protected Entry(int hash, K key, V value, Entry<K,V> next)
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
protected Object clone()
return new Entry<>(hash, key, value,
(next==null ? null : (Entry<K,V>) next.clone()));
// set get方法
public boolean equals(Object o)
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry)o;
return key.equals(e.getKey()) && value.equals(e.getValue());
public int hashCode()
return (Objects.hashCode(key) ^ Objects.hashCode(value));
public String toString()
return key.toString()+"="+value.toString();
Hashtable 中的Holder内部类
- Holder用来加载当虚拟机完全启动后才初始化的因子
- 由于String类型的key的hashCode方法可能产生更多的hash碰撞,所以JDK7中设定了阈值,当超过阈值后使用一种特殊的hashCode计算方法,JDK1.8中已经去除相应机制
- 初始化hashSeed时,首先判断虚拟机是否完全启动,然后根据是否使用altHashing决定hashSeed的值
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
private static class Holder
static final int ALTERNATIVE_HASHING_THRESHOLD;
static
String altThreshold = java.security.AccessController.doPrivileged(
new sun.security.action.GetPropertyAction(
"jdk.map.althashing.threshold"));
int threshold;
try
threshold = (null != altThreshold)
? Integer.parseInt(altThreshold)
: ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;
// disable alternative hashing if -1
if (threshold == -1)
threshold = Integer.MAX_VALUE;
if (threshold < 0)
throw new IllegalArgumentException("value must be positive integer.");
catch(IllegalArgumentException failed)
throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
ALTERNATIVE_HASHING_THRESHOLD = threshold;
final boolean initHashSeedAsNeeded(int capacity)
boolean currentAltHashing = hashSeed != 0;
boolean useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean switching = currentAltHashing ^ useAltHashing;
if (switching)
hashSeed = useAltHashing
? sun.misc.Hashing.randomHashSeed(this)
: 0;
return switching;
插入元素 put方法
- 与HashMap最大的区别在于整个put方法是被synchronized包围的,整个方法是同步的
- 计算key的hash值,如果使用alternative hashing还需要与hashSeed进行抑或,进一步打乱
- 与Integer.maxvalue按位与,确保hash值为正的,对table.length取余计算index值
- table.index位置可能已有元素(产生hash碰撞),采用头插法,将元素插入到index位置的头部
- 如果元素个数超过threshold,进行扩容(rehash()),扩容至原来的2倍多一的大小
- 由于table.length变化,index需要重新计算
- 将原table中的元素transfer到新的table中,将头插法添加新元素
注意(e.hash == hash) && e.key.equals(key)
在判断是插入还是更新时,先判断hash值是否相等,如果hash值不等,即便equals返回true也会执行插入操作,而不是更新操作
public synchronized V put(K key, V value)
// Make sure the value is not null
if (value == null)
throw new NullPointerException();
// Makes sure the key is not already in the hashtable.
Entry tab[] = table;
int hash = hash(key);
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next)
if ((e.hash == hash) && e.key.equals(key))
V old = e.value;
e.value = value;
return old;
modCount++;
if (count >= threshold)
// Rehash the table if the threshold is exceeded
rehash();
tab = table;
hash = hash(key);
index = (hash & 0x7FFFFFFF) % tab.length;
// Creates the new entry.
Entry<K,V> e = tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
return null;
private int hash(Object k)
// hashSeed will be zero if alternative hashing is disabled.
