application.properties และ YAML

Spring Boot reads externalized configuration from application.properties or application.yml, placed under src/main/resources. This keeps tunable values out of your compiled code.

At startup Boot loads these files into the Environment, making every key available for injection, conditional beans, and auto-configuration.

• One file for the whole app, or split per profile
• Values can be overridden later by env vars or CLI args

The classic format is a flat list of key=value lines. Nested concepts are expressed with dotted keys.

It is simple and diff-friendly, but verbose when you have deep structures because each key repeats the full path.

application.yml expresses the same data as a hierarchy, so shared prefixes are written once. It is easier to read for deeply nested config like spring.jpa.properties.hibernate.*.

Indentation is significant: use spaces, never tabs, and keep two spaces per level.

YAML shines for collections. A sequence uses - items; a map is just nested keys. Spring binds these directly to List and Map properties.

You can interpolate one property into another with ${...}. This avoids repeating literals and lets a single base value drive several derived keys.

Inside a placeholder you can supply a fallback after a colon: ${KEY:default}. Spring uses the default when the referenced property or environment variable is absent.

For one-off properties, inject directly with @Value. SpEL placeholders resolve against the Environment at bean creation time.

Use @Value for a handful of keys; prefer @ConfigurationProperties when a feature has many related settings.

Functionally they are interchangeable; the choice is about ergonomics.

• .properties — trivial parsing, great for small/flat config, no indentation traps
• .yml — concise for nested trees, native lists/maps, but indentation-sensitive

If both files exist, .properties takes precedence over .yml for the same key, which can surprise teams. Pick one convention.

A single YAML file can hold several documents separated by ---. Combined with spring.config.activate.on-profile, this lets you keep common and profile-specific values together.

Spring converts string values into rich types. Duration accepts suffixes like 30s or 5m, and DataSize accepts 10MB. Enums and List are also converted automatically.

Checked-in application.yml is the wrong home for passwords and API keys. Keep secrets in environment variables, a vault, or a secrets manager, and reference them via placeholders.

• Commit non-sensitive defaults only
• Inject secrets at deploy time

Test your understanding of property precedence.

You learned how Spring Boot externalizes configuration.

• application.properties and application.yml both feed the Environment
• YAML is concise for nested trees and native lists/maps
• Use ${...} for references and ${KEY:default} for fallbacks
• Spring converts to types like Duration and DataSize
• Keep secrets out of committed files