Java集合框架08--Hashtable和源码分析

2023年2月26日 306次阅读来源: java集合源码分析

原文链接：http://blog.csdn.net/eson_15/article/details/51208166

上一章我们学习了HashMap的源码，这一节我们来讨论一下HashTable，HashTable和HashMap在某种程度上是类似的。我们依然遵循以下步骤：先对HashTable有个整体的认识，然后学习它的源码，深入剖析HashTable。

1.HashTable简介

首先看一下HashTable的继承关系

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java.lang.Object
↳ java.util.Dictionary<K, V>
↳ java.util.Hashtable<K, V>
public class Hashtable<K,V> extends Dictionary<K,V>
implements Map<K,V>, Cloneable, java.io.Serializable { }

java.lang.Object
   ↳     java.util.Dictionary<K, V>
         ↳     java.util.Hashtable<K, V>

public class Hashtable<K,V> extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable { }

我们可以看出，HashTable不但继承了Dictionary，而且实现了Map、Cloneable和Serializable接口，所以HashTable也可以实例化。HashTable和hashMap不同，
HashTable是线程安全的（等会我们在源码中就能看出）。下面我们先总览一下HashTable都有哪些API，然后我们详细分析它们。

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synchronized void clear()
synchronized Object clone()
boolean contains(Object value)
synchronized boolean containsKey(Object key)
synchronized boolean containsValue(Object value)
synchronized Enumeration<V> elements()
synchronized Set<Entry<K, V>> entrySet()
synchronized boolean equals(Object object)
synchronized V get(Object key)
synchronized int hashCode()
synchronized boolean isEmpty()
synchronized Set<K> keySet()
synchronized Enumeration<K> keys()
synchronized V put(K key, V value)
synchronized void putAll(Map<? extends K, ? extends V> map)
synchronized V remove(Object key)
synchronized int size()
synchronized String toString()
synchronized Collection<V> values()

synchronized void                clear()
synchronized Object              clone()
             boolean             contains(Object value)
synchronized boolean             containsKey(Object key)
synchronized boolean             containsValue(Object value)
synchronized Enumeration<V>      elements()
synchronized Set<Entry<K, V>>    entrySet()
synchronized boolean             equals(Object object)
synchronized V                   get(Object key)
synchronized int                 hashCode()
synchronized boolean             isEmpty()
synchronized Set<K>              keySet()
synchronized Enumeration<K>      keys()
synchronized V                   put(K key, V value)
synchronized void                putAll(Map<? extends K, ? extends V> map)
synchronized V                   remove(Object key)
synchronized int                 size()
synchronized String              toString()
synchronized Collection<V>       values()

从HashTable的API中可以看出，HashTable之所以是线程安全的，是因为方法上都加了synchronized关键字。

2. HashTable的数据结构

2.1 存储结构

和HashMap一样，HashTable内部也维护了一个数组，数组中存放的是Entry<K,V>实体，数组定义如下：

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private transient Entry<K,V>[] table;

private transient Entry<K,V>[] table;

然后我们看看Entry实体的定义：

2.2 Entry实体

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/**
* Entry实体类的定义
*/
private static class Entry<K,V> implements Map.Entry<K,V> {
int hash; //哈希值
final K key;
V value;
Entry<K,V> next; //指向的下一个Entry，即链表的下一个节点
//构造方法
protected Entry(int hash, K key, V value, Entry<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
//由于HashTable实现了Cloneable接口，所以支持克隆操作
protected Object clone() {
return new Entry<>(hash, key, value, (next==null ? null : (Entry<K,V>) next.clone()));
}
//下面对Map.Entry的具体操作了
public K getKey() { //拿到key
return key;
}
public V getValue() { //拿到value
return value;
}
public V setValue(V value) { //设置value
if (value == null) //从这里可以看出，HashTable中的value是不允许为空的！
throw new NullPointerException();
V oldValue = this.value;
this.value = value;
return oldValue;
}
//判断两个Entry是否相等
public boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry)o;
//必须两个Entry的key和value均相等才行
return key.equals(e.getKey()) && value.equals(e.getValue());
}
public int hashCode() { //计算hashCode
return (Objects.hashCode(key) ^ Objects.hashCode(value));
}
public String toString() { //重写toString方法
return key.toString()+“=”+value.toString();
}
}

/**
 * Entry实体类的定义
 */
private static class Entry<K,V> implements Map.Entry<K,V> {
    int hash; //哈希值
    final K key;
    V value;
    Entry<K,V> next; //指向的下一个Entry，即链表的下一个节点

