LinkedHashMap源码解析

先看到成员变量

	
	//为了实现有序访问HashMap中的数据，使用一个额外双向链表来维护数据的顺序
     
    //双向链表的头节点
    transient LinkedHashMap.Entry<K,V> head;

	//双向链表的尾节点
    transient LinkedHashMap.Entry<K,V> tail;
	
	//默认为false,可以构造函数设置
	//为true时，被访问的数据会移动到双向链表的尾部
	//为false时，则按照插入的顺序维持双向链表
    final boolean accessOrder;

看到父类HashMap的put方法，之前的文章已经讲过HashMap，这里着重看LinkedHashMap的方法

	public V put(K key, V value) {
		//第五个参数(evict)为true 
        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) {
                    	//看这里，LinkedHashMap重写了该方法，返回的是LinkedHashMap的node，含有before和after属性，来维持一个双向链表。			
                    	//实例化node,并将node加入到双向链表中
                        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;
                //看这里，新插入的节点的value替换了老的节点(e)的value，如果accessOrder为true 移动e到双向链表的尾部
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        //看这里，可能会从HashMap移除双向链表中的头节点
        afterNodeInsertion(evict);
        return null;
    }

newNode方法

	Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
		//返回LinkedHashMap的node,比起HashMap的node,多了before和after属性
        LinkedHashMap.Entry<K,V> p =
            new LinkedHashMap.Entry<K,V>(hash, key, value, e);
        //将新插入的节点，放在双向链表的队尾
        linkNodeLast(p);
        return p;
    }
    private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
    	//获取尾节点赋值给last
        LinkedHashMap.Entry<K,V> last = tail;
        //将尾节点修改为p
        tail = p;
        //如果尾节点为空，说明双向链表还没初始化，设置头节点为p
        if (last == null)
            head = p;
        else {
        	//设置p的前驱结点为last
            p.before = last;
            //设置last的后继节点为p
            last.after = p;
        }
    }

afterNodeAccess方法

    void afterNodeAccess(Node<K,V> e) { // move node to last
        LinkedHashMap.Entry<K,V> last;
        //如果accessOrder 为 true 且 尾节点不是e
        //tail尾节点赋值给last
        if (accessOrder && (last = tail) != e) {
        	//e赋值给p
        	//p的前驱节点为b
        	//p的后继节点为a 
            LinkedHashMap.Entry<K,V> p =
                (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
            //由于要移动到尾部，设置后继节点为空
            p.after = null;
            //如果b没有值，说明自己是头节点
            if (b == null)
            	//将头节点设置赋值为a
                head = a;
            //前驱节点b存在
            else
            	//将b的后继节点设置为a
                b.after = a;
            //如果a不为空
            if (a != null)
            	//将a的前取节点设置为b
                a.before = b;
            //如果a为空，没有后继节点
            else
            	//b赋值给last
                last = b;
            //如果last为空    
            if (last == null)
            	//将头节点设置为p
                head = p;
            //如果last不为空  
            else {
            	//将p的前驱节点色设置为last
                p.before = last;
                //将last的后继节点设置为p
                last.after = p;
            }
            //将尾节点设置为p
            tail = p;
            //modCount加一
            ++modCount;
        }
    }

afterNodeInsertion方法

    void afterNodeInsertion(boolean evict) { // possibly remove eldest
        LinkedHashMap.Entry<K,V> first;
        //如果evict为true , 且first赋值为头结点 不为空 ， 且 removeEldestEntry返回为true
        if (evict && (first = head) != null && removeEldestEntry(first)) {
        	//获取头结点
            K key = first.key;
            //调用父类的removeNode从HashMap中移除该节点
            removeNode(hash(key), key, null, false, true);
        }
    }	
    //可以继承LinkedHashMap重写该方法返回true 
	protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
        return false;
    }
	
	//com.mysql.jdbc.util.LRUCache继承了LinkedHashMap重写了removeEldestEntry
	public class LRUCache extends LinkedHashMap<Object, Object> {
    private static final long serialVersionUID = 1L;
    protected int maxElements;

	public LRUCache(int maxSize) {
		//将accessorder设置为true!
		super(maxSize, 0.75F, true);
		//设置LRUCache中最多存放的数据量
		this.maxElements = maxSize;
	}

