application.properties và application.yml là gì? Externalized config hoạt động thế nào?

File config externalize settings ra ngoài code — environment-specific values (DB URL, port, log level...) thay đổi không cần rebuild.

Hai format:

# application.properties
server.port=8080
spring.datasource.url=jdbc:postgresql://localhost/db

# application.yml — hierarchical, cleaner cho nested config
server:
  port: 8080
spring:
  datasource:
    url: jdbc:postgresql://localhost/db

Truy cập trong code:

@Value("${server.port}")
private int port;

// Recommended — typed binding
@ConfigurationProperties(prefix = "app")
record AppProps(String name, int maxRetries) {}

Profiles: application-dev.yml, application-prod.yml → activate qua spring.profiles.active=prod. Deploy 1 JAR cho mọi env.

Thứ tự ưu tiên (cao → thấp): command-line args → env vars → application-{profile}.yml → application.yml. Override dễ ngoài JAR mà không sửa source.

Secrets không nên commit: dùng env vars, Vault, AWS Secrets Manager, hoặc Spring Cloud Config server.

Config files externalize settings outside code — environment-specific values (DB URLs, ports, logging) change without rebuilding.

Two formats:

# application.properties
server.port=8080
spring.datasource.url=jdbc:postgresql://localhost/db

# application.yml — hierarchical, cleaner for nested config
server:
  port: 8080
spring:
  datasource:
    url: jdbc:postgresql://localhost/db

Reading them in code:

@Value("${server.port}")
private int port;

// Recommended — typed binding
@ConfigurationProperties(prefix = "app")
record AppProps(String name, int maxRetries) {}

Profiles: application-dev.yml, application-prod.yml → activate via spring.profiles.active=prod. Deploy a single JAR across environments.

Precedence (highest → lowest): command-line args → env vars → application-{profile}.yml → application.yml. Easy to override outside the JAR without changing source.

Secrets should not be committed: use env vars, Vault, AWS Secrets Manager, or Spring Cloud Config server.