All language subtitles for 017 Primary - Overview_en

Instructor: This video will cover the primary annotation.

Now resolving the issue with multiple coach implementations,

we saw the example in the previous set of videos,

of, by making use of the qualifier annotation.

We were super specific, by mentioning a coach by name.

However, there's an alternate solution available.

Instead of specifying a coach by name,

using a qualifier annotation,

I could say, you know what?

I simply need a coach.

I don't care which coach.

And I could say, Hey, if there are multiple coaches

out there, then you coaches figure it out.

You all tell me who's the primary coach.

I really don't care.

I simply need a coach to help me out, here.

That's it.

So as before, we had our multiple coach implementations,

our track coach, baseball coach,

tennis coach, and cricket coach.

And then the one thing to notice here is,

that now we make use of this new annotation primary.

So this annotation basically says,

out of the multiple coach implementations, this is

going to be the primary implementation that you should use.

Now we can resolve this using the primary.

So here's our code for our demo controller.

And now one thing to notice here, with our demo controller,

and our constructor injection, is that there's no need

to use the qualifier annotation.

Because now, if there's multiple coaches,

we know the primary coach, based on that primary annotation,

that's on the track coach.

All right, so notice the difference, here.

There's one caveat, here.

When you are using the primary annotation, and you probably

was wondering about this, and thought about this.

When using the primary annotation can have only one

for multiple implementations.

If you mark multiple classes with primary, then hmm,

we have a little problem.

And when you run your application, if you try to

mark multiple classes with primary,

then, in the error messages it'll say, Hey,

more than one primary bean found.

So spring will say, I don't know what

you want me to do, here.

There's more than one primary.

I'm not sure which one to use, so I'm not going to start.

All right.

So just have to be aware of that.

You can only mark one class with primary.

Now, you may also wonder, well, what about

mixing primary and qualifier at the same time?

Can I use both of those in the same class?

The answer is yes, but you're asking for trouble.

Not really, but you have to be aware of this is,

that qualifier has the higher priority.

So even if you mention a given class as the primary class,

that can be overridden, or have higher priority,

by making use of the qualifier annotation.

So in this example here, qualifier has a higher priority.

So even though there's a primary coach, track coach,

this example will actually make use of cricket coach.

All right, so just something to be aware of.

And so you may wonder, well, which one should I use?

Should I use primary, or should I use qualifier?

Well, primary leads it up to the implementation classes.

You could have issues of multiple primary classes,

leading to an error.

Qualifier allows you to be very specific

on which bean you want.

And so in general, I recommend using qualifier,

and the reasoning is, is that it's more specific,

and it also has a higher priority,

compared to primary annotation.

This all looks pretty good.

Let's go ahead and move into the video, and let's go ahead

and write some code using the primary annotation.