unit-test-config-properties_skill

Overview

This skill provides concise, battle-tested patterns for unit testing Spring @ConfigurationProperties classes using ApplicationContextRunner and @ConfigurationPropertiesTest techniques. It helps validate property binding, type conversion, nested structures, defaults, validation constraints, and profile-specific wiring without starting a full Spring context. Use it to keep configuration tests fast, focused, and reliable.

How this skill works

The patterns use ApplicationContextRunner to bootstrap minimal contexts with explicit property values and beans, then assert bound properties or context failures. Tests cover simple bindings, nested objects and collections, type conversions (Duration, DataSize, Charset), validation via @Validated, default-value behavior, and profile-specific user configurations. Failures are asserted by checking context.hasFailed() and inspecting the failure message when relevant.

When to use it

• Verify @ConfigurationProperties binding and prefixes without loading the full application
• Assert type conversions for Duration, DataSize, lists, maps, and enums
• Validate @Validated constraints (@NotBlank, @Min, @Max, @Email, @Positive) on config classes
• Test nested properties, lists of sub-objects, and default values
• Confirm profile-specific beans or settings load correctly in prod/dev scenarios

Best practices

• Use ApplicationContextRunner.withPropertyValues(...) and .withBean(...) to isolate tests
• Cover all property paths including nested fields and indexed list entries
• Assert both success and failure cases for validation constraints
• Test default values by not providing properties and then by overriding them
• Include edge cases: empty strings, nulls, type mismatches and missing required props
• Keep tests focused and fast — each runner instance should validate one scenario

Example use cases

• Bind and assert security settings (jwtSecret, expiration, two-factor flag) from properties
• Validate server configuration fails when host is blank or port is out of range
• Verify nested database pool settings and a list of replica entries bind correctly
• Confirm Duration and DataSize strings convert to Java types as expected
• Load dev vs prod configuration via spring.profiles.active and verify bean values