return hashSeed ^ k.hashCode();
protected void rehash()
int oldCapacity = table.length;
Entry<K,V>[] oldMap = table;
// overflow-conscious code
int newCapacity = (oldCapacity << 1) + 1;
if (newCapacity - MAX_ARRAY_SIZE > 0)
if (oldCapacity == MAX_ARRAY_SIZE)
// Keep running with MAX_ARRAY_SIZE buckets
return;
newCapacity = MAX_ARRAY_SIZE;
Entry<K,V>[] newMap = new Entry[newCapacity];
modCount++;
threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
boolean rehash = initHashSeedAsNeeded(newCapacity);
table = newMap;
for (int i = oldCapacity ; i-- > 0 ;)
for (Entry<K,V> old = oldMap[i] ; old != null ; )
Entry<K,V> e = old;
old = old.next;
if (rehash)
e.hash = hash(e.key);
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = newMap[index];
newMap[index] = e;
查询方法 get
- 定位到table指定位置,然后顺链表查找
- 注意get方法也是同步的,在put方法执行完之前,get方法也需要等待
public synchronized V get(Object key)
Entry tab[] = table;
int hash = hash(key);
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next)
if ((e.hash == hash) && e.key.equals(key))
return e.value;
return null;
查找算法 containsKey containsValue
- 查询方法也是同步的,需要等待put方法执行完
- 对key的查询可以用hash算法直接定位到table数组指定的位置
- 对value的查询,需要遍历整个table数组和所有链表节点,因此时间复杂度是与(capacity和size)成正比
public synchronized boolean containsKey(Object key)
Entry tab[] = table;
int hash = hash(key);
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next)
if ((e.hash == hash) && e.key.equals(key))
return true;
return false;
public boolean containsValue(Object value)
return contains(value);
public synchronized boolean contains(Object value)
if (value == null)
throw new NullPointerException();
Entry tab[] = table;
for (int i = tab.length ; i-- > 0 ;)
for (Entry<K,V> e = tab[i] ; e != null ; e = e.next)
if (e.value.equals(value))
return true;
return false;
删除
- 首先定位到table指定位置
- 注意删除对应位置头结点时的情况
public synchronized V remove(Object key)
Entry tab[] = table;
int hash = hash(key);
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index], prev = null ; e != null ; prev = e, e = e.next)
if ((e.hash == hash) && e.key.equals(key))
modCount++;
if (prev != null)
prev.next = e.next;
else
tab[index] = e.next;
count--;
V oldValue = e.value;
e.value = null;
return oldValue;
return null;
浅拷贝 clone
- 由于没有对key和value进行克隆,所以当通过原map修改key和value的属性时,新map中的key和value也会改变
- 与HashMap不同的是HashMap为对每个节点重建了Entry(同样没有克隆key和value),HashTable只是重建了table中的每个头结点
public synchronized Object clone()
try
Hashtable<K,V> t = (Hashtable<K,V>) super.clone();
t.table = new Entry[table.length];
for (int i = table.length ; i-- > 0 ; )
t.table[i] = (table[i] != null)
? (Entry<K,V>) table[i].clone() : null;
t.keySet = null;
t.entrySet = null;
t.values = null;
t.modCount = 0;
return t;
catch (CloneNotSupportedException e)
// this shouldn't happen, since we are Cloneable
throw new InternalError();
视图 KeySet ValueSet EntrySet
- 视图是针对于HashTable 的table 进行的操作,与通过HashTable操作效果相同
- 与HashMap不同,contains,remove方法又重新写了一遍,而在HashMap中是直接调用的HashMap的已有方法,HashMap中的实现更简洁
private class EntrySet extends AbstractSet<Map.Entry<K,V>>
public Iterator<Map.Entry<K,V>> iterator()
return getIterator(ENTRIES);
public boolean add(Map.Entry<K,V> o)
return super.add(o);
public boolean contains(Object o)
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
Object key = entry.getKey();
Entry[] tab = table;
int hash = hash(key);
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index]; e != null; e = e.next)
if (e.hash==hash && e.equals(entry))
return true;
return false;
public boolean remove(Object o)
if (!(o instanceof Map.Entry))
return false;
Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
K key = entry.getKey();
Entry[] tab = table;
int hash = hash(key);
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index], prev = null; e != null;
prev = e, e = e.next)
if (e.hash==hash && e.equals(entry))
modCount++;
if (prev != null)
prev.next = e.next;
else
tab[index] = e.next;
count--;
e.value = null;
return true;
return false;
public int size()
return count;
public void clear()
Hashtable.this.clear();
迭代器
- 由于rehash等因素,迭代次序并不保证不变
- 查找下一个元素算法:如果当前链表已经到尾节点,从数组中顺次查找下一个非空节点,头结点作为next()
- 通过模拟枚举变量KEYS,VALUES,ENTRYS,同时实现了三种视图的Iterator
- Enumerator是已经被废弃的迭代元素的方法,相比于Iterator他缺少了remove方法,且方法名更长
- Hashtable同时对这两种接口进行了适配
private class Enumerator<T> implements Enumeration<T>, Iterator<T>
Entry[] table = Hashtable.this.table;
int index = table.length;
Entry<K,V> entry = null;
Entry<K,V> lastReturned = null;
int type;
/**
* Indicates whether this Enumerator is serving as an Iterator
* or an Enumeration. (true -> Iterator).
