Java实现LRU缓存

我就是我 2022-05-29 01:10 225阅读 0赞

# 原文地址：https://www.cnblogs.com/lzrabbit/p/3734850.html #

# [LRU缓存实现(Java)][LRU_Java] #

*  [LRU Cache的LinkedHashMap实现][LRU Cache_LinkedHashMap]
 *  [LRU Cache的链表+HashMap实现][LRU Cache_HashMap]
 *  [LinkedHashMap的FIFO实现][LinkedHashMap_FIFO]
 *  [调用示例][Link 1]

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

![复制代码][copycode.gif]

//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;
    }

![复制代码][copycode.gif]

## 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

![复制代码][copycode.gif]

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();
        }
    }

![复制代码][copycode.gif]

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

![复制代码][copycode.gif]

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;
        }
    };

![复制代码][copycode.gif]

## 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

![复制代码][copycode.gif]

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();
        }
    }

![复制代码][copycode.gif]

#  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

![复制代码][copycode.gif]

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;
        }
    }

![复制代码][copycode.gif]

# 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

![复制代码][copycode.gif]

final int cacheSize = 5;
    LinkedHashMap<Integer, String> lru = new LinkedHashMap<Integer, String>() {
        @Override
        protected boolean removeEldestEntry(Map.Entry<Integer, String> eldest) {
        return size() > cacheSize;
        }
    };

![复制代码][copycode.gif]

# 调用示例 #

测试代码

![ContractedBlock.gif][] ![ExpandedBlockStart.gif][]

![复制代码][copycode.gif]

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();
        }
    
    }

![复制代码][copycode.gif]

运行结果

![ContractedBlock.gif][] ![ExpandedBlockStart.gif][]

![复制代码][copycode.gif]

"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

![复制代码][copycode.gif]

[LRU_Java]: http://www.cnblogs.com/lzrabbit/p/3734850.html
[LRU Cache_LinkedHashMap]: https://www.cnblogs.com/lzrabbit/p/3734850.html#f1
[LRU Cache_HashMap]: https://www.cnblogs.com/lzrabbit/p/3734850.html#f2
[LinkedHashMap_FIFO]: https://www.cnblogs.com/lzrabbit/p/3734850.html#f3
[Link 1]: https://www.cnblogs.com/lzrabbit/p/3734850.html#f4
[copycode.gif]: /images/20220529/8f833c67ad0e4d9eac32b68ac84dda54.png
[ContractedBlock.gif]: https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif
[ExpandedBlockStart.gif]: /images/20220529/8ca8f10ef5cb4fad8ebdbcdbf064fe38.png