【Java集合类源码分析】Hashtable源码分析
一、Hashtable简介
Hashtable是JDK1.0引入的,同样是基于哈希表(拉链法)实现的,每个元素都是一个key-value对,其内部通过单链表解决冲突,容量不足(超过了阈值)时,同样会进行扩容。
public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, java.io.Serializable
Hashtable继承了Dictionary抽象类,实现了Map接口。
Hashtable实现了实现了Cloneable接口,支持克隆;实现了Serializable接口,支持序列化,能够通过序列化传输。
Hashtable线程安全,可在多线程环境下使用。
二、Hashtable源码分析
public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable {
/** * 存储数据的Entry数组。必要时调整大小 * Hashtable同样采用链表法解决冲突,每一个Entry本质上是一个单向链表 */
private transient Entry<?,?>[] table;
/** * Hashtable中包含的键值映射的数量 */
private transient int count;
/** * 阀值,用于判断是否需要调整Hashtable的容量(阀值=容量*加载因子) * @serial */
private int threshold;
/** * 加载因子 * @serial */
private float loadFactor;
/** * Hashtable被修改的次数,用于实现fail-fast机制 */
private transient int modCount = 0;
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = 1421746759512286392L;
/** * 构造具有指定的初始容量和加载因子的空HashMap * @param initialCapacity 初始容量 * @param loadFactor 加载因子 */
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;
//设置加载因子
this.loadFactor = loadFactor;
//创建Entry数组
table = new Entry<?,?>[initialCapacity];
//设置阀值
threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}
/** * 构造具有指定初始容量和默认加载因子(0.75)的空Hashtable * @param initialCapacity 初始容量 */
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
/** * 构造一个默认初始容量(11)和默认加载因子(0.75)空的Hashtable * 注意:Hashtable的默认初始容量为11,HashMap的默认初始容量为16 */
public Hashtable() {
this(11, 0.75f);
}
/** * 构造一个新的Hashtable包含指定的map */
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
/** * 返回Hashtable中键值对的数量 */
public synchronized int size() {
return count;
}
/** * 如果Hashtable不包含键值对,则返回true */
public synchronized boolean isEmpty() {
return count == 0;
}
/** * 返回Hashtable中key的枚举对象 */
public synchronized Enumeration<K> keys() {
return this.<K>getEnumeration(KEYS);
}
/** * 返回Hashtable中value的枚举对象 */
public synchronized Enumeration<V> elements() {
return this.<V>getEnumeration(VALUES);
}
/** * 判断Hashtable是否包含指定value */
public synchronized boolean contains(Object value) {
if (value == null) { //说明Hashtable中的value不能为null
throw new NullPointerException();
}
Entry<?,?> tab[] = table;
//从后向前遍历
for (int i = tab.length ; i-- > 0 ;) {
for (Entry<?,?> e = tab[i] ; e != null ; e = e.next) {
if (e.value.equals(value)) {
return true;
}
}
}
return false;
}
/** * 判断Hashtable是否包含指定value */
public boolean containsValue(Object value) {
return contains(value);
}
/** * 判断Hashtable是否包含指定key */
public synchronized boolean containsKey(Object key) {
Entry<?,?> tab[] = table;
//计算hash值(Hashtable直接用key的hashCode)
int hash = key.hashCode();
//计算索引值,hash值可能为负数,&0x7FFFFFFF是为了将负的hash值转化为正值
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return true;
}
}
return false;
}
/** * 返回key对应的value,没有的话返回null */
@SuppressWarnings("unchecked")
public synchronized V get(Object key) {
Entry<?,?> tab[] = table;
//获取key的hashCode值
int hash = key.hashCode();
//根据hash值计算索引值以确定桶的位置
int index = (hash & 0x7FFFFFFF) % tab.length;
//遍历所在桶中的链表以查找键值等于key的元素
for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
//判断key是否相同
if ((e.hash == hash) && e.key.equals(key)) {
return (V)e.value;
}
}
return null;
}
/** * 可分配的数组的最大大小 */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/** * 重新调整Hashtable的大小 * 当Hashtable中的键数达到其阈值时,将自动调用此方法 */
@SuppressWarnings("unchecked")
protected void rehash() {
int oldCapacity = table.length;
Entry<?,?>[] oldMap = table;
//创建新容量大小的Entry数组(新容量=旧容量*2+1)
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<?,?>[] newMap = new Entry<?,?>[newCapacity];
modCount++;
//设置下一次需要扩容的阀值
threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
table = newMap;
//将旧Hashtable中的元素复制到新Hashtable中
for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry<K,V> old = (Entry<K,V>)oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
old = old.next;
//重新计算索引值(重新组织各个Entry位置)
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = (Entry<K,V>)newMap[index];
newMap[index] = e;
}
}
}
/** * 向Hashtable中存储键值对 */
public synchronized V put(K key, V value) {