    //构造方法
    protected Entry(int hash, K key, V value, Entry<K,V> next) {
        this.hash = hash;
        this.key =  key;
        this.value = value;
        this.next = next;
    }
    //由于HashTable实现了Cloneable接口，所以支持克隆操作
    protected Object clone() {
        return new Entry<>(hash, key, value, (next==null ? null : (Entry<K,V>) next.clone()));
    }

    //下面对Map.Entry的具体操作了

    public K getKey() { //拿到key
        return key;
    }

    public V getValue() { //拿到value
        return value;
    }

    public V setValue(V value) { //设置value
        if (value == null) //从这里可以看出，HashTable中的value是不允许为空的！
            throw new NullPointerException();

        V oldValue = this.value;
        this.value = value;
        return oldValue;
    }
    //判断两个Entry是否相等
    public boolean equals(Object o) {
        if (!(o instanceof Map.Entry))
            return false;
        Map.Entry<?,?> e = (Map.Entry)o;
        //必须两个Entry的key和value均相等才行
        return key.equals(e.getKey()) && value.equals(e.getValue());
    }

    public int hashCode() { //计算hashCode
        return (Objects.hashCode(key) ^ Objects.hashCode(value));
    }

    public String toString() { //重写toString方法
        return key.toString()+"="+value.toString();
    }
}

从Entry实体的源码中可以看出，HashTable其实就是个存储Entry的数组，Entry中包含了键值对以及下一个Entry（用来处理冲突的），形成链表。而且Entry中的value是不允许为nul的。好了，我们对HashTable整体上了解了后，下面开始详细分析HashTable中的源码。

3.HashTable源码分析（基于JDK1.7）

3.1 成员属性

首先我们看看HashTable都有哪些关键属性：

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private transient Entry<K,V>[] table;
private transient int count;//记录HashTable中有多少Entry实体
//阈值，用于判断是否需要调整Hashtable的容量（threshold = 容量*加载因子）
private int threshold;
private float loadFactor; // 加载因子
private transient int modCount = 0; // Hashtable被改变的次数，用于fail-fast
// 序列版本号
private static final long serialVersionUID = 1421746759512286392L;
//最大的门限阈值，不能超过这个
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;

private transient Entry<K,V>[] table;

private transient int count;//记录HashTable中有多少Entry实体

//阈值，用于判断是否需要调整Hashtable的容量（threshold = 容量*加载因子）
private int threshold;

private float loadFactor; // 加载因子


private transient int modCount = 0; // Hashtable被改变的次数，用于fail-fast

// 序列版本号
private static final long serialVersionUID = 1421746759512286392L;

//最大的门限阈值，不能超过这个
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;

这写成员属性的功能和HashMap基本上都一样的，这里就不再赘述了，详细信息可以看下上一篇博文
HashMap对应的该部分。下面看看HashTable的几个构造方法：

3.2 构造方法

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//参数为数组容量和加载因子的构造方法
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]; //初始化数组
//初始化门限 = 容量 * 加载因子
threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
initHashSeedAsNeeded(initialCapacity);
}
//参数为初始容量的构造方法
public Hashtable(int initialCapacity) {
this(initialCapacity, 0.75f); //我们可以看出，默认加载因子为0.75
}
//默认构造方法
public Hashtable() { //可以看出，默认容量为11，加载因子为0.75
this(11, 0.75f);
}
//包含“子Map”的构造函数
public Hashtable(Map<? extends K, ? extends V> t) {
this(Math.max(2*t.size(), 11), 0.75f);//先比较容量，如果Map的2倍容量大于11，则使用新的容量
putAll(t);
}

//参数为数组容量和加载因子的构造方法
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]; //初始化数组
    //初始化门限 = 容量 * 加载因子
    threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    initHashSeedAsNeeded(initialCapacity);
}

//参数为初始容量的构造方法
public Hashtable(int initialCapacity) {
    this(initialCapacity, 0.75f); //我们可以看出，默认加载因子为0.75
} 

//默认构造方法
public Hashtable() { //可以看出，默认容量为11，加载因子为0.75
    this(11, 0.75f);
}

//包含“子Map”的构造函数
public Hashtable(Map<? extends K, ? extends V> t) {
    this(Math.max(2*t.size(), 11), 0.75f);//先比较容量，如果Map的2倍容量大于11，则使用新的容量
    putAll(t);
}

