Java8集合框架——LinkedHashMap源码分析

本文的结构如下：

一、LinkedHashMap 的 Javadoc 文档注释和简要说明
二、LinkedHashMap 的内部实现：一些扩展属性和构造函数
三、LinkedHashMap 的 put 操作和扩容
四、LinkedHashMap 的 get 操作
五、LinkedHashMap 的 remove 操作

一、LinkedHashMap 的 Javadoc 文档注释和简要说明

　　先膜拜下 LinkedHashMap 的 Javadoc，只能说很佩服，这文档注释把 LinkedHashMap 的主要特点都罗列出来了。看懂这注释，然后再对照源码，可以理解个七七八八八，也不会奇怪说各路总结那么多，都是哪来的。以下是 Javadoc 的几点摘抄：

LinkedHashMap 是 Map 接口的 hash table 和 linked list 实现类，内部所有节点维护了双链表，迭代顺序可预测，默认按照插入顺序进行迭代输出（已存在的 k 重新 put 不影响顺序，因为 m.containsKey(k) 会先返回 true ），这种特性对于需要有序的 Map 参数来说很有用，而且效率优于 TreeMap。
LinkedHashMap 还提供了构造器用于指定按照访问顺序进行迭代输出，即按照最近最少访问到最近访问的访问顺序：from least-recently accessed to most-recently (access-order)。这种特性适合做 LRU 缓存（least-recently used cache），即继承 LinkedHashMap ，重写 removeEldestEntry(Map.Entry) 方法来指定什么时候移除的策略。
LinkedHashMap 继承了 HashMap，基本操作(add, contains and remove)可以认为是O(1)，因需要维护双链表，性能可能会略低于 HashMap，但是有一个例外：LinkedHashMap 的迭代只与实际大小有关（毕竟可以依靠双链表进行迭代），而 HashMap 的迭代则与容量有关，性能会相对低于 LinkedHashMap。
同样不适合多线程操作，需要额外进行同步，比如使用 Collections.synchronizedMap 。
迭代器也是 fail-fast，而且并不保证出现有并发修改就百分百抛出 ConcurrentModificationException，而是尽可能检查到，因此只适用于检测 bug（抛出 ConcurrentModificationException 说明有问题，但是没有抛出来不能说明没问题）。

　　可以看出，LinkedHashMap 有 2个主要用途：

有序的 HashMap
LRU cache

LinkedHashMap 的 Javadoc：

/**
 * <p>Hash table and linked list implementation of the <tt>Map</tt> interface,
 * with predictable iteration order.  This implementation differs from
 * <tt>HashMap</tt> in that it maintains a doubly-linked list running through
 * all of its entries.  This linked list defines the iteration ordering,
 * which is normally the order in which keys were inserted into the map
 * (<i>insertion-order</i>).  Note that insertion order is not affected
 * if a key is <i>re-inserted</i> into the map.  (A key <tt>k</tt> is
 * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when
 * <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to
 * the invocation.)
 *
 * <p>This implementation spares its clients from the unspecified, generally
 * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}),
 * without incurring the increased cost associated with {@link TreeMap}.  It
 * can be used to produce a copy of a map that has the same order as the
 * original, regardless of the original map's implementation:
 * <pre>
 *     void foo(Map m) {
 *         Map copy = new LinkedHashMap(m);
 *         ...
 *     }
 * </pre>
 * This technique is particularly useful if a module takes a map on input,
 * copies it, and later returns results whose order is determined by that of
 * the copy.  (Clients generally appreciate having things returned in the same
 * order they were presented.)
 *
 * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is
 * provided to create a linked hash map whose order of iteration is the order
 * in which its entries were last accessed, from least-recently accessed to
 * most-recently (<i>access-order</i>).  This kind of map is well-suited to
 * building LRU caches.  Invoking the {@code put}, {@code putIfAbsent},
 * {@code get}, {@code getOrDefault}, {@code compute}, {@code computeIfAbsent},
 * {@code computeIfPresent}, or {@code merge} methods results
 * in an access to the corresponding entry (assuming it exists after the
 * invocation completes). The {@code replace} methods only result in an access
 * of the entry if the value is replaced.  The {@code putAll} method generates one
 * entry access for each mapping in the specified map, in the order that
 * key-value mappings are provided by the specified map's entry set iterator.
 * <i>No other methods generate entry accesses.</i>  In particular, operations
 * on collection-views do <i>not</i> affect the order of iteration of the
 * backing map.
 *
 * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to
 * impose a policy for removing stale mappings automatically when new mappings
 * are added to the map.
 *
 * <p>This class provides all of the optional <tt>Map</tt> operations, and
 * permits null elements.  Like <tt>HashMap</tt>, it provides constant-time
 * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and
 * <tt>remove</tt>), assuming the hash function disperses elements
 * properly among the buckets.  Performance is likely to be just slightly
 * below that of <tt>HashMap</tt>, due to the added expense of maintaining the
 * linked list, with one exception: Iteration over the collection-views
 * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i>
 * of the map, regardless of its capacity.  Iteration over a <tt>HashMap</tt>
 * is likely to be more expensive, requiring time proportional to its
 * <i>capacity</i>.
 *
 * <p>A linked hash map has two parameters that affect its performance:
 * <i>initial capacity</i> and <i>load factor</i>.  They are defined precisely
 * as for <tt>HashMap</tt>.  Note, however, that the penalty for choosing an
 * excessively high value for initial capacity is less severe for this class
 * than for <tt>HashMap</tt>, as iteration times for this class are unaffected
 * by capacity.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a linked hash map concurrently, and at least
 * one of the threads modifies the map structurally, it <em>must</em> be
 * synchronized externally.  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the map.
 *
 * If no such object exists, the map should be "wrapped" using the
 * {@link Collections#synchronizedMap Collections.synchronizedMap}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the map:<pre>
 *   Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre>
 *
 * A structural modification is any operation that adds or deletes one or more
 * mappings or, in the case of access-ordered linked hash maps, affects
 * iteration order.  In insertion-ordered linked hash maps, merely changing
 * the value associated with a key that is already contained in the map is not
 * a structural modification.  <strong>In access-ordered linked hash maps,
 * merely querying the map with <tt>get</tt> is a structural modification.
 * </strong>)
 *
 * <p>The iterators returned by the <tt>iterator</tt> method of the collections
 * returned by all of this class's collection view methods are
 * <em>fail-fast</em>: if the map is structurally modified at any time after
 * the iterator is created, in any way except through the iterator's own
 * <tt>remove</tt> method, the iterator will throw a {@link
 * ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:   <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>The spliterators returned by the spliterator method of the collections
 * returned by all of this class's collection view methods are
 * <em><a href="Spliterator.html#binding">late-binding</a></em>,
 * <em>fail-fast</em>, and additionally report {@link Spliterator#ORDERED}.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @implNote
 * The spliterators returned by the spliterator method of the collections
 * returned by all of this class's collection view methods are created from
 * the iterators of the corresponding collections.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 *
 * @author  Josh Bloch
 * @see     Object#hashCode()
 * @see     Collection
 * @see     Map
 * @see     HashMap
 * @see     TreeMap
 * @see     Hashtable
 * @since   1.4
 */

