为了分析ConcurrentHashMap，决定先分析一下Hashtable，两者都是线程安全的，必然会有不同的区别，Hashtable和HashMap也有很大的区别。
我们先来看看Hashtable吧。
成员变量：
//存储单链表表头的数组，和HashMap中类似
private transient Entry[] table;
//Hashtable中实际元素的数量
private transient int count;
//Hashtable的临界值(容量* 加载因子).
private int threshold;
//加载因子
private float loadFactor;
//修改次数，提供fast-fail机制
private transient int modCount = 0;
//Hashtable的版本号
private static final long serialVersionUID = 1421746759512286392L;
构造函数：
//构造一个新的Hashtable，有指定的初始容量和加载因子。注意到没有设置Hashtable的最大容量是多少。
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)(initialCapacity * loadFactor);
}
//指定初始容量，和HashMap一样，加载因子是0.75
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f);
}
//默认的无参构造，发现默认容量大小是11
public Hashtable() {
this(11, 0.75f);
}
//将指定Map集合传递给Hashtable，默认加载因子是0.75
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
Hashtable解决冲突的单链表：
private static class Entry<K,V> implements Map.Entry<K,V> {
int hash;
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;
}
//复制，为了Hashtable的克隆，后面会看到
protected Object clone() {
return new Entry<K,V>(hash, key, value,
(next==null ? null : (Entry<K,V>) next.clone()));
}
// Map.Entry Ops
public K getKey() {
return key;
}
public V getValue() {
return value;
}
public V setValue(V value) {
//如果为空抛出异常
if (value == null)
throw new NullPointerException();
V oldValue = this.value;
this.value = value;
//设置后返回旧值
return oldValue;
}
//若key和value都相等才返回true
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()));
}
//哈希值为键的哈希值异或值的哈希值
public int hashCode() {
return hash ^ (value==null ? 0 : value.hashCode());
}
public String toString() {
return key.toString()+"="+value.toString();
}
}
其他方法：
//使用的同步函数进行控制
public synchronized int size() {
return count;
}
//判断是否为空
public synchronized boolean isEmpty() {
return count == 0;
}
//返回所有key的枚举对象
public synchronized Enumeration<K> keys() {
return this.<K>getEnumeration(KEYS);
}
//返回所有value的枚举对象
public synchronized Enumeration<V> elements() {
return this.<V>getEnumeration(VALUES);
}
//判断Hashtable中是否包含指定的对象
public synchronized boolean contains(Object value) {
//如果指定的对象时null，则抛出异常，说明Hashtable中不支持null键
if (value == null) {
throw new NullPointerException();
}
//双重for遍历寻找，从后向前遍历数组，逐一遍历链表
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;
}
//与上面一样，因为实现了Map接口，所以需要实现此方法
public boolean containsValue(Object value) {
return contains(value);
}
//判断是否包含于指定key相同的键
public synchronized boolean containsKey(Object key) {
Entry tab[] = table;
int hash = key.hashCode(); //这里的hash值直接采用key的hash值
//数组中的索引值计算方法不同，此处与上0x7FFFFFFF是为了将负数（hashCode有可能是负数）转为正数
//然后对数组长度取模
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;
}
//提取与指定key相同的键，若没有返回null
public synchronized V get(Object key) {
Entry tab[] = table;
int hash = key.hashCode();//取出key的hash值
int index = (hash & 0x7FFFFFFF) % tab.length; //计算索引
//搜索
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;
}
//调整Hashtable的大小
protected void rehash() {
int oldCapacity = table.length;
Entry[] oldMap = table;
//直接调整为原来的大小的2倍，至于为什么加1，在put某一个数的时候，发现需要调整，那么调整完后，还要把这个数给添加进去，1就是给那个数留的空间。
int newCapacity = oldCapacity * 2 + 1;
Entry[] newMap = new Entry[newCapacity];
modCount++; //修改次数加1
threshold = (int)(newCapacity * loadFactor); //重新设置临界值
table = newMap; //重新赋引用
//将旧值全部复制到新的Hashtable中
for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry<K,V> old = oldMap[i] ; old != null ; ) {
