=======================================
Day-01 : Spring Boot & Microservices
=======================================

1) What is Spring Boot ?

2) Why Spring Boot ?

3) Advantages with Spring Boot

4) What type of apps we can build using boot ?


=======================
What is Spring Boot ?
======================

=> Spring Boot is an approach to develop spring framework based applications with less configurations.

=> Spring Boot is an extension for existing spring framework.

=> Spring Boot internally uses spring framework only.

=> Spring Boot supports rapid application development.

	Spring Boot = (Spring + Auto Config) - xml config

=======================================================
What type of applications we can create using boot ?
=======================================================

=> By using this spring boot we can develop several types of applications

			a) stand-alone app
			b) web application ( C2B )
			c) distributed application (B2B)

			  (webservices / rest apis)

========================
Spring Boot Advantages	
========================

1) POM starters

2) Dependency Version Conflicts solution

3) Embedded Servers

4) Auto Configuration

5) Actuators....

===========================
What is POM starter ?
===========================

=> pom starters are used to simplify maven or gradle configurations

=> We have several pom starters

			a) spring-boot-starter-web

			b) spring-boot-starter-data-jpa

			c) spring-boot-starter-mail

			d) spring-boot-starter-security

Note: when we add pom starter boot will take care of depenency version.

==================
Embedded Servers
==================

=> Spring Boot will provide servers to run our application

		a) tomcat (default)
		b) jetty
		c) netty

===================
Auto Configuration 
===================

=> It is one of the most imp feature in spring boot

=> Boot will identify configurations required for the application based on pom starters and it will provide that configuration in runtme.	

===============
Actuators	
==============

=> To monitor and manage our application

=> Production Ready Features

		/health
		/beans
		/mappings
		/configProps
		/heapdump
		/threaddemp


=======================================
Day-02 : Developing Spring Boot Apps
=======================================

=> We can develop in 2 ways

		a) Spring Initializr website (start.spring.io)

				-> Create Project
				-> Download as zip file
				-> Extract zip file
				-> Import into IDE as Maven project

		b) IDE (STS/IntelliJ)

===========================================
Spring Boot Application Folder Structure
===========================================

src/main/java   => To write application source code

	- Application.java  (Start class & Entry Point)

src/main/resources	=> To keep application config file

	- application.properties

src/test/java		=> To write junit test cases

	- ApplicationTest.java

src/test/resources	=> To keep unit test config files

Maven Dependencies  => libraries downloaded

target  => byte code (.class files)

pom.xml => Maven config file


======================================
What is start class in Spring Boot ?
======================================

=> Start class is entry point for boot application execution

=> It is also called as main class in spring boot

=> When we create boot application, start class will be created by default.

@SpringBootApplication
public class Application {

	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}

}

===========================================
What is @SpringBootApplication annotation?
==========================================

=> This is used at start class of spring boot...

=> This is equal to 3 annotations

	a) @SpringBootConfiguration
	b) @EnableAutoConfiguration
	c) @ComponentScan


======================================
What is @ComponentScan annotation ?
======================================

=> It is used to scan the project and identify spring beans available in the application.

=> Component Scanning works based on base-package of application.

Note: The package which contains start class of spring boot is called as base package.

=> The packages which are starting with base package name are called as sub packages.


			in.ashokit
				- Application.java

			in.ashokit.service
			in.ashokit.dao
			in.ashokit.controller
			in.ashokit.util

			com.ashokit ---------- will not be scanned

=============================================================
Q) Can we configure more than one base package in the app ?
==============================================================

@SpringBootApplication
@ComponentScan(basePackages = { "in.ashokit", "com.tcs" })
public class Application {

	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}

}



===============================================
Q) How to represent java class as Spring Bean ?
===============================================

@Component : To represent java class as spring bean
	
@Service : To represent java class as spring bean

@Repository : To represent java class as spring bean

@Configuration : To represent java class as configuration class

@Bean : To customize objs creation process ( method level )

@Controller  : To handle http requests in web app (c2b)

@RestController : To handle http requests in rest apis

@ComponentScan : To scan spring beans in our applications


=======================================================
Q) What is run ( ) method in spring boot start class ?
=======================================================

=> It is entrypoint for boot application execution

		- Bootstrap context
		- starting listeners
		- prepare Environment
		- printing banner
		- create context (IOC)
		- prepare context
		- call runners
		- return ioc

================================
What is banner in Spring Boot ?	
================================

=> Logo which is printing on console is called banner

=> By default spring logo will be printed on console

=> We can customize banner text by creating "banner.txt"

		File location : src/main/resources	

=> Banner works based on modes

			a) console (default)

			2) log (print in log file)

			3) off

=> We can banner mode like below in application.properties file

			spring.main.banner-mode=off



===============================================
How IoC will be started in boot application ?
===============================================

=> run ( ) method is responsible to start IOC in boot app

=> run ( ) method will use predefined class to start IOC based on pom starter

default app (standalone) : spring-boot-starter

		Class : AnnotationConfigApplicationContext	

web app : spring-boot-starter-web (tomcat)

		Class:  AnnotationConfigServletWebServerApplicationContext

reactive app : spring-boot-starter-webflux (netty)

	Class :  AnnotationConfigReactiveWebServerApplicationContext


=================================
What is Runner in Spring Boot ?	
=================================

=> Runner is used to execute logic only once when application started.

=> We have 2 types of runners in spring boot

			a) ApplicationRunner
			b) CommandLineRunner

@Component
public class MyAppRunner implements ApplicationRunner{
	
	@Override
	public void run(ApplicationArguments args) throws Exception {
		System.out.println("my runner executed....");
		// logic
	}
}

Usecases : insert data in db, delete staging tables data, setup cache.



