Question

I don’t see any difference between two ways, @Qualifier is always used with @Autowired.

@Autowired
@Qualifier("alpha")

VS

@Resource(name="alpha")

Anyone could let me know the difference? Thanks!

Answer 1

@Autowired can be used alone . If it is used alone , it will be wired by type . So problems arises if more than one bean of the same type are declared in the container as @Autowired does not know which beans to use to inject. As a result , use @Qualifier together with @Autowired to clarify which beans to be actually wired by specifying the bean name (wired by name)

@Resource is wired by name too . So if @Autowired is used together with @Qualifier , it is the same as the @Resource.

The difference are that @Autowired and @Qualifier are the spring annotation while @Resource is the standard java annotation (from JSR-250) . Besides , @Resource only supports for fields and setter injection while @Autowired supports fields , setter ,constructors and multi-argument methods injection.

It is suggested to use @Resource for fields and setter injection. Stick with @Qualifier and @Autowired for constructor or a multi-argument method injection.

See this:

If you intend to express annotation-driven injection by name, do not
primarily use @Autowired – even if is technically capable of referring
to a bean name through @Qualifier values. Instead, prefer the JSR-250
@Resource annotation which is semantically defined to identify a
specific target component by its unique name, with the declared type
being irrelevant for the matching process.