Java Springboot

Configuration

Formats, profiles, .env and secret basics, and the short @ConfigurationProperties example already live in create-spring-boot-java-project (references/configuration.md); read it for those and keep one format (properties or YAML) as the project already does. This file adds what is needed when changing an existing application. Verified on Boot 4.1.1.

Type-safe properties (records)

@ConfigurationProperties(prefix = "app.mail")
@Validated
public record MailProperties(@NotBlank String from, @DefaultValue("3") @Positive int retries,
		@DefaultValue("5s") Duration timeout) {}

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• Records bind through their canonical constructor (constructor binding is the default for records). Use @DefaultValue (org.springframework.boot.context.properties.bind.DefaultValue) for optional values; Duration, DataSize, enums and List/Map bind natively (5s, PT5S, 10MB).

• Register with @ConfigurationPropertiesScan on the application class (or @EnableConfigurationProperties(MailProperties.class)). Verified: the record is a bean with retries == 3 and timeout == 5s when only from is set.

• @Validated + jakarta.validation constraints make the application fail at startup on invalid configuration, which is what you want. Validation needs the validation starter (spring-boot-starter-validation).

• Prefer @ConfigurationProperties to scattered @Value; use @Value only for one-off values. Add spring-boot-configuration-processor (optional/annotation-processor path, as start.spring.io does) for IDE metadata.

• Immutability: no setters; nested config as nested records.

• Inject the record where needed; do not read Environment directly in business code.

Profiles

• Keep production-safe defaults in application.properties; profile files (application-dev.properties) only override what differs. Activate with SPRING_PROFILES_ACTIVE.

• Group related profiles with spring.profiles.group.prod=prod-db,prod-observability when a deployment needs several.

• Use @Profile sparingly for beans; do not branch application logic on profile names.

• Do not ship a test profile in src/main; tests use src/test/resources or @TestPropertySource (see spring-boot-testing).

Config import and secrets

spring.config.import=optional:file:.env[.properties],optional:configtree:/run/secrets/

• optional: prevents startup failure when the file is absent (local development); drop it for values that must exist in production.

• Kubernetes/Docker secrets mounted as files: configtree: maps each file to a property (file name = key).

• Environment variables override files through relaxed binding (APP_MAIL_FROM -> app.mail.from).

• Never commit .env or real credentials; commit .env.sample. Do not log property values holding secrets, and do not expose /actuator/env and /actuator/configprops publicly (spring-observability, spring-security).

• Externalized config sources like Vault or Spring Cloud Config are out of scope unless the project already uses them; follow its existing import setup.

Dev-time services

Docker Compose

Add spring-boot-docker-compose (start.spring.io adds it, optional/development-only). With a compose.yaml at the project root, ./mvnw spring-boot:run starts the services and wires connection details automatically. Pin image tags (start.spring.io generates postgres:latest; replace it, see create-spring-boot-java-project references/database.md). Not executed in this skill's verification: only the generated files were inspected.

Testcontainers at dev time

start.spring.io (with the testcontainers dependency) generates TestcontainersConfiguration and a TestDemoApplication in src/test:

public class TestDemoApplication {
	public static void main(String[] args) {
		SpringApplication.from(DemoApplication::main).with(TestcontainersConfiguration.class).run(args);
	}
}

Run it with ./mvnw spring-boot:test-run (Gradle: ./gradlew bootTestRun). Verified on Boot 4.1.1: PostgreSQL and Kafka containers started from the @ServiceConnection beans, the app reached UP on /actuator/health, and stopping the app removed the containers (Ryuk). Make TestcontainersConfiguration public if it is used from other packages (see create-spring-boot-java-project references/spring-boot-4.md). Choose either Compose or Testcontainers for dev-time services, not both, to avoid two databases fighting over the same properties.

Boot 3 vs 4

Same property binding rules. Boot 4 changes that touch configuration: Jackson properties continue under spring.jackson.* for Jackson 3 (Jackson 2 compatibility lives in a separate module; consult the migration guide), modular auto-configuration (classes moved to org.springframework.boot.<technology>.autoconfigure, which matters only if you reference or exclude auto-configurations by class name: spring.autoconfigure.exclude=org.springframework.boot.jdbc.autoconfigure.DataSourceAutoConfiguration). Use the spring-boot-4-migration skill for the move itself.