//Hashtable中不能插入value为null的元素
if (value == null) {
throw new NullPointerException();
}
Entry<?,?> tab[] = table;
//获取key的hashCode值
int hash = key.hashCode();
//根据hash值计算索引值以确定桶的位置
int index = (hash & 0x7FFFFFFF) % tab.length;
//遍历所在桶中的链表以查找键值等于key的元素
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
//已存在,新value替换旧value,并返回旧value
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}
//不存在,调用addEntry方法
addEntry(hash, key, value, index);
return null;
}
private void addEntry(int hash, K key, V value, int index) {
modCount++;
Entry<?,?> tab[] = table;
//Hashtable实际容量超过了阀值
if (count >= threshold) {
// 重新调整Hashtable的大小(新容量为旧容量的两倍+1),并重新组织内部Entry
rehash();
tab = table;
hash = key.hashCode();
//扩容后重新计算索引值
index = (hash & 0x7FFFFFFF) % tab.length;
}
//创建一个新的Entry<K,V>对象,然后添加到这个桶的链表头部
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>) tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
}
/** * 删除键为key的元素,并返回value(如果存在) */
public synchronized V remove(Object key) {
Entry<?,?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>)tab[index];
//本质是删除单链表中的结点(单链表需要保留待删除结点的前一个结点)
for(Entry<K,V> 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;
}
/** * 将t中的全部元素都添加到Hashtable中 */
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());
}
/** * 清空Hashtable,将Entry数组中的元素设为null */
public synchronized void clear() {
Entry<?,?> tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}
/** * 返回Hashtable实例的浅副本,键和值本身不会克隆 */
public synchronized Object clone() {
try {
Hashtable<?,?> t = (Hashtable<?,?>)super.clone();
t.table = new Entry<?,?>[table.length];
for (int i = table.length ; i-- > 0 ; ) {
t.table[i] = (table[i] != null)
? (Entry<?,?>) 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(e);
}
}
public synchronized String toString() {
int max = size() - 1;
if (max == -1)
return "{}";
StringBuilder sb = new StringBuilder();
Iterator<Map.Entry<K,V>> it = entrySet().iterator();
sb.append('{');
for (int i = 0; ; i++) {
Map.Entry<K,V> e = it.next();
K key = e.getKey();
V value = e.getValue();
sb.append(key == this ? "(this Map)" : key.toString());
sb.append('=');
sb.append(value == this ? "(this Map)" : value.toString());
if (i == max)
return sb.append('}').toString();
sb.append(", ");
}
}
/** * 获取Hashtable的枚举类对象 * 若Hashtable的实际大小为0,则返回"空枚举类"对象 * 否则,返回正常Enumerator对象 */
private <T> Enumeration<T> getEnumeration(int type) {
if (count == 0) {
return Collections.emptyEnumeration();
} else {
return new Enumerator<>(type, false);
}
}
/** * 获取Hashtable的迭代器、 * 若Hashtable的实际大小为0,则返回"空迭代器"对象 * 否则,返回正常Enumerator对象(Enumerator实现了迭代器和枚举两个接口) */
private <T> Iterator<T> getIterator(int type) {
if (count == 0) {
return Collections.emptyIterator();
} else {
return new Enumerator<>(type, true);
}
}
/** 返回key对应的集合,不能重复 */
private transient volatile Set<K> keySet;
/** 返回entrySet对应的集合,不能重复 */
private transient volatile Set<Map.Entry<K,V>> entrySet;
/** 返回value对应的集合,能够重复 */
private transient volatile Collection<V> values;
/** * 返回一个被synchronizedSet封装后的KeySet对象 * synchronizedSet封装的目的是对KeySet的所有的方法都添加synchronized,实现多线程同步 */
public Set<K> keySet() {
if (keySet == null)
keySet = Collections.synchronizedSet(new KeySet(), this);
return keySet;
}
/** * 返回key对应的集合 * KeySet继承于AbstractSet,该集合中没有重复的key */
private class KeySet extends AbstractSet<K> {
public Iterator<K> iterator() {
return getIterator(KEYS);
}
public int size() {
return count;
}
public boolean contains(Object o) {
return containsKey(o);
}
public boolean remove(Object o) {
return Hashtable.this.remove(o) != null;
}
public void clear() {
Hashtable.this.clear();
}
}
/** * 返回一个被synchronizedSet封装后的EntrySet对象 * synchronizedSet封装的目的是对EntrySet的所有的方法都添加synchronized,实现多线程同步 */
public Set<Map.Entry<K,V>> entrySet() {
if (entrySet==null)
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
}
/** * 返回Entry对应的集合 * EntrySet继承于AbstractSet,该集合中没有重复的EntrySet */
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 = key.hashCode();
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<?,?> entry = (Map.Entry<?,?>) o;
