Spring Boot application.properties Explained — A Complete Beginner's Guide

# ── Server Configuration ──────────────────────────────────────────
# Tell the embedded Tomcat server to listen on port 8081
# (default is 8080, but 8080 is often already taken on dev machines)
server.port=8081

# ── Application Identity ──────────────────────────────────────────
# A human-readable name for your application
# Spring Boot displays this in startup logs
spring.application.name=bookstore-api

# ── Database Connection ───────────────────────────────────────────
# The JDBC URL tells Spring where your database lives
spring.datasource.url=jdbc:postgresql://localhost:5432/bookstore_db

# The database username (never use 'root' in production)
spring.datasource.username=bookstore_user

# The database password
# We'll learn how to keep this out of source control shortly
spring.datasource.password=securepassword123

# ── JPA / Hibernate ───────────────────────────────────────────────
# Print every SQL statement to the console — great for debugging,
# turn this OFF in production (it floods your logs)
spring.jpa.show-sql=true

# ── Logging Level ─────────────────────────────────────────────────
# Set the log level for your own packages to DEBUG so you can see
# detailed output while you're developing
logging.level.com.thecodeforge.bookstore=DEBUG

// ─────────────────────────────────────────────────────────────────────────────
// FILE 1: application.properties (in src/main/resources)
// ─────────────────────────────────────────────────────────────────────────────
//
// bookstore.page-size=20
// bookstore.currency=GBP
// bookstore.payment.api-url=https://api.payments.example.com/v2
// bookstore.payment.timeout-seconds=30
//
// ─────────────────────────────────────────────────────────────────────────────
// FILE 2: BookstoreProperties.java
// Using @ConfigurationProperties to map a whole group of related properties
// onto a single, type-safe Java object. This is the recommended approach
// for anything more than a single isolated value.
// ─────────────────────────────────────────────────────────────────────────────

package com.thecodeforge.bookstore.config;

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;

// @ConfigurationProperties(prefix = "bookstore") means:
// "Go find every property that starts with 'bookstore.' and map it onto
//  the fields of this class."  The prefix is stripped — so 'bookstore.currency'
// maps to the field named 'currency'.
@Component
@ConfigurationProperties(prefix = "bookstore")
public class BookstoreProperties {

    // Maps to bookstore.page-size=20 in application.properties
    // Spring Boot auto-converts kebab-case (page-size) to camelCase (pageSize)
    private int pageSize;

    // Maps to bookstore.currency=GBP
    private String currency;

    // Spring Boot maps nested properties to nested objects automatically.
    // bookstore.payment.api-url maps to payment.apiUrl below.
    private Payment payment = new Payment();

    // ── Getters and setters are REQUIRED for @ConfigurationProperties ─────────
    // Spring uses them to inject the values. Lombok's @Data works too.
    public int getPageSize() { return pageSize; }
    public void setPageSize(int pageSize) { this.pageSize = pageSize; }

    public String getCurrency() { return currency; }
    public void setCurrency(String currency) { this.currency = currency; }

    public Payment getPayment() { return payment; }
    public void setPayment(Payment payment) { this.payment = payment; }

    // Nested static class for the 'bookstore.payment.*' group of properties
    public static class Payment {
        private String apiUrl;
        private int timeoutSeconds;

        public String getApiUrl() { return apiUrl; }
        public void setApiUrl(String apiUrl) { this.apiUrl = apiUrl; }

        public int getTimeoutSeconds() { return timeoutSeconds; }
        public void setTimeoutSeconds(int timeoutSeconds) { this.timeoutSeconds = timeoutSeconds; }
    }
}

// ─────────────────────────────────────────────────────────────────────────────
// FILE 3: BookSearchService.java
// Demonstrates BOTH @Value (for a single property) and injecting
// the BookstoreProperties object (for the full group)
// ─────────────────────────────────────────────────────────────────────────────

package com.thecodeforge.bookstore.service;

import com.thecodeforge.bookstore.config.BookstoreProperties;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;

@Service
public class BookSearchService {

    // @Value injects a single property directly.
    // The ":GBP" part is a DEFAULT VALUE — if 'bookstore.currency' is missing
    // from application.properties, Spring uses "GBP" instead of crashing.
    @Value("${bookstore.currency:GBP}")
    private String currency;

