Application Properties

🔍 What Is It?

Spring Boot's externalized configuration system lets you configure your application without changing code.

Configuration sources (in priority order, highest first):

1. Command-line arguments (--server.port=9090)
2. SPRING_APPLICATION_JSON environment variable
3. OS environment variables (SERVER_PORT=9090)
4. application-{profile}.properties (profile-specific)
5. application.properties / application.yml
6. @PropertySource annotations
7. Default values in code

Configuration formats:

• →application.properties — key=value pairs (simpler)
• →application.yml — YAML (hierarchical, less repetition)

Binding approaches:

• →@Value("${property.name}") — inject single value
• →@ConfigurationProperties(prefix="app") — bind a group of properties to a class (preferred)

🎯 Why Do We Need It?

Without externalized configuration:

• →Database URLs would be hardcoded in source code
• →The same JAR couldn't run in dev, staging, and production
• →Changing a port would require recompilation
• →Secrets would be committed to Git

With Spring Boot's config system:

• →One JAR runs in all environments — only config changes
• →Secrets come from environment variables or Vault at runtime
• →Profiles activate environment-specific config automatically
• →Teams can change behavior without touching code

🏗️ What Happens in Real Projects?

In production microservices:

• →application.properties contains non-sensitive defaults
• →application-prod.properties overrides for production
• →Database passwords come from environment variables (Kubernetes Secrets)
• →Feature flags come from a Config Server (Spring Cloud Config)
• →Logging levels changed at runtime via Actuator without restart

⚙️ How It Works

Property relaxed binding:

Spring Boot normalizes property names — these are all equivalent:

• →spring.datasource.url (properties)
• →SPRING_DATASOURCE_URL (env var)
• →--spring.datasource.url (command line)

@ConfigurationProperties binding:

Spring reads all properties with the given prefix and binds them to the class fields using relaxed binding. Supports nested objects, lists, and maps. Much safer than @Value — validates at startup, not at runtime.

💻 Example

Configuration in Practice — From simple properties to type-safe configuration classes.

spring:
  application:
    name: order-service
  datasource:
    url: jdbc:postgresql://localhost:5432/orders
    username: postgres
    password: ${DB_PASSWORD}
    hikari:
      maximum-pool-size: 20
      minimum-idle: 5
      connection-timeout: 30000
  jpa:
    hibernate:
      ddl-auto: validate
    show-sql: false

server:
  port: 8080
  shutdown: graceful

app:
  jwt:
    secret: ${JWT_SECRET}
    expiration-ms: 86400000
  cors:
    allowed-origins:
      - https://app.company.com
  features:
    new-checkout: false

🤔 Common Confusions

🚨 Common Production Issues

✅ Best Practices

🏛️ Architect's Note

💬 Frequently Asked Questions

🎯 Key Takeaways

🔗 Related Topics

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