基于LinkedHashMap实现LRU缓存调度算法原理

引言

本文就 《基于LinkedHashMap实现LRU缓存调度算法原理及应用 》一文作为材料，记录一些常见问题，备忘。

延伸出两道常见的Java面试题：

1. 插入Entry节点到table表的链表中时，Hashmap 和LinkedHashmap使用头茶法还是尾茶法？遍历map的时候，Entry.Entryset()获取的set集合,是按照从头到尾还是从尾到头的顺序存储的？
   
2. 实现LRU算法最合适的数据结构？

如果读者可以打出来，不用继续看下边的资料了。初学者请继续阅读。相信你读完之后可以找到问题的答案。

LinkedHashMap基础

LinkedHashMap继承了HashMap底层是通过Hash表+单向链表实现Hash算法，内部自己维护了一套元素访问顺序的列表。 

 /**
   * The head of the doubly linked list.
   */
  private transient Entry header;
  .....
 /**
   * LinkedHashMap entry.
   */
  private static class Entry extends HashMap.Entry {
      // These fields comprise the doubly linked list used for iteration.
      Entry before, after;

HashMap构造函数中回调了子类的init方法实现对元素初始化 

 void init() {
     header = new Entry(-1, null, null, null);
     header.before = header.after = header;
 }

LinkedHashMap中有一个属性可以执行列表元素的排序算法 

 /**
   * The iteration ordering method for this linked hash map: true
   * for access-order, false for insertion-order.
   *
   * @serial
   */
  private final boolean accessOrder;

accessOrder为true使用访问顺序排序，false使用插入顺序排序那么在哪里可以设置这个值。 

 /**
   * Constructs an empty LinkedHashMap 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 - true for
   *         access-order, false 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;
  }

LRU算法

使用有访问顺序排序方式实现LRU,那么哪里LinkedHashMap是如何实现LRU的呢？ 

    //LinkedHashMap方法
    public V get(Object key) {
        Entry e = (Entry)getEntry(key);
        if (e == null)
            return null;
        e.recordAccess(this);
        return e.value;
    }
    //HashMap方法
    public V put(K key, V value) {
 if (key == null)
     return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry 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);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

当调用get或者put方法的时候，如果K-V已经存在，会回调Entry.recordAccess()方法 
我们再看一下LinkedHashMap的Entry实现 

 /**
   * 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 m) {
      LinkedHashMap lm = (LinkedHashMap)m;
      if (lm.accessOrder) {
          lm.modCount++;
          remove();
          addBefore(lm.header);
      }
  }

  /**
   * Remove this entry from the linked list.
   */
  private void remove() {
      before.after = after;
      after.before = before;
  }

  /**
   * Insert this entry before the specified existing entry in the list.
   */
  private void addBefore(Entry existingEntry) {
      after  = existingEntry;
      before = existingEntry.before;
      before.after = this;
      after.before = this;
  }

recordAccess方法会accessOrder为true会先调用remove清楚的当前首尾元素的指向关系，之后调用addBefore方法，将当前元素加入header之前。 

当有新元素加入Map的时候会调用Entry的addEntry方法，会调用removeEldestEntry方法，这里就是实现LRU元素过期机制的地方，默认的情况下removeEldestEntry方法只返回false表示元素永远不过期。 

   /**
     * 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
        Entry eldest = header.after;
        if (removeEldestEntry(eldest)) {
            removeEntryForKey(eldest.key);
        } else {
            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 old = table[bucketIndex];
 Entry e = new Entry(hash, key, value, old);
        table[bucketIndex] = e;
        e.addBefore(header);
        size++;
    }

    protected boolean removeEldestEntry(Map.Entry eldest) {
        return false;
    }

基本的原理已经介绍完了，那基于LinkedHashMap我们看一下是该如何实现呢？ 

Java代码  

import java.util.LinkedHashMap;


public class URLLinkedListHashMap extends LinkedHashMap {

	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;

	/** 最大数据存储容量 */  
    private static final int  LRU_MAX_CAPACITY     = 1024;  

    /** 存储数据容量  */  
    private int               capacity;  

    /** 
     * 默认构造方法 
     */  
    public URLLinkedListHashMap() {  
        super();  
    }  

    /** 
     * 带参数构造方法 
     * @param initialCapacity   容量 
     * @param loadFactor        装载因子 
     * @param isLRU             是否使用lru算法，true：使用（按方案顺序排序）;false：不使用（按存储顺序排序） 
     */  
    public URLLinkedListHashMap(int initialCapacity, float loadFactor, boolean isLRU) {  
        super(initialCapacity, loadFactor, isLRU);  
        capacity = LRU_MAX_CAPACITY;  
    }  
    
    public URLLinkedListHashMap(int initialCapacity, float loadFactor, boolean isLRU,int lruCapacity) {  
        super(initialCapacity, loadFactor, isLRU);  
        this.capacity = lruCapacity;  
    } 
    
    @Override
    protected boolean removeEldestEntry(java.util.Map.Entry eldest) {
    	// TODO Auto-generated method stub
    	return super.removeEldestEntry(eldest);
    }

}

测试代码： 

Java代码  

import java.util.LinkedHashMap;
import java.util.Map.Entry;


public class LRUTest {

    public static void main(String[] args) {

        LinkedHashMap map = new URLLinkedListHashMap(16, 0.75f, false);
        map.put("a", "a"); //a  a
        map.put("b", "b"); //a  a b
        map.put("c", "c"); //a  a b c
        map.put("a", "a"); //   b c a     
        map.put("d", "d"); //b  b c a d
        map.put("a", "a"); //   b c d a
        map.put("b", "b"); //   c d a b     
        map.put("f", "f"); //c  c d a b f
        map.put("g", "g"); //c  c d a b f g

        
        map.get("d"); //c a b f g d
        for (Entry entry : map.entrySet()) {
            System.out.print(entry.getValue() + ", ");
        }
        
        System.out.println();

        map.get("a"); //c b f g d a
        for (Entry entry : map.entrySet()) {
            System.out.print(entry.getValue() + ", ");
        }
        System.out.println();

        map.get("c"); //b f g d a c
        for (Entry entry : map.entrySet()) {
            System.out.print(entry.getValue() + ", ");
        }
        System.out.println();

        map.get("b"); //f g d a c b
        for (Entry entry : map.entrySet()) {
            System.out.print(entry.getValue() + ", ");
        }
        System.out.println();

        map.put("h", "h"); //f  f g d a c b h
        for (Entry entry : map.entrySet()) {
            System.out.print(entry.getValue() + ", ");
        }
        System.out.println();
    }


}

答案：

1. 插入Entry节点到table表的链表中时，Hashmap 和LinkedHashmap使用头茶法。遍历map的时候，Entry.Entryset()获取的set集合,是按照从尾到头的顺序存储的，采用FIFO原理打印。
   
2. 实现LRU算法最合适的数据结构是LinkedHashmap

部分转自： http://woming66.iteye.com/blog/1284326