Spring Boot 4 Configuration Properties

Spring Boot 4 baseline

The examples use Java 17 and Jakarta APIs supported by this Boot version. Keep dependencies managed by the project's Boot BOM.

Bind and validate

Inspect existing property prefixes and external deployment configuration before renaming keys. Use a typed ConfigurationProperties record for a related group of settings. Register it with EnableConfigurationProperties or ConfigurationPropertiesScan; a record is not automatically a bean. Use Validated and Jakarta constraints, and include spring-boot-starter-validation. Validate nested objects with Valid. A single record constructor needs no ConstructorBinding annotation.

Use explicit units for durations and sizes. Provide defaults only when they are operationally safe; missing credentials or required endpoints should fail startup. Do not assume NotNull rejects zero or negative Duration values; add a duration constraint or an explicit invariant.

ClientProperties and its configuration show a validated prefix with explicit duration units. Do not turn a full API token into a record component: generated toString can expose secrets. Resolve secrets at the integration boundary or use a redacting value type.

Deployment and overrides

Document the public property keys and validate actual environment-variable binding in a test. Use environment variables or a mounted secret/config tree according to the deployment platform. Higher-precedence property sources can override packaged defaults; test the intended profile and override path. Avoid depending on YAML list merging: lists are generally replaced. Keep profile activation outside documents activated by that same profile.

Do not publish configprops/env actuator endpoints publicly or disable sanitization to debug. Binding is normally at startup; changing a file or secret does not imply live refresh. Define restart/rotation behavior when configuration changes.

Secret rotation and exposure checks

Document who owns each secret, how consumers reload it, and when the old credential is revoked. Configuration binding at startup does not automatically refresh a connection pool, HTTP client, signing key or cached credential. Test the real restart/reload path before a rotation rollout. Where a provider supports overlapping keys, keep the overlap bounded and verify that consumers accept the new key before removing the old one. A compromised key may require immediate revocation through the incident owner rather than a normal overlap window.

Check config files, build output, container layers and CI logs for accidental credential exposure. Use synthetic canary values in tests, never production secrets. Test that binding failures, generated toString methods and startup logs do not echo credentials. If exposure is found, coordinate revocation and evidence handling; deleting a file does not invalidate its credential. Do not publish secret-bearing diagnostics or rewrite repository history without approval.

Verification

Use ApplicationContextRunner with EnableConfigurationProperties to assert valid binding and startup failure on missing or invalid settings. Test nested validation, unit conversion and the override path you actually deploy. Include at least one invalid duration test.

Gotchas

• Agent defines a properties record but never registers it - enable scanning or explicit registration.
• Agent adds Validated without a validation provider - include the validation starter.
• Agent uses a bare numeric timeout - use explicit units.
• Agent adds a default credential - fail fast and inject secrets externally.
• Agent logs the full bound configuration - redact secret-bearing values.

Official sources

• Boot external configuration
• Configuration property validation

// BAD: misspelled keys go unnoticed, units are unclear, and the token has a default.
class ClientSettings {
    @Value("${app.client.timout:2000}")
    String timeout;

    @Value("${app.client.token:development-secret}")
    String token;
}

package com.example.config;

import java.net.URI;
import java.time.Duration;
import jakarta.validation.constraints.Max;
import jakarta.validation.constraints.Min;
import jakarta.validation.constraints.NotNull;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.validation.annotation.Validated;

@Validated
@ConfigurationProperties("app.client")
public record ClientProperties(@NotNull URI baseUrl, @NotNull Duration timeout,
        @Min(1) @Max(5) int maxAttempts) {
    public ClientProperties {
        if (timeout != null && (timeout.isZero() || timeout.isNegative())) {
            throw new IllegalArgumentException("app.client.timeout must be positive");
        }
    }
}

app:
  client:
    base-url: https://orders.example.com
    timeout: 2s
    max-attempts: 3

claude plugin marketplace add rrezartprebreza/spring-boot-skills
claude plugin install spring-boot-4-skills@spring-boot-skills

codex plugin marketplace add rrezartprebreza/spring-boot-skills
codex plugin add spring-boot-4-skills@spring-boot-skills