我们可以看到，如果我们不指定数组容量和加载因子，HashTable会自动初始化容量为11，加载因子为0.75。加载因子和HashMap是相同的。

3.3 存取方法

和HashMap的分析一样，HashTable的存取部分重点分析put和get方法，其他的方法我放到代码中分析。首先看看HashTable是如何存储数据的：

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public synchronized V put(K key, V value) {
//确保value不为空
if (value == null) {
throw new NullPointerException();
}
Entry tab[] = table;
int hash = hash(key); //计算哈希值
int index = (hash & 0x7FFFFFFF) % tab.length; //根据哈希值计算在数组中的索引
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) { //如果对应的key已经存在
V old = e.value;
e.value = value; //替换掉原来的value
return old;
}
}
//否则新添加一个Entry
modCount++;
if (count >= threshold) { //判断数组中的Entry数量是否已经达到阈值
rehash(); //如果达到了，扩容
tab = table;
hash = hash(key); //重新计算哈希值
index = (hash & 0x7FFFFFFF) % tab.length; //重新计算在新的数组中的索引
}
//创建一个新的Entry
Entry<K,V> e = tab[index];
//存到对应的位置，并将其next置为原来该位置的Entry，这样就与原来的连上了
tab[index] = new Entry<>(hash, key, value, e);
count++;
return null;
}

public synchronized V put(K key, V value) {
    //确保value不为空
    if (value == null) {
        throw new NullPointerException();
    }

    Entry tab[] = table;
    int hash = hash(key); //计算哈希值
    int index = (hash & 0x7FFFFFFF) % tab.length; //根据哈希值计算在数组中的索引
    for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) { //如果对应的key已经存在
            V old = e.value;
            e.value = value; //替换掉原来的value
            return old;
        }
    }
    //否则新添加一个Entry
    modCount++;
    if (count >= threshold) { //判断数组中的Entry数量是否已经达到阈值
        rehash(); //如果达到了，扩容

        tab = table;
        hash = hash(key); //重新计算哈希值
        index = (hash & 0x7FFFFFFF) % tab.length; //重新计算在新的数组中的索引
    }

    //创建一个新的Entry
    Entry<K,V> e = tab[index];
    //存到对应的位置，并将其next置为原来该位置的Entry，这样就与原来的连上了
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
    return null;
}

put方法中，首先检测value是否为null，如果为null则会抛出NullPointerException异常。然后往下走，跟HashMap的过程一样，先计算哈希值，再根据哈希值计算在数组中的索引位置，不过这里计算索引位置的方法和HashMap不同，HashMap里使用的是 hash & (length-1)的方法，其实本质上跟这里用的(hash & 0x7FFFFFFF) % table.length一样的效果，但是HashMap中的方法效率要高，至于它们两为啥本质一样的，可以参见我的上一博客：
HashMap，那里分析的很详细。HashTable中的很好理解，直接取余就是索引值，地球人都知道~

然后便开始往数组中存数据了，如果当前的key已经在里面了，那么直接替换原来旧的value，如果不存在，先判断数组中的Entry数量有没有达到门限值，达到了就要调用rehash方法进行扩容，然后重新计算当前key在新的数组中的索引值，然后在该位置添加进去即可。下面我们看一下rehash方法：

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private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE – 8;
protected void rehash() {
int oldCapacity = table.length;
Entry<K,V>[] oldMap = table; //保存旧数组
int newCapacity = (oldCapacity << 1) + 1; //新数组容量 = 2 * 旧容量 + 1
if (newCapacity – MAX_ARRAY_SIZE > 0) {
if (oldCapacity == MAX_ARRAY_SIZE)
return;
newCapacity = MAX_ARRAY_SIZE; //不能超出最大值
}
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;//重新计算在新的数组中的索引
//第一次newMap[index]为空，后面每次的nex都是当前的Entry，这样才能连上
e.next = newMap[index];
newMap[index] = e;//然后将该Entry放到当前位置
}
}
}

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

protected void rehash() {
    int oldCapacity = table.length;
    Entry<K,V>[] oldMap = table; //保存旧数组

    int newCapacity = (oldCapacity << 1) + 1; //新数组容量 = 2 * 旧容量 + 1
    if (newCapacity - MAX_ARRAY_SIZE > 0) {
        if (oldCapacity == MAX_ARRAY_SIZE) 
            return;
        newCapacity = MAX_ARRAY_SIZE; //不能超出最大值
    }
    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;//重新计算在新的数组中的索引
            //第一次newMap[index]为空，后面每次的nex都是当前的Entry，这样才能连上
            e.next = newMap[index];
            newMap[index] = e;//然后将该Entry放到当前位置
        }
    }
}

