Difference between @Primary vs @Autowired with @Qualifier annotations

Question

So, if I understood correctly, both are the way to determine which bean to autowire if there are multiple candidates. So what exactly is the difference?

Answer 1

Read @Primary as the "default".

If a bean has @Autowired without any @Qualifier, and multiple beans of the type exist, the candidate bean marked @Primary will be chosen, i.e. it is the default selection when no other information is available, i.e. when @Qualifier is missing.

A good use case is that initially you only had one bean of the type, so none of the code used @Qualifier. When you then add another bean, you then also add @Qualifier to both the old and the new bean, so any @Autowired can choose which one it wants. By also adding @Primary to the old original bean, you don't have to add @Qualifier to all the existing @Autowired. They are "grandfathered" in, so to speak.

@Primary is also good if e.g. 95% of @Autowired wants a particular bean. That way, only the @Autowired that wants the other bean(s) need to specify @Qualifier. That way, you have primary beans that all autowired wants, and @Qualifier is only used to request an "alternate" bean.

Answer 2

@Qualifier should be used in conjuction with @Autowired always. This will indicate the bean name which needs to be Autowired in case of multiple beans with same type is present in the application context.(so that spring can autowire by name.)

@Primary should be used in conjuction with @Bean / @Autowired which indicates which bean should be given higher preference, when there are multiple beans of same type is present.

One of the classic use cases where you would use @Primary is when the framework(example spring-data) expects a bean of some type (example EntityManager) but you have multiple datasources and you would have configured multiple Entity Managers. In such cases @Qualifier doesn't quite help.

Answer 3

@Qualifier

If there are more than one instances available for an injection point then we can use @Qualifier annotation to resolve an ambiguity. As @Qualifier is used at the injection point, there might be two situations where we don't want to or cannot use @Qualifier.

1. When autowiring mode is Autowire.BY_TYPE. Then, of course, we cannot use @Qualifier because we actually don't have user-defined injection point specified as @Autowired or @Inject
2. We want to do bean selection (i.e. resolve the ambiguity) at configuration time rather than during beans development time.

The solution to above problems is to use @Primary annotation.

@Primary

This Indicates that a particular bean should be given preference when multiple beans are candidates to be autowired to a single-valued dependency. If exactly one 'primary' bean exists among the candidates, it will be the autowired value.