Entry<K,V> e = old;
old = old.next;
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = newMap[index];
newMap[index] = e;
}
}
}
//往里面put键值对
public synchronized V put(K key, V value) {
//同样，如果是null则抛出异常
if (value == null) {
throw new NullPointerException();
}
// 如果key存在当前的Hashtable中，用对应的旧值替换新值
Entry tab[] = table;
int hash = key.hashCode();
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++; //修改次数加1
//如果Hashtable中总数量大于临界值
if (count >= threshold) {
//扩容
rehash();
tab = table;
index = (hash & 0x7FFFFFFF) % tab.length;
}
// 将旧值放到新值的后面，新值放到头结点
Entry<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
return null;
}
//移除指定key键的元素
public synchronized V remove(Object key) {
Entry tab[] = table;
int hash = key.hashCode(); //获取Hash值
int index = (hash & 0x7FFFFFFF) % tab.length;//获取索引
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 {
//如果删除头结点，将下一个直接赋给数组
tab[index] = e.next;
}
count--;
V oldValue = e.value;
e.value = null;
return oldValue;
}
}
return null;
}
//将指定Map集合中的元素全部放入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
public synchronized void clear() {
Entry tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null; //置为null
count = 0;
}
//克隆一个Hashtable，浅克隆
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();
}
}
//重写的toString，而HashMap的是继承自AbstractMap，最后做判断的时候略微不一样
//HashMap是直接用迭代器判断hashNext，而Hashtable是判断个数
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(", ");
}
}
先看三个常量：
// 可以看到0是键，1是值，2是键值对
private static final int KEYS = 0;
private static final int VALUES = 1;
private static final int ENTRIES = 2;
遍历方法：
//返回Hashtable的枚举类对象，传入的参数是类型，表示是键还是值，还是键值
private <T> Enumeration<T> getEnumeration(int type) {
if (count == 0) {
return (Enumeration<T>)emptyEnumerator;
} else {
return new Enumerator<T>(type, false);
}
}
//返回迭代器
private <T> Iterator<T> getIterator(int type) {
if (count == 0) {
return (Iterator<T>) emptyIterator;
} else {
return new Enumerator<T>(type, true);
}
}
//Hashtable中所有键的集合
private transient volatile Set<K> keySet = null;
//Hashtable中所有键值对的集合
private transient volatile Set<Map.Entry<K,V>> entrySet = null;
private transient volatile Collection<V> values = null; //所有值的集合
//返回所有键的集合，调用Collections的synchronizedSet,这样就给keySet中的所有方法全部加上synchronized 实现同步
public Set<K> keySet() {
if (keySet == null)
keySet = Collections.synchronizedSet(new KeySet(), this);
return keySet;
}
//定义的keySet类继承AbstractSet
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Hashtable中的键值对集合，自动给
public Set<Map.Entry<K,V>> entrySet() {
if (entrySet==null)
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
}
//定义的EntrySet类
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);
}
//判断entrySet中是否包含指定的对象
// 同样要获取哈希值，计算按索引
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;
//查找Entry对象是否在entryset
for (Entry e = tab[index]; e != null; e = e.next)
if (e.hash==hash && e.equals(entry))
return true;
return false;
}
//将存在的对象删除
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 = key.hashCode();
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();
}
}
//返回Hashtable中所有值的集合，用synchronized修饰，保证其所有方法都是同步的
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();
}
}
//依次遍历Hashtable的entrySet的集合
public synchronized boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Map))
return false;
Map<K,V> t = (Map<K,V>) 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;
