LinkedHashMap 初探
代码中遇到LRUCache的数据结构，仔细研究一下底层就是一个LinkedHashMap的基础数据结构，于是决定简单的总结一下LinkedHashMap的源码。
public class LinkedHashMap<K,V>
extends HashMap<K,V>
implements Map<K,V>可以看出LinkedHashMap是继承于HashMap类的同时也是对Map结构的一种实现。那LinkedHashMap内部实现结构与HashMap相比有什么不同呢，用一句话来概括一下：
LinkedHashMap在HashMap原有的Hash表结构基础上将HashMap中的所有节点维护为一个双向循环链表的结构，从而似的LinkedHashMap能够支持按照key-value的插入顺序（Insertion-order）或者访问顺序（Access-order）去遍历访问。
1.对HashMap的Entry类重写
LinkedHashMap中对HashMap中的Entry类进行了重写，这是实现双向循环链表的基本数据结构。
/**
* LinkedHashMap entry.
*/
private static class Entry<K,V> extends HashMap.Entry<K,V> {
// These fields comprise the doubly linked list used for iteration.
Entry<K,V> before, after; //双向循环链表的前驱节点和后继节点指针
Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
super(hash, key, value, next); //构造函数直接调用父类的构造函数
}
/**
* Removes this entry from the linked list.
*/
private void remove() { //删除操作
before.after = after;
after.before = before;
}
/**
* Inserts this entry before the specified existing entry in the list.
*/
private void addBefore(Entry<K,V> existingEntry) { //插入操作
after = existingEntry;
before = existingEntry.before;
before.after = this;
after.before = this;
}
/**
* 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) { //重写父类的recordAccess函数，对双向链表进行维护
LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
if (lm.accessOrder) {
lm.modCount++;
remove();
addBefore(lm.header); //这里很重要的一点：每次将最近访问的节点移动到双向链表的虚拟header节点之前
}
}
void recordRemoval(HashMap<K,V> m) { //重写父类的recordRemoval函数
remove();
}
}
2.构造函数
这里主要说明参数最多的一个构造函数，其他构造函数无非就是将相关数据域设为默认值。
/**
* 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; //用于判断当前LinkedHashMap的双向循环链表维护方式，即AccessOrder和InsertionOrder
}
可以看到这里主要时调用父类的构造函数，同时初始化双向循环链表的维护方式。
/**
* 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; //初始化双向循环链表的虚拟header节点
}这里需要说明的一点时header的声明：
/**
* The head of the doubly linked list.
*/
private transient Entry<K,V> header;生明中使用了transient关键字，也就是说当一个LinkedHashMap被串行化的时候，header节点是不会被包括在内的，也就是说，header节点并不存储真正有意义的数据域。
3.put操作
LinkedHashMap并没有重写HashMap中的put操作，而是重写了addEntry函数。/**
* 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)) { //是否删除最最少访问节点
removeEntryForKey(eldest.key); //这里被淘汰的节点始终时header节点的after节点
} else { //与HashMap中的处理方式相同，增大capacity重建Hash
if (size >= threshold)
resize(2 * table.length);
}
}这里比较有意思的函数是
removeEldestEntry(eldest)如果你有兴趣看这段注释的话，你会发现他其实视为了给程序员实现Cache提供的一个可扩展接口，也就是说在你自己的Cache类中继承并重写这个函数，就可以实现指定策略的Cache。如将accessOrder设为true，同时按照注释中的示例重写方式，就实现了LRU；如果使用默认accessOrder为false，同时按照注释中的示例重写方式就会保证最早被插入的优先被删除。
/**
* Returns <tt>true</tt> if this map should remove its eldest entry.
* This method is invoked by <tt>put</tt> and <tt>putAll</tt> after
* inserting a new entry into the map. It provides the implementor
* with the opportunity to remove the eldest entry each time a new one
* is added. This is useful if the map represents a cache: it allows
* the map to reduce memory consumption by deleting stale entries.
*
* <p>Sample use: this override will allow the map to grow up to 100
* entries and then delete the eldest entry each time a new entry is
* added, maintaining a steady state of 100 entries.
* <pre>
* private static final int MAX_ENTRIES = 100;
*
* protected boolean removeEldestEntry(Map.Entry eldest) {
* return size() > MAX_ENTRIES;
* }
* </pre>
*
* <p>This method typically does not modify the map in any way,
* instead allowing the map to modify itself as directed by its
* return value. It <i>is</i> permitted for this method to modify
* the map directly, but if it does so, it <i>must</i> return
* <tt>false</tt> (indicating that the map should not attempt any
* further modification). The effects of returning <tt>true</tt>
* after modifying the map from within this method are unspecified.
*
* <p>This implementation merely returns <tt>false</tt> (so that this
* map acts like a normal map - the eldest element is never removed).
*
* @param eldest The least recently inserted entry in the map, or if
* this is an access-ordered map, the least recently accessed
* entry. This is the entry that will be removed it this
* method returns <tt>true</tt>. If the map was empty prior
* to the <tt>put</tt> or <tt>putAll</tt> invocation resulting
* in this invocation, this will be the entry that was just
* inserted; in other words, if the map contains a single
* entry, the eldest entry is also the newest.
* @return <tt>true</tt> if the eldest entry should be removed
* from the map; <tt>false</tt> if it should be retained.
*/
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
return false;
}通过createEntry实现了新的key-value插入，
/**
* 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; //三步保证插入始终在bucketIndex的邻接链表头
e.addBefore(header); //维护双向循环链表，这里保证新插入的节点始终在header的前面
size++;
}
到这里就是先了put的全部功能
4.get操作
LinkedHashMap重写了HashMap的get操作，没什么好说，按照key查询的时间复杂度没变
/**
* 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); //调用父类的getKey
if (e == null)
return null;
e.recordAccess(this); //调用重写过得recordAccess在适当情况下维护双向链表
return e.value;
}
5.被优化的一些函数
containsValue通过直接遍历双向链表，对原有的HashMap进行了优化
/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value.
*
* @param value value whose presence in this map is to be tested
* @return <tt>true</tt> if this map maps one or more keys to the
* specified value
*/
public boolean containsValue(Object value) {
// Overridden to take advantage of faster iterator
if (value==null) {
for (Entry e = header.after; e != header; e = e.after)
if (e.value==null)
return true;
} else {
for (Entry e = header.after; e != header; e = e.after)
if (value.equals(e.value))
return true;
}
return false;
}
6.小结
LinkedHashMap通过对Map中的Entry重写定义，在原有的Hash表数据结构基础上维护了一个双向循环链表，从而记录了Map中的数据插入/访问顺序，同时对Iterator进行了重写。
LinkedHashMap同样没有任何线程同步操作，所以不支持多线程并发访问。