Spring Boot Basics For Day 1 Leaners

1. Creating a Spring Boot Project

Created a Spring Boot project using Spring Initializr.

Configuration:

• Project: Maven

• Language: Java

• Spring Boot: 4.1.0

• Group: com.test

• Artifact: demo

• Package: com.test.demo

• Java: 21

• Dependency: Spring Web

Generated project structure:

src
 ├── main
 │   ├── java
 │   │   └── com.test.demo
 │   │       └── DemoApplication.java
 │   └── resources
 │       └── application.properties
 └── test

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2. Running the Application

Start the application:

./mvnw spring-boot:run

If successful, Spring Boot starts an embedded web server.

Default URL:

http://localhost:8080

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3. Basic Java Object Creation

Class:

public class TestClass {

    public String test() {
        return "Test";
    }
}

Creating an object and calling a method:

String value = new TestClass().test();

System.out.println(value);

Output:

Test

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4. Printing to Console

Inside main():

System.out.println("hello");

Output:

hello

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5. Creating a Simple REST Endpoint

Controller:

@RestController
public class TestClass {

    @RequestMapping("/")
    public String test() {
        return "Test";
    }
}

Access:

http://localhost:8080/

Response:

Test

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6. What @RestController Does

@RestController

Marks a class as a web controller.

Methods inside it can handle HTTP requests and return responses.

Example:

@RequestMapping("/")
public String test() {
    return "Test";
}

When a browser visits /, this method executes.

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7. Understanding the Root Path

@RequestMapping("/")

/ means the root URL.

Examples:

/          -> homepage
/users     -> users endpoint
/products  -> products endpoint

Example:

@RequestMapping("/hello")
public String hello() {
    return "Hello";
}

URL:

http://localhost:8080/hello

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8. application.properties

Location:

src/main/resources/application.properties

Used for application configuration.

Example:

server.port=9090

Now the application runs on:

http://localhost:9090

instead of:

http://localhost:8080

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9. Important Files

DemoApplication.java

Application entry point.

@SpringBootApplication
public class DemoApplication {

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

application.properties

Stores application configuration.

pom.xml

Stores project dependencies and build configuration.

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10. Commands Learned

Run application:

./mvnw spring-boot:run

Build project:

./mvnw clean package

Run tests:

./mvnw test

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Summary

Learned:

• Create a Spring Boot project

• Run a Spring Boot application

• Create Java objects using new

• Call methods on objects

• Print output using System.out.println()

• Create a basic REST endpoint

• Use @RestController

• Use @RequestMapping

• Access endpoints from the browser

• Change server port using application.properties

• Understand basic project structure