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Spring Boot Framework

What is Spring Boot ?

• Spring Boot makes it easy to create stand-alone, production-grade Spring grade applications that we can just run.

What is Spring ?

• Application Framework
• Dependecy Injection
• Provides a template which lets us build an enterprise application.
• Programming and Configuration Model
• Focus on business problem and let string handle other things.
• Infrastructure Support
  • Connecting to Database (R-DBMS database, MongoDB etc)
• Problems with Spring
  • Huge Framework
  • Multiple Setup Steps
  • Multiple Configuration Steps
  • Multiple Build & Deploy Steps

Introduction

Spring Boot

• Opinionated
• Convention over Configuration
• Stand-alone
• Production Ready

Maven

• Dependency management tool
• Mangaes dependency using pom.xml

Creating a Project

Step-1: Create a simple Maven Project

Step-2: Convert to Spring Project using Spring Boot Convention

Put the below dependency in pom.xml.

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.6.RELEASE</version>
</parent>

Step-3: Converting it to a Web Application

Add the web dependencies to make it a web application.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <version>2.2.6.RELEASE</version>
</dependency>

Step-4: Use the appropriate Java version

<properties>
    <java.version>1.8</java.version>
</properties>

Starting the Spring Boot App

Create a class App.java with below code.

package com.kodefork.articles;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class App {
    public static void main(String[] args){
        // Create a servlet container and run
        SpringApplication.run(App.class, args);
    }
}

Notes:

• SpringApplication has a static method run() that does all the magic, it takes class App and starts spring boot application as it it annotated by **@SpringBootApplication **.

Starting Spring Boot

• Sets up default configuration.
• Starts Spring application context.
• Performs class path scan.

• Starts tomcat server.

• Application Context
  • Spring is a container for all the code that runs on application server, and this container is called application context.
  • Application server has business services, controllers and data services and spring acts as container for all these services.
  • This container is k/a application context.
• Spring boot starter creates this application context.
• Spring boot also performs a class path scan.
• While scanning it looks for special annotations like if class has
  • @Service annotation it is a service
  • @Controller annotation it is controller
• After scanning it treats class as the given annotation.

Adding a Controller

Controller

• A Java Class
• Marked with annotations
• Has info about
  • What URL access triggers it ?
  • What method to run when accessed ?
• Also provides information about what URL access needs to trigger that controller to execute.

Spring MVC Framework

• The web layer in spring boot application leverages a spring framework Spring MVC a child project of while Spring Framework.
• It lets us build server-side code which maps to URLs and provides responses.
• Responses can be :
  • REST Api response in form of JSON.
  • A full HTML page.
  • A JSP or FTL Response.

Writing a REST Controller

• To make a class REST Controller just put @RestController annotation above the class.
• Use @RequestMapping annotation to map method to a particular request url.
• By default, @RequestMapping maps only to GET method, to map other method we need to specify it in annotation.

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Let’s perform a quick side-by-side comparison, so we can have a bird’s eye view of the constraints’ functionality and easily spot their differences:

• @NotNull: a constrained CharSequence, Collection, Map, or Array is valid as long as it’s not null, but it can be empty
• @NotEmpty: a constrained CharSequence, Collection, Map, or Array is valid as long as it’s not null and its size/length is greater than zero
• @NotBlank: a constrained String is valid as long as it’s not null and the trimmed length is greater than zero