LRU缓存

用缓存来存放之前读取过的数据，这样，再次读取的时候，可以直接在缓存里面取，而不用再重新查找一遍，这样系统的反应能力会有很大提高。但是，当我们读取的个数特别大的时候，我们不可能把所有已经读取的数据都放在缓存里，毕竟内存大小是一定的，所以一般把最近常读取的放在缓存里。

LRU(Least Recently Used)缓存利用了这样的一种思想, 把最新读取的数据放在最前面，缓存中存储的是读取最频繁的数据，以能够提高系统的性能。

LinkedHashMap实现LRU

LinkedHashMap支持按照访问顺序的存储，也就是说，最近读取的会放在最前面，最不常读取的会放在最后。其次，LinkedHashMap 有一个方法用于判断是否需要移除最不常读取的数，原始方法默认不需要移除，所以，LRU 需要 override 这个方法，使得当缓存里存放的数据个数超过规定个数后，就把最不常用的移除掉。

import java.util.LinkedHashMap;
import java.util.Collection;
import java.util.Map;
import java.util.ArrayList;

/**
 * An LRU cache, based on <code>LinkedHashMap</code>.
 *
 * <p>
 * This cache has a fixed maximum number of elements (<code>cacheSize</code>).
 * If the cache is full and another entry is added, the LRU (least recently
 * used) entry is dropped.
 *
 * <p>
 * This class is thread-safe. All methods of this class are synchronized.
 *
 * <p>
 * Author: Christian d'Heureuse, Inventec Informatik AG, Zurich, Switzerland<br>
 * Multi-licensed: EPL / LGPL / GPL / AL / BSD.
 */
public class LRUCache<K, V> {
    private static final float hashTableLoadFactor = 0.75f;
    private LinkedHashMap<K, V> map;
    private int cacheSize;

    /**
     * Creates a new LRU cache. 在该方法中，new LinkedHashMap<K,V>(hashTableCapacity,
     * hashTableLoadFactor, true)中，true代表使用访问顺序
     *
     * @param cacheSize
     *            the maximum number of entries that will be kept in this cache.
     */
    public LRUCache(int cacheSize) {
        this.cacheSize = cacheSize;
        int hashTableCapacity = (int) Math
                .ceil(cacheSize / hashTableLoadFactor) + 1;
        map = new LinkedHashMap<K, V>(hashTableCapacity, hashTableLoadFactor,
                true) {
            // (an anonymous inner class)
            private static final long serialVersionUID = 1;

            @Override
            protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
                return size() > LRUCache.this.cacheSize;
            }
        };
    }

    /**
     * Retrieves an entry from the cache.<br>
     * The retrieved entry becomes the MRU (most recently used) entry.
     *
     * @param key
     *            the key whose associated value is to be returned.
     * @return the value associated to this key, or null if no value with this
     *         key exists in the cache.
     */
    public synchronized V get(K key) {
        return map.get(key);
    }

    /**
     * Adds an entry to this cache. The new entry becomes the MRU (most recently
     * used) entry. If an entry with the specified key already exists in the
     * cache, it is replaced by the new entry. If the cache is full, the LRU
     * (least recently used) entry is removed from the cache.
     *
     * @param key
     *            the key with which the specified value is to be associated.
     * @param value
     *            a value to be associated with the specified key.
     */
    public synchronized void put(K key, V value) {
        map.put(key, value);
    }

    /**
     * Clears the cache.
     */
    public synchronized void clear() {
        map.clear();
    }

    /**
     * Returns the number of used entries in the cache.
     *
     * @return the number of entries currently in the cache.
     */
    public synchronized int usedEntries() {
        return map.size();
    }

    /**
     * Returns a <code>Collection</code> that contains a copy of all cache
     * entries.
     *
     * @return a <code>Collection</code> with a copy of the cache content.
     */
    public synchronized Collection<Map.Entry<K, V>> getAll() {
        return new ArrayList<Map.Entry<K, V>>(map.entrySet());
    }

    // Test routine for the LRUCache class.
    public static void main(String[] args) {
        LRUCache<String, String> c = new LRUCache<String, String>(3);
        c.put("1", "one"); // 1
        c.put("2", "two"); // 2 1
        c.put("3", "three"); // 3 2 1
        c.put("4", "four"); // 4 3 2
        if (c.get("2") == null)
            throw new Error(); // 2 4 3
        c.put("5", "five"); // 5 2 4
        c.put("4", "second four"); // 4 5 2
        // Verify cache content.
        if (c.usedEntries() != 3)
            throw new Error();
        if (!c.get("4").equals("second four"))
            throw new Error();
        if (!c.get("5").equals("five"))
            throw new Error();
        if (!c.get("2").equals("two"))
            throw new Error();
        // List cache content.
        for (Map.Entry<String, String> e : c.getAll())
            System.out.println(e.getKey() + " : " + e.getValue());
    }
}