java LRU(Least Recently Used )详解及实例代码
java LRU(Least Recently Used )详解 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,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.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,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 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,"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,"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>() { @Override protected boolean removeEldestEntry(Map.Entry<Integer,String> eldest) { return size() > cacheSize; } }; lru.put(1,"77"); lru.get(4); System.out.println(lru.toString()); System.out.println(); } } 运行结果 "C:Program Files (x86)Javajdk1.6.0_10binjava" -Didea.launcher.port=7535 "-Didea.launcher.bin.path=C:Program Files (x86)JetBrainsIntelliJ IDEA 13.0.2bin" -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_10jrelibresources.jar;C:Program Files (x86)Javajdk1.6.0_10jrelibrt.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.Algorithmtargettest-classes;D:SVNprojectsJavaJava.Algorithmtargetclasses;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=11,5=11,6=66,7=77} over... 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