HashTable源码解读

一:总述

底层实现原理是用数组+链表，与HashMap一样，但HashTable是线程安全的，HashMap是非线程安全的

下面是其结构图（与hashMap类似）

二：属性说明

    /**
     * The hash table data.
     */
    private transient Entry<?,?>[] table;//数据保存的数组,与HashMap一样

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

    /**
     * The table is rehashed when its size exceeds this threshold.  (The
     * value of this field is (int)(capacity * loadFactor).)
     *
     * @serial
     */
    private int threshold;//加载因子，默认为0.75

    /**
     * The load factor for the hashtable.
     *
     * @serial
     */
    private float loadFactor;//扩容门槛相对于 数组的比例计算公式 table.length * loadFactor = threshold

    /**
     * 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;

三：构造方法

由构造方法可得知，hashtable没有size规定默认为11，在hashMap中默认为16，并且hashMap必须是大于16的2的次方（特殊指定除外，但是hashMap自动会计算得出大于指定值得2的次方）

    /**
     * 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() {
　　　　 //默认数组大小为11，加载因子为0.75
        this(11, 0.75f);
    }

四：扩容方法

rehash()

　　他也是和hashmap差不多 就是直接扩容两倍，但是hashTable中会在加1,并且扩容后要重新计算每个元素对应的数组位子，相对于hashMap来说性能会有点差距，hashMap少一步取余的计算,他们在插入数据时都是放在链表的头位子上。

    /**
     * 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;

　　　　　// todo hashMap中的length 都市2的次方倍数并且扩容都是 * 2的 但是hashTable 实在原来的基础上* 2 还要加1
        // 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++;
　　　　 // 这里计算 下次扩容的门槛数量
        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;
　　　　　　　　　　// 这里是重新计算hash 但是hashMap中优化的比较好，不需要重新计算， 根据二进制来 判断hash和老长度 与运算 是否
                int index = (e.hash & 0x7FFFFFFF) % newCapacity;
　　　　　　　　　　// 这里相同hashMap和 hashTable都是放在第一个
                e.next = (Entry<K,V>)newMap[index];
                newMap[index] = e;
            }
        }
    }

五：addEntry方法

这里面就是实际的插入过程，但是这里面判断了是否需要扩容，没有判断是否已经包含，是否包含都是在调用这个方法前判断的。

    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++;
    }

六：put()方法

方法前加了synchronized。所有对外提供的方法上基本独有这个关键字。hashTable中value不能为空，HashMap中是可以的。而且如果key已经存在就直接覆盖老的value

    /**
     * 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
　　
        //不能插入空值
        if (value == null) {
            throw new NullPointerException();
        }
　　　　　
　　     // 查找key对应的数组下标 以便获取所在的链表
        // Makes sure the key is not already in the hashtable.
        Entry<?,?> tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;

　　　　 //这里判断是否存在当前key
        @SuppressWarnings("unchecked")
        Entry<K,V> entry = (Entry<K,V>)tab[index];
        for(; entry != null ; entry = entry.next) {
　　　　　　　// 这里也不太一样 hashMap中可以设置判断value是否相等类判断是否覆盖老value 
　　　　     // hashMap中相当于 就有一个cas的原理可供选择
            if ((entry.hash == hash) && entry.key.equals(key)) {
                V old = entry.value;
                entry.value = value;
                return old;
            }
        }

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

七：get()方法

比较简单，就是根据key计算数组下标，在遍历链表查找是否相同的key

    /**
     * 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;
    }

八：remove()方法

　　根据key计算数组下标，在遍历链表并记录当前元素的上一个元素，查找是否相同的key，将上一个元素的next节点=当前元素的下一个节点（将当前元素的next节点赋值给上一个节点的next节点）

/**
     * 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 <code>null</code> if the key did not have a mapping
     * @throws  NullPointerException  if the key is <code>null</code>
     *
     *
     * 删除 置顶key的元素  源码相对hashMap简单很多
     */
    public synchronized V remove(Object key) {
        Entry<?,?> tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        Entry<K,V> e = (Entry<K,V>)tab[index];
        for(Entry<K,V> 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;
    }

九：与hashMap的区别

　　HashMap允许key和value为null，Hashtable不允许。 HashMap的默认初始容量为16，Hashtable为11。 HashMap的扩容为原来的2倍，Hashtable的扩容为原来的2倍加1。 HashMap是非线程安全的，Hashtable是线程安全的。