Getting Started with Spring Boot

Spring Boot is a framework that creates ready-to-run web applications without dozens of lines of XML with a “convention over configuration” philosophy. The server appears in one line, java -jar, including the built-in Tomcat.

Learning Objectives

• Know the project structure provided by Spring Initializr
• Know the role of @SpringBootApplication
• Understand the concept of dependency injection (DI)
• Change basic settings such as port and log with application.properties
• Run as mvn spring-boot:run

Key concepts

1) Spring Initializr

If you access start.spring.io in your browser and select dependencies, a project zip will be created. We recommend the following options:

• Project: Maven
• Language: Java
• Spring Boot: 3.3.x
• Java: 21
• Dependencies: Spring Web (+ 26 H2/JPA)

2) Standard project structure

spring-boot-app/
├── pom.xml
├── src/
│   ├── main/
│   │   ├── java/com/codingnow/lecture/spring23/Application.java
│   │   └── resources/application.properties
│   └── test/
│       └── java/com/codingnow/lecture/spring23/ApplicationTests.java

3) @SpringBootApplication

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

This one annotation is a meta annotation that combines 3 (@Configuration, @EnableAutoConfiguration, @ComponentScan). Automatically finds and registers beans in the same package/subpackage.

4) Illustration of Dependency Injection (DI)

@Service
class HelloService {
    String greet() { return "Hello, Spring!"; }
}

@RestController
class HelloController {
    private final HelloService service;
    HelloController(HelloService service) { this.service = service; }
    @GetMapping("/hello") String hello() { return service.greet(); }
}

Spring creates a HelloService object and automatically passes it to the controller constructor. The core of DI is a pattern that the caller does not directly new.

5) application.properties

server.port=8081
spring.application.name=lecture-spring23
logging.level.root=INFO

See as example

Example 1 — pom.xml : Spring Boot dependency

pom.xml (Summary):

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3.3.6</version>
</parent>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

spring-boot-starter-web One dependency includes all built-in Tomcat, Jackson (JSON), and Spring MVC.

Example 2 — Application.java: Entry point

package com.codingnow.lecture.spring23;

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

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

Example 3 — HelloService.java + HelloController.java: First case of DI

HelloService.java :

package com.codingnow.lecture.spring23;

import org.springframework.stereotype.Service;

@Service
public class HelloService {
    public String greet(String name) {
        return "Hello, " + name + "!";
    }
}

HelloController.java :

package com.codingnow.lecture.spring23;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloController {
    private final HelloService service;

    public HelloController(HelloService service) {
        this.service = service;
    }

    @GetMapping("/hello")
    public String hello(@RequestParam(defaultValue = "Spring") String name) {
        return service.greet(name);
    }
}

Example 4 — application.properties

server.port=8081
spring.application.name=lecture-spring23

run

cd 06_Spring_Boot/23_Spring_Boot_Start
mvn spring-boot:run

In another terminal:

curl http://localhost:8081/hello
# Hello, Spring!

curl "http://localhost:8081/hello?name=Jisoo"
#Hello, Jisoo!

Common Mistakes

1. Application.java cannot find components in other packages → Put them in the top package.
2. Port conflict (8080 occupied) → change server.port
3. Execution does not end with mvn spring-boot:run (exit with Ctrl+C)
4. Use @Autowired field without constructor injection (difficult to test)
5. Specify Spring Boot dependency versions one by one (managed by parent)

Cleanup

• The pair @SpringBootApplication + main is the entry point
• DI is a pattern of “not creating objects directly”
• Environment settings with application.properties
• Can be run immediately with mvn spring-boot:run

Try it yourself

cd 06_Spring_Boot/23_Spring_Boot_Start
mvn spring-boot:run
#another terminal
curl http://localhost:8081/hello