实现 Java 本地缓存的方法解析

public class Cache implements Comparable<Cache>{
  // 键
  private Object key;
  // 缓存值
  private Object value;
  // 最后一次访问时间
  private long accessTime;
  // 创建时间
  private long writeTime;
  // 存活时间
  private long expireTime;
  // 命中次数
  private Integer hitCount;
  ...getter/setter()...

/**
 * 添加缓存
 *
 * @param key
 * @param value
 */
public void put(K key, V value,long expire) {
  checkNotNull(key);
  checkNotNull(value);
  // 当缓存存在时，更新缓存
  if (concurrentHashMap.containsKey(key)){
    Cache cache = concurrentHashMap.get(key);
    cache.setHitCount(cache.getHitCount()+1);
    cache.setWriteTime(System.currentTimeMillis());
    cache.setAccessTime(System.currentTimeMillis());
    cache.setExpireTime(expire);
    cache.setValue(value);
    return;
  }
  // 已经达到最大缓存
  if (isFull()) {
    Object kickedKey = getKickedKey();
    if (kickedKey !=null){
      // 移除最少使用的缓存
      concurrentHashMap.remove(kickedKey);
    }else {
      return;
    }
  }
  Cache cache = new Cache();
  cache.setKey(key);
  cache.setValue(value);
  cache.setWriteTime(System.currentTimeMillis());
  cache.setAccessTime(System.currentTimeMillis());
  cache.setHitCount(1);
  cache.setExpireTime(expire);
  concurrentHashMap.put(key, cache);
}

/**
 * 获取缓存
 *
 * @param key
 * @return
 */
public Object get(K key) {
  checkNotNull(key);
  if (concurrentHashMap.isEmpty()) return null;
  if (!concurrentHashMap.containsKey(key)) return null;
  Cache cache = concurrentHashMap.get(key);
  if (cache == null) return null;
  cache.setHitCount(cache.getHitCount()+1);
  cache.setAccessTime(System.currentTimeMillis());
  return cache.getValue();
}

  /**
   * 获取最少使用的缓存
   * @return
   */
  private Object getKickedKey() {
    Cache min = Collections.min(concurrentHashMap.values());
    return min.getKey();
  }

/**
 * 处理过期缓存
 */
class TimeoutTimerThread implements Runnable {
  public void run() {
    while (true) {
      try {
        TimeUnit.SECONDS.sleep(60);
        expireCache();
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
  }

  /**
   * 创建多久后，缓存失效
   *
   * @throws Exception
   */
  private void expireCache() throws Exception {
    System.out.println("检测缓存是否过期缓存");
    for (Object key : concurrentHashMap.keySet()) {
      Cache cache = concurrentHashMap.get(key);
      long timoutTime = TimeUnit.NANOSECONDS.toSeconds(System.nanoTime()
          - cache.getWriteTime());
      if (cache.getExpireTime() > timoutTime) {
        continue;
      }
      System.out.println(" 清除过期缓存 ： " + key);
      //清除过期缓存
      concurrentHashMap.remove(key);
    }
  }
}