Java底层类和源码分析系列-HashTable底层架构和源码分析

几个要点

  • HashTable底层的数据结构基于链表(O(n))的数组;
  • HashTable不允许空key和空value;
  • HashMap元素非按照写入时顺序排序,而是按Key的hash取n模来排序(算法优化采用(n - 1) & hash)
  • HashTable线程安全类,但它是直接在方法上使用synchronized,是利用阻塞式的内部锁(整表上锁)保证线程安全的,其并发效率低。可考虑Concurrent包,如ConcurrentHashMap;
  • Hashtable和HashMap一样,都有初始容量和加载因子两个影响性能的参数,并且加载因子默认也是0.75;
  • 初始容量initialCapacity是11,不要求必须是2的指数倍数,而不是HashMap的16;
  • hash算法直接使用object的hashcode(键的哈希码),没有HashMap的优化(高低16位异或);
  • Hashtable在初始化时就创建了数组,HashMap是懒加载;
  • 定位算法是(e.hash & 0x7FFFFFFF) % tab.length。由于HashMap中桶的个数必须是2的指数倍数,因此得到桶索引处的方法可以优化为hash & (length-1);
  • Hashtable插入效率低下,每次插入都要遍历一次链表,每次都是O(n),效率比HashMap要低下,HashMap一般为直接定位则为O(1),红黑树内少量元素的O(LogN),链表内内少量元素的O(n),链表而且是低于8的;
  • Hashtable读取效率也低下,每次读取都要遍历一次链表,每次都是O(n),HashMap几乎为O(1),直接定位h & (length-1),加上红黑树内的O(LogN),所以几乎为O(1);

类定义

public class Hashtable<K,V>
    extends Dictionary<K,V>
    implements Map<K,V>, Cloneable, java.io.Serializable

属性

  • Entry<?,?>[] table :Entry类型的数组,用于存储Hashtable中的键值对;
  • int count :存储hashtable中有多少个键值对
  • int threshold :当count值大于该值是,哈希表扩大容量,进行rehash()
  • float loadFactor :threshold=哈希表的初始大小*loadFactor,初始容量默认为11,loadFactor值默认为0.75
  • int modCount :此HashTable结构修改次数,每次添加,更新,删除元素时,这个值就加1。实现"fail-fast"机制,在并发集合中对Hashtable进行迭代操作时,若其他线程对Hashtable进行结构性的修改,迭代器会通过比较expectedModCount和modCount是否一致,如果不一致则抛出ConcurrentModificationException异常。

构造器

  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);
    }

    public Hashtable(int initialCapacity) {
        this(initialCapacity, 0.75f);
    }

    public Hashtable() {
        this(11, 0.75f);
    }

    public Hashtable(Map<? extends K, ? extends V> t) {
        this(Math.max(2*t.size(), 11), 0.75f);
        putAll(t);
    }

modCount一致性

public static void main(String[] args) {
         Hashtable<Integer, String> tb = new Hashtable<Integer,String>();
         tb.put(1, "BUPT");
         tb.put(2, "PKU");
         tb.put(3, "THU");
         Iterator<Entry<Integer, String>> iter = tb.entrySet().iterator();
         while(iter.hasNext()){
             Entry<?, ?> entry = (Entry<?, ?>) iter.next(); //此处会抛出异常
             System.out.println(entry.getValue());
             if("THU".equals(entry.getValue())){
                 tb.remove(entry.getKey());
             }
         }
    }
/* 输出结果如下:
THU
Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.Hashtable$Enumerator.next(Hashtable.java:1367)
    at ali.Main.main(Main.java:16) */

ConcurrentModificationException异常,每一次修改hashtable中的数据都更新modCount的值,而迭代器Enumerator<T>的next方法会判断modCount != expectedModCount

     public T next() {
            //首先判断modCount和expectedModCount是否相等
            //由于在主程序中Hashtable对象通过tb.remove()方法修改了modCount的值,使得expectedModCount和modCount不相等而抛出异常
            //解决办法就是将tb.remove()方法替换为iter.remove()方法
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            return nextElement();
        }
//该方法在remove元素的同时修改了modCount和expectedModCount的值 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; @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 == lastReturned) { modCount++; expectedModCount++; if (prev == null) tab[index] = e.next; else prev.next = e.next; count--; lastReturned = null; return; } } throw new ConcurrentModificationException(); } }

主要方法

put

可以看出,每次插入都要遍历一次链表,每次都是O(n),效率比HashMap要低下,HashMap一般为红黑树内O(LogN),链表内O(n),直接定位则为1

public synchronized V put(K key, V value) {
        //值不允许为null
        if (value == null) {
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry<?,?> tab[] = table;
        //得到键的hash
        int hash = key.hashCode();
        //得到对应hash在数组中的桶索引
        int index = (hash & 0x7FFFFFFF) % tab.length;
        @SuppressWarnings("unchecked")
        //得到桶中链表头节点
        Entry<K,V> entry = (Entry<K,V>)tab[index];
        //从头开始遍历
        for(; entry != null ; entry = entry.next) {
            //一旦hash值相等并且键相等,替换旧值
            if ((entry.hash == hash) && entry.key.equals(key)) {
                V old = entry.value;
                entry.value = value;
                return old;
            }
        }
        //如果没有找到相同键,那么添加新节点
        addEntry(hash, key, value, index);
        return null;
    }

addEntry

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

        Entry<?,?> tab[] = table;
        //如果尺寸超过了阈值,进行rehash
        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++;
    }

get

public synchronized V get(Object key) {
        Entry<?,?> tab[] = table;
        int hash = key.hashCode();
        int index = (hash & 0x7FFFFFFF) % tab.length;
    //遍历所有元素,哈希值和key一致,则返回
        for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                return (V)e.value;
            }
        }
        return null;
    }

 

posted @ 2020-04-11 12:32  昕友软件开发  阅读(519)  评论(0编辑  收藏  举报
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