自各儿动手写写：LinkedHashMap源码浅析
此系列文章中，上一篇是关于HashMap的源码剖析，这篇文章将向大家剖析一下LinkedHashMap的源码！
四. LinkedHashMap
我们知道从API的描述中可以看出HashMap与LinkedHashMap最大的不同在于，后者维护者一个运行于所有条目的双向链表。有了这个双向链表，就可以在迭代的时候按照插入的顺序迭代出元素(当然也可以通过LRU算法迭代元素，下面会讲到)。
1. 类结构
public class LinkedHashMap<K, V> extends HashMap<K, V> implements Map<K, V>
我们可以看出LinkedHashMap继承了HashMap！
2. 几个重要的成员变量
/**
* The head of the doubly linked list.
*/
private transient Entry<K, V> header;
/**
* The iteration ordering method for this linked hash map: <tt>true</tt>
* for access-order, <tt>false</tt> for insertion-order.
*
* @serial
*/
private final boolean accessOrder;
对于父类HashMap中的成员变量这里就不讲了，可参考http://boy00fly.iteye.com/blog/1139845
其中Entry类是LinkedHashMap的内部类定义，代码片段如下：
//继承了HashMap.Entry private static class Entry<K, V> extends HashMap.Entry<K, V> //新增的两个成员变量 Entry<K, V> before, after;
可以看得出来新增的这两个变量就是双向链表实现的关键！！分别指向当前Entry的前一个、后一个Entry。
header就是指向双向链表的头部！
accessOrder：true则按照LRU算法迭代整个LinkedHashMap，false则按照元素的插入顺序迭代！
3. 几个重要的构造函数
/**
* Constructs an empty <tt>LinkedHashMap</tt> instance with the
* specified initial capacity, load factor and ordering mode.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @param accessOrder the ordering mode - <tt>true</tt> for
* access-order, <tt>false</tt> for insertion-order
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)
{
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
}
是不是很简单，也可参考http://boy00fly.iteye.com/blog/1139845这篇文章的构造函数分析。其中accessOrder就是我们上面所讲的控制迭代顺序的开关，只有此构造函数有这个参数，其他的构造函数默认就是false。
4. 几个重要的方法
/**
* Called by superclass constructors and pseudoconstructors (clone,
* readObject) before any entries are inserted into the map. Initializes
* the chain.
*/
void init()
{
header = new Entry<K, V>(-1, null, null, null);
header.before = header.after = header;
}
此方法是在父类构造函数初始化的时候调用的，LinkedHashMap重写了init方法。代码中表达的意思也很明确了，这是双向链表的初始化状态。
/**
* This override alters behavior of superclass put method. It causes newly
* allocated entry to get inserted at the end of the linked list and
* removes the eldest entry if appropriate.
*/
void addEntry(int hash, K key, V value, int bucketIndex)
{
createEntry(hash, key, value, bucketIndex);
// Remove eldest entry if instructed, else grow capacity if appropriate
Entry<K, V> eldest = header.after;
if (removeEldestEntry(eldest))
{
removeEntryForKey(eldest.key);
}
else
{
if (size >= threshold)
resize(2 * table.length);
}
}
/**
* This override differs from addEntry in that it doesn't resize the
* table or remove the eldest entry.
*/
void createEntry(int hash, K key, V value, int bucketIndex)
{
HashMap.Entry<K, V> old = table[bucketIndex];
Entry<K, V> e = new Entry<K, V>(hash, key, value, old);
table[bucketIndex] = e;
e.addBefore(header);
size++;
}
/**
* Inserts this entry before the specified existing entry in the list.
*/
private void addBefore(Entry<K, V> existingEntry)
{
after = existingEntry;
before = existingEntry.before;
before.after = this;
after.before = this;
}
addEntry方法是父类HashMap的一个方法，被put相关的方法所调用即在新增元素的时候调用。
我们通过形象的图来看一个基本的流程：
（从初始化到添加了3个元素过程中，各个元素before、after引用变化，画的有点丑，呵呵，不过意思能够表达清楚，代码的内容就不再累述了，大体和HashMap类似！）
再介绍一个get方法
/**
* 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==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link #containsKey containsKey} operation may be used to
* distinguish these two cases.
*/
public V get(Object key)
{
Entry<K, V> e = (Entry<K, V>)getEntry(key);
if (e == null)
return null;
e.recordAccess(this);
return e.value;
}
/**
* This method is invoked by the superclass whenever the value
* of a pre-existing entry is read by Map.get or modified by Map.set.
* If the enclosing Map is access-ordered, it moves the entry
* to the end of the list; otherwise, it does nothing.
*/
void recordAccess(HashMap<K, V> m)
{
LinkedHashMap<K, V> lm = (LinkedHashMap<K, V>)m;
if (lm.accessOrder)
{
lm.modCount++;
remove();
addBefore(lm.header);
}
}
/**
* Removes this entry from the linked list.
*/
private void remove()
{
before.after = after;
after.before = before;
}
这里我们来看一下recordAccess方法！
上面我们不是讲过accessOrder这个参数值控制着LinkedHashMap的迭代顺序嘛，这里我们来看一下。
当accessOrder为true时，remove方法就是将当前元素从双向链表中移除，
addBefore方法再将当前元素插入到链表的头部去，这样最近读到的元素，在迭代的时候是优先被迭代出来的！
这就是所谓的LRU算法（Least Recently Used）：最近最少使用算法。
当accessOrder为false时，不做任何事情，就按照插入顺序迭代出来！
还有些其他的方法这里也不再累述了，重点的都在上面阐述了！