LRU缓存实现(Java)

LRU缓存实现(Java)

 

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

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//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;

}
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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

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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();

    }

}
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 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

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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;

    }

};
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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

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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();

    }

}
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 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

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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;

    }

}
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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

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final int cacheSize = 5;

LinkedHashMap<Integer, String> lru = new LinkedHashMap<Integer, String>() {

    @Override

    protected boolean removeEldestEntry(Map.Entry<Integer, String> eldest) {

    return size() > cacheSize;

    }

};
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调用示例

测试代码

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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();

    }



}
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运行结果

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"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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