Question

I’m using spring data jpa with querydsl. I have a method that returns query results in pages containing total count. getting the total count is expensive and I would like to cache it. how is that possible?

My naive approach

@Cacheable("queryCount")
private long getCount(JPAQuery query){
    return query.count();
}

does not work (to make it work they way wanted the actually key for the cache should not be the whole query, just the criteria). Anyway tested it, did not work and then I found this: Spring 3.1 @Cacheable – method still executed

The way I understand this I can only cache the public interface methods. However in said method I would need to cache a property of the return value, eg.

Page<T> findByComplexProperty(...)

I would need to cache

page.getTotalElements();

Annotating the whole method works (it is cached) but not the way I would like. Assume getting total count takes 30 seconds. Hence for every new page request user needs to wait 30 sec. if he goes back a page, then the cache is used but I would want the count to be only run exactly once and then count is fetched from cache.

How can I do that?

Answer 1

My solution was to autowire the cache manager in the class creating the complex query:

@Autowired
private CacheManager cacheManager;

and then create a simple private method getCount

private long getCount(JPAQuery query) {
    Predicate whereClause = query.getMetadata().getWhere();
    String key = whereClause.toString();        
    Cache cache = this.cacheManager.getCache(QUERY_CACHE);        
    Cache.ValueWrapper value = cache.get(key);
    if (value == null) {
        Long result = query.count();
        cache.put(key, result);
        return result;
    }
    return (Long)value.get();
}