java中LRU缓存实现

Java  /  管理员 发布于 8年前   208

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<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) {@Overrideprotected 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>() {@Overrideprotected 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 Mainstart...===========================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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