LRU缓存机制
前言
LRU（Least Recently Used）即最近最少使用、最近最久未使用，是一种缓存解决方案。
缓存思想在计算机科学中应用广泛，其核心是以空间换时间，实现方式主要采用特定的数据结构，在计算机存储层次结构中，低一层的存储器都可以看做是高一层的缓存。比如Cache是内存的缓存，内存是硬盘的缓存，硬盘是网络的缓存等等。
第一次接触到LRU是在操作系统的课程上，在提到OS页面调度算法的时候提到的LRU页面置换算法，页面置换是基于时间和空间局部性原理的基础上，而缓存类似于LRU这样的缓存技术在这样的场景下得到应用。
LRU在数据库缓存中也有广泛应用，例如Redis缓存淘汰策略。在使用next.js的时候，访问页面时，为了加快访问速度也会用到。
LRU：如果一个数据最近没有被访问到，那么它将来被访问的几率也很小，也就是说当限定的内存空间已没有其他空间可用时，应该把最久没有访问到的数据去除掉。
LRU为了实现时间复杂度O(1)，采用HashMap+双向链表的数据结构，实get和put操作，如下图所示。
Java中可以采用LinkedHashMap源实现LRU，Python中可以调用OrderedDict()实现。
例题 LeetCode146. LRU 缓存机制
Java实现
下面用是用Java实现的几种方式，分别是
调用Java的LinkedHashMap实现LRU
package LRU;
import java.util.*;
/**
* @author Orust
* @create 2021/5/24 16:07
*/
// 146. LRU 缓存机制: https://leetcode-cn.com/problems/lru-cache/
// LinkedHashMap源码中写道: This kind of map is well-suited to building LRU caches.
// put和get的时间复杂度均为O(1)
// 空间复杂度为O(capacity) capacity为LRU缓存大小
// 广泛应用与OS内存页面调度，Redis缓存淘汰策略
// 样例输入
// ["LRUCache_4","put","put","put","get","put","put","get","put","put","get","put","get","get","get","put","put","get","put","get"]
// [[10],[7,28],[7,1],[8,15],[6],[10,27],[8,10],[8],[6,29],[1,9],[6],[10,7],[1],[2],[13],[8,30],[1,5],[1],[13,2],[12]]
// 预期结果
// [null,null,null,null,-1,null,null,10,null,null,29,null,9,-1,-1,null,null,5,null,-1]
// 调用Java的LinkedHashMap实现LRU
public class LRUCache {
int capacity;
LinkedHashMap<Integer, Integer> cache;
public LRUCache(int capacity) {
this.capacity = capacity;
cache = new LinkedHashMap<>();
}
public int get(int key) {
if (cache.containsKey(key)) {
int num = cache.remove(key);
cache.put(key, num);
return num;
} else return -1; // 未找到当前元素
}
public void put(int key, int value) {
if (cache.containsKey(key)) {
cache.remove(key);
} else if (cache.size() == capacity) {
// cache.remove(cache.entrySet().iterator().next().getKey());
cache.remove(cache.keySet().iterator().next());
}
cache.put(key, value);
}
public static void main(String[] args) {
LRUCache obj = new LRUCache(10);
obj.put(7, 28);
obj.put(7, 1);
obj.put(8, 15);
System.out.println(obj.get(6));
obj.put(10, 27);
obj.put(8, 10);
System.out.println(obj.get(8));
obj.put(6, 29);
obj.put(1, 9);
System.out.println(obj.get(6));
obj.put(10, 7);
System.out.println(obj.get(1));
System.out.println(obj.get(2));
System.out.println(obj.get(13));
obj.put(8, 30);
obj.put(1, 5);
System.out.println(obj.get(1));
obj.put(13, 2);
System.out.println(obj.get(12));
}
}
HashMap + 定义双向链表 实现LRU
package LRU;
import java.util.*;
/**
* @author Orust
* @create 2021/5/24 16:18
*/
// HashMap + 定义双向链表 实现LRU
public class LRUCache_2 {
int capacity;
HashMap<Integer, Node> map;
Node head;
Node tail;
private static class Node {
int key;
int value;
Node prev;
Node next;
public Node(int key, int value) {
this.key = key;
this.value = value;
}
}
public LRUCache_2(int capacity) {
this.capacity = capacity;
map = new HashMap<>();
}
public int get(int key) {
if (map.containsKey(key)) {
Node node = map.get(key);
moveToHead(node);
return node.value;
} else return -1;
}
public void put(int key, int value) {
if (map.containsKey(key)) {
Node node = map.get(key);
node.value = value;
map.put(key, node);
moveToHead(node);
} else {
if (map.size() == capacity)
map.remove(popTail());
Node node = new Node(key, value);
map.put(key, node);
addToHead(node);
}
}
private void addToHead(Node node) {
if (tail == null) { // 当前链表为空
head = node;
tail = node;
} else {
head.prev = node;
node.next = head;
node.prev = null;
head = node;
}
}
private void moveToHead(Node node) {
if (node == head) return; // 判断是否是头尾节点并删除节点
node.prev.next = node.next;
if (node == tail) tail = tail.prev;
else node.next.prev = node.prev;
addToHead(node);
}
private int popTail() {
if (tail == null) throw new NullPointerException("NullPointerException!");
int key = tail.key;
if (tail == head) {
head = null;
tail = null;
} else {
tail = tail.prev;
tail.next = null;
}
return key;
}
public static void main(String[] args) {
LRUCache_2 obj = new LRUCache_2(10);
obj.put(7, 28);
obj.put(7, 1);
obj.put(8, 15);
System.out.println(obj.get(6));
obj.put(10, 27);
obj.put(8, 10);
System.out.println(obj.get(8));
obj.put(6, 29);
obj.put(1, 9);
System.out.println(obj.get(6));
obj.put(10, 7);
System.out.println(obj.get(1));
System.out.println(obj.get(2));
System.out.println(obj.get(13));
obj.put(8, 30);
obj.put(1, 5);
System.out.println(obj.get(1));
obj.put(13, 2);
System.out.println(obj.get(12));
}
}
HashMap + 定义静态内部类 MyLinkedList + 泛型 实现LRU
package LRU;
import java.util.*;
/**
* @author Orust
* @create 2021/5/24 17:03
*/
// HashMap + 定义静态内部类MyLinkedList + 泛型 实现LRU
public class LRUCache_3<K, V> {
int capacity;
Map<K, Node<K, V>> map = new HashMap<>();
MyLinkedList<K, V> cache = new MyLinkedList<>();
public LRUCache_3(int capacity) {
this.capacity = capacity;
}
public V get(K key) {
if (map.containsKey(key)) {
Node<K, V> node = map.get(key);
cache.moveToHead(node);
return node.value;
} else return null;
}
public void put(K key, V value) {
if (map.containsKey(key)) {
Node<K, V> node = map.get(key);
node.value = value;
map.put(key, node);
cache.moveToHead(node);
} else {
if (map.size() == capacity)
map.remove(cache.popTail());
Node<K, V> node = new Node<>(key, value);
map.put(key, node);
cache.addToHead(node);
}
}
private static class Node<K, V> {
K key;
V value;
Node<K, V> prev;
Node<K, V> next;
Node(K key, V value) {
this.key = key;
this.value = value;
}
}
private static class MyLinkedList<K, V> {
Node<K, V> head; // head是最近使用的数据
Node<K, V> tail;
private void addToHead(Node<K, V> node) {
if (tail == null) { // 当前链表为空
head = node;
tail = node;
} else {
head.prev = node;
node.next = head;
node.prev = null;
head = node;
}
}
private void moveToHead(Node<K, V> node) {
if (node == head) return; // 判断是否是头尾节点并删除节点
node.prev.next = node.next;
if (node == tail) tail = tail.prev;
else node.next.prev = node.prev;
addToHead(node);
}
private K popTail() {
if (tail == null) throw new NullPointerException("NullPointerException!");
K key = tail.key;
if (tail == head) {
head = null;
tail = null;
} else {
tail = tail.prev;
tail.next = null;
}
return key;
}
}
public static void main(String[] args) {
LRUCache_3<String, Integer> obj = new LRUCache_3<>(10);
obj.put("7", 28);
obj.put("7", 1);
obj.put("8", 15);
System.out.println(obj.get("6"));
obj.put("10", 27);
obj.put("8", 10);
System.out.println(obj.get("8"));
obj.put("6", 29);
obj.put("1", 9);
System.out.println(obj.get("6"));
obj.put("10", 7);
System.out.println(obj.get("1"));
System.out.println(obj.get("2"));
System.out.println(obj.get("13"));
obj.put("8", 30);
obj.put("1", 5);
System.out.println(obj.get("1"));
obj.put("13", 2);
System.out.println(obj.get("12"));
}
}
在tomcat6.x版本中被使用的LRU实现方式
package LRU;
import java.util.*;
/**
* @author Orust
* @create 2021/5/25 10:53
*/
// 文章链接 https://www.iteye.com/blog/flychao88-1977653
// 在tomcat6.x版本中被使用的LRU实现方式
public class LRUCache_4 {