All language subtitles for 014 Qualifiers - Overview_en

Instructor: In this video will cover Annotation

Autowiring and Qualifiers.

For auto wiring,

we're injecting a coach implementation.

So, Spring will scan for components,

check to see if anyone implements a given coach interface.

If so, let's inject them.

But if we have multiple implementations, which one,

like what algorithm will Spring use to determine

which coach that it should implement? Hmm.

So, here's a diagram here

of our multiple coach implementations.

So, we have coach, cricket coach,

baseball coach, track coach, tennis coach, et cetera.

And then, we have the actual source code

for these implementations.

So, we have our cricket coach, baseball, track and tennis.

All implement the coach interface.

So, when we ask for a coach implementation,

which one will Spring pick?

Well, we have a little problem.

So, this is the error message that you'll actually

encounter when you run your application.

Spring will say there's a parameter zero of the constructor

and the controller required a single bean,

but four were found.

Okay so, I need a coach,

but there's too many of them out here.

And at this point, Spring will not start up.

The application will not start

because there's too much ambiguity.

Spring can't figure out which one you want.

So, one solution here is to be specific

and that's by making use of the qualifier annotation.

And so, here's our coding here for our demo controller.

And everything looks the same except

for this one new entry here for qualifier.

So, here we give qualifier

and then we specify the bean ID of cricket coach.

Now the bean ID has the same name

as the class except for the first character as lowercase.

All right. So, that's how we come

up with cricket coach here, starting with lower case.

And this will actually resolve the issue

because now we're being very specific.

We're saying, hey, use cricket coach

as the injection for this given item.

And there's other bean IDs out there that we could use

such as baseball coach, track coach, or tennis coach.

But in this scenario here,

we're making use of the cricket coach.

Now that's for constructor injection.

For setter injection, you can do a similar thing.

You can also use the qualifier annotation.

And then, here's our code examples,

our normal setter method set coach

and then we specify qualifier annotation

and then give cricket coach.

Again, the bean ID is the same name as the class

except for the first character is lowercase.

All righty, this looks pretty good.

Let's go ahead and move into our (indistinct)

and let's write the code.