Object key = entry.getKey();
Entry<?,?>[] tab = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>)tab[index];
for(Entry<K,V> 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();
}
}
/** * 返回一个被synchronizedSet封装后的ValueCollection对象 * synchronizedSet封装的目的是对ValueCollection的所有的方法都添加synchronized,实现多线程同步 */
public Collection<V> values() {
if (values==null)
values = Collections.synchronizedCollection(new ValueCollection(),
this);
return values;
}
private class ValueCollection extends AbstractCollection<V> {
public Iterator<V> iterator() {
return getIterator(VALUES);
}
public int size() {
return count;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
Hashtable.this.clear();
}
}
// Comparison and hashing
/** * 覆写equals函数 */
public synchronized boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Map))
return false;
Map<?,?> t = (Map<?,?>) o;
if (t.size() != size())
return false;
try {
Iterator<Map.Entry<K,V>> i = entrySet().iterator();
while (i.hasNext()) {
Map.Entry<K,V> e = i.next();
K key = e.getKey();
V value = e.getValue();
if (value == null) {
if (!(t.get(key)==null && t.containsKey(key)))
return false;
} else {
if (!value.equals(t.get(key)))
return false;
}
}
} catch (ClassCastException unused) {
return false;
} catch (NullPointerException unused) {
return false;
}
return true;
}
/** * 覆写hashCode函数 */
public synchronized int hashCode() {
int h = 0;
if (count == 0 || loadFactor < 0)
return h; // Returns zero
loadFactor = -loadFactor; // Mark hashCode computation in progress
Entry<?,?>[] tab = table;
for (Entry<?,?> entry : tab) {
while (entry != null) {
h += entry.hashCode();
entry = entry.next;
}
}
loadFactor = -loadFactor; // Mark hashCode computation complete
return h;
}
/** * 将Hashtable实例的状态保存到流中(即序列化) */
private void writeObject(java.io.ObjectOutputStream s)
throws IOException {
Entry<Object, Object> entryStack = null;
synchronized (this) {
// Write out the threshold and loadFactor
s.defaultWriteObject();
// Write out the length and 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<?,?> 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;
}
}
/** * 从流中重构Hashtable实例(即反序列化) */
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
// Read in the threshold and loadFactor
s.defaultReadObject();
// Validate loadFactor (ignore threshold - it will be re-computed)
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new StreamCorruptedException("Illegal Load: " + loadFactor);
// Read the original length of the array and number of elements
int origlength = s.readInt();
int elements = s.readInt();
// Validate # of elements
if (elements < 0)
throw new StreamCorruptedException("Illegal # of Elements: " + elements);
// Clamp original length to be more than elements / loadFactor
// (this is the invariant enforced with auto-growth)
origlength = Math.max(origlength, (int)(elements / loadFactor) + 1);
// Compute new length with a bit of room 5% + 3 to grow but
// no larger than the clamped original length. 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 + elements / 20) / loadFactor) + 3;
if (length > elements && (length & 1) == 0)
length--;
length = Math.min(length, origlength);
table = new Entry<?,?>[length];
threshold = (int)Math.min(length * loadFactor, MAX_ARRAY_SIZE + 1);
count = 0;
// Read the number of elements and then all the key/value objects
for (; elements > 0; elements--) {
@SuppressWarnings("unchecked")
K key = (K)s.readObject();
@SuppressWarnings("unchecked")
V value = (V)s.readObject();
// sync is eliminated for performance
reconstitutionPut(table, key, value);
}
}
private void reconstitutionPut(Entry<?,?>[] tab, K key, V value)
throws StreamCorruptedException
{
if (value == null) {
throw new java.io.StreamCorruptedException();
}
// Makes sure the key is not already in the hashtable.
// This should not happen in deserialized version.
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
throw new java.io.StreamCorruptedException();
}
}
// Creates the new entry.