    // Injecting the whole BookstoreProperties object via constructor injection
    // (preferred over field injection — easier to test)
    private final BookstoreProperties bookstoreProperties;

    public BookSearchService(BookstoreProperties bookstoreProperties) {
        this.bookstoreProperties = bookstoreProperties;
    }

    public void printConfiguration() {
        System.out.println("=== Bookstore Configuration ===");
        System.out.println("Currency     : " + currency);
        System.out.println("Page size    : " + bookstoreProperties.getPageSize());
        System.out.println("Payment URL  : " + bookstoreProperties.getPayment().getApiUrl());
        System.out.println("Timeout (sec): " + bookstoreProperties.getPayment().getTimeoutSeconds());
    }
}

# ─────────────────────────────────────────────────────────────────────────────
# FILE 1: application.properties  (the shared baseline — every environment uses this)
# ─────────────────────────────────────────────────────────────────────────────
#
# spring.application.name=bookstore-api
# bookstore.page-size=20
# bookstore.currency=GBP
#
# Set which profile is active during LOCAL development.
# In CI/CD pipelines and production servers, NEVER set this here.
# Use an environment variable instead: SPRING_PROFILES_ACTIVE=prod
# spring.profiles.active=dev
#

# ─────────────────────────────────────────────────────────────────────────────
# FILE 2: application-dev.properties  (only loaded when profile 'dev' is active)
# ─────────────────────────────────────────────────────────────────────────────
#
# Local developer machine settings
#
# server.port=8081
# spring.datasource.url=jdbc:h2:mem:bookstore_dev_db
# spring.datasource.username=sa
# spring.datasource.password=
# spring.jpa.show-sql=true
# logging.level.com.thecodeforge.bookstore=DEBUG
#

# ─────────────────────────────────────────────────────────────────────────────
# FILE 3: application-prod.properties  (only loaded when profile 'prod' is active)
# ─────────────────────────────────────────────────────────────────────────────

# Production runs on the standard HTTPS port
server.port=443

# Real PostgreSQL database in the cloud.
# Notice the URL, username and password are DIFFERENT from dev.
spring.datasource.url=jdbc:postgresql://prod-db.us-east-1.rds.amazonaws.com:5432/bookstore_prod
spring.datasource.username=bookstore_prod_user
# In real production you'd reference an environment variable here:
# spring.datasource.password=${DB_PASSWORD}
spring.datasource.password=${DB_PASSWORD}

# Turn OFF SQL logging in production — it's a security risk and noise
spring.jpa.show-sql=false

# Only show WARN and ERROR in production logs — DEBUG is too verbose
logging.level.com.thecodeforge.bookstore=WARN
logging.level.root=WARN

# ─────────────────────────────────────────────────────────────────────────────
# SAME CONFIGURATION as the application.properties examples above,
# but written in YAML format.
# ─────────────────────────────────────────────────────────────────────────────

# Notice how hierarchy replaces repeated prefixes.
# 'spring.datasource.url' becomes:
#   spring:
#     datasource:
#       url: ...
# Each level of indentation = one dot in the properties key.

spring:
  application:
    name: bookstore-api          # Same as: spring.application.name=bookstore-api

  datasource:
    url: jdbc:postgresql://localhost:5432/bookstore_db
    username: bookstore_user
    password: securepassword123

  jpa:
    show-sql: true               # Same as: spring.jpa.show-sql=true

server:
  port: 8081                     # Same as: server.port=8081

bookstore:
  page-size: 20                  # Same as: bookstore.page-size=20
  currency: GBP
  payment:
    api-url: https://api.payments.example.com/v2
    timeout-seconds: 30

logging:
  level:
    com.thecodeforge.bookstore: DEBUG

# ─────────────────────────────────────────────────────────────────────────────
# Profile-specific YAML blocks — bonus YAML-only feature!
# You can define multiple profiles in ONE yml file using '---' as a separator.
# This is impossible with .properties files (each profile needs its own file).
# ─────────────────────────────────────────────────────────────────────────────
---
spring:
  config:
    activate:
      on-profile: prod           # Everything below only applies to 'prod' profile

  datasource:
    url: jdbc:postgresql://prod-db.amazonaws.com:5432/bookstore_prod
    password: ${DB_PASSWORD}     # Read from environment variable

server:
  port: 443

logging:
  level:
    root: WARN