Add fallback option default false to @Qualifier [SPR-13623]

[spring-projects-issues]

Yanming Zhou opened SPR-13623 and commented

It's identical to current implementation If fallback=false, so it's backward compatible.
fallback=true is useful for override base platform in particular app.

@Component
public class DefaultEchoService implements EchoService {
}

@Qualifier(value = "customEchoService", fallback = true)
@Autowired
private EchoService echoService;

Spring will inject DefaultEchoService as echoService, If we have another EchoService named "customEchoService", spring should inject it instead.

@Component("customEchoService")
public class CustomEchoService implements EchoService {
}

---

No further details from SPR-13623

[spring-projects-issues]

Stéphane Nicoll commented

Just to make sure I understand this properly. You'd like that when fallback is true and there is no a candidate bean named customEchoService it should inject what exactly? Should it be named default + beanType.getSimpleName()?.

That looks a bit weird to me and it changes the semantic of Qualifier quite significantly.

[spring-projects-issues]

Yanming Zhou commented

If fallback is true and there is no candidate named "customEchoService", It should act as no @Qualifier, It will inject "defaultEchoService" with this case but regardless of the name.

[spring-projects-issues]

Stéphane Nicoll commented

It's still not clear, sorry. It will inject "defaultEchoService" because that's the only bean of type EchoseService in the context?

[spring-projects-issues]

Yanming Zhou commented

@Qualifier(value = "customEchoService", fallback = true)
@Autowired
private EchoService echoService;

is equals to

@Autowired
private EchoService echoService;

if there is no "customEchoService"

[spring-projects-issues]

Stéphane Nicoll commented

your "equals to" makes no sense without more context. What if I have two EchoService beans in the context?

[spring-projects-issues]

Yanming Zhou commented

"equals to" means exactly identical. just ignore @Qualifier and keep the spring rule.
It will inject primary candidate or the only candidate.
It will inject nothing if there is no candidate and required=false.
It will throws exception if there is no candidate or more than one candidate and no a primary.

[spring-projects-issues]

Stéphane Nicoll commented

why don't you start with that in your original description rather than having a defaultEchoService that insinuates that default is some magic prefix? Anyway, your request is clear now.

[spring-projects-issues]

Yanming Zhou commented

Should it be resolved in 4.3 ?

[spring-projects-issues]

Stéphane Nicoll commented

We just discussed this issue and we're not keen on implementing this. This significantly complexify the semantic of @Qualifier and your "fallback" may mean something different to someone else. If you need to have such fine tune autowiring arrangement, we advise you to inject the list of candidates and figure out which one you have to use.

[spring-projects-issues]

Yanming Zhou commented

Inject the list or map will contaminate clean code. How about add a dedicated annotation? Such like @FallbackQualifier or @AlternativeQualifier. It's useful for certain scene, If spring add such feature, Developers will realize its value.

[spring-projects-issues]

Yanming Zhou commented

Workaround :

	@Autowired(required = false)
	@Qualifier("cacheStringRedisTemplate")
	private RedisTemplate<String, String> cacheStringRedisTemplate;

	@Autowired
	@Qualifier("stringRedisTemplate")
	private RedisTemplate<String, String> stringRedisTemplate;


	@PostConstruct
	public void afterPropertiesSet() {
		if (cacheStringRedisTemplate != null)
			stringRedisTemplate = cacheStringRedisTemplate;
	}

Wish:

@Autowired
@Qualifier(value = "cacheStringRedisTemplate", fallback = "stringRedisTemplate")
private RedisTemplate<String, String> stringRedisTemplate;

@Autowired
@Qualifier("cacheStringRedisTemplate")
@FallbackQualifier("stringRedisTemplate")
private RedisTemplate<String, String> stringRedisTemplate;

@Autowired
@CandidateQualifier("cacheStringRedisTemplate")
@Qualifier("stringRedisTemplate")
private RedisTemplate<String, String> stringRedisTemplate;