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>)tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
}
/** * Entry<K,V>是单向链表,是Hashtable<K,V>的组织形式 */
private static class Entry<K,V> implements Map.Entry<K,V> {
final 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;
}
@SuppressWarnings("unchecked")
protected Object clone() {
return new Entry<>(hash, key, value,
(next==null ? null : (Entry<K,V>) next.clone()));
}
public K getKey() {
return key;
}
public V getValue() {
return value;
}
public V setValue(V value) {
if (value == null)
throw new NullPointerException();
V oldValue = this.value;
this.value = value;
return oldValue;
}
public boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
return (key==null ? e.getKey()==null : key.equals(e.getKey())) &&
(value==null ? e.getValue()==null : value.equals(e.getValue()));
}
public int hashCode() {
return hash ^ Objects.hashCode(value);
}
public String toString() {
return key.toString()+"="+value.toString();
}
}
// Types of Enumerations/Iterations
private static final int KEYS = 0;
private static final int VALUES = 1;
private static final int ENTRIES = 2;
/** * Enumerator的作用是提供 * 通过elements()遍历Hashtable的接口 * 通过entrySet()遍历Hashtable的接口 */
private class Enumerator<T> implements Enumeration<T>, Iterator<T> {
Entry<?,?>[] table = Hashtable.this.table;
int index = table.length;
Entry<?,?> entry;
Entry<?,?> lastReturned;
int type;
/** * Enumerator是迭代器(Iterator)还是枚举类(Enumeration)的标志 * true:迭代器;false:枚举类 */
boolean iterator;
/** * 在将Enumerator当作迭代器使用时会用到,用来实现fail-fast机制(检测并发修改) */
protected int expectedModCount = modCount;
Enumerator(int type, boolean iterator) {
this.type = type;
this.iterator = iterator;
}
/** * 判断是否存在下一个元素 * 从遍历table的数组的末尾向前查找,直到找到不为null的Entry */
public boolean hasMoreElements() {
Entry<?,?> 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;
}
/** * 获取下一个元素 * 从后向前遍历数组,若某位置不为null(即所对应的单链表的头结点不为null),则遍历该单链表 */
@SuppressWarnings("unchecked")
public T nextElement() {
Entry<?,?> 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<?,?> 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;
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>)tab[index];
for(Entry<K,V> 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();
}
}
}
}
三、总结
1、Hashtable的容量为任意正整数(默认容量为11),而HashMap的容量始终为2的整数次幂(默认容量为16),两者的默认加载因子都是0.75。
2、Hashtable中key和value都不允许为null(会抛NullPointerException异常),而HashMap中key和value都允许为null(key只能有一个为null,而value可以有多个为null)。
3、扩容时,Hashtable的新容量为旧容量的2倍加1,而HashMap的新容量为旧容量的2倍。
4、Hashtable直接使用了key的hashCode(),而HashMap重新计算了key的hash值。Hashtable在计算索引时,先将hash值与上0x7FFFFFFF,这是为了保证hash值始终为正数(只有符号位改变,而后面的位都不变),再对数组长度取模,而HashMap使用的是h&(length-1),用位与运算代替取模提高了效率。
5、Hashtable不仅支持Iterator迭代器遍历而且支持Enumeration遍历。
6、“Hashtable和HashMap的区别主要是前者是同步的,后者是快速失败机制保证不会出现多线程并发错误(Fast-Fail)。”,这是一个被很多文章转载过的概念,但其描述并不准确,容易引起误会。实质上,Fast-Fail与同步保护的是两种不同情况下的并发,两者不能拿来做比较。
Hashtable是同步的,在执行get,put,remove,size,clear等一次性读写操作时,使用了同步机制,避免了多个线程同时读写Hashtable。但同步机制并不能避免在Iterator或Enumeration遍历过程中其他线程对Hashtable的put、remove、clear操作,这些写操作都会被毫无阻拦地成功执行。
快速失败机制主要目的在于使Iterator遍历数组的线程能及时发现其他线程对Map的修改(如put、remove、clear等),因此,Fast-Fail并不能保证所有情况下的多线程并发错误,只能保护Iterator遍历过程中的iterator.next()与写并发。
其次,Hashtable的iterator遍历方式也是支持Fast-Fail的,不能说它没有快速失败机制。写一个简单的例程就可以证明这一点,一个线程做Iterator遍历,另一个线程向Hashtable中put新的key和value,很容易就会观察到Fast-Fail机制报告ConcurrentModificationException。