到这里put方法就分析完了，还有个putAll方法，是将整个Map加到当前HashTable中，内部也是遍历每个Entry，然后调用上面的put方法而已，简单看一下吧：

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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());
}

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());
}

到这里，是不是感觉HashTable其实很简单，比HashMap简单多了。下面来看看get方法，也很简单，我觉得已经不用再分析了……

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public synchronized V get(Object key) {
Entry tab[] = table;
int hash = hash(key); //哈希值
int index = (hash & 0x7FFFFFFF) % tab.length; //索引值
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return e.value; //拿到value
}
}
return null;
}

public synchronized V get(Object key) {
    Entry tab[] = table;
    int hash = hash(key); //哈希值
    int index = (hash & 0x7FFFFFFF) % tab.length; //索引值
    for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            return e.value; //拿到value
        }
    }
    return null;
}

3.4 其他方法

上面分析完了存取方法，剩下来的其他方法我放到代码里分析了，也很简单：

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//返回数组中Entry数
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);
}
//内部私有方法，返回枚举对象
private <T> Enumeration<T> getEnumeration(int type) {
if (count == 0) {
return Collections.emptyEnumeration();
} else {
return new Enumerator<>(type, false); //new一个Enumeration对象，见下面：
}
}
// Types of Enumerations/Iterations
private static final int KEYS = 0;
private static final int VALUES = 1;
private static final int ENTRIES = 2;
//私有内部类，实现了Enumeration接口和Iterator接口
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;
//该字段用来决定是使用iterator还是Enumeration
boolean iterator; //false表示使用Enumeration
//fail-fast
protected int expectedModCount = modCount;
Enumerator(int type, boolean iterator) {
this.type = type;
this.iterator = iterator;
}
public boolean hasMoreElements() { //判断是否含有下一个元素
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() { //获得下一个元素
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;
//根据传进来的关键字决定返回什么
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) { //保证了线程安全
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();
}
}
}
//判断HashTable中是否包含value值
public synchronized boolean contains(Object value) {
if (value == null) { //value不能为空
throw new NullPointerException();
}
Entry tab[] = table;
//从后向前遍历table数组中的元素（Entry）
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;
}
public boolean containsValue(Object value) {
return contains(value);
}
//判断HashTable中是否包含key
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;
}
//删除HashTable中键为key的Entry，并返回value
public synchronized V remove(Object key) {
Entry tab[] = table;
int hash = hash(key);
int index = (hash & 0x7FFFFFFF) % tab.length;
//找到key对应的Entry，然后在链表中找到要删除的节点，删除之。
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;
}
//清空HashTable
public synchronized void clear() {
Entry tab[] = table;
modCount++;
for (int index = tab.length; –index >= 0; )
tab[index] = null; //将HashTable中数组值全部设置为null
count = 0;
}
//克隆一个HashTable，并以Object的形式返回
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toString方法：{, ,}
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(”, ”);
}
}
// Hashtable的“key的集合”。它是一个Set，意味着没有重复元素
private transient volatile Set<K> keySet = null;
// Hashtable的“key-value的集合”。它是一个Set，意味着没有重复元素
private transient volatile Set<Map.Entry<K,V>> entrySet = null;
// Hashtable的“value的集合”。它是一个Collection，意味着可以有重复元素
private transient volatile Collection<V> values = null;
//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;
}
private class KeySet extends AbstractSet<K> {
public Iterator<K> iterator() {
return getIterator(KEYS); //返回一个迭代器，装有HashTable的信息
//从这里也可以看出，获取到了key的Set集合后，要想取数据，只能通过迭代器
}
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();
}
}
// 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，返回正常的Enumerator的对象。(由上面代码可知，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);
}
}
//返回一个被synchronizedSet封装后的entrySet对象
public Set<Map.Entry<K,V>> entrySet() {
if (entrySet==null)
entrySet = Collections.synchronizedSet(new EntrySet(), this);
return entrySet;
}
//跟keySet类似
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);
}
// 查找EntrySet中是否包含Object(o)
// 首先，在table中找到o对应的Entry(Entry是一个单向链表)
// 然后，查找Entry链表中是否存在Object
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;
}
// 删除元素Object(o)
// 首先，在table中找到o对应的Entry(Entry是一个单向链表)
// 然后，删除链表中的元素Object
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();
}
}
// 返回一个被synchronizedCollection封装后的ValueCollection对象
// synchronizedCollection封装的目的是对ValueCollection的所有方法都添加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); //同上
}
public int size() {
return count;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
Hashtable.this.clear();
}
}
//重写equals()方法
// 若两个Hashtable的所有key-value键值对都相等，则判断它们两个相等
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;
}
//计算哈希值
//若HashTable的实际大小为0或者加载因子<0，则返回0
//否则返回“HashTable中的每个Entry的key和value的异或值的总和”
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<K,V> entry : tab)
while (entry != null) {
h += entry.hashCode();
entry = entry.next;
}
loadFactor = -loadFactor; // Mark hashCode computation complete
return h;
}
// java.io.Serializable的写入函数
// 将Hashtable的“总的容量，实际容量，所有的Entry”都写入到输出流中
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;
}
}
// java.io.Serializable的读取函数：根据写入方式读出
// 将Hashtable的“总的容量，实际容量，所有的Entry”依次读出
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<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;
}
private void reconstitutionPut(Entry<K,V>[] 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 = 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)) {
throw new java.io.StreamCorruptedException();
}
}
// Creates the new entry.
Entry<K,V> e = tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
}

