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。
