Hashtable 源码

问题

1. Hashtable 允许 null 吗？
2. Hashtable 是线程安全的吗？如何实现的？

概括

Hashtable 是一个散列表，它存储的内容是键值对（key-value）映射。Hashtable 是线程安全的、无序的，key 和 value 都不可以为 null。

继承结构

• 实现了 Dictionary<K,V> 接口

• 实现了 Map<K,V> 接口

存储结构

• Hashtable 的实现采用了 数组 + 链表 的数据结构；

源码实现

属性

View Code

/**
 * The hash table data.
 */
private transient Entry<?,?>[] table;

/**
 * The total number of entries in the hash table.
 */
private transient int count;

/**
 * The table is rehashed when its size exceeds this threshold.  (The
 * value of this field is (int)(capacity * loadFactor).)
 *
 * @serial
 */
private int threshold;

/**
 * The load factor for the hashtable.
 *
 * @serial
 */
private float loadFactor;

/**
 * The number of times this Hashtable has been structurally modified
 * Structural modifications are those that change the number of entries in
 * the Hashtable or otherwise modify its internal structure (e.g.,
 * rehash).  This field is used to make iterators on Collection-views of
 * the Hashtable fail-fast.  (See ConcurrentModificationException).
 */
private transient int modCount = 0;

HashTable采用 拉链法 实现哈希表；

• table

  table 是一个 Entry[] 数组类型，Entry 代表了 拉链 的节点；每一个Entry代表了一个键值对，哈希表的 key-value键值对 都是存储在 Entry 数组中的；

• count

  count 是 Hashtable 的大小，注意这个大小并不是 Hashtable 的容器大小，而是他所包含 Entry 键值对的数量；

• threshold

  threshold 是 Hashtable 的阈值，用于判断是否需要调整 Hashtable 的容量；threshold 的值等于 容量 * 加载因子；

• loadFactor

  loadFactor 加载因子；

• modCount

  modCount 是用来实现 “fail-fast” 机制的（也就是快速失败）；所谓快速失败就是在并发集合中，其进行迭代操作时，若有其他线程对其进行结构性的修改，这时迭代器会立马感知到，并且立即抛出 ConcurrentModificationException 异常，而不是等到迭代完成之后才告诉你（你已经出错了）

构造方法

View Code

/**
 * Constructs a new, empty hashtable with the specified initial
 * capacity and the specified load factor.
 *
 * @param      initialCapacity   the initial capacity of the hashtable.
 * @param      loadFactor        the load factor of the hashtable.
 * @exception  IllegalArgumentException  if the initial capacity is less
 *             than zero, or if the load factor is nonpositive.
 */
public Hashtable(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal Load: "+loadFactor);

    if (initialCapacity==0)
        initialCapacity = 1;
    this.loadFactor = loadFactor;
    table = new Entry<?,?>[initialCapacity];
    threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}

/**
 * Constructs a new, empty hashtable with the specified initial capacity
 * and default load factor (0.75).
 *
 * @param     initialCapacity   the initial capacity of the hashtable.
 * @exception IllegalArgumentException if the initial capacity is less
 *              than zero.
 */
public Hashtable(int initialCapacity) {
    this(initialCapacity, 0.75f);
}

/**
 * Constructs a new, empty hashtable with a default initial capacity (11)
 * and load factor (0.75).
 */
public Hashtable() {
    this(11, 0.75f);
}

/**
 * Constructs a new hashtable with the same mappings as the given
 * Map.  The hashtable is created with an initial capacity sufficient to
 * hold the mappings in the given Map and a default load factor (0.75).
 *
 * @param t the map whose mappings are to be placed in this map.
 * @throws NullPointerException if the specified map is null.
 * @since   1.2
 */
public Hashtable(Map<? extends K, ? extends V> t) {
    this(Math.max(2*t.size(), 11), 0.75f);
    putAll(t);
}

四个构造函数；

• 用指定初始容量和指定加载因子构造一个新的空哈希表；
• 用指定初始容量和默认的加载因子 (0.75) 构造一个新的空哈希表；
• 默认构造函数，容量为11，加载因子为0.75；
• 构造一个与给定的 Map 具有相同映射关系的新哈希表；

操作方法

put(K key, V value)