====================
Day-5 : Autowiring
====================		

=> It is used to perform dependency injection

=> Injecting one bean object into another bean object is called as dependency injection.	

=> We can perform dependency injection in 3 ways

				1) setter injection
				2) constructor injection
				3) field injection

Note: IoC container is responsible to perform dependency injection.

=> IOC will manage and colloborate bean objects	

=======================
What is Spring Bean ?	
=======================

=> The java class which is managed by IoC is called as spring bean.


====================================
What is Constructor injection ?		
===================================

=> Injecting dependent bean object into target bean object using target class constructor is called as CI.

=> To enable this we will write @Autowired at constructor level.

Note: When we have only one parameterized constructor in target class then @Autowired is optional.


@Service
public class UserService {

	private UserDao userDao;

	public UserService() {
		System.out.println("UserService:: default - constructor");
	}

	@Autowired
	public UserService(UserDao userDao) {
		System.out.println("UserService:: param - Constructor");
		this.userDao = userDao;
	}

	public void getName(int id) {
		String findName = userDao.findName(id);
		System.out.println("Name ::" + findName);
	}

}	


====================================
What is setter injection ?		
===================================

=> Injecting dependent bean object into target bean object using target class setter method is called as SI.

=> To enable this we will write @Autowired at setter method level.

@Service
public class UserService {

	private UserDao userDao;

	public UserService() {
		System.out.println("UserService:: default - constructor");
	}

	@Autowired
	public void setUserDao(UserDao userDao) {
		this.userDao = userDao;
	}

	public void getName(int id) {
		String findName = userDao.findName(id);
		System.out.println("Name ::" + findName);
	}

}

===========================
What is field injection ?
===========================

=> Injecting dependent bean object into target bean object using target bean variable is called as field injection.

Note: Internally IOC will use reflection api to perform field injection.

@Service
public class UserService {

	@Autowired
	private UserDao userDao;

	public void getName(int id) {
		String findName = userDao.findName(id);
		System.out.println("Name ::" + findName);
	}

}

Note: Using reflection api we can access private variables outside of the class.


public class User {

	private int age = 20;

	public int getAge() {
		return age;
	}
}

public class Test {

	public static void main(String[] args) throws Exception {

		Class<?> clz = Class.forName("in.ashokit.bean.User");

		Object object = clz.newInstance();


		Field ageField = clz.getDeclaredField("age"); // loading variable
		ageField.setAccessible(true); // making variable accessible
		ageField.set(object, 50); // setting value to variable
		
		
		User u = (User) object;
		int age = u.getAge();
		System.out.println("User Age :: " + age);

	}

}

==================
Autowiring modes
==================

=> Autowiring works based on modes

1) byName  : Based on variable name it will identify dependent bean

2) byType : Based on variable datatype it will identify dependent bean.


==================================
What is @Qualifier annotation ?
==================================

=> It is used to identify dependent bean based on bean name

@Component
public class Robot {

	@Autowired
	@Qualifier("chip64")
	private IChip chip;

}

Note: IChip is an interface which is having 32bit & 64bit implementation classes.

=> Based on above configuration it will inject the bean whose name is "chip64" 


===============================================
Q) What is Ambiguity problem in Autowiring ?
===============================================

=> If one interface having more than one impl class then we will get ambiguity problem in "byType" mode.

=> To resolve byType ambiguity problem we can use @Primary annotation.

=> The bean which is having @Primary annotation will get highest priority for autowiring.


=====================
What is Bean scope
=====================

=> Bean scope represents how many objects should be created for spring bean.

=> By default every spring bean is singleton (only one object)

=> We can customize bean scope using @Scope annotation

=> prototype scope represents every time new object.


Note: For singleton beans  objects will be created when ioc container starting.


@Component
@Scope("prototype")
public class Robot {
		//logic
}

==========================================================

@SpringBootApplication

	- @SpringBootConfiguration
	- @EnableAutoConfiguration
	- @ComponentScan

@Configuration
@Bean
@Component
@Service
@Repository
@Autowired
@Qualifier
@Primary
@Scope

==================================
CI ---> target class constructor
==================================

=> dependent bean is mandatory to create target bean

=> if dependent bean not available then target bean can't created.

=> first dependent bean obj will be created then only target bean object will be created.

==================================
SI --> target class setter method
==================================

=> First target bean object will be created then dependent object obj will be created.

=> After target bean creation using target bean obj setter method will be called (optional)

==================================
FI --> target class variable
==================================

=> IOC will use reflection api to perform FI.

=> Field Inections is simple to write.

=> IOC violating oops principles.

=> It is giving chance to violate Single Responsibility principle.


=========
Summary
==========

1) What is Spring Boot

2) Spring Boot Advantages

			- pom starters
			- auto configuration
			- embedded servers
			- actuators

3) First Boot Application			

4) Boot Application Folder Structure (Maven)

5) What is start class in Spring Boot

			- @SpringBootApplication
			- SpringApplication.run ()

6) What is Component Scanning

7) How run ( ) method works internally

			- bootstrap our app
			- start listeners
			- prepare env
			- print banner
			- create IOC
			- call runners
			- return IOC		

8) How to customize banner in boot app

9) How ioc will start in boot application ?

10) Runners in Spring Boot

			 - Application Runner
			 - Commandline Runner

11) Spring Boot Annotations

			 - @SpringBootApplication
			 - @Configuration
			 - @Bean
			 - @Component
			 - @Service
			 - @Repository			 

12) Bean Scopes (@Scope)

			 - singleton
			 - prototype
			 - request 
			 - session

13) Autowiring
			
			- byName
			- byType
			- @Autowired
			- @Qualifier
			- @Primary

14) What is Dependency Injection

			- Constructor Injection
			- Setter Injection
			- Field Injection

15) What is IOC container