详解Java实现LRU缓存

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，废话不多说，下面来说下Java版的LRU缓存实现

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，具体使用什么方式看个人喜好

//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<K,V> 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;

/**
 * Created by liuzhao on 14-5-15.
 */
public class LRUCache2<K, V> extends LinkedHashMap<K, V> {
 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<K, V> 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<String, String> map = new LinkedHashMap<String, String>((int) Math.ceil(cacheSize / 0.75f) + 1, 0.75f, true) {
 @Override
 protected boolean removeEldestEntry(Map.Entry<String, String> 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;

/**
 * Created by liuzhao on 14-5-13.
 */
public class LRUCache3<K, V> {

 private final int MAX_CACHE_SIZE;
 private final float DEFAULT_LOAD_FACTOR = 0.75f;
 LinkedHashMap<K, V> 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<Map.Entry<K, V>> 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;

/**
 * Created by liuzhao on 14-5-12.
 */
public class LRUCache1<K, V> {

 private final int MAX_CACHE_SIZE;
 private Entry first;
 private Entry last;

 private HashMap<K, Entry<K, V>> hashMap;

 public LRUCache1(int cacheSize) {
 MAX_CACHE_SIZE = cacheSize;
 hashMap = new HashMap<K, Entry<K, V>>();
 }

 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<K, V> 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<K, V> 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<K, V> {
 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<Integer, String> lru = new LinkedHashMap<Integer, String>() {
 @Override
 protected boolean removeEldestEntry(Map.Entry<Integer, String> 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;

/**
 * Created by liuzhao on 14-5-15.
 */
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<Integer, String> 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 <T> void lruCache2() {
 System.out.println();
 System.out.println("===========================LRU LinkedHashMap(inheritance)实现===========================");
 LRUCache2<Integer, String> 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<Integer, String> 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<Integer, String> lru = new LinkedHashMap<Integer, String>() {
  @Override
  protected boolean removeEldestEntry(Map.Entry<Integer, String> 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)\Java\jdk1.6.0_10\bin\java" -Didea.launcher.port=7535 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA 13.0.2\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\charsets.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\deploy.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\javaws.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\jce.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\jsse.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\management-agent.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\plugin.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\resources.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\rt.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\ext\dnsns.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\ext\localedata.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\ext\sunjce_provider.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\ext\sunmscapi.jar;C:\Program Files (x86)\Java\jdk1.6.0_10\jre\lib\ext\sunpkcs11.jar;D:\SVN\projects\Java\Java.Algorithm\target\test-classes;D:\SVN\projects\Java\Java.Algorithm\target\classes;C:\Program Files (x86)\JetBrains\IntelliJ IDEA 13.0.2\lib\idea_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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