Add support for deserializing objects with generics

[JosRoseboom]

Currently there is no RedisSerializer that can deserialize an object with generics like Set<Person>. This is for example a problem when using redis as cache in spring boot:

 @Cacheable("personCache)
 public Set<Person> getPersons()  {
   // expensive code
 }

Using for example Jackson2JsonRedisSerializer, we could specify Set as type, but it won't handle the Person type. It deserializes to Set<LinkedHashMap> with the LinkedHashMap having the properties of the person. A cache hit results in a ClassCastException because a LinkedHashMap cannot be cast to a Person (similarly, a Set of enums is deserialized as a Set of Strings).

Classes using a generic (Person in this case) should not be forced to implement Serializable, so Jackson based serialization in itself is nice.

Note: Created this issue to refer to in pull request: pull request on the way

[mp911de]

Generics are not retained for values, therefore the cache implementation cannot know anything about the type definition. Spring's Cache API does not support generic type hints either, see https://github.com/spring-projects/spring-framework/blob/main/spring-context/src/main/java/org/springframework/cache/Cache.java#L82.

I think using Jackson should work when using type hints, other than that, there's not much we can do here. I'll leave this one open once your PR arrives.

[JosRoseboom]

Hi Mark,

thanks for your fast response. I created a pull request: #2375

One of the tests I added shows a serializer that could make the Set<Person> case in my original post work:

@Test // GH-2374
void testSetOfPersons() {
	PersonObjectFactory personFactory = new PersonObjectFactory();
	Set<Person> personSet = Set.of(personFactory.instance(), personFactory.instance());
	JacksonTypeReference2JsonRedisSerializer<Set<Person>> personSetSerializer = 
                  new JacksonTypeReference2JsonRedisSerializer<>(new TypeReference<>() {});

	assertThat(personSetSerializer.deserialize(personSetSerializer.serialize(personSet))).isEqualTo(personSet);
}

In a Spring project that uses Redis as cache for @Cacheable, in a configuration Bean for redis cache, I could do something like:

@Configuration
public class RedisCacheConfig {

    @Bean
    public RedisCacheManagerBuilderCustomizer redisCacheManagerBuilderCustomizer() {
        JacksonTypeReference2JsonRedisSerializer<Set<Person>> serializer = new JacksonTypeReference2JsonRedisSerializer<>(new TypeReference<>() {
        });
        RedisCacheConfiguration conf = RedisCacheConfiguration.defaultCacheConfig()
                .serializeValuesWith(RedisSerializationContext.SerializationPair.fromSerializer(serializer));
        return builder -> builder.withCacheConfiguration("personCache", conf);
    }

}

Now JacksonTypeReference2JsonRedisSerializer<Set<Person>> deserializes the bytes of the redis DB to a Set<Person> instead of a Set<LinkedHashMap> and the ClassCastException is gone.

Currently solved it in a project this way and it would be nice to make it a bit easier for others to handle this case, hence this PR.

[mp911de]

Closing as per https://github.com/spring-projects/spring-data-redis/pull/2375#issuecomment-1541619462