如何告诉Spring缓存不要在@Cacheable注释中缓存空值

Question

Is there a way to specify that if the method returns null value, then don't cache the result in @Cacheable annotation for a method like this?

@Cacheable(value="defaultCache", key="#pk")
public Person findPerson(int pk) {
   return getSession.getPerson(pk);
}

Update: here is the JIRA issue submitted regarding caching null value last November, which hasn't resolved yet: [#SPR-8871] @Cachable condition should allow referencing return value - Spring Projects Issue Tracker

Answer 1

Hooray, as of Spring 3.2 the framework allows for this using Spring SPEL and unless . Note from the java doc surrounding Cacheable:

http://static.springsource.org/spring/docs/3.2.x/javadoc-api/org/springframework/cache/annotation/Cacheable.html

public abstract String unless

Spring Expression Language (SpEL) attribute used to veto method caching.

Unlike condition(), this expression is evaluated after the method has been called and can therefore refer to the result. Default is "", meaning that caching is never vetoed.

The important aspect is that unless is evaluated after the method has been called. This makes perfect sense because the method will never get executed if the key is already in the cache.

So in the above example you would simply annotate as follows (#result is available to test the return value of a method):

@Cacheable(value="defaultCache", key="#pk", unless="#result == null")
public Person findPerson(int pk) {
   return getSession.getPerson(pk);
}

I would imagine this condition arises from the use of pluggable cache implementations such as Ehcache which allows caching of nulls. Depending on your use case scenario this may or may not be desirable.

Answer 2

update this answer is outdated now, for Spring 3.2 and later see Tech Trip's answer , OP: feel free to mark it as accepted.

I don't think that it's possible(even though there's conditional Cache eviction in Spring that can be executed after the method invocation with @CacheEvict parameter beforeInvocation set to false, which is default value) examining the CacheAspectSupport class shows that the returned value is not stored anywhere before the inspectAfterCacheEvicts(ops.get(EVICT)); call.

protected Object execute(Invoker invoker, Object target, Method method, Object[] args) {
    // check whether aspect is enabled
    // to cope with cases where the AJ is pulled in automatically
    if (!this.initialized) {
        return invoker.invoke();
    }

    // get backing class
    Class<?> targetClass = AopProxyUtils.ultimateTargetClass(target);
    if (targetClass == null && target != null) {
        targetClass = target.getClass();
    }
    final Collection<CacheOperation> cacheOp = getCacheOperationSource().getCacheOperations(method, targetClass);

    // analyze caching information
    if (!CollectionUtils.isEmpty(cacheOp)) {
        Map<String, Collection<CacheOperationContext>> ops = createOperationContext(cacheOp, method, args, target, targetClass);

        // start with evictions
        inspectBeforeCacheEvicts(ops.get(EVICT));

        // follow up with cacheable
        CacheStatus status = inspectCacheables(ops.get(CACHEABLE));

        Object retVal = null;
        Map<CacheOperationContext, Object> updates = inspectCacheUpdates(ops.get(UPDATE));

        if (status != null) {
            if (status.updateRequired) {
                updates.putAll(status.cUpdates);
            }
            // return cached object
            else {
                return status.retVal;
            }
        }

        retVal = invoker.invoke();

        inspectAfterCacheEvicts(ops.get(EVICT));

        if (!updates.isEmpty()) {
            update(updates, retVal);
        }

        return retVal;
    }

    return invoker.invoke();
}

Answer 3

If Spring annotation

@Cacheable(value="defaultCache", key="#pk",unless="#result!=null")

does not work ,you can try:

@CachePut(value="defaultCache", key="#pk",unless="#result==null")

It works for me.