二、LinkedHashMap 的内部实现：一些扩展属性和构造函数

　　LinkedHashMap 继承了 HashMap，这里重点说下 LinkedHashMap 在内部属性和构造函数方面扩展的部分。

1、扩展的属性和内部类

　　可以初步看出内部的一些变化，比如增加了首节点和尾节点的记录，内部节点元素增加了 before 和 after 节点。这些都是维持双链表需要用到的。另外就是 accessOrder ，用于指定是否按照访问顺序(设置为 true) 排序（默认 false 是插入顺序）。

    /**
     * HashMap.Node subclass for normal LinkedHashMap entries.
     * LinkedHashMap 的内部节点实现类，这里增加了 before 和 after 节点，用于维护 doubly-linked list
     * 这里继承了 HashMap.Node ，保证新节点的类型一致，都是 HashMap.Node
     */
    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);
        }
    }

    /**
     * The head (eldest) of the doubly linked list.
     * 首节点元素(最早插入/最近最早访问过的)
     */
    transient LinkedHashMap.Entry<K,V> head;

    /**
     * The tail (youngest) of the doubly linked list.
     * 尾节点元素(最晚插入/最近访问的)
     */
    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.
     * 迭代器的顺序控制
     * true:根据访问顺序
     * false:默认场景，根据插入顺序
     * @serial
     */
    final boolean accessOrder;

2、构造函数

　　和 HashMap 构造函数的差别主要是 accessOrder 的设置。

    /**
     * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
     * with the specified initial capacity and load factor.
     *
     * 指定 初始容量 和 负载因子 ，同时默认为 插入顺序
     * @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).
     *
     * 指定 初始容量 ，默认负载因子 0.75，同时默认为 插入顺序
     * @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).
     *
     * 空构造函数，默认初始容量 16，默认负载因子 0.75，同时默认为 插入顺序
     */
    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.
     *
     * 通过指定 Map 构造默认为 插入顺序 的 LinkedHashMap
     * @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.
     *
     * 指定 初始容量、负载因子、排序模式
     * @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;
    }

三、LinkedHashMap 的 put 操作和扩容

　　put 操作直接继承自 HashMap，由于 LinkedHashMap 会涉及到双向链表的处理，这里有几个注意点/改动点需要说明下：

1、重写新节点创建函数 Node<K,V> newNode(int hash, K key, V value, Node<K,V> e)，维护双链表

　　LinkedHashMap 的节点会有双向链表，因此在这里进行了处理，很明显，新节点即使最后访问也是最新插入的，直接就丢到最后去没毛病，因此链接到了链表最后/最新处。

