【源码】LinkedHashMap源码剖析

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注：以下源码基于jdk1.7.0_11

public class LinkedHashMap<K,V>
    extends HashMap<K,V>
    implements Map<K,V>

 private transient Entry<K,V> header;//内部双向链表的头结点
    /**
     *代表这个链表的排序方式，true代表按照访问顺序，false代表按照插入顺序。
     */
 private final boolean accessOrder;

public LinkedHashMap(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor);
        accessOrder = false;
    }
    public LinkedHashMap(int initialCapacity) {
        super(initialCapacity);
        accessOrder = false;
    }
    public LinkedHashMap() {
        super();
        accessOrder = false;
    }
    public LinkedHashMap(Map<? extends K, ? extends V> m) {
        super(m);
        accessOrder = false;
    }
   public LinkedHashMap(int initialCapacity,
                         float loadFactor,
                         boolean accessOrder) {
        super(initialCapacity, loadFactor);
        this.accessOrder = accessOrder;
    }

public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
       ... ...
        init();
    }

 @Override
    void init() {
        header = new Entry<>(-1, null, null, null);//初始化双向链表
        header.before = header.after = header;//不光是双向链表，还是循环链表
    }

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;//前驱、后继指针
        Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
            super(hash, key, value, next);
        }
        /**
         * Removes this entry from the linked list.
         */
        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.
         */
        void recordAccess(HashMap<K,V> m) {
            LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
            if (lm.accessOrder) {
                lm.modCount++;
                remove();
                addBefore(lm.header);
            }
        }
        void recordRemoval(HashMap<K,V> m) {
            remove();
        }
    }

 public V put(K key, V value) {//HashMap的put方法
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key);
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);//发生覆盖操作时，会调用此方法
                return oldValue;
            }
        }
      ... ...
    }

 public V get(Object key) {
        Entry<K,V> e = (Entry<K,V>)getEntry(key);
        if (e == null)
            return null;
        e.recordAccess(this);
        return e.value;
    }

void addEntry(int hash, K key, V value, int bucketIndex) {
        super.addEntry(hash, key, value, bucketIndex);
        // Remove eldest entry if instructed
        Entry<K,V> eldest = header.after;//标记最少访问的对象
        if (removeEldestEntry(eldest)) {//判断是否需要删除这个对象---->可由子类实现来提供缓存功能
            removeEntryForKey(eldest.key);
        }
    }
    void createEntry(int hash, K key, V value, int bucketIndex) {
        HashMap.Entry<K,V> old = table[bucketIndex];
        Entry<K,V> e = new Entry<>(hash, key, value, old);
        table[bucketIndex] = e;
        e.addBefore(header);//添加到链表尾部
        size++;
    }

 private abstract class LinkedHashIterator<T> implements Iterator<T> {
        Entry<K,V> nextEntry    = header.after;//指向链表首部
        Entry<K,V> lastReturned = null;
        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();
            Entry<K,V> e = lastReturned = nextEntry;
            nextEntry = e.after;
            return e;
        }
    }

【源码】LinkedHashMap源码剖析,布布扣,bubuko.com

【源码】LinkedHashMap源码剖析

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原文地址：http://blog.csdn.net/chdjj/article/details/38677315