//返回数组中Entry数
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);
}

//内部私有方法，返回枚举对象
private <T> Enumeration<T> getEnumeration(int type) {
    if (count == 0) {
        return Collections.emptyEnumeration();
    } else {
        return new Enumerator<>(type, false); //new一个Enumeration对象，见下面：
    }
}

// Types of Enumerations/Iterations
private static final int KEYS = 0;
private static final int VALUES = 1;
private static final int ENTRIES = 2;

//私有内部类，实现了Enumeration接口和Iterator接口
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;

    //该字段用来决定是使用iterator还是Enumeration
    boolean iterator; //false表示使用Enumeration

    //fail-fast
    protected int expectedModCount = modCount;

    Enumerator(int type, boolean iterator) {
        this.type = type;
        this.iterator = iterator;
    }

    public boolean hasMoreElements() { //判断是否含有下一个元素
        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() { //获得下一个元素
        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;
            //根据传进来的关键字决定返回什么
            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) { //保证了线程安全
            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();
        }
    }
}

//判断HashTable中是否包含value值
public synchronized boolean contains(Object value) {
    if (value == null) { //value不能为空
        throw new NullPointerException();
    }

    Entry tab[] = table;
    //从后向前遍历table数组中的元素（Entry）
    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;
}

public boolean containsValue(Object value) {
    return contains(value);
}

//判断HashTable中是否包含key
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;
}

//删除HashTable中键为key的Entry，并返回value
public synchronized V remove(Object key) {
    Entry tab[] = table;
    int hash = hash(key);
    int index = (hash & 0x7FFFFFFF) % tab.length;
    //找到key对应的Entry，然后在链表中找到要删除的节点，删除之。
    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;
}

//清空HashTable
public synchronized void clear() {
    Entry tab[] = table;
    modCount++;
    for (int index = tab.length; --index >= 0; )
        tab[index] = null; //将HashTable中数组值全部设置为null
    count = 0;
}

//克隆一个HashTable，并以Object的形式返回
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toString方法：{, ,}
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(", ");
    }
}   

// Hashtable的“key的集合”。它是一个Set，意味着没有重复元素
private transient volatile Set<K> keySet = null;
// Hashtable的“key-value的集合”。它是一个Set，意味着没有重复元素
private transient volatile Set<Map.Entry<K,V>> entrySet = null;
// Hashtable的“value的集合”。它是一个Collection，意味着可以有重复元素
private transient volatile Collection<V> values = null;

//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;
}


private class KeySet extends AbstractSet<K> {
    public Iterator<K> iterator() {
        return getIterator(KEYS); //返回一个迭代器，装有HashTable的信息
        //从这里也可以看出，获取到了key的Set集合后，要想取数据，只能通过迭代器
    }
    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();
    }
}

// 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，返回正常的Enumerator的对象。(由上面代码可知，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);
    }
}

//返回一个被synchronizedSet封装后的entrySet对象
public Set<Map.Entry<K,V>> entrySet() {
    if (entrySet==null)
        entrySet = Collections.synchronizedSet(new EntrySet(), this);
    return entrySet;
}
//跟keySet类似
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);
    }

