LinkedHashMap源码分析
一、简介
LinkedHashMap继承了HashMap，并通过双向列表来维护元素的顺序，本文结合HashMap的源码探究一下LinkedHashMap是如何维护元素的顺序的。
1.1 构造方法
public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V> {
/**
* The iteration ordering method for this linked hash map: <tt>true</tt>
* for access-order, <tt>false</tt> for insertion-order.
*
* @serial
*/
final boolean accessOrder;
/**
* Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
* with the specified initial capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public LinkedHashMap(int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor);
accessOrder = false;
}
/**
* Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
* with the specified initial capacity and a default load factor (0.75).
*
* @param initialCapacity the initial capacity
* @throws IllegalArgumentException if the initial capacity is negative
*/
public LinkedHashMap(int initialCapacity) {
super(initialCapacity);
accessOrder = false;
}
/**
* Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
* with the default initial capacity (16) and load factor (0.75).
*/
public LinkedHashMap() {
super();
accessOrder = false;
}
/**
* Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with
* the same mappings as the specified map. The <tt>LinkedHashMap</tt>
* instance is created with a default load factor (0.75) and an initial
* capacity sufficient to hold the mappings in the specified map.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public LinkedHashMap(Map<? extends K, ? extends V> m) {
super();
accessOrder = false;
putMapEntries(m, false);
}
/**
* 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;
}
}LinkedHashMap的构造方法和HashMap相比多了一个accessOrder参数，LinkedHashMap基于双向列表来记录元素的顺序。元素的排序方式根据accessOrder参数来进行控制：
- accessOrder = false ：根据元素的插入顺序排序
- accessOrder = true ： 根据元素的访问顺序排序
1.2 主要属性
/**
* HashMap.Node subclass for normal LinkedHashMap entries.
*/
static class Entry<K,V> extends HashMap.Node<K,V> {
Entry<K,V> before, after;
Entry(int hash, K key, V value, Node<K,V> next) {
super(hash, key, value, next);
}
}
private static final long serialVersionUID = 3801124242820219131L;
/**
* The head (eldest) of the doubly linked list.
*/
transient LinkedHashMap.Entry<K,V> head;
/**
* The tail (youngest) of the doubly linked list.
*/
transient LinkedHashMap.Entry<K,V> tail;
/**
* The iteration ordering method for this linked hash map: <tt>true</tt>
* for access-order, <tt>false</tt> for insertion-order.
*
* @serial
*/
final boolean accessOrder;- LinkedHashMap的静态内部类
Entry<K,V>继承了HashMap的HashMap.Node<K,V>,Entry内部新增了before和after两个属性，实现了双向列表。 head和tail两个字段用于记录队列的头节点和尾节点。accessOrder控制列表的顺序是根据插入顺序排序还是根据访问顺序进行排序。
1.3 主要方法
LinkedHashMap并没有重写父类HashMap的put方法，而是重写了HashMap中空的钩子方法，HashMap源码如下：
// Callbacks to allow LinkedHashMap post-actions
void afterNodeAccess(Node<K,V> p) { }
void afterNodeInsertion(boolean evict) { }
void afterNodeRemoval(Node<K,V> p) { } 这些钩子方法在HashMap的put等方法中进行了调用，如：
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}afterNodeInsertion(boolean evict)方法
这个方法的主要作用就是在元素添加到Map中后，判断是否要移除最老的元素，默认返回false。
void afterNodeInsertion(boolean evict) { // possibly remove eldest
LinkedHashMap.Entry<K,V> first;
if (evict && (first = head) != null && removeEldestEntry(first)) {
K key = first.key;
removeNode(hash(key), key, null, false, true);
}
}我们来看一下removeEldestEntry(first)方法的实现：
/**
* 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;
}当我们将LinkedHashMap用作LRU缓存，我们需要限制缓存的大小，这时我们需要重写LinkedHashMap的removeEldestEntry方法，当元素个数超过设置的最大值后，则移除最老的元素，代码如下：
private static final int MAX_ENTRIES = 100;
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > MAX_ENTRIES;
}afterNodeAccess(Node<K,V> e)方法
首先来看一下afterNodeAccess方法被调用的时机：
- LinkedHashMap中调用
public V get(Object key)方法时，且accessOrder=true - LinkedHashMap中调用
public V getOrDefault(Object key, V defaultValue)方法时，且accessOrder=true - HashMap中调用
put方法添加一个Key已经存在的元素 - HashMap中调用
public boolean replace(K key, V oldValue, V newValue)方法时 - HashMap中调用
public V replace(K key, V value)方法时 - HashMap中调用
public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)方法时，对应Key的值存且在不为null - HashMap中调用
public V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)方法时 - HashMap中调用
public V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)方法时 - HashMap中调用
public V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)方法时
该方法的主要作用是在accessOrder=true的情况下，将e节点转移至双向队列末尾。
void afterNodeAccess(Node<K,V> e) { // move node to last
LinkedHashMap.Entry<K,V> last;
if (accessOrder && (last = tail) != e) {
LinkedHashMap.Entry<K,V> p =
(LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
p.after = null;
if (b == null)
head = a;
else
b.after = a;
if (a != null)
a.before = b;
else
last = b;
if (last == null)
head = p;
else {
p.before = last;
last.after = p;
}
tail = p;
++modCount;
}
}afterNodeRemoval(Node<K,V> e)方法
该方法的作用是在元素被移除后将列表中的元素也进行移除。
void afterNodeRemoval(Node<K,V> e) { // unlink
LinkedHashMap.Entry<K,V> p =
(LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
p.before = p.after = null;
if (b == null)
head = a;
else
b.after = a;
if (a == null)
tail = b;
else
a.before = b;
}总结
LinkedHashMap继承了HashMap，通过Entry对象构建了一个双向队列，并通过重写HashMap中预留的空的钩子方法维护元素的顺序。当accessOrder=false，默认根据元素的插入顺序排序；accessOrder=true，根据元素的访问顺序进行排序。
根据LinkedHashMap的特点，能够很方便的实现一个LRU缓存算法，如下：
public class LinkedHashMapLRUCache extends LinkedHashMap {
private Integer CACHE_MAX_COUNT;
public LinkedHashMapLRUCache(int initialCapacity) {
super(initialCapacity, 0.75f, true);
CACHE_MAX_COUNT = initialCapacity;
}
@Override
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > CACHE_MAX_COUNT;
}
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