HashTable源代码剖析

<span style="font-size:14px;font-weight: normal;">public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable {

    //transient不能被序列化  数据部分
    private transient Entry[] table;

  // 元素个数
    private transient int count;

//当HashTable的大小超过这个阈值时重Hash
    private int threshold;

//装载因子 过大会导致冲突机会变大  过小会导致空间浪费
    private float loadFactor;

  //fail-fast机制  保证迭代时，其它线程不干扰
    private transient int modCount = 0;

//构造函数 初始容量仅仅要大于0即可，不同于HashMap（系统优化为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)(initialCapacity * loadFactor);
    }

     //默认0.75的装载因子
    public Hashtable(int initialCapacity) {
	this(initialCapacity, 0.75f);
    }

   //默认的构造函数
    public Hashtable() {
	this(11, 0.75f);
    }

  //map的初始容量必须大于等于11
    public Hashtable(Map<? extends K, ? extends V> t) {
	this(Math.max(2*t.size(), 11), 0.75f);
	putAll(t);
    }
  //synchronized线程安全的原因
    public synchronized int size() {
	return count;
    }

  //是否为空
    public synchronized boolean isEmpty() {
	return count == 0;
    }

   //返回枚举迭代器 keys
    public synchronized Enumeration<K> keys() {
	return this.<K>getEnumeration(KEYS);
    }

      //返回枚举迭代器 values
    public synchronized Enumeration<V> elements() {
	return this.<V>getEnumeration(VALUES);
    }

    //是否包括此元素 从最后一列循环
    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;
    }

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

  //与查看是否包括value的方法不同 直接依据hash值找到对应的一列
    public synchronized boolean containsKey(Object key) {
	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)) {
		return true;
	    }
	}
	return false;
    }

    public synchronized V get(Object key) {
	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)) {
		return e.value;
	    }
	}
	return null;
    }

 //重哈希新的大小 oldCapacity * 2 + 1
    protected void rehash() {
	int oldCapacity = table.length;
	Entry[] oldMap = table;

	int newCapacity = oldCapacity * 2 + 1;
	Entry[] newMap = new Entry[newCapacity];

	modCount++;
	threshold = (int)(newCapacity * loadFactor);
	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;

		int index = (e.hash & 0x7FFFFFFF) % newCapacity;
		e.next = newMap[index];
		newMap[index] = e;
	    }
	}
    }
   //加入元素  hashTable不能加入空元素  与HashMap不同
    public synchronized V put(K key, V value) {
	// Make sure the value is not null
	if (value == null) {
	    throw new NullPointerException();
	}

	// 看该KEY是否已经存在
	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++;
 //大于阈值重哈希
	if (count >= threshold) {
	    rehash();

            tab = table;
            index = (hash & 0x7FFFFFFF) % tab.length;
	}

	// 新加入的元素放在链表的第一个位置
	Entry<K,V> e = tab[index];
	tab[index] = new Entry<K,V>(hash, key, value, e);
	count++;
	return null;
    }

    public synchronized V remove(Object key) {
	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) {
// 这里就是重要的 比較像等时要同一时候比較equals和哈希值  覆写当中一个还有一个也要复写
	    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;
    }

  //批量加入
    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
    public synchronized void clear() {
	Entry tab[] = table;
	modCount++;
	for (int index = tab.length; --index >= 0; )
	    tab[index] = null;
	count = 0;
    }

//复制
    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();
	}
    }

    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(", ");
	}
    }

    private <T> Enumeration<T> getEnumeration(int type) {
	if (count == 0) {
	    return (Enumeration<T>)emptyEnumerator;
	} else {
	    return new Enumerator<T>(type, false);
	}
    }

    private <T> Iterator<T> getIterator(int type) {
	if (count == 0) {
	    return (Iterator<T>) emptyIterator;
	} else {
	    return new Enumerator<T>(type, true);
	}
    }

    private transient volatile Set<K> keySet = null;
    private transient volatile Set<Map.Entry<K,V>> entrySet = null;
    private transient volatile Collection<V> values = null;

   //得到set集合
    public Set<K> keySet() {
	if (keySet == null)
	    keySet = Collections.synchronizedSet(new KeySet(), this);
	return keySet;
    }
   //KeySet类  key的集合
    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();
        }
    }

    public Set<Map.Entry<K,V>> entrySet() {
	if (entrySet==null)
	    entrySet = Collections.synchronizedSet(new EntrySet(), this);
	return entrySet;
    }
//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);
	}

