Spring Boot Application Properties

Application properties are the configuration settings for your Spring Boot app. They control the server port, database connections, logging level, and much more — all from a single file without touching Java code.

Where the File Lives

src/
└── main/
    └── resources/
        └── application.properties   ← This file

Spring Boot automatically loads this file at startup. Every setting you place here overrides the built-in defaults.

Basic Syntax

key=value

# Examples
server.port=9090
spring.application.name=my-app
logging.level.root=INFO

YAML Alternative

You can use application.yml instead. YAML uses indentation instead of dots to create hierarchy:

Properties format:               YAML format:
──────────────────────           ──────────────────────
server.port=9090            →    server:
spring.application.name=app →      port: 9090
                                 spring:
                                   application:
                                     name: app

Both files do the same job. Use whichever format your team prefers. You cannot use both in the same project — pick one.

Most Commonly Used Properties

Server Settings

server.port=8080                      # HTTP port
server.servlet.context-path=/api      # App runs at /api/*
server.error.include-message=always   # Show error messages

Datasource Settings

spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=secret
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver

JPA Settings

spring.jpa.hibernate.ddl-auto=update    # auto-create/update tables
spring.jpa.show-sql=true                # Print SQL to console
spring.jpa.properties.hibernate.format_sql=true

Logging Settings

logging.level.root=INFO
logging.level.com.estudy247=DEBUG       # Debug only your package
logging.file.name=app.log              # Write logs to a file

Custom Properties

Define your own keys and read them in Java using @Value:

# In application.properties
app.max-upload-size=10MB
app.admin-email=admin@estudy247.com

// In Java
@Component
public class FileUploadService {

    @Value("${app.max-upload-size}")
    private String maxUploadSize;

    @Value("${app.admin-email}")
    private String adminEmail;
}

Binding Multiple Properties to a Class

For a group of related properties, use @ConfigurationProperties instead of multiple @Value annotations:

# application.properties
app.mail.host=smtp.gmail.com
app.mail.port=587
app.mail.username=noreply@estudy247.com

@Component
@ConfigurationProperties(prefix = "app.mail")
public class MailProperties {

    private String host;
    private int port;
    private String username;

    // Getters and setters
}

Spring maps each property to the matching field automatically. You get a single typed object instead of scattered @Value annotations.

Property Priority Diagram

Highest priority (wins)
        │
        ▼
1. Command-line arguments       java -jar app.jar --server.port=9090
2. Environment variables        SERVER_PORT=9090
3. application-{profile}.properties
4. application.properties       ← Default file
        │
        ▼
Lowest priority (can be overridden)

This means you can override any setting at deployment time using environment variables — without changing any code or config files.

Hiding Sensitive Values

Never commit database passwords or API keys to version control. Use environment variables or a secrets manager instead:

# application.properties (safe to commit)
spring.datasource.password=${DB_PASSWORD}

# Your server environment (never commit this)
export DB_PASSWORD=actual_secret_password

Summary

• application.properties or application.yml controls all app settings
• Use @Value to read a single custom property in Java
• Use @ConfigurationProperties to bind a group of related properties to a class
• Environment variables and command-line args override file-based properties
• Keep secrets in environment variables — never commit them to code