Java8-Hashtable实现原理

你真的了解Hashtable吗？
我们知道Hashtable与HashMap 的区别主要是线程安全，那除了这个区别还有什么区别吗？接下来我们带着这个问题一起去探究一下。Hashtable 采用的是 数组+链表 形式存储数据，例如的：

Hashtable数据结构

Hashtable 是绝对线程安全，所以每个方法都会使用同步串，

put实现

  /**
 * Maps the specified key to the specified
 * value in this hashtable. Neither the key nor the
 * value can be null. 

 *
 * The value can be retrieved by calling the get 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 null if it did not have one
 * @exception  NullPointerException  if the key or value is
 *               null
 * @see     Object#equals(Object)
 * @see     #get(Object)
 */
public synchronized V put(K key, V value) {
    // 进来的第一步是校验 value 不能为空
    // 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;
    // 直接使用key.hashCode(),表明key 也不能为空，不然也会报NPE的
    int hash = key.hashCode();
    // 这里的index 算法是，hash --> 去除最高位，然后和长度取余
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry entry = (Entry)tab[index];
    // 从这里可以看得出来，它采用的数据结构只有 数组+ 链式，这里是链式的查找逻辑
    for(; entry != null ; entry = entry.next) {
        if ((entry.hash == hash) && entry.key.equals(key)) {
            V old = entry.value;
            // 这里也没有 其他控制，直接替换数据
            entry.value = value;
            return old;
        }
    }
    // 这里是添加 节点的入口
    addEntry(hash, key, value, index);
    return null;
}

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

    Entry tab[] = table;
    // 如果容量 操作了容量控制因子，则开启扩容操作
    if (count >= threshold) {
        // Rehash the table if the threshold is exceeded
        rehash();

        tab = table;
        // 扩容之后 需要重新计算 hash 值 和 index 位置
        hash = key.hashCode();
        index = (hash & 0x7FFFFFFF) % tab.length;
    }

    // Creates the new entry.
    @SuppressWarnings("unchecked")
    Entry e = (Entry) tab[index];
    // 这里需要注意的问题，新插入的数据永远在 链表第一个，优点栈的感觉
    tab[index] = new Entry<>(hash, key, value, e);
    count++;
}

接下来我们来看一下get的实现

get

  /**
 * Returns the value to which the specified key is mapped,
 * or {@code null} if this map contains no mapping for the key.
 *
 * 
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();
     // hash 去除最高位，对数组长度取余
    int index = (hash & 0x7FFFFFFF) % tab.length;
    // 链表数据查询数据
    for (Entry e = tab[index] ; e != null ; e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            return (V)e.value;
        }
    }
    return null;
}

接下来我们看一下删除的实现，

remove实现

  /**
 * Removes the key (and its corresponding value) from this
 * hashtable. This method does nothing if the key is not in the hashtable.
 *
 * @param   key   the key that needs to be removed
 * @return  the value to which the key had been mapped in this hashtable,
 *          or null if the key did not have a mapping
 * @throws  NullPointerException  if the key is null
 */
public synchronized V remove(Object key) {
    Entry tab[] = table;
    int hash = key.hashCode();
    int index = (hash & 0x7FFFFFFF) % tab.length;
    @SuppressWarnings("unchecked")
    Entry e = (Entry)tab[index];
    // 链式结构 查找删除
    for(Entry prev = null ; e != null ; prev = e, e = e.next) {
        if ((e.hash == hash) && e.key.equals(key)) {
            modCount++;
            // 修改链式节点 next 指向删除数据
            if (prev != null) {
                prev.next = e.next;
            } else {
                tab[index] = e.next;
            }
            count--;
            V oldValue = e.value;
            e.value = null;
            return oldValue;
        }
    }
    return null;
}

再来看一下rehash的实现

rehash实现

  /**
 * 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
   // 扩容算法：直接扩大2倍 + 1
    int newCapacity = (oldCapacity << 1) + 1;
   // 为了避免 oom 对超过最大值进行重新赋值
    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++;
   // 最大容量 为：（数组长度 * 扩容因子  ， 最大的数组长度 + 1 ）最小值
    threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
    table = newMap;

    for (int i = oldCapacity ; i-- > 0 ;) {
        // 对 数组链表数据 进行重新 hash  index 计算，rehash 之后 会使得 最早插入的数据 回到 链表的 第一位
        for (Entry old = (Entry)oldMap[i] ; old != null ; ) {
            Entry e = old;
            old = old.next;

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

好了，就到这里了。