        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<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();
        }
    }

    public Collection<V> values() {
	if (values==null)
	    values = Collections.synchronizedCollection(new ValueCollection(),
                                                        this);
        return values;
    }
    //values类，含有values的迭代器
    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();
        }
    }

    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 {
//迭代一方，再还有一方中进行查找
            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;
    }
//  h += e.key.hashCode() ^ e.value.hashCode()
    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 (int i = 0; i < tab.length; i++)
            for (Entry e = tab[i]; e != null; e = e.next)
                h += e.key.hashCode() ^ e.value.hashCode();
        loadFactor = -loadFactor;  // Mark hashCode computation complete

	return h;
    }
 //序列化  table不能序列化，之序列化里面的key value
    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;
	    }
	}
    }

    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;
    }

    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<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<K,V>(hash, key, value, e);
        count++;
    }

    /**
     * Hashtable collision list.
     */
    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;
	}
//java的clone是浅复制
	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) {
	    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()));
	}
      //哈希码 通过 ^
	public int hashCode() {
	    return hash ^ (value==null ? 0 : value.hashCode());
	}

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

 //枚举迭代器
    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;

	boolean iterator;

	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 */
//循环直到一个没有空的列
	    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();
	    }
	}
    }

    private static class EmptyEnumerator implements Enumeration<Object> {

	EmptyEnumerator() {
	}

	public boolean hasMoreElements() {
	    return false;
	}

	public Object nextElement() {
	    throw new NoSuchElementException("Hashtable Enumerator");
	}
    }

    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");
	}

    }

}</span>

几点总结

针对Hashtable，我们相同给出几点比較重要的总结。但要结合与HashMap的比較来总结。

1、二者的存储结构和解决冲突的方法都是同样的。

2、HashTable在不指定容量的情况下的默认容量为11，而HashMap为16。Hashtable不要求底层数组的容量一定要为2的整数次幂，而HashMap则要求一定为2的整数次幂。

3、Hashtable中key和value都不同意为null，而HashMap中key和value都同意为null（key仅仅能有一个为null，而value则能够有多个为null）。可是假设在Hashtable中有类似put(null,null)的操作，编译相同能够通过，由于key和value都是Object类型。但执行时会抛出NullPointerException异常，这是JDK的规范规定的。

我们来看下ContainsKey方法和ContainsValue的源代码：

[java] view
plaincopy

1. // 推断Hashtable是否包括“值(value)”
2. public synchronized boolean contains(Object value) {
3. //注意。Hashtable中的value不能是null，
4. // 若是null的话，抛出异常!
5. if (value == null) {
6. throw new NullPointerException();
7. }
8. // 从后向前遍历table数组中的元素(Entry)
9. // 对于每一个Entry(单向链表)。逐个遍历。推断节点的值是否等于value
10. Entry tab[] = table;
11. for (int i = tab.length ; i-- > 0 ;) {
12. for (Entry<K,V> e = tab[i] ; e != null ; e = e.next) {
13. if (e.value.equals(value)) {
14. return true;
15. }
16. }
17. }
18. return false;
19. }
20. public boolean containsValue(Object value) {
21. return contains(value);
22. }
23. // 推断Hashtable是否包括key
24. public synchronized boolean containsKey(Object key) {
25. Entry tab[] = table;
26. /计算hash值，直接用key的hashCode取代
27. int hash = key.hashCode();
28. // 计算在数组中的索引值
29. int index = (hash & 0x7FFFFFFF) % tab.length;
30. // 找到“key相应的Entry(链表)”，然后在链表中找出“哈希值”和“键值”与key都相等的元素
31. for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
32. if ((e.hash == hash) && e.key.equals(key)) {
33. return true;
34. }
35. }
36. return false;
37. }

非常明显。假设value为null，会直接抛出NullPointerException异常。但源代码中并没有对key是否为null推断，有点小不解。只是NullPointerException属于RuntimeException异常。是能够由JVM自己主动抛出的，或许对key的值在JVM中有所限制吧。

4、Hashtable扩容时。将容量变为原来的2倍加1。而HashMap扩容时。将容量变为原来的2倍。

5、Hashtable计算hash值。直接用key的hashCode()，而HashMap又一次计算了key的hash值。Hashtable在求hash值相应的位置索引时，用取模运算。而HashMap在求位置索引时，则用与运算。且这里一般先用hash&0x7FFFFFFF后，再对length取模。&0x7FFFFFFF的目的是为了将负的hash值转化为正值，由于hash值有可能为负数。而&0x7FFFFFFF后。仅仅有符号外改变。而后面的位都不变。