Spring Generic Factory

Problem Statement

I want to be able to create multiple objects of the generic type without having to create each instance by hand and wire it in via@Bean.

In the code below, I am trying to create DefaultRedisClient and CacheReadService for the different class types: Person and Blog. That's just an example. In reality, there could be tens of classes/objects I would like to read from the cache, and for each one of those, I am duplicating the code with slight modifications.

To create this block of code again and again with only the CacheKey and CacheValue type changing along with a couple other things seems wrong.

@Configuration
public class RedisCacheReadServiceConfig  {

    @Bean
    public DefaultRedisClient< PersonCacheValue > personRedisClient(
            @Autowired @Lazy
            RedisTemplate< String, String > redisTemplate) {

        return new DefaultRedisClient<>(redisTemplate, PERSON.getName());
    }

    @Bean
    public CacheReadService< PersonCacheKey, PersonCacheValue > personCacheReadServiceRedisImpl(
            @Lazy @Autowired
            DefaultRedisClient< PersonCacheValue > personRedisClient,
            @Lazy @Autowired
            PersonCacheFallback personCacheFallback,
            @Lazy @Autowired
            RedisClientFactory< PersonCacheValue > redisClientFactory) {

        return new DefaultCacheReadService<>(personRedisClient, PERSON, personCacheFallback);
    }

    @Bean
    public DefaultRedisClient< BlogCacheValue > blogRedisClient(
            @Autowired @Lazy
            RedisTemplate< String, String > redisTemplate) {

        return new DefaultRedisClient<>(redisTemplate, BLOG.getName());
    }

    @Bean
    public CacheReadService<
            BlogCacheKey,
            BlogCacheValue > blogCacheReadServiceRedisImpl(
            @Lazy @Autowired
            DefaultRedisClient< BlogCacheValue > blogRedisClient,
            @Lazy @Autowired
            BlogCacheFallback blogCacheFallback,
            @Lazy @Autowired
            RedisClientFactory< BlogCacheValue > redisClientFactory) {

        return new DefaultCacheReadService<>(blogRedisClient, BLOG, blogCacheFallback);
    }
}

Solution

The fact of the matter is that there's a better way. Since Spring 4.0, Spring will automatically consider generics as a form of @Qualifier. And because of that, we can have a factory pattern that uses generic types, such as @Qualifier. Please consider the example below:

/**
 * A factory that generates / provides DefaultRedisClient < V >
 *
 * @param  CacheValue type
 */
@Service
public class RedisClientFactory< V extends CacheValue > {

    private final RedisTemplate< String, String > redisTemplate;

    @Autowired
    public RedisClientFactory(@Lazy RedisTemplate< String, String > redisTemplate) {

        this.redisTemplate = redisTemplate;
    }

    public DefaultRedisClient< V > getRedisClient(final String domainName) {

        return new DefaultRedisClient<>(redisTemplate, domainName);
    }
}

With the RedisClientFactory above, our main config becomes simpler because instead of creating DefaultRedisClient beans for different CacheValue types, we have delegated that work to the RedisClientFactory and now can simply auto-wire DefaultRedisClient<V> by specifying the generic class type V. See line number 7 and 19 below. Spring is injecting a generic RedisClientFactory of the given type. Once we have that, we can simply call the getRedisClient method to create the RedisClient we want.

@Configuration
public class RedisCacheReadServiceConfig  {

    @Bean
    public CacheReadService< PersonCacheKey, PersonCacheValue > personCacheReadServiceRedisImpl(
            @Lazy @Autowired
            PersonCacheFallback personCacheFallback,
            @Lazy @Autowired
            RedisClientFactory< PersonCacheValue > redisClientFactory) {

        return new DefaultCacheReadService<>(redisClientFactory.getRedisClient(PERSON.getName()), BLOG, personCacheFallback);
    }

    @Bean
    public CacheReadService < BlogCacheKey, BlogCacheValue > blogCacheReadServiceRedisImpl(
            @Lazy @Autowired
            BlogCacheFallback blogCacheFallback,
            @Lazy @Autowired
            RedisClientFactory< BlogCacheValue > redisClientFactory) {

        return new DefaultCacheReadService<>(redisClientFactory.getRedisClient(BLOG.getName()), BLOG, blogCacheFallback);
    }
}

But, we can go one step further and create a factory class to create CacheReadService for different CacheKey and CacheValue. See below:

• In line 9, we are injecting the genericRedisClientFactory<V>

• In line 19, we use the generic RedisClientFactory<V> to create a generic DefaultRedisClient<V>.

• In line 20, it is a pretty straightforward creation of CacheReadService<K,V>.

@Service
public class CacheReadServiceFactory< K extends CacheKey, V extends CacheValue > {

  private final RedisClientFactory redisClientFactory;
  private final CacheFallback cacheFallback;

  @Autowired
  public CacheReadServiceFactory(
    @Lazy RedisClientFactory< V > redisClientFactory,
    @Lazy final CacheFallback< K, V > cacheFallback) {

    this.redisClientFactory = redisClientFactory;
    this.cacheFallback = cacheFallback;
  }

  public CacheReadService< K, V > getCacheReadService(
    final CacheDomainType cacheDomainType) {

    final DefaultRedisClient< V > redisClient = redisClientFactory.getRedisClient(cacheDomainType.getName());
    return new DefaultCacheReadService<>(redisClient, cacheDomainType, cacheFallback);
  }
}

Now, because of the CacheReadServiceFactory (above), our configuration class (below) becomes super simple because of Spring and being able to use generics for auto-wiring (since Spring 4.x).

Final Solution

/**
 * Final Config class
 */
@Configuration
public class RedisCacheReadServiceConfig {

  @Bean
  public personCacheReadService(
    @Lazy @Autowired
    final CacheReadServiceFactory< PersonCacheKey, PersonCacheValue > cacheReadServiceFactory
  ) {

    return cacheReadServiceFactory.getCacheReadService(PERSON);
  }

  @Bean
  public blogCacheReadService(
    @Lazy @Autowired
    final CacheReadServiceFactory< BlogCacheKey, BlogCacheValue > cacheReadServiceFactory
  ) {

    return cacheReadServiceFactory.getCacheReadService(BLOG);
  }
}

Please feel free to leave questions or comments below!

References

1: https://spring.io/blog/2013/12/03/spring-framework-4-0-and-java-generics