View Code

/**
 * Maps the specified <code>key</code> to the specified
 * <code>value</code> in this hashtable. Neither the key nor the
 * value can be <code>null</code>. <p>
 *
 * The value can be retrieved by calling the <code>get</code> method
 * with a key that is equal to the original key.
 *
 * @param      key     the hashtable key
 * @param      value   the value
 * @return     the previous value of the specified key in this hashtable,
 *             or <code>null</code> if it did not have one
 * @exception  NullPointerException  if the key or value is
 *               <code>null</code>
 * @see     Object#equals(Object)
 * @see     #get(Object)
 */
public synchronized V put(K key, V value) {
    // Make sure the value is not null
    if (value == null) {
        throw new NullPointerException();
    }

    // Makes sure the key is not already in the hashtable.
    Entry<?,?> tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry<K,V> entry = (Entry<K,V>)tab[index];
    for(; entry != null ; entry = entry.next) {
        if ((entry.hash == hash) && entry.key.equals(key)) {
            V old = entry.value;
            entry.value = value;
            return old;
        }
    }

    addEntry(hash, key, value, index);
    return null;
}

private void addEntry(int hash, K key, V value, int index) {
    modCount++;

    Entry<?,?> tab[] = table;
    if (count >= threshold) {
        // Rehash the table if the threshold is exceeded
        rehash();

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

    // Creates the new entry.
    @SuppressWarnings("unchecked")
    Entry<K,V> e = (Entry<K,V>) tab[index];
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
}

• 元素添加方式使用了同步关键字，是线程安全的；
• 添加过程先计算 key 对应的位置，如果该位置已经存在元素，则替换原来的值；
• 如果该位置没有元素，则添加到对应的位置；
• 如果元素个数达到扩容阀值，先进行扩容，重新计算hash；

rehash()

View Code

/**
 * Increases the capacity of and internally reorganizes this
 * hashtable, in order to accommodate and access its entries more
 * efficiently.  This method is called automatically when the
 * number of keys in the hashtable exceeds this hashtable's capacity
 * and load factor.
 */
@SuppressWarnings("unchecked")
protected void rehash() {
    int oldCapacity = table.length;
    Entry<?,?>[] oldMap = table;

    // overflow-conscious code
    int newCapacity = (oldCapacity << 1) + 1;
    if (newCapacity - MAX_ARRAY_SIZE > 0) {
        if (oldCapacity == MAX_ARRAY_SIZE)
            // Keep running with MAX_ARRAY_SIZE buckets
            return;
        newCapacity = MAX_ARRAY_SIZE;
    }
    Entry<?,?>[] newMap = new Entry<?,?>[newCapacity];

    modCount++;
    threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    table = newMap;

    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;

            int index = (e.hash & 0x7FFFFFFF) % newCapacity;
            e.next = (Entry<K,V>)newMap[index];
            newMap[index] = e;
        }
    }
}

• 增加元素过程中，如果达到阀值，重新计算 hash；
• 容量扩大两倍+1，并拷贝旧数组中的元素到新数组；

get(Object key)

View Code

/**
 * Returns the value to which the specified key is mapped,
 * or {@code null} if this map contains no mapping for the key.
 *
 * <p>More formally, if this map contains a mapping from a key
 * {@code k} to a value {@code v} such that {@code (key.equals(k))},
 * then this method returns {@code v}; otherwise it returns
 * {@code null}.  (There can be at most one such mapping.)
 *
 * @param key the key whose associated value is to be returned
 * @return the value to which the specified key is mapped, or
 *         {@code null} if this map contains no mapping for the key
 * @throws NullPointerException if the specified key is null
 * @see     #put(Object, Object)
 */
@SuppressWarnings("unchecked")
public synchronized V get(Object key) {
    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.key.equals(key)) {
            return (V)e.value;
        }
    }
    return null;
}

• 计算散列表中的位置，找到则遍历链表；

  总结

• Hashtable 是基于 数组 + 链表 的存储实现；

• Hashtable 的元素操作都是同步的，是线程安全的；

• Hashtable 的初始容量不必为 2 的 n 次幂；

• Hashtable 散列算法相对与 HashMap 性能更低一些；

  Hashtable 的位置计算方式： key.hashCode 转为整型范围内最大的数值 (hash & 0x7FFFFFFF)，再对数组长度取模，取模运算效率相对较低；

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