Environment variables in configuration in Spring Boot

Introduction

Environment variables let you set values outside your code. This helps keep your app flexible and secure.

You want to change database passwords without changing code.

You need to set different settings for development and production.

You want to keep API keys secret and not in your code.

You want to configure ports or URLs depending on where the app runs.

Syntax

Spring Boot

spring.datasource.url=${DB_URL}
spring.datasource.username=${DB_USER}
spring.datasource.password=${DB_PASS}

Use ${VARIABLE_NAME} to refer to environment variables in application.properties or application.yml.

If the environment variable is missing, Spring Boot may fail to start or use a default if provided.

Examples

Basic example using environment variables for database connection.

Spring Boot

spring.datasource.url=${DB_URL}
spring.datasource.username=${DB_USER}
spring.datasource.password=${DB_PASS}

Use :8080 to provide a default value if PORT is not set.

Spring Boot

server.port=${PORT:8080}

Set app name from environment or use 'MyApp' as default.

Spring Boot

app.name=${APP_NAME:MyApp}

Sample Program

This Spring Boot app reads an environment variable APP_GREETING to show a greeting message. If the variable is not set, it shows 'Hello, World!'.

Run the app and visit /greet to see the message.

Spring Boot

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.beans.factory.annotation.Value;
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 ConfigController {

    @Value("${APP_GREETING:Hello, World!}")
    private String greeting;

    @GetMapping("/greet")
    public String greet() {
        return greeting;
    }
}

Important Notes

Environment variables are case-sensitive on most systems.

Use application.yml for cleaner configuration if preferred.

Remember to set environment variables before starting your Spring Boot app.

Summary

Environment variables let you configure apps without changing code.

Use ${VAR_NAME} syntax in Spring Boot config files.

You can provide default values with : after the variable name.