Unit Test Config Properties Agent Skill

Unit Testing Configuration Properties and Profiles

Overview

This skill provides patterns for unit testing @ConfigurationProperties bindings, environment-specific configurations, and property validation using JUnit 5. Covers testing property name mapping, type conversions, validation constraints, nested structures, and profile-specific configurations without full Spring context startup.

Key validation checkpoints:

• Property prefix matches between @ConfigurationProperties and test properties
• Validation triggers on @Validated classes with invalid values
• Type conversions work for Duration, DataSize, collections, and maps

When to Use

• Testing @ConfigurationProperties property binding
• Testing property name mapping and type conversions
• Validating configuration with @NotBlank, @Min, @Max, @Email constraints
• Testing environment-specific configurations (dev, prod)
• Testing nested property structures and collections
• Verifying default values when properties are not specified
• Fast configuration tests without Spring context startup

Instructions

• Set up test dependencies: Add spring-boot-starter-test and AssertJ dependencies
• Use ApplicationContextRunner: Test property bindings without starting full Spring context
• Define property prefixes: Ensure @ConfigurationProperties(prefix = "...") matches test property paths
• Test all property paths: Verify each property including nested structures and collections
• Test validation constraints: Use context.hasFailed() to verify @Validated properties reject invalid values
• Test type conversions: Verify Duration (30s), DataSize (50MB), collections, and maps convert correctly
• Test default values: Verify properties have correct defaults when not specified in test properties
• Test profile-specific configs: Use @Profile with ApplicationContextRunner for environment-specific configurations
• Test edge cases: Include empty strings, null values, and type mismatches

Troubleshooting flow:

• If properties don't bind → Check prefix matches (kebab-case to camelCase conversion)
• If validation doesn't trigger → Verify @Validated annotation is present
• If context fails to start → Check dependencies and @ConfigurationProperties class structure

Examples

Setup: Test Dependencies

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-configuration-processor</artifactId>
  <scope>provided</scope>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.assertj</groupId>
  <artifactId>assertj-core</artifactId>
  <scope>test</scope>
</dependency>

Basic Pattern: Property Binding

@ConfigurationProperties(prefix = "app.security")
@Data
public class SecurityProperties {
  private String jwtSecret;
  private long jwtExpirationMs;
  private int maxLoginAttempts;
  private boolean enableTwoFactor;
}

class SecurityPropertiesTest {

  @Test
  void shouldBindPropertiesFromEnvironment() {
    new ApplicationContextRunner()
      .withPropertyValues(
        "app.security.jwtSecret=my-secret-key",
        "app.security.jwtExpirationMs=3600000",
        "app.security.maxLoginAttempts=5",
        "app.security.enableTwoFactor=true"
      )
      .withBean(SecurityProperties.class)
      .run(context -> {
        SecurityProperties props = context.getBean(SecurityProperties.class);
        assertThat(props.getJwtSecret()).isEqualTo("my-secret-key");
        assertThat(props.getJwtExpirationMs()).isEqualTo(3600000L);
        assertThat(props.getMaxLoginAttempts()).isEqualTo(5);
        assertThat(props.isEnableTwoFactor()).isTrue();
      });
  }

  @Test
  void shouldUseDefaultValuesWhenPropertiesNotProvided() {
    new ApplicationContextRunner()
      .withPropertyValues("app.security.jwtSecret=key")
      .withBean(SecurityProperties.class)
      .run(context -> {
        SecurityProperties props = context.getBean(SecurityProperties.class);
        assertThat(props.getJwtSecret()).isEqualTo("key");
        assertThat(props.getMaxLoginAttempts()).isZero();
      });
  }
}

Validation Testing

@ConfigurationProperties(prefix = "app.server")
@Data
@Validated
public class ServerProperties {
  @NotBlank
  private String host;

  @Min(1)
  @Max(65535)
  private int port = 8080;

  @Positive
  private int threadPoolSize;
}

class ConfigurationValidationTest {

  @Test
  void shouldFailValidationWhenHostIsBlank() {
    new ApplicationContextRunner()
      .withPropertyValues(
        "app.server.host=",
        "app.server.port=8080",
        "app.server.threadPoolSize=10"
      )
      .withBean(ServerProperties.class)
      .run(context -> {
        assertThat(context).hasFailed()
          .getFailure()
          .hasMessageContaining("host");
      });
  }

  @Test
  void shouldPassValidationWithValidConfiguration() {
    new ApplicationContextRunner()
      .withPropertyValues(
        "app.server.host=localhost",
        "app.server.port=8080",
        "app.server.threadPoolSize=10"
      )
      .withBean(ServerProperties.class)
      .run(context -> {
        assertThat(context).hasNotFailed();
        assertThat(context.getBean(ServerProperties.class).getHost()).isEqualTo("localhost");
      });
  }
}

Type Conversion Testing

@ConfigurationProperties(prefix = "app.features")
@Data
public class FeatureProperties {
  private Duration cacheExpiry = Duration.ofMinutes(10);
  private DataSize maxUploadSize = DataSize.ofMegabytes(100);
  private List<String> enabledFeatures;
  private Map<String, String> featureFlags;
}

class TypeConversionTest {

  @Test
  void shouldConvertDurationFromString() {
    new ApplicationContextRunner()
      .withPropertyValues("app.features.cacheExpiry=30s")
      .withBean(FeatureProperties.class)
      .run(context -> {
        assertThat(context.getBean(FeatureProperties.class).getCacheExpiry())
          .isEqualTo(Duration.ofSeconds(30));
      });
  }

  @Test
  void shouldConvertCommaDelimitedList() {
    new ApplicationContextRunner()
      .withPropertyValues("app.features.enabledFeatures=feature1,feature2")
      .withBean(FeatureProperties.class)
      .run(context -> {
        assertThat(context.getBean(FeatureProperties.class).getEnabledFeatures())
          .containsExactly("feature1", "feature2");
      });
  }
}

For nested properties, profile-specific configurations, collection binding, and advanced validation patterns, see references/advanced-examples.md.

Best Practices

• Test all property bindings including nested structures and collections
• Test validation constraints for all @NotBlank, @Min, @Max, @Email, @Positive annotations
• Test both default and custom values to verify fallback behavior
• Use ApplicationContextRunner for fast context-free testing
• Test profile-specific configurations separately with @Profile
• Verify type conversions for Duration, DataSize, collections, and maps
• Test edge cases: empty strings, null values, type mismatches, out-of-range values

Constraints and Warnings

• Kebab-case to camelCase: Property app.my-property maps to myProperty in Java
• Loose binding: Spring Boot uses loose binding by default; use strict binding if needed
• @Validated required: Add @Validated annotation to enable constraint validation
• @ConstructorBinding: All parameters must be bindable when using constructor binding
• List indexing: Use [0], [1] notation; ensure sequential indexing for lists
• Duration format: Accepts ISO-8601 (PT30S) or simple syntax (30s, 1m, 2h)
• Context isolation: Each ApplicationContextRunner creates a new context with no shared state
• Profile activation: Use spring.profiles.active=profileName in withPropertyValues() for profile tests

Troubleshooting

| Issue | Cause | Solution |

|-------|-------|----------|

| Properties not binding | Prefix mismatch | Verify @ConfigurationProperties(prefix="...") matches property paths |

| Validation not triggered | Missing @Validated | Add @Validated annotation to configuration class |

| Context fails to start | Missing dependencies | Ensure spring-boot-starter-test is in test scope |

| Nested properties null | Inner class missing | Use @Data on nested classes or provide getters/setters |

| Collection binding fails | Wrong indexing | Use [0], [1] notation, not (0), (1) |