IntroductionToSpring

#what is Spring??
-> spring is a farmework of java for wed developement 
-> It is light weight and works on POJO (plain old java object)

when spring was first launched it was just a single module that was used for dependency injection but later demand increased
and spring developed itself and now it has multiple modules which are used in
microservies
cloud applications
web app
event driver
batch

#Community
spring is a very famous in indystry but why?
it is beacuse it has 
good feature 
wide community
good documentation
go to spring docs on google and you can read and reffer different topics and modules of spring


#springsetup

1.IntelliJ IDEA-> for community version there is no extension

2.Eclipse-> go to help->eclipse marketplace->search->Springtool5->install->confirm->next->next
restart->new project -> spring 

3.VS Code-> open extension -> spring boot extension,springboot dashboard,spring initializer install all


#Core components of spring
There are two core components of spring 
->IOC inverse of control
-> DI dependency injection

what is IOC?
-> when working on java we have to create, manage and destroy multiple objects
-> To reduce these IOC was created which stands for Inversion of control i.e you are giving control of creating
managing and destroying objects to spring itself mean you dont have to always think of object management spring will
take care of it and you can focus on business logic 
->So basically there is a IOC container where spring stores object

what is DI?
DI Stands for the dependecy injection which is second core component of spring
it is a design pattern and its work is to get the object from IOC Container and put it in your application
that is injecting objects in application

                        
#Spring vs SpringBoot
Spring- when spring was released we have to manually configure all the dependecies through pom.xml or annotations
these use to create more manual work so it was lengthy process just to write simple code like hello world

SpringBoot is built on top of spring i.e it provides everything in a predefined manner means all dependencies and 
annotation so we dont have to manually import everything making it smooth and easy to code.
its like just select what you want and springboot and it will give you structure with all configurations directly

springboot is a opinionated framework build on top of spring 

#first springboot code

eclipse-> go to new project->spring->spring starter project->name,type=maven,package=jar,language=java->next you can see list of dependecies which can be added for now no need so -> next->finish

Intellij-> intellij has no spring extension and support so we have to first go to 
Spring.starter.io which is official link of spring and after going select all configuration (name ,lang,java version etc)
then click generate it will provide you a zip file and download it then go to that file -> extract all-> you will get a folder
click on the folder and open with intellij

#First dependency injection code

lets talk about 
SpringApplication.run(Springapplication.class,args)-> in these the run() returns a object of ConfigurableApplicationContext that is extended of ApplicationContext.
see JAB we have to talk to IOC container to get objects/beans we need object of ApplicationContext
so code becomes 
ApplicationContext context= SpringApplication.run(...)
now you got object of ApplicationContext now using these object we can getbeans
see human-> object but when control in hand of spring we call object as beans
so laptop lap=context.getBean(laptop.class) (.getbean() to get object from ioc container and we have to always mention (.class)that we are using)
But see for each class you want for spring to create object and store in IOC you have to annotate it with @Component
If you dont spring wont track that class and create object


#Autowired

consider you are calling a object of Laptop and using its methods eg run() but in laptop run method there is CPU.start() method of CPU class and Motherboard.run() so basically if we have to run laptop we have run [CPU.start() and Motherboard.run()] but for that we have to create Object of CPU and Motherboard but how can we?? so do we have to add all in IOC container?? 
yes we have to annotate all classes with @Component but we dont have to class CPU cpu =getBean(CPU.class) cause we have concept of Autowired which is a annotation you just have to annotate it

eg 

public class Laptop
{
    @Autowired
    CPU cpu

    @Autowired
    Motherboard mb

    public void run()
    {
        cpu.start()
        mb.run()
    }
}

so autowrired means it links the main class->laptop and laptop->cpu and motherboard in future if you want to link cpu->electricity you can
do that also so we dont always have to use .getBean()
if one class is dependent on other for execution we use annotation @Autowired