今天，用Cacheable注解时，发现空值，也会被缓存下来。下次另一个系统如果更新了值，这边从缓存取，还是空值，会有问题。所以一方面，希望另一个系统更新时，能清缓存，另一方面不希望缓存中有太多垃圾数据。

参考http://stackoverflow.com/questions/12113725/how-do-i-tell-spring-cache-not-to-cache-null-value-in-cacheable-annotation的做法，对程序做修改

    @Cacheable(key = "''+#id", unless="#result == null")
    public XXXPO get(int id) {
       //get from db
    }

这样，当方法返回空值时，就不会被缓存起来。见unless解释：

/**
 * Spring Expression Language (SpEL) attribute used to veto method caching.
 * <p>Unlike {@link #condition()}, this expression is evaluated after the method
 * has been called and can therefore refer to the {@code result}. Default is "",
 * meaning that caching is never vetoed.
 * @since 3.2
 */

大意是使用result的值，来决定是否来否定方法缓存。因此可以用来做条件判断