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;
    }
}