Application.properties basics in Spring Boot

Introduction

The application.properties file helps you set up your Spring Boot app's settings in one place. It makes changing app behavior easy without touching code.

You want to set the server port number for your app.

You need to configure database connection details like URL and username.

You want to enable or disable debug logging.

You want to add custom settings your app can read.

You want to change email or API keys without changing code.

Syntax

Spring Boot

property.name=value

Each line sets one property with a name and a value separated by =.

Use dot notation to organize properties, like server.port=8080.

Examples

Sets the app to run on port 8080.

Spring Boot

server.port=8080

Configures the database connection URL.

Spring Boot

spring.datasource.url=jdbc:mysql://localhost:3306/mydb

Turns on debug logging for Spring framework classes.

Spring Boot

logging.level.org.springframework=DEBUG

Defines a custom property your app can read.

Spring Boot

app.custom.message=Hello from properties!

Sample Program

This Spring Boot app reads a custom message from application.properties and shows it at the home page.

Spring Boot

package com.example.demo;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

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

@RestController
class HelloController {

    @Value("${app.custom.message}")
    private String message;

    @GetMapping("/")
    public String hello() {
        return message;
    }
}

Important Notes

Remember to place application.properties in src/main/resources folder.

Changes in application.properties require app restart to take effect.

You can override properties with environment variables or command line args.

Summary

application.properties stores app settings in key=value lines.

It helps configure ports, databases, logging, and custom values.

Spring Boot reads these properties automatically when the app starts.