    // 查找EntrySet中是否包含Object(o)
    // 首先，在table中找到o对应的Entry(Entry是一个单向链表)
    // 然后，查找Entry链表中是否存在Object
    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;
    }
    // 删除元素Object(o)
    // 首先，在table中找到o对应的Entry(Entry是一个单向链表)
    // 然后，删除链表中的元素Object
    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();
    }
}

// 返回一个被synchronizedCollection封装后的ValueCollection对象
// synchronizedCollection封装的目的是对ValueCollection的所有方法都添加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); //同上
    }
    public int size() {
        return count;
    }
    public boolean contains(Object o) {
        return containsValue(o);
    }
    public void clear() {
        Hashtable.this.clear();
    }
}

//重写equals()方法
// 若两个Hashtable的所有key-value键值对都相等，则判断它们两个相等
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;
}

//计算哈希值
//若HashTable的实际大小为0或者加载因子<0，则返回0
//否则返回“HashTable中的每个Entry的key和value的异或值的总和”
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<K,V> entry : tab)
        while (entry != null) {
            h += entry.hashCode();
            entry = entry.next;
        }
    loadFactor = -loadFactor;  // Mark hashCode computation complete

    return h;
}

// java.io.Serializable的写入函数
// 将Hashtable的“总的容量，实际容量，所有的Entry”都写入到输出流中
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;
    }
}

// java.io.Serializable的读取函数：根据写入方式读出
// 将Hashtable的“总的容量，实际容量，所有的Entry”依次读出
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<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;
}

private void reconstitutionPut(Entry<K,V>[] 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 = 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)) {
            throw new java.io.StreamCorruptedException();
        }
    }
    // Creates the new entry.
    Entry<K,V> e = tab[index];
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
}

4.HashTable的遍历方式

Hashtable的遍历方式比较简单，一般分两步：

1. 获得Entry或key或value的集合；

2. 通过Iterator迭代器或者Enumeration遍历此集合。

4.1 遍历HashTable的Entry （效率高）

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// 假设table是HashTable对象
// table中的key是String类型，value是Integer类型
Integer value = null;
Iterator iter = table.entrySet().iterator();
while(iter.hasNext()) {
Map.Entry entry = (Map.Entry)iter.next();
// 获取key
key = (String)entry.getKey();
// 获取value
value = (Integer)entry.getValue();
}

// 假设table是HashTable对象
// table中的key是String类型，value是Integer类型
Integer value = null;
Iterator iter = table.entrySet().iterator();
while(iter.hasNext()) {
    Map.Entry entry = (Map.Entry)iter.next();
    // 获取key
    key = (String)entry.getKey();
    // 获取value
    value = (Integer)entry.getValue();
}

4.2 遍历HashTable的key

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String key = null;
Integer value = null;
Iterator iter = table.keySet().iterator();
while (iter.hasNext()) {
// 获取key
key = (String)iter.next();
// 根据key，获取value
value = (Integer)table.get(key);
}

String key = null;
Integer value = null;
Iterator iter = table.keySet().iterator();
while (iter.hasNext()) {
    // 获取key
    key = (String)iter.next();
    // 根据key，获取value
    value = (Integer)table.get(key);
}

4.3 遍历HashTable的value

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Integer value = null;
Collection c = table.values();
Iterator iter= c.iterator();
while (iter.hasNext()) {
value = (Integer)iter.next();
}

Integer value = null;
Collection c = table.values();
Iterator iter= c.iterator();
while (iter.hasNext()) {
    value = (Integer)iter.next();
}

4.5 通过Enumeration遍历HashTable的key（效率高）

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Enumeration enu = table.keys();
while(enu.hasMoreElements()) {
System.out.println(enu.nextElement());
}

Enumeration enu = table.keys();
while(enu.hasMoreElements()) {
    System.out.println(enu.nextElement());
}

4.6 通过Enumeration遍历HashTable的value （效率高）

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Enumeration enu = table.elements();
while(enu.hasMoreElements()) {
System.out.println(enu.nextElement());
}

Enumeration enu = table.elements();
while(enu.hasMoreElements()) {
    System.out.println(enu.nextElement());
}

HashTable的遍历就介绍到这吧，至此，HashTable的源码就讨论完了，如有错误之处，欢迎留言指正~

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    原文作者：java集合源码分析
    原文地址: https://blog.csdn.net/PORSCHE_GT3RS/article/details/78598980
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