// 创建新节点 并将 新节点 链接 到最后
Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
    LinkedHashMap.Entry<K,V> p =
        new LinkedHashMap.Entry<K,V>(hash, key, value, e);
    linkNodeLast(p);        // 将 新节点 链接 到最后
    return p;
}

// link at the end of list
// 将 新节点 链接 到最后
private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
    LinkedHashMap.Entry<K,V> last = tail;
    tail = p;
    if (last == null)
        head = p;
    else {
        p.before = last;
        last.after = p;
    }
}

2、HashMap 中留下来的三个回调函数， LinkedHashMap 都进行了重写

　　put 操作中有使用到的是 afterNodeAccess(Node<K,V> p) 和 afterNodeInsertion(boolean evict)。

afterNodeAccess(Node<K,V> p) ：k 存在的时候进行的操作。如果是根据访问控制顺序，需要将访问到的节点的链接到最后去；
afterNodeInsertion(boolean evict) ：k 不存在的时候进行的操作。 LRU cache 中可以进行实际的移除节点操作

// Callbacks to allow LinkedHashMap post-actions
void afterNodeAccess(Node<K,V> p) { }            // 访问节点后需要进行的操作，如果指定了根据访问顺序控制，则在这里将节点挪到最后
void afterNodeInsertion(boolean evict) { }       // 插入节点后需要进行的操作，比如 LRU cache 中移除最早的节点
void afterNodeRemoval(Node<K,V> p) { }           // 移除指定节点

　　在 LinkedHashMap 中的实现如下：

// 移除 e 节点元素后的操作，对于 HashMap ，removeNode 函数已经是移除了节点，这里是 LinkedHashMap 处理节点中和双向链表有关的的 before 和 after
void afterNodeRemoval(Node<K,V> e) { // unlink
    LinkedHashMap.Entry<K,V> p =
        (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
    // 移除 e 节点本身的链接
    p.before = p.after = null;
    if (b == null)        // 重置 e 节点上一个节点的 after 链接
        head = a;
    else
        b.after = a;
    if (a == null)        // 重置 e 节点下一个节点的 before 链接
        tail = b;
    else
        a.before = b;
}

// 是否移除最早插入/访问的节点元素
void afterNodeInsertion(boolean evict) { // possibly remove eldest
    LinkedHashMap.Entry<K,V> first;
    // 最简单的 LRU cache 其实就是重写 removeEldestEntry 什么时候返回 true 的逻辑（比如超过容量限制），然后移除最早插入/访问的节点
    if (evict && (first = head) != null && removeEldestEntry(first)) {
        K key = first.key;
        removeNode(hash(key), key, null, false, true);
    }
}

// 节点访问后是否将节点挪到最后
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)        // 重置 e 节点上一个节点的 after 链接
            head = a;
        else
            b.after = a;
        if (a != null)        // 重置 e 节点下一个节点的 before 链接
            a.before = b;
        else
            last = b;
        if (last == null)        // 只有一个 e 节点的场景
            head = p;
        else {
            p.before = last;    // 把 e 节点挪到最后
            last.after = p;
        }
        tail = p;            // 尾节点处理
        ++modCount;
    }
}

　　这里再看看 removeEldestEntry(Map.Entry<K,V> eldest)，这个方法是实现 LRU cache 的关键所在，文档注释中其实已经写明了简要应用，也就是检查 Map 的实际大小是否大于规定的容量，超过就是返回true，需要进行节点移除，保证集合不超过规定的上限。

/**
 * 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() &gt; 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;
}

3、还有一个比较骚的操作就是 HashMap 内部红黑树节点 TreeNode 是直接继承 LinkedHashMap.Entry，因此这方面的红黑树转化、扩容等等基本上可以说是无缝对接。

static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {...}

　　红黑树转化和扩容其实只是涉及到内部节点的挪动，双向链表是不用改动的，因此不需要进行操作。

四、LinkedHashMap 的 get 操作

　　增加了 afterNodeAccess(Node<K,V> p) 的调用，对于访问顺序控制 LinkedHashMap，需要将访问的节点挪到最后去。其他的和 HashMap 一样。

    /**
     * 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) {
        Node<K,V> e;
        if ((e = getNode(hash(key), key)) == null)
            return null;
        if (accessOrder)
            afterNodeAccess(e);        // 增加访问节点后需要进行的操作，如果指定了根据访问顺序控制，则在这里将节点挪到最后
        return e.value;
    }

五、LinkedHashMap 的 remove 操作

　　节点的移除使用的是 HashMap 的 remove(Object key) ，移除其实是一样的，只是 LinkedHashMap 在最后需要处理双链表，这里使用的是扩展了 afterNodeRemoval(Node<K,V> p) 来进行处理。这个方法在 LinkedHashMap 的实现可以翻看本文前面的介绍。