LinkedHashMap 是 Map 接口的哈希表和链接列表实现，具有可预知的迭代顺序。此实现与 HashMap 的不同之处在于，后者维护着一个运行于所有条目的双重链接列表。此链接列表定义了迭代顺序，该迭代顺序通常就是将键插入到映射中的顺序（插入顺序）。
注意，如果在映射中重新插入 键，则插入顺序不受影响。
1.LinkedHashMap数据成员
/**
* The head of the doubly linked list.
*/
// 双向链接的头指针
private transient Entry<K,V> header;
/**
* The iteration ordering method for this linked hash map: <tt>true</tt>
* for access-order, <tt>false</tt> for insertion-order.
*
* @serial
*/
// 迭代的排序方式：设置为true则按访问顺序，否则按插入顺序
private final boolean accessOrder;
2.LinkedHashMap构造方法
/**
* 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;
}
/**
* Called by superclass constructors and pseudoconstructors (clone,
* readObject) before any entries are inserted into the map. Initializes
* the chain.
*/
// 被父类HashMap的构造函数调用
void init() {
// 初始化双向链接的头指针
header = new Entry<K,V>(-1, null, null, null);
header.before = header.after = header;
}
3.LinkedHashMap的Entry
添加了before,after 两个引用，用来构造双向链表
/**
* 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;
Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
super(hash, key, value, next);
}
/**
* Removes this entry from the linked list.
*/
// 从双向链表中移除
private void remove() {
before.after = after;
after.before = before;
}
/**
* Inserts this entry before the specified existing entry in the list.
*/
private void addBefore(Entry<K,V> existingEntry) {
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.
*/
// Override了父类的recordAccess方法，用来记录访问顺序
void recordAccess(HashMap<K,V> m) {
LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
if (lm.accessOrder) {
lm.modCount++;
remove();
// 添加到链表的头部之前（即末尾）
// put 方法修改会引起顺序的变更
addBefore(lm.header);
}
}
void recordRemoval(HashMap<K,V> m) {
remove();
}
}
addEntry方法
/**
* 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
// 根据方法判断是否移除最老的元素，如果返回true，可以实现一个简单的LRUCache
Entry<K,V> eldest = header.after;
if (removeEldestEntry(eldest)) {
removeEntryForKey(eldest.key);
} else {
// 大小超过threshold，进行rehash 扩容
if (size >= threshold)
resize(2 * table.length);
}
}
/**
* 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;
// 创建一个entry，并添加到双向链表的末尾
e.addBefore(header);
size++;
}
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
return false;
}
可以重写 removeEldestEntry(Map.Entry) 方法来实施策略，以便在将新映射关系添加到映射时自动移除旧的映射关系。
示例用法：此重写允许映射增加到 100 个条目，然后每次添加新条目时删除最旧的条目，始终维持 100 个条目的稳定状态。
private static final int MAX_ENTRIES = 100;
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > MAX_ENTRIES;
}
transfer方法
/**
* Transfers all entries to new table array. This method is called
* by superclass resize. It is overridden for performance, as it is
* faster to iterate using our linked list.
*/
void transfer(HashMap.Entry[] newTable) {
// 由于记录了双向链表，遍历所有entry变得更加简单
int newCapacity = newTable.length;
for (Entry<K,V> e = header.after; e != header; e = e.after) {
int index = indexFor(e.hash, newCapacity);
e.next = newTable[index];
newTable[index] = e;
}
}
迭代器的实现
private abstract class LinkedHashIterator<T> implements Iterator<T> {
// 第一次next元素 是 header.after
Entry<K,V> nextEntry = header.after;
Entry<K,V> lastReturned = null;
/**
* The modCount value that the iterator believes that the backing
* List should have. If this expectation is violated, the iterator
* has detected concurrent modification.
*/
int expectedModCount = modCount;
public boolean hasNext() {
return nextEntry != header;
}
public void remove() {
if (lastReturned == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
LinkedHashMap.this.remove(lastReturned.key);
lastReturned = null;
expectedModCount = modCount;
}
Entry<K,V> nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (nextEntry == header)
throw new NoSuchElementException();
// 通过迭代链表的指针进行遍历
Entry<K,V> e = lastReturned = nextEntry;
nextEntry = e.after;
return e;
}
}
private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> {
public Map.Entry<K,V> next() { return nextEntry(); }
}