Module 3: Your First Application

Level: Beginner
Time: ~60 minutes

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Learning Objectives

• Understand the default project structure
• Know the role of the main class and @SpringBootApplication
• Create a simple REST controller and return JSON
• Use application.properties for basic configuration

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3.1 Project Structure

After generating from Spring Initializr you typically have:

demo/
├── src/
│   ├── main/
│   │   ├── java/
│   │   │   └── com/example/demo/
│   │   │       └── DemoApplication.java    ← Main class
│   │   └── resources/
│   │       ├── application.properties      ← Config
│   │       ├── static/                     ← CSS, JS, images
│   │       └── templates/                  ← View templates (if used)
│   └── test/
│       └── java/
│           └── com/example/demo/
│               └── DemoApplicationTests.java
├── pom.xml   (or build.gradle)
└── mvnw / gradlew (wrappers)

• src/main/java — application code
• src/main/resources — config, static files, templates
• src/test/java — tests

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3.2 The Main Class

Example:

package com.example.demo;

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

@SpringBootApplication
public class DemoApplication {

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

What @SpringBootApplication Does

It is a composition of three annotations:

1. @SpringBootConfiguration — marks this class as a configuration source (like @Configuration).
2. @EnableAutoConfiguration — enables Spring Boot auto-configuration (e.g. embedded server, default MVC config).
3. @ComponentScan — scans the package of this class and sub-packages for @Component, @Service, @Controller, etc.

So the class that has @SpringBootApplication is both the entry point and the root of component scanning.

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3.3 Your First REST Controller

Create a new class in the same package (or a sub-package, e.g. controller):

package com.example.demo.controller;

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

@RestController
public class HelloController {

    @GetMapping("/hello")
    public String hello() {
        return "Hello, Spring Boot!";
    }
}

• @RestController — combines @Controller and @ResponseBody: return values are written directly to the HTTP response (e.g. JSON or plain text).
• **@GetMapping("/hello")** — maps HTTP GET requests for/hello` to this method.

Run the application and open: http://localhost:8080/hello
You should see: Hello, Spring Boot!

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3.4 Returning JSON

Return an object; Spring (with Jackson on the classpath) serializes it to JSON:

package com.example.demo.controller;

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

import java.util.Map;

@RestController
public class HelloController {

    @GetMapping("/hello")
    public String hello() {
        return "Hello, Spring Boot!";
    }

    @GetMapping("/info")
    public Map<String, Object> info() {
        return Map.of(
            "application", "Demo",
            "version", "1.0",
            "status", "running"
        );
    }
}

Request: GET http://localhost:8080/info
Response (example): {"application":"Demo","version":"1.0","status":"running"}

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3.5 Using a DTO (Data Transfer Object)

For a clearer API, use a dedicated class:

package com.example.demo.dto;

public class AppInfo {
    private String application;
    private String version;
    private String status;

    public AppInfo(String application, String version, String status) {
        this.application = application;
        this.version = version;
        this.status = status;
    }

    // Getters and setters (required for JSON serialization)
    public String getApplication() { return application; }
    public void setApplication(String application) { this.application = application; }
    public String getVersion() { return version; }
    public void setVersion(String version) { this.version = version; }
    public String getStatus() { return status; }
    public void setStatus(String status) { this.status = status; }
}

Controller:

@GetMapping("/info")
public AppInfo info() {
    return new AppInfo("Demo", "1.0", "running");
}

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3.6 Configuration: application.properties

In src/main/resources/application.properties you can override defaults:

# Server
server.port=8080

# Application name (used in Actuator, logging)
spring.application.name=demo

# Custom property (we can inject this later)
app.welcome.message=Welcome to our API!

Change server.port, restart, and verify the app listens on the new port.

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3.7 Summary

• Main class: @SpringBootApplication + SpringApplication.run(...).
• REST endpoint: @RestController + @GetMapping (or other @*Mapping) methods.
• JSON: Return objects; Spring uses Jackson to serialize to JSON.
• Config: application.properties in src/main/resources.

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Exercises

1. Add a GET endpoint /greet?name=YourName that returns "Hello, YourName!".
2. Create a small DTO (e.g. Product with id, name, price) and an endpoint that returns one product as JSON.
3. Move the welcome message to application.properties and inject it into a controller to return it from an endpoint.