如何实现Guava缓存来存储和获取不同类型的对象？

Question

Right now my cache looks like the following:

public class TestCache {

    private LoadingCache<String, List<ObjectABC>> cache;

    TestCache() {
        cache = CacheBuilder.newBuilder().expireAfterAccess(10, TimeUnit.MINUTES).maximumSize(25)
                .build(new CacheLoader<String, List<ObjectABC>>(
                ) {
                    @Override
                    public List<ObjectABC> load(String key) throws Exception {
                        // TODO Auto-generated method stub
                        return addCache(key);
                    }

                });
    }

    private List<ObjectABC> addCache(String key) {
    final JoiObjectMapper mapper = new JoiObjectMapper();

        final Collection<File> allConfigFiles = FileUtils.listFiles(new File(key), null, true);
        final List<ObjectABC> configsList = new ArrayList<>();

        allConfigFiles.forEach(configFile -> {
            try {
                     configsList.add(mapper.readValue(configFile, new TypeReference<ObjectABC>() {
                      }));
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        });

        return configsList;
    }

    public List<ObjectABC> getEntry(String key) {
         try {
            return cache.get(key);
        } catch (ExecutionException e) {
            throw new NonRetriableException(String.format(
                    "Exception occured while trying to get data from cache for the key : {} Exception: {}",
                    key.toString(), e));
        }
    }
}

In the above code, when I pass a String key (which is path to a local folder) it takes all the files present in that location and maps them to ObjectABC using ObjectMapper .

Now my problem is that I want to instead have a generic loading cache like
LoadingCache<String, List<Object>> .

And I want to map files in different folders to different Objects, eg map files in /root/Desktop/folder1 to List<ObjectABC> and map files in /root/Desktop/folder2 to List<ObjectDEF> and be able to store and retrieve that information from the cache.

How can I pass to the cache the information of which object to use for mapping?

Answer 1

You can create a custom class wrapping a LoadingCache<Key<?>, Object> like that:

class HeterogeneousCache {

    private final LoadingCache<Key<?>, Object> cache;

    public <T> T get(Key<T> key) throws ExecutionException {
        return key.getType().cast(cache.get(key));
    }
}

@Value // provides constructor, getters, equals, hashCode
class Key<T> {

    private final String identifier;
    private final Class<T> type;
}

(I used Lombok's @Value annotation for simplicity)

Of course, this is just a stub and you might need to adapt this to your needs. The main problem might be that you can't get a Class<List<ObjectABC>> - you can only get a Class<List> . The easiest way out of this is to wrap the List<ObjectABC> in some custom type. The harder way (not recommended) is to use Guava's TypeToken .

---

Attribution : This answer is based on the post by Frank Appel entitled How to Map Distinct Value Types Using Java Generics , which itself is based on Joshua Bloch 's typesafe hetereogeneous containers from Effective Java .

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Edit : A Complete Solution

Since the OP wants List<T> as result, and since he needs instances of TypeReference<T> , I replaced Class<T> with TypeReference<T> in Key<T> :

@Value // provides constructor, getters, equals, hashCode
class Key<T> {
    private final String identifier;
    private final TypeReference<T> typeReference;
}

Here's how CustomHeterogeneousCache looks now:

class CustomHeterogeneousCache {

    private final LoadingCache<Key<?>, List<?>> cache = CacheBuilder.newBuilder()
            .expireAfterAccess(10, TimeUnit.MINUTES)
            .maximumSize(25)
            .build(CacheLoader.from(this::computeEntry));

    @SuppressWarnings("unchecked")
    public <T> List<T> getEntry(Key<T> key) {
        return (List<T>) cache.getUnchecked(key);
    }

    private <T> List<T> computeEntry(Key<T> key) {
        final JoiObjectMapper mapper = new JoiObjectMapper();
        final Collection<File> allConfigFiles = FileUtils.listFiles(new File(key.getIdentifier()), null, true);
        return allConfigFiles.stream()
                .map(configFile -> {
                    try {
                        return mapper.readValue(configFile, key.getTypeReference());
                    } catch (Exception e) {
                        throw new RuntimeException(e);
                    }
                })
                .collect(Collectors.toList());
    }
}

Since implementations of TypeReference do not have value semantics, the user must make sure that every Key is created once, and then only referenced, eg:

class Keys {
    public static final Key<ObjectABC> ABC = new Key<>("/root/Desktop/folder1", new TypeReference<ObjectABC>() {
    });
    public static final Key<ObjectDEF> DEF = new Key<>("/root/Desktop/folder2", new TypeReference<ObjectDEF>() {
    });
}