java中怎樣實現LRU緩存

java中怎樣實現LRU緩存？針對這個問題，這篇文章給出了相對應的分析和解答，希望能幫助更多想解決這個問題的朋友找到更加簡單易行的辦法。

LRU是Least Recently Used 的縮寫，翻譯過來就是“最近最少使用”，最近最少使用算法（LRU）是大部分操作系統為最大化頁面命中率而廣泛采用的一種頁面置換算法。該算法的思路是，發生缺頁中斷時，選擇未使用時間最長的頁面置換出去。[1]從程序運行的原理來看，最近最少使用算法是比較接近理想的一種頁面置換算法，這種算法既充分利用了內存中頁面調用的歷史信息，又正確反映了程序的局部問題。

比如我們緩存10000條數據，當數據小于10000時可以隨意添加，當超過10000時就需要把新的數據添加進來，同時要把過期數據刪除，以確保我們最大緩存10000條，那怎么確定刪除哪條過期數據呢，采用LRU算法實現的話就是將最老的數據刪掉。

下面來說下Java版的LRU緩存實現：

Java里面實現LRU緩存通常有兩種選擇，一種是使用LinkedHashMap，一種是自己設計數據結構，使用鏈表+HashMap

LRU Cache的LinkedHashMap實現

LinkedHashMap自身已經實現了順序存儲，默認情況下是按照元素的添加順序存儲，也可以啟用按照訪問順序存儲，即最近讀取的數據放在最前面，最早讀取的數據放在最后面，然后它還有一個判斷是否刪除最老數據的方法，默認是返回false，即不刪除數據。

我們使用LinkedHashMap實現LRU緩存的方法就是對LinkedHashMap實現簡單的擴展，擴展方式有兩種，一種是inheritance，一種是delegation。

//LinkedHashMap的一個構造函數，當參數accessOrder為true時，即會按照訪問順序排序，最近訪問的放在最前，最早訪問的放在后面
public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) {
        super(initialCapacity, loadFactor);
        this.accessOrder = accessOrder;
}

//LinkedHashMap自帶的判斷是否刪除最老的元素方法，默認返回false，即不刪除老數據
//我們要做的就是重寫這個方法，當滿足一定條件時刪除老數據
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
        return false;
}

LRU緩存LinkedHashMap(delegation)實現

delegation方式實現更加優雅一些，但是由于沒有實現Map接口，所以線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;
        //根據cacheSize和加載因子計算hashmap的capactiy，+1確保當達到cacheSize上限時不會觸發hashmap的擴容，
        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緩存LinkedHashMap(inheritance)實現

采用inheritance方式實現比較簡單，而且實現了Map接口，在多線程環境使用時可以使用 Collections.synchronizedMap()方法實現線程安全操作

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();
    }
}

這樣算是比較標準的實現吧，實際使用中這樣寫還是有些繁瑣，更實用的方法時像下面這樣寫，省去了單獨見一個類的麻煩

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 Cache的鏈表+HashMap實現

注：此實現為非線程安全，若在多線程環境下使用需要在相關方法上添加synchronized以實現線程安全操作

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實現

FIFO是First Input First Output的縮寫，也就是常說的先入先出，默認情況下LinkedHashMap就是按照添加順序保存，我們只需重寫下removeEldestEntry方法即可輕松實現一個FIFO緩存，簡化版的實現代碼如下

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

調用示例

測試代碼

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 鏈表實現===========================");
        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)實現===========================");
        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)實現===========================");
        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默認實現===========================");
        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();
    }

}

運行結果

"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 鏈表實現===========================
5:11 4:11 3:11 2:11 1:11 
4:11 7:77 2:11 6:66 5:11 


===========================LRU LinkedHashMap(inheritance)實現===========================
1:11 2:11 3:11 4:11 5:11 
5:11 6:66 2:11 7:77 4:11 


===========================LRU LinkedHashMap(delegation)實現===========================
1:11 2:11 3:11 4:11 5:11 
5:11 6:66 2:11 7:77 4:11 


===========================FIFO LinkedHashMap默認實現===========================
{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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