Spring Boot in Action Skill

Apply the practices from Craig Walls' "Spring Boot in Action" to review existing code and write new Spring Boot applications. This skill operates in two modes: Review Mode (analyze code for violations of Spring Boot idioms) and Write Mode (produce clean, idiomatic Spring Boot from scratch).

The core philosophy: Spring Boot removes boilerplate through auto-configuration, starter dependencies, and sensible defaults. Fight the framework only when necessary — and when you do, prefer application.properties over code.

Reference Files

• practices-catalog.md — Before/after examples for auto-configuration, starters, properties, profiles, security, testing, Actuator, and deployment

How to Use This Skill

Before responding, read practices-catalog.md for the topic at hand. For configuration issues read the properties/profiles section. For test code read the testing section. For a full review, read all sections.

---

Mode 1: Code Review

When the user asks you to review Spring Boot code, follow this process:

Step 1: Identify the Layer

Determine whether the code is a controller, service, repository, configuration class, or test. Review focus shifts by layer.

Step 2: Analyze the Code

Check these areas in order of severity:

1. Auto-Configuration (Ch 2, 3): Is auto-configuration being fought manually? Look for @Bean definitions that replicate what Spring Boot already provides (DataSource, Jackson, Security, etc.). Remove manual config where auto-config suffices.

2. Starter Dependencies (Ch 2): Are dependencies declared individually instead of using starters? spring-boot-starter-web, spring-boot-starter-data-jpa, spring-boot-starter-security etc. bundle correct transitive dependencies and version-manage them.

3. Externalized Configuration (Ch 3): Are values hardcoded that belong in application.properties? Ports, URLs, credentials, timeouts should all be externalized. Use @ConfigurationProperties for type-safe config objects; use @Value only for single values.

4. Profiles (Ch 3): Is environment-specific config (dev DB vs prod DB) handled with if statements or system properties? Use @Profile and application-{profile}.properties instead.

5. Security (Ch 3): Is WebSecurityConfigurerAdapter extended when simple property-based config would suffice? Is HTTP Basic enabled in production? Are actuator endpoints exposed without auth?

6. Testing (Ch 4):

   • Use @SpringBootTest for full integration tests, not raw new MyService()
   • Use @WebMvcTest for controller-only tests (no full context)
   • Use @DataJpaTest for repository tests (in-memory DB, no web layer)
   • Use MockMvc for controller assertions without starting a server
   • Use @MockBean to replace real beans with mocks in slice tests
   • Avoid @SpringBootTest(webEnvironment = RANDOM_PORT) unless testing the full HTTP stack

7. Actuator (Ch 7): Is the application missing health/metrics endpoints? Is /actuator fully exposed without security? Are custom health indicators implemented for critical dependencies?

8. Deployment (Ch 8): Is spring.profiles.active set for production? Is database migration (Flyway/Liquibase) configured? Is the app packaged as a self-contained JAR (preferred) or WAR?

9. General Idioms:

   • Constructor injection over field injection (@Autowired on fields)
   • @RestController = @Controller + @ResponseBody — use it for REST APIs
   • Return ResponseEntity<T> from controllers when status codes matter
   • Optional<T> from repository methods, never null

Step 3: Report Findings

For each issue, report:

• Chapter reference (e.g., "Ch 3: Externalized Configuration")
• Location in the code
• What's wrong (the anti-pattern)
• How to fix it (the Spring Boot idiomatic way)
• Priority: Critical (security/bugs), Important (maintainability), Suggestion (polish)

Step 4: Provide Fixed Code

Offer a corrected version with comments explaining each change.

---

Mode 2: Writing New Code

When the user asks you to write new Spring Boot code, apply these core principles:

Project Bootstrap (Ch 1, 2)

1. Start with Spring Initializr (Ch 1). Use start.spring.io or spring init CLI. Select starters upfront — don't add raw dependencies manually.

2. Use starters, not individual dependencies (Ch 2). spring-boot-starter-web includes Tomcat, Spring MVC, Jackson, and logging at compatible versions. Never declare spring-webmvc + jackson-databind + tomcat-embed-core separately.

3. The main class is the only required boilerplate (Ch 2):

   @SpringBootApplication
   public class MyApp {
       public static void main(String[] args) {
           SpringApplication.run(MyApp.class, args);
       }
   }
   

   @SpringBootApplication = @Configuration + @EnableAutoConfiguration + @ComponentScan.

Configuration (Ch 3)

4. Externalize all environment-specific values (Ch 3). Nothing deployment-specific belongs in code. Use application.properties / application.yml for defaults.

5. Use @ConfigurationProperties for grouped config (Ch 3). Bind a prefix to a POJO — type-safe, IDE-friendly, testable:

   @ConfigurationProperties(prefix = "app.mail")
   @Component
   public class MailProperties {
       private String host;
       private int port = 25;
       // getters + setters
   }
   

6. Use profiles for environment differences (Ch 3). application-dev.properties overrides application.properties when spring.profiles.active=dev. Never use if (env.equals("production")) in code.

7. Override auto-configuration surgically (Ch 3). Use spring.* properties first. Only define a @Bean when properties are insufficient. Annotate with @ConditionalOnMissingBean if providing a fallback.

8. Customize error pages declaratively (Ch 3). Place error/404.html, error/500.html in src/main/resources/templates/error/. No custom ErrorController needed for basic cases.

Security (Ch 3)

9. Extend WebSecurityConfigurerAdapter only for custom rules (Ch 3). For simple HTTP Basic with custom users, spring.security.user.name / spring.security.user.password properties suffice.

10. Always secure Actuator endpoints in production (Ch 7). Expose only health and info publicly; require authentication for env, beans, mappings, shutdown.

REST Controllers (Ch 2)

11. Use @RestController for API endpoints (Ch 2). Eliminates @ResponseBody on every method.

12. Return ResponseEntity<T> when HTTP status matters (Ch 2). ResponseEntity.ok(body), ResponseEntity.notFound().build(), ResponseEntity.status(201).body(created).

13. Use constructor injection, not field injection (Ch 2). Constructor injection makes dependencies explicit and enables testing without Spring context:

    // Prefer this:
    @RestController
    public class BookController {
        private final BookRepository repo;
        public BookController(BookRepository repo) { this.repo = repo; }
    }
    

14. Use Optional from repository queries (Ch 2). repo.findById(id).orElseThrow(() -> new ResