LRU是Least Recently Used 的缩写,翻译过来就是“最近最少使用”,LRU缓存就是使用这种原理实现,简单的说就是缓存一定量的数据,当超过设定的阈值时就把一些过期的数据删除掉。比如我们缓存10000条数据,当数据小于1000
LRU是Least Recently Used 的缩写,翻译过来就是“最近最少使用”,LRU缓存就是使用这种原理实现,简单的说就是缓存一定量的数据,当超过设定的阈值时就把一些过期的数据删除掉。
比如我们缓存10000条数据,当数据小于10000时可以随意添加,当超过10000时就需要把新的数据添加进来,同时要把过期数据删除,以确保我们最大缓存10000条,那怎么确定删除哪条过期数据呢,采用LRU算法实现的话就是将最老的数据删掉。
下面来说下Java版的LRU缓存实现:(推荐:java视频教程)
Java里面实现LRU缓存通常有两种选择,一种是使用LinkedHashMap,一种是自己设计数据结构,使用链表+HashMap
LRU Cache的LinkedHashMap实现
LinkedHashMap自身已经实现了顺序存储,默认情况下是按照元素的添加顺序存储,也可以启用按照访问顺序存储,即最近读取的数据放在最前面,最早读取的数据放在最后面,然后它还有一个判断是否删除最老数据的方法,默认是返回false,即不删除数据。
我们使用LinkedHashMap实现LRU缓存的方法就是对LinkedHashMap实现简单的扩展,扩展方式有两种,一种是inheritance,一种是delegation。
//LinkedHashMap的一个构造函数,当参数accessOrder为true时,即会按照访问顺序排序,最近访问的放在最前,最早访问的放在后面
public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) {
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
}
//LinkedHashMap自带的判断是否删除最老的元素方法,默认返回false,即不删除老数据
//我们要做的就是重写这个方法,当满足一定条件时删除老数据
protected boolean removeEldestEntry(Map.Entry eldest) {
return false;
}
LRU缓存LinkedHashMap(inheritance)实现
采用inheritance方式实现比较简单,而且实现了Map接口,在多线程环境使用时可以使用 Collections.synchronizedMap()方法实现线程安全操作
package cn.lzrabbit.structure.lru;
import java.util.LinkedHashMap;
import java.util.Map;
public class LRUCache2 extends LinkedHashMap {
private final int MAX_CACHE_SIZE;
public LRUCache2(int cacheSize) {
super((int) Math.ceil(cacheSize / 0.75) + 1, 0.75f, true);
MAX_CACHE_SIZE = cacheSize;
}
@Override
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > MAX_CACHE_SIZE;
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
for (Map.Entry entry : entrySet()) {
sb.append(String.fORMat("%s:%s ", entry.geTKEy(), entry.getValue()));
}
return sb.toString();
}
}
这样算是比较标准的实现吧,实际使用中这样写还是有些繁琐,更实用的方法时像下面这样写,省去了单独见一个类的麻烦
final int cacheSize = 100;
Map map = new LinkedHashMap((int) Math.ceil(cacheSize / 0.75f) + 1, 0.75f, true) {
@Override
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > cacheSize;
}
};
LRU缓存LinkedHashMap(delegation)实现
delegation方式实现更加优雅一些,但是由于没有实现Map接口,所以线程同步就需要自己搞定了
package cn.lzrabbit.structure.lru;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Set;
public class LRUCache3 {
private final int MAX_CACHE_SIZE;
private final float DEFAULT_LOAD_FACTOR = 0.75f;
LinkedHashMap map;
public LRUCache3(int cacheSize) {
MAX_CACHE_SIZE = cacheSize;
//根据cacheSize和加载因子计算hashmap的capactiy,+1确保当达到cacheSize上限时不会触发hashmap的扩容,
int capacity = (int) Math.ceil(MAX_CACHE_SIZE / DEFAULT_LOAD_FACTOR) + 1;
map = new LinkedHashMap(capacity, DEFAULT_LOAD_FACTOR, true) {
@Override
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > MAX_CACHE_SIZE;
}
};
}
public synchronized void put(K key, V value) {
map.put(key, value);
}
public synchronized V get(K key) {
return map.get(key);
}
public synchronized void remove(K key) {
map.remove(key);
}
public synchronized Set> getAll() {
return map.entrySet();
}
public synchronized int size() {
return map.size();
}
public synchronized void clear() {
map.clear();
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
for (Map.Entry entry : map.entrySet()) {
sb.append(String.format("%s:%s ", entry.getKey(), entry.getValue()));
}
return sb.toString();
}
}
LRU Cache的链表+HashMap实现
注:此实现为非线程安全,若在多线程环境下使用需要在相关方法上添加synchronized以实现线程安全操作
package cn.lzrabbit.structure.lru;
import java.util.HashMap;
public class LRUCache1 {
private final int MAX_CACHE_SIZE;
private Entry first;
private Entry last;
private HashMap> hashMap;
public LRUCache1(int cacheSize) {
MAX_CACHE_SIZE = cacheSize;
hashMap = new HashMap>();
}
public void put(K key, V value) {
Entry entry = getEntry(key);
if (entry == null) {
if (hashMap.size() >= MAX_CACHE_SIZE) {
hashMap.remove(last.key);
removeLast();
}
entry = new Entry();
entry.key = key;
}
entry.value = value;
moveToFirst(entry);
hashMap.put(key, entry);
}
public V get(K key) {
Entry entry = getEntry(key);
if (entry == null) return null;
moveToFirst(entry);
return entry.value;
}
public void remove(K key) {
Entry entry = getEntry(key);
if (entry != null) {
if (entry.pre != null) entry.pre.next = entry.next;
if (entry.next != null) entry.next.pre = entry.pre;
if (entry == first) first = entry.next;
if (entry == last) last = entry.pre;
}
hashMap.remove(key);
}
private void moveToFirst(Entry entry) {
if (entry == first) return;
if (entry.pre != null) entry.pre.next = entry.next;
if (entry.next != null) entry.next.pre = entry.pre;
if (entry == last) last = last.pre;
if (first == null || last == null) {
first = last = entry;
return;
}
entry.next = first;
first.pre = entry;
first = entry;
entry.pre = null;
}
private void removeLast() {
if (last != null) {
last = last.pre;
if (last == null) first = null;
else last.next = null;
}
}
private Entry getEntry(K key) {
return hashMap.get(key);
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
Entry entry = first;
while (entry != null) {
sb.append(String.format("%s:%s ", entry.key, entry.value));
entry = entry.next;
}
return sb.toString();
}
class Entry {
public Entry pre;
public Entry next;
public K key;
public V value;
}
}
LinkedHashMap的FIFO实现
FIFO是First Input First Output的缩写,也就是常说的先入先出,默认情况下LinkedHashMap就是按照添加顺序保存,我们只需重写下removeEldestEntry方法即可轻松实现一个FIFO缓存,简化版的实现代码如下
final int cacheSize = 5;
LinkedHashMap lru = new LinkedHashMap() {
@Override
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > cacheSize;
}
};
调用示例
测试代码
package cn.lzrabbit.structure.lru;
import cn.lzrabbit.ITest;
import java.util.LinkedHashMap;
import java.util.Map;
public class LRUCacheTest {
public static void main(String[] args) throws Exception {
System.out.println("start...");
lruCache1();
lruCache2();
lruCache3();
lruCache4();
System.out.println("over...");
}
static void lruCache1() {
System.out.println();
System.out.println("===========================LRU 链表实现===========================");
LRUCache1 lru = new LRUCache1(5);
lru.put(1, "11");
lru.put(2, "11");
lru.put(3, "11");
lru.put(4, "11");
lru.put(5, "11");
System.out.println(lru.toString());
lru.put(6, "66");
lru.get(2);
lru.put(7, "77");
lru.get(4);
System.out.println(lru.toString());
System.out.println();
}
static void lruCache2() {
System.out.println();
System.out.println("===========================LRU LinkedHashMap(inheritance)实现===========================");
LRUCache2 lru = new LRUCache2(5);
lru.put(1, "11");
lru.put(2, "11");
lru.put(3, "11");
lru.put(4, "11");
lru.put(5, "11");
System.out.println(lru.toString());
lru.put(6, "66");
lru.get(2);
lru.put(7, "77");
lru.get(4);
System.out.println(lru.toString());
System.out.println();
}
static void lruCache3() {
System.out.println();
System.out.println("===========================LRU LinkedHashMap(delegation)实现===========================");
LRUCache3 lru = new LRUCache3(5);
lru.put(1, "11");
lru.put(2, "11");
lru.put(3, "11");
lru.put(4, "11");
lru.put(5, "11");
System.out.println(lru.toString());
lru.put(6, "66");
lru.get(2);
lru.put(7, "77");
lru.get(4);
System.out.println(lru.toString());
System.out.println();
}
static void lruCache4() {
System.out.println();
System.out.println("===========================FIFO LinkedHashMap默认实现===========================");
final int cacheSize = 5;
LinkedHashMap lru = new LinkedHashMap() {
@Override
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > cacheSize;
}
};
lru.put(1, "11");
lru.put(2, "11");
lru.put(3, "11");
lru.put(4, "11");
lru.put(5, "11");
System.out.println(lru.toString());
lru.put(6, "66");
lru.get(2);
lru.put(7, "77");
lru.get(4);
System.out.println(lru.toString());
System.out.println();
}
}
运行结果
"C:Program Files (x86)Javajdk1.6.0_10injava" -Didea.launcher.port=7535 "-Didea.launcher.bin.path=C:Program Files (x86)JetBrainsIntelliJ IDEA 13.0.2in" -Dfile.encoding=UTF-8 -classpath "C:Program Files (x86)Javajdk1.6.0_10jrelibcharsets.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibdeploy.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibjavaws.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibjce.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibjsse.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibmanagement-agent.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibplugin.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibesources.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibt.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibextdnsns.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibextlocaledata.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibextsunjce_provider.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibextsunmscapi.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibextsunpkcs11.jar;D:SVNprojectsJavaJava.AlGorithm arget est-classes;D:SVNprojectsJavaJava.Algorithm argetclasses;C:Program Files (x86)JetBrainsIntelliJ IDEA 13.0.2libidea_rt.jar" com.intellij.rt.execution.application.AppMain Main
start...
===========================LRU 链表实现===========================
5:11 4:11 3:11 2:11 1:11
4:11 7:77 2:11 6:66 5:11
===========================LRU LinkedHashMap(inheritance)实现===========================
1:11 2:11 3:11 4:11 5:11
5:11 6:66 2:11 7:77 4:11
===========================LRU LinkedHashMap(delegation)实现===========================
1:11 2:11 3:11 4:11 5:11
5:11 6:66 2:11 7:77 4:11
===========================FIFO LinkedHashMap默认实现===========================
{1=11, 2=11, 3=11, 4=11, 5=11}
{3=11, 4=11, 5=11, 6=66, 7=77}
over...
Process finished with exit code 0
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