浅谈java如何实现Redis的LRU缓存机制

package thread;
import java.util.LinkedHashMap;
import java.util.Map;
 
public class LRUCacheTest {
    int capacity;
    Map<Integer,Integer> map;
 
    public LRUCacheTest(int capacity){
        this.capacity = capacity;
        map = new LinkedHashMap<>();
    }
 
    public int get(int key){
        //没有找到
       if(!map.containsKey(key)){
          return -1;
       }
       Integer value = map.remove(key);
       map.put(key,value);
       return value;
    }
 
    public void put(int key,int value){
        if(map.containsKey(key)){
           map.remove(key);
           map.put(key,value);
           return;
        }
        map.put(key,value);
        //超出capacity，删除最久没用的即第一个,或者可以复写removeEldestEntry方法
        if(map.size() > capacity){
           map.remove(map.entrySet().iterator().next().getKey());
        }
 
    }
 
    public static void main(String[] args) {
        LRUCacheTest lruCache = new LRUCacheTest(10);
        for (int i = 0; i < 10; i++) {
            lruCache.map.put(i,i);
            System.out.print(lruCache.map.size()+"\t");
        }
        System.out.println();
        System.out.println(lruCache.map);
        lruCache.put(10,200);
        System.out.println(lruCache.map);
        lruCache.put(11,100);
        System.out.println(lruCache.map);
        lruCache.get(2);
        System.out.println(lruCache.map);
    }
 
}

package thread;
import java.util.LinkedHashMap;
import java.util.Map;
 
public class LRUByLinkedHashMap extends LinkedHashMap<Integer,Integer> {
    /**
     * LRU中最大元素数量
     */
    private int maxSize;
 
    public LRUByLinkedHashMap(int maxSize) {
        // 容量为最大值/0.75，即最大负载容量为maxSize
        // accessOrder=true  根据查询排序，即最近被使用的放到后面
        super((int) Math.ceil(maxSize / 0.75) + 1, 0.75f, true);
        this.maxSize = maxSize;
    }
 
    /**
     * 此方法为钩子方法，map插入元素时会调用此方法
     * 此方法返回true则证明删除最老的因子
     * @param eldest
     * @return
     */
    @Override
    protected boolean removeEldestEntry(Map.Entry<Integer, Integer> eldest) {
        return size() > maxSize;
    }
 
    public static void main(String[] args) {
        LRUByLinkedHashMap hashMap = new LRUByLinkedHashMap(10);
        for (int i = 0; i < 10; i++) {
            hashMap.put(i,i);
            System.out.print(hashMap.size()+"\t");
        }
        System.out.println();
        System.out.println(hashMap);
        hashMap.put(10,200);
        System.out.println(hashMap);
        hashMap.put(11,100);
        System.out.println(hashMap);
        hashMap.get(10);
        System.out.println(hashMap);
    }
 
}

package thread;
import java.util.HashMap;
import java.util.Map;
 
public class LRURedis {
    private int capacity;
    private Map<Integer,ListNode> map;
    private ListNode head;
    private ListNode tail;
 
    public LRURedis(int capacity){
      this.capacity = capacity;
      map = new HashMap<>();
      head = new ListNode(-1,-1);
      tail = new ListNode(-1,-1);
      head.next = tail;
      tail.pre = head;
    }
 
    public int get(int key){
        if(!map.containsKey(key)){
           return -1;
        }
        ListNode node = map.get(key);
        node.pre.next = node.next;
        node.next.pre = node.pre;
        return node.val;
    }
 
    public void put(int key,int value){
        if (get(key)!=-1){
            map.get(key).val = value;
            return;
        }
 
        ListNode node = new ListNode(key,value);
        map.put(key,node);
        moveToTail(node);
 
        if (map.size() > capacity){
            map.remove(head.next.key);
            head.next = head.next.next;
            head.next.pre = head;
        }
 
    }
 
    //把节点移动到尾巴
    private void moveToTail(ListNode node) {
        node.pre = tail.pre;
        tail.pre = node;
        node.pre.next = node;
        node.next = tail;
    }
 
    //定义双向链表节点
    private class ListNode{
        int key;
        int val;
        ListNode pre;
        ListNode next;
 
        //初始化双向链表
        public ListNode(int key,int val){
          this.key = key;
          this.val = val;
          pre = null;
          next = null;
        }
    }
}