google guava checking for item in cache

Question

I'm trying to use google guava cache on a program but am not quite getting how it works.

I'm loading up the cache and then at a later stage i'm trying to check if an item exists in the cache, my code below doesnt quite work

The getIfPresent returns null if it doesnt exist but the load which calls it bombs out after with the error

Exception in thread "main" com.google.common.cache.CacheLoader$InvalidCacheLoadException: CacheLoader returned null for key

 private static LoadingCache<String, Image> imageCache
          = CacheBuilder.newBuilder()
                .build(new CacheLoader<String, Image>() {

            @Override
            public Image load(String key) throws Exception {                    
                if (getImage(key) != null) {                    
                    return getImage(key);                       
                }               
                return null;
            }                 
          });           

public static Image getImage(String key) throws ExecutionException {

    return imageCache.getIfPresent(key);

}

this means i cant check for the presense of the item in the cache like so

    try {
        readImage = imageCache.get(fileName);
    } catch (ExecutionException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    }

    if (readImage != null) {


    }

can someone explain to me what i'm doing wrong here?

Your cache loader is calling a method which calls the cache which calls the cache loader... Your load method should actually do the loading of the image. — Andy Turner

Omar Hrynkiewicz Omar Hrynkiewicz · Accepted Answer · 2015-06-01T21:10:29

If you need manage null values in your Loader, use Guava Optional

@Override
public Optional<Image> load(String key) throws Exception {
    return Optional.fromNullable(getImage(key));
}

Image getImage(String key) throws ExecutionException {
    //your code to get image from database, or other source
    return yourCodeToGetImageFromTheSource(key);
}

Your client code, can be:

try {
    Optional<Image> imageCached = imageCache.get(fileName);
} catch (ExecutionException e1) {
    // TODO error handling
}

if (imageCached.isPresent()) {
    Image img = imageCached.get();
} else {
    //your code when img is null
}

google guava checking for item in cache

3 Answers