如何实现一个缓存服务

　　场景：我们对于需要大量计算的场景，希望将结果缓存起来，然后我们一起来实现一个缓存服务。即对于一个相同的输入，它的输出是不变的（也可以短时间不变）。

实现说明：这里实现采用GuavaCache+装饰器模式。

首先设计一个缓存服务接口。

public interface CacheableService<I, O> {

    /**
     * 计算服务
     * @param i
     * @return
     * @throws Exception 
     */
    O doService(I i) throws Exception;
}

这里定义了一个缓存服务接口，这里的key和Hashmap的key一样，需要覆写equals和hashcode方法。

public class CacheableServiceWrapper<I , O> implements
        CacheableService<I, O>,
        GlobalResource {

    /**
     * 日志
     */
    private final static Logger LOGGER = LoggerFactory
            .getLogger(CacheableServiceWrapper.class);

    /**
     * 缓存大小
     */
    private int MAX_CACHE_SIZE = 20;

    /**
     * 出现异常的时候重试,默认不重试
     */
    private boolean retryOnExp = false;

    /**
     * 重试次数,默认为0,即不重试
     */
    private int retryTimes = 0;

    /**
     * 默认30分钟
     */
    private long expireTimeWhenAccess = 30 * 60;

    /**
     * 缓存
     */
    private LoadingCache<I, Future<O>> cache = null;

    private CacheableService<I, O> cacheableService = null;

    /**
     * Calculate o.
     *
     * @param i the
     * @return the o
     * @throws Exception the exception
     */
    public O doService(final I i) throws Exception {

        Assert.notNull(cacheableService, "请设置好实例");

        int currentTimes = 0;
        while (currentTimes <= retryTimes) {
            try {
                Future<O> oFuture = cache.get(i);
                return oFuture.get();

            } catch (Exception e) {
                if (!retryOnExp) {
                    throw e;
                }
                currentTimes++;
                LoggerUtils.info(LOGGER, "第", currentTimes, "重试,key=", i);
            }
        }
        throw new Exception("任务执行失败");
    }


    /**
     * 提交计算任务
     *
     * @param i
     * @return
     */
    private Future<O> createTask(final I i) {
        Assert.notNull(cacheableService, "请设置好实例");

        LoggerUtils.info(LOGGER, "提交任务,key=", i);
        LoggerUtils.info(LOGGER, "当前cache=", JSON.toJSONString(cache));

        Future<O> resultFuture = THREAD_POOL.submit(new Callable<O>() {

            public O call() throws Exception {
                return cacheableService.doService(i);
            }
        });
        return resultFuture;

    }

    /**
     * 构造函数
     */
    public CacheableServiceWrapper(CacheableService<I, O> cacheableService,
                                            int maxCacheSize, long expireTime) {
        this.cacheableService = cacheableService;
        this.MAX_CACHE_SIZE = maxCacheSize;
        this.expireTimeWhenAccess = expireTime;
        cache = CacheBuilder.newBuilder().maximumSize(MAX_CACHE_SIZE)
                .expireAfterAccess(expireTimeWhenAccess, TimeUnit.SECONDS)
                .build(new CacheLoader<I, Future<O>>() {
                    public Future<O> load(I key) throws ExecutionException {
                        LoggerUtils.warn(LOGGER, "get Element from cacheLoader");
                        return createTask(key);
                    }

                    ;
                });
    }

    /**
     * 构造函数
     */
    public CacheableServiceWrapper(CacheableService<I, O> cacheableService) {
        this.cacheableService = cacheableService;
        cache = CacheBuilder.newBuilder().maximumSize(MAX_CACHE_SIZE)
                .expireAfterAccess(expireTimeWhenAccess, TimeUnit.SECONDS)
                .build(new CacheLoader<I, Future<O>>() {
                    public Future<O> load(I key) throws ExecutionException {
                        LoggerUtils.warn(LOGGER, "get Element from cacheLoader");
                        return createTask(key);
                    }

                    ;
                });
    }

    /**
     * Setter method for property <tt>retryTimes</tt>.
     *
     * @param retryTimes value to be assigned to property retryTimes
     */
    public void setRetryTimes(int retryTimes) {
        this.retryTimes = retryTimes;
    }

    /**
     * Setter method for property <tt>retryOnExp</tt>.
     *
     * @param retryOnExp value to be assigned to property retryOnExp
     */
    public void setRetryOnExp(boolean retryOnExp) {
        this.retryOnExp = retryOnExp;
    }

}

缓存服务装饰器