*/
boolean iterator;
/**
* The modCount value that the iterator believes that the backing
* Hashtable should have. If this expectation is violated, the iterator
* has detected concurrent modification.
*/
protected int expectedModCount = modCount;
Enumerator(int type, boolean iterator)
this.type = type;
this.iterator = iterator;
public boolean hasMoreElements()
Entry<K,V> e = entry;
int i = index;
Entry[] t = table;
/* Use locals for faster loop iteration */
while (e == null && i > 0)
e = t[--i];
entry = e;
index = i;
return e != null;
public T nextElement()
Entry<K,V> et = entry;
int i = index;
Entry[] t = table;
/* Use locals for faster loop iteration */
while (et == null && i > 0)
et = t[--i];
entry = et;
index = i;
if (et != null)
Entry<K,V> e = lastReturned = entry;
entry = e.next;
return type == KEYS ? (T)e.key : (type == VALUES ? (T)e.value : (T)e);
throw new NoSuchElementException("Hashtable Enumerator");
// Iterator methods
public boolean hasNext()
return hasMoreElements();
public T next()
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
return nextElement();
public void remove()
if (!iterator)
throw new UnsupportedOperationException();
if (lastReturned == null)
throw new IllegalStateException("Hashtable Enumerator");
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
synchronized(Hashtable.this)
Entry[] tab = Hashtable.this.table;
int index = (lastReturned.hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index], prev = null; e != null;
prev = e, e = e.next)
if (e == lastReturned)
modCount++;
expectedModCount++;
if (prev == null)
tab[index] = e.next;
else
prev.next = e.next;
count--;
lastReturned = null;
return;
throw new ConcurrentModificationException();
序列化
- 与HashMap实现相同,key与value分别写出,在对端逐个读入Key和value,然后加入新Map进行关联
- 由于count在可以传输得到,所以预先确定了table的容量,减少了扩容的开销
private void writeObject(java.io.ObjectOutputStream s)
throws IOException
Entry<K, V> entryStack = null;
synchronized (this)
// Write out the length, threshold, loadfactor
s.defaultWriteObject();
// Write out length, count of elements
s.writeInt(table.length);
s.writeInt(count);
// Stack copies of the entries in the table
for (int index = 0; index < table.length; index++)
Entry<K,V> entry = table[index];
while (entry != null)
entryStack =
new Entry<>(0, entry.key, entry.value, entryStack);
entry = entry.next;
// Write out the key/value objects from the stacked entries
while (entryStack != null)
s.writeObject(entryStack.key);
s.writeObject(entryStack.value);
entryStack = entryStack.next;
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
// Read in the length, threshold, and loadfactor
s.defaultReadObject();
// Read the original length of the array and number of elements
int origlength = s.readInt();
int elements = s.readInt();
// Compute new size with a bit of room 5% to grow but
// no larger than the original size. Make the length
// odd if it's large enough, this helps distribute the entries.
// Guard against the length ending up zero, that's not valid.
int length = (int)(elements * loadFactor) + (elements / 20) + 3;
if (length > elements && (length & 1) == 0)
length--;
if (origlength > 0 && length > origlength)
length = origlength;
Entry<K,V>[] newTable = new Entry[length];
threshold = (int) Math.min(length * loadFactor, MAX_ARRAY_SIZE + 1);
count = 0;
initHashSeedAsNeeded(length);
// Read the number of elements and then all the key/value objects
for (; elements > 0; elements--)
K key = (K)s.readObject();
V value = (V)s.readObject();
// synch could be eliminated for performance
reconstitutionPut(newTable, key, value);
this.table = newTable;
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