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() > 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。