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;
} 