}
//这是hashCode的计算
public synchronized int hashCode() {
int h = 0;
//若Hashtable为空，或者加载因子小于0，则返回0
if (count == 0 || loadFactor < 0)
return h; // Returns zero
loadFactor = -loadFactor; // Mark hashCode computation in progress
Entry[] tab = table;
for (int i = 0; i < tab.length; i++)
for (Entry e = tab[i]; e != null; e = e.next)
//每个key的hash值和value的hash值异或相加
h += e.key.hashCode() ^ e.value.hashCode();
loadFactor = -loadFactor; // Mark hashCode computation complete
return h;
}
//序列化，将Hashtable的实际长度，哈希数组的长度，以及每个链表的值都写入
private synchronized void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
// Write out the length, threshold, loadfactor
s.defaultWriteObject();
// Write out length, count of elements and then the key/value objects
s.writeInt(table.length);
s.writeInt(count);
for (int index = table.length-1; index >= 0; index--) {
Entry entry = table[index];
while (entry != null) {
s.writeObject(entry.key);
s.writeObject(entry.value);
entry = entry.next;
}
}
}
//将流中的数据依次读取出
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[] table = new Entry[length];
count = 0;
// 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(table, key, value);
}
this.table = table;
}
Hashtable中的枚举类：
//给Hashtable提供了枚举遍历和迭代器遍历的好处
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;
//代表是迭代器还是枚举类，true代表迭代器，false代表枚举类
boolean iterator;
//提供fast-fail机制
protected int expectedModCount = modCount;
Enumerator(int type, boolean iterator) {
this.type = type;
this.iterator = iterator;
}
//向前遍历，是否还有别的数，直到找到null为止
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");
}
// 迭代器的方法
public boolean hasNext() {
return hasMoreElements();
}
public T next() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
return nextElement();
}
//修改动作要加锁
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;
//首先找到索引
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();
}
}
}
//创建一个空的枚举对象
private static Enumeration emptyEnumerator = new EmptyEnumerator();
//创建一个空的迭代器对象
private static Iterator emptyIterator = new EmptyIterator();
//如果Hashtable大小为0，但是还要用枚举遍历，就返回一个空的枚举对象
private static class EmptyEnumerator implements Enumeration<Object> {
EmptyEnumerator() {
}
public boolean hasMoreElements() {
return false;
}
public Object nextElement() {
throw new NoSuchElementException("Hashtable Enumerator");
}
}
//<span style="font-family: 宋体;">如果Hashtable大小为0，但是还要用迭代器遍历，就返回一个空的迭代器对象</span>
private static class EmptyIterator implements Iterator<Object> {
EmptyIterator() {
}
public boolean hasNext() {
return false;
}
public Object next() {
throw new NoSuchElementException("Hashtable Iterator");
}
public void remove() {
throw new IllegalStateException("Hashtable Iterator");
}
}
总结：
首先我们可以看到Hashtable中的方法都加了synchronized修饰，这就保证了线程的安全性，但是这在一定程度上也降低了效率，没有HashMap的效率高。
HashMap和Hashtable的几点区别：
1、在指定参数构造函数时Hashtable如果指定的参数大于2的30次方，则不会替换；而HashMap则会用2的30次方替换。当然，不出意外的话，两个都会堆溢出。
2、HashMap的默认容量大小是16，而Hashtable的是11
3、Hashtable中不支持<null,null>,会抛出空指针异常，但是HashMap中支持
4、增加、删除、查询等一系列操作，Hashtable会直接采用key的hash值，而HashMap则是重新计算Hash值
5、HashMap中的遍历方式只支持迭代器；Hashtable中遍历方式有迭代器和枚举。
6、二者在扩容时采用的策略不一样：HashMap中在扩容时是先将值添加到链表上，然后判断下一次是否需要扩容；而Hashtable是在添加的时候
决定是否需要扩容，所以需要多分出来一个元素的空间。
7、Hashtable继承自Dictionary，而HashMap继承自AbstractMap。
......
如果要从源码上说区别，感觉是说不完了，点到为止吧。
若有遗漏重要的，还请大家指教。
版权声明：本文为博主原创文章，转载请注明出处。
时间： 2024-10-05 05:58:27