	@Override
	protected boolean removeEldestEntry(Entry<Object, Object> eldest) {
		//当HashMap中的元素个数超过了maxElements，返回true，从HashMap中移除双向链表头结点
		return (size() > this.maxElements);
	}
}

再来get方法，LinkedHashMap重写了父类的get方法

	public V get(Object key) {
        Node<K,V> e;
        //调用父类的getNode从HashMap中找到该节点
        if ((e = getNode(hash(key), key)) == null)
            return null;
        //如果accessOrder为true , 调用afterNodeAccess方法将该节点移动到双向链表的尾部
        if (accessOrder)
            afterNodeAccess(e);
        return e.value;
    }

再来看到父类的remove方法

    public V remove(Object key) {
        Node<K,V> e;
        return (e = removeNode(hash(key), key, null, false, true)) == null ?
            null : e.value;
    }
    final Node<K,V> removeNode(int hash, Object key, Object value,
                               boolean matchValue, boolean movable) {
        Node<K,V>[] tab; Node<K,V> p; int n, index;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (p = tab[index = (n - 1) & hash]) != null) {
            Node<K,V> node = null, e; K k; V v;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            else if ((e = p.next) != null) {
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                else {
                    do {
                        if (e.hash == hash &&
                            ((k = e.key) == key ||
                             (key != null && key.equals(k)))) {
                            node = e;
                            break;
                        }
                        p = e;
                    } while ((e = e.next) != null);
                }
            }
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                //看这里，HashMap中对node的引用已经断开了，现在要断开双向链表对node的引用
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }

afterNodeRemoval方法

	void afterNodeRemoval(Node<K,V> e) { // unlink
		//将p赋值为e 
		//b赋值为p的前驱结点
		//a赋值为p的后继节点
        LinkedHashMap.Entry<K,V> p =
            (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
        //将p的前驱结点和后继节点置空
        p.before = p.after = null;
        //如果b为空，说明p是头结点，将头结点设置为a
        if (b == null)
            head = a;
        //p不是头结点
        else
        	//将b的后继节点设置为a
            b.after = a;
        //如果a为空，说明p是尾节点，将尾节点设置为b
        if (a == null)
            tail = b;
        //p不是尾节点
        else
        	//将a的前驱结点设置为b
            a.before = b;
    }

最后看到LinkedHashMap的LinkedEntrySet的iterator方法

    public final Iterator<Map.Entry<K,V>> iterator() {
        return new LinkedEntryIterator();
    }
    //继承了LinkedHashIterator
    final class LinkedEntryIterator extends LinkedHashIterator
        implements Iterator<Map.Entry<K,V>> {
        //next方法调用父类的nextNode方法
        public final Map.Entry<K,V> next() { return nextNode(); }
    }
	
	abstract class LinkedHashIterator {
        LinkedHashMap.Entry<K,V> next;
        LinkedHashMap.Entry<K,V> current;
        int expectedModCount;
		
        LinkedHashIterator() {
        	//next赋值为头节点
            next = head;
            //expectedModCount赋值为modCount
            expectedModCount = modCount;
            current = null;
        }

        public final boolean hasNext() {
            return next != null;
        }
		
		//这里不是遍历table去取到数据返回， 而是遍历双向链表返回数据 
        final LinkedHashMap.Entry<K,V> nextNode() {
        	//e赋值为next
            LinkedHashMap.Entry<K,V> e = next;
            //modCount != expectedModCount 抛异常
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (e == null)
                throw new NoSuchElementException();
            //current 赋值为 e
            current = e;
            //修改next为下一个节点
            next = e.after;
            return e;
        }
		
		//移除current节点
        public final void remove() {
            Node<K,V> p = current;
            if (p == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            current = null;
            K key = p.key;
            //移除该节点 ， modCount加一了
            removeNode(hash(key), key, null, false, false);
            //这里会重新修改expectedModCount的值为modCount
            expectedModCount = modCount;
        }
    }

到此LinkedHashMap主要的方法就分析完了。