上文描述了solr3.6.0怎么采用maven管理的方式在eclipse中搭建开发环境，在solr中，为了提高搜索性能，采用了缓存机制，这里描述的是LRU缓存，这里用到了 LinkedHashMap类
要基于LinkedHashMap来实现LRU缓存，我们可以选择inheritance, 也可以选择 delegation， 下面是基于delegation的实现方式：
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.
*
* @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"); //
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());
} } // end class LRUCache
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