07 - Spring Getting Started/008 DI Using Spring Boot_en.srt

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(gentle music)

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-: Now let's try to implement dependency injection in Spring.

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Now the thing is we started with Spring Boot,

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but then the actual sense or what happens

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behind the scene will make much more sense once we do this

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using Spring framework.

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Now, why we are doing in Spring Boot is

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because to understand what Spring Boot can do.

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In terms of dependency injection, of course Spring Boot

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can do lot of different things,

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but we are starting with the first step.

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Okay, now when I say what we wanna do here,

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before I do that, I have one more update.

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So in the last video we talked about the same thing.

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We ran this application but it was using Java 17.

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So what I did is I have installed Java 21

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and now we are going

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to run all the application using Java 21.

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Okay, so what dependency I'm talking about?

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See this is our main code, right?

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So we have a main application here.

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In this if you can see,

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what we are doing is we have a main method,

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and then we are simply saying Springapplication.run.

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Basically it activates your Spring framework.

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It also has something called a container,

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where you can create your Spring beans.

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As I mentioned before, Spring is responsible

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to create the objects

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and these objects are basically called beans.

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So any object which is created by Spring, so created,

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managed by Spring will be called beans.

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Nothing fancy, it's just normal object, different name.

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Okay so this is how you run the container,

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but we are not using container in this code.

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We are simply running Hello World.

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So nothing about containers till now,

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but I want to use container now.

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So what do is I want to create a simple class here.

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Very simple class.

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Let's say I want to say maybe alien, okay?

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Now if you're not sure why I always create

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or work with alien, basically I believe

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that programmers are aliens, okay?

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It's because we don't live in this world,

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mentally or virtually, we live in virtual world, right?

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So we are aliens anyway, nothing fancy.

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Imagine every time I say alien you say programmer.

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That's fine.

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Okay so we got alien here

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and in this alien, what I'll do is I want

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to do a very simple thing, okay?

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First of all, I have to deactivate my Github copilot.

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It's giving some suggestion, okay?

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So what I'll do now is I will create a very

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simple method here.

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I will say public void code because there's a programmer

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or as as alien, we code.

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And then what I'm going to do here is

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I'm going to do a very simple thing.

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I will simply print something

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and let's say, I'll say coding, that's it.

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I'm just saying code. So printIn Coding.

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But now I want to call this from my main method.

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So let me just keep this here.

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And okay, so we got both the codes parallelly here

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and now if you can see if I will remove this Hello World,

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I want to create the object of alien

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and we know how to do that, right?

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We can simply create alien object here and alien object

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and then I can simply call code.

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That's the only way you can call this method

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because this is a non-static method,

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and if you want to call it, you need object.

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And that's what we are doing here, we are saying alien obj,

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got a new alien, we got the object created.

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And then using the reference we are calling the method.

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Let's see if this works.

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I'll right click here, see run on this

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and you will get the output.

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Output says okay, it's taking some time,

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but output says coding.

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So at least this is working.

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But now the idea is I don't want

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to create this object by myself.

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What I want is I want Spring to give me this object.

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That means new alien is not something I want.

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And that's tricky, right, because if you don't say

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new alien, how it'll create the object

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and now you are saying we don't want to create the object,

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we want Spring to create it.

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How Spring will know that you want to create the object?

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Now that's funny, right? Okay.

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For that, what you have to do is first of all,

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if you say Spring creates the object, let's say

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with this code, I'm assuming that Spring will create

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the object for me because we are using Spring framework,

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where exactly it'll create the object.

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So it'll create the object inside the container.

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Remember I mentioned about the containers?

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So imagine you have a JVM, inside JVM,

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you have a special area

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or the special box which is running,

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which is your container.

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So every time your Spring framework creates the object

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for you, it'll be available in the container.

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That means I have to find a way to get to that container.

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And the way you can do that is

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by using application context, okay?

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That means we have to get object for it now.

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The thing is if you look at this particular method here,

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which is Springapplication.run,

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and if you go to run method,

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and if you can see run returns the object

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of configurable application context, okay?

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This configurable application context

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extends application context.

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That means if you can simply use this run method return

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value, you got the application context object.

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Okay, let me just do that now.

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So I will say application context,

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and I will say this is name as context itself equal to.

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So what we're doing is this run returns this object context,

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which means now you have a way to communicate

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with your IOC container.

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Okay now how will you communicate?

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So it's very simple.

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You will simply say, hey context, that means hey container,

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give me the object.

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And remember, the object is not object in Spring.

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The object is called bean. Okay?

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So you have to say getBean. So you will say getBean.

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And then here you mention which class object

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you want, of course, right?

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You have to mention because in your project you might be

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having multiple classes.

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So how will your Spring framework know what you want?

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So mention, hey, you know, I want the object of alien,

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which you have created inside your container.

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Your Spring will say, okay, I got it.

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You basically need this Alien.class object.

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I mean you need object of this class,

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but the IOC container will be able

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to give you this object only if the object

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is there in the container.

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We are assuming it'll be there, but is it?

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Let's try, let's rerun this

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and let's see what it says.

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Okay, your Spring is not happy, it's giving you some error.

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It says no such bean definition. Can you see that?

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We got the exception here.

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And it says no qualifying bean of type

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com.telusko.app.alien okay?

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So basically it's not able to find this bean.

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But you'll be saying then why we are using Spring

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framework, if it cannot create the object for us?

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Actually it can.

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The only thing is see in your project,

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let's say you have a hundred classes,

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do you really want Spring to create all the object for you?

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Of course not, right?

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So there should be a way

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that you should be able to tell Spring framework

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which object you want and which you don't want.

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So Spring by default says,

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"I will not create any object for you.

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Only when you mention that you want the object

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or you want me to create the object,

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then only I will create."

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How will you tell your Spring framework you want the object?

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And for that you have to use annotations.

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So on top of it you will simply use annotation,

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which is called component.

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Now making this class as a component, you are making sure

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that your Spring knows that Spring has

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to manage this particular object.

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So create the object, assemble the object, and manage it.

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Everything will be done by Spring just by this annotation.

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I know you don't believe me.

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Let's see, let's run this and here we go.

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Okay, so you can see we got the output coding.

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Basically what is happening here is you have not

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created any object.

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Spring is doing it.

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Just because you say component,

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the moment your Spring framework loads, it says,

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I mean Spring Boot here, it says,

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"Okay, I can see a component on top of alien.

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That means it is my responsibility to create this object."

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I mean you try out; create another class, let's say for

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a laptop, and then don't make component

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and try to say getBean; it'll not work,

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because only those objects will be created

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where you have component on top of your class

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and that's how basically your

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dependency injection is happening okay?

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So Spring is taking this object in the container

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and it is also getting injected here

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because we are saying getBean.

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Okay, now can I create this object multiple times?

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Let's see, let's see.

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I will say one Alien obj1 context.getBean

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and I will say alien.class.

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Let me say obj1.code.

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Now will this work? That's the first question.

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Let's run this and see what happens. Run.

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And you can see, yes,

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so coding is getting printed two times.

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That means this is working, right?

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But the question is, are we getting the same object

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or we getting different object?

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Okay, now that's tricky and I don't want to answer that now.

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Maybe we'll do that once we move towards

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understanding Spring framework.

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You know, sometimes surprises are fun,

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suspense is fun basically.

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And yeah, that's on this video.

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But then in the next video, let's also discuss

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what if you have a different object?

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What if alien is dependent on some other object?