【java源码一带一路系列】之Hashtable

摘要：从上往下看的源码，标注了路上一些景点。感兴趣的同学可以继续跟，它返回的是一个内部实现的类实现了。此外，循环中的新旧数据迁移的行代码也很经典。一词成功引起了笔者注意。与之对应的是深拷贝。里面的对象会在原来的对象和它的副本之间共享。

从上往下看的源码，标注了路上一些“景点”。皮皮虾，我们走。

/**
 * 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 t) {
    this(Math.max(2*t.size(), 11), 0.75f);
    putAll(t);
}

initialCapacity不指定的话是默认11；

这里多带带列出这个初始化方法，主要是因为这个2*t.size()，如果你还记得的话，在第二路文中介绍HashMap.resize()时，它就是按2扩容的。

/**
 * Returns an enumeration of the keys in this hashtable.
 *
 * @return  an enumeration of the keys in this hashtable.
 * @see     Enumeration
 * @see     #elements()
 * @see     #keySet()
 * @see     Map
 */
public synchronized Enumeration keys() {
    return this.getEnumeration(KEYS);
}

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

看到synchronized想说的是HashTable是同步，所以大多数方法都可见进行了同步处理，这点与HashMap不同（其他差别：HashTable的key、value都不允许为null）。该方法通过Enumeration遍历Hashtable的键（KEYS是定义的全局变量），类似的，还有通过Enumeration遍历Hashtable的值。感兴趣的同学可以继续跟getEnumeration()，它返回的是一个hashtable内部实现的Enumerator类（实现了Enumeration, Iterator）。
应用实例如下：

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

/**
 * The maximum size of array to allocate.
 * Some VMs reserve some header words in an array.
 * Attempts to allocate larger arrays may result in
 * OutOfMemoryError: Requested array size exceeds VM limit
 */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

这里好奇的是“-8”从何而来？从stackoverflow得到的答案是（译）：

数组(ARRAY)需要用8bytes来存储（2^31 = 2,147,483,648 ）大小(size)。

/**
 * 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 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;
        }
    }
}

hashtable的新容量是：2倍+1，即始终为奇数。不同于前2回讲过的hashmap中resize()则是2倍。why?我们知道“除2以外所有的素数都是奇数”，而当哈希表的大小为素数时，简单的取模哈希的结果分布更加均匀，从而降低哈希冲突。

“0x7FFFFFFF”即二进制的32个1，按位与运算后使得结果范围在区间[0,2147483647]内，可以理解为取正。

此外，for循环中的新旧数据迁移的5行代码也很经典。

 /**
 * Creates a shallow copy of this hashtable. All the structure of the
 * hashtable itself is copied, but the keys and values are not cloned.
 * This is a relatively expensive operation.
 *
 * @return  a clone of the hashtable
 */
public synchronized Object clone() {
    try {
        Hashtable t = (Hashtable)super.clone();
        t.table = new Entry[table.length];
        for (int i = table.length ; i-- > 0 ; ) {
            t.table[i] = (table[i] != null)
                ? (Entry) 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(e);
    }
}

“shallow copy”一词成功引起了笔者注意。what is it?浅拷贝非java独有的概念，其他编程语言同样存在，如C#、C++、IOS、python等。与之对应的是深拷贝(deep copy)。两者的区别是：

对象的浅拷贝会对“主”对象进行拷贝，但不会复制主对象里面的对象。"里面的对象“会在原来的对象和它的副本之间共享。深拷贝是一个整个独立的对象拷贝。[参考文献2]

“I have a pen, I have a apple."（唱起来了） 我，笔，苹果是三个对象，我引用了笔和苹果，此时对我进行拷贝，效果如图：

public class Apple {
    String color;

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    } 
}

Apple apple = new Apple();
apple.setColor("red");

Hashtable ht = new Hashtable();
ht.put("apple", apple);
System.out.println(((Apple)ht.get("apple")).getColor());

Hashtable htc = (Hashtable) ht.clone();
System.out.println(((Apple)htc.get("apple")).getColor());

((Apple)htc.get("apple")).setColor("green");
System.out.println(((Apple)ht.get("apple")).getColor());
System.out.println(((Apple)htc.get("apple")).getColor());

//输出结果：
//red
//red
//green
//green
//浅拷贝的hashtable共用一个苹果

/**
 * Returns a string representation of this Hashtable object
 * in the form of a set of entries, enclosed in braces and separated
 * by the ASCII characters ", " (comma and space). Each
 * entry is rendered as the key, an equals sign =, and the
 * associated element, where the toString method is used to
 * convert the key and element to strings.
 *
 * @return  a string representation of this hashtable
 */
public synchronized String toString() {
    int max = size() - 1;
    if (max == -1)
        return "{}";

    StringBuilder sb = new StringBuilder();
    Iterator> it = entrySet().iterator();

    sb.append("{");
    for (int i = 0; ; i++) {
        Map.Entry 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(", ");
    }
}

"(this Map)"，即，ht.put(ht, ht)将输出{(this Map)=(this Map)}。

还有同步处理用到的volatile关键字（各个线程在访问该关键字所修饰变量前必须从主存（共享内存）中读取该变量的值）、Collections.synchronizedSet()（创建同步的集合对象）有缘再续。

更多有意思的内容，欢迎访问笔者小站： rebey.cn

1.HashMap和HashTable到底哪不同？,2016-07-05;

2.Java 中的浅拷贝与深拷贝(Shallow vs. Deep Copy in Java ),中英;

3.Java基础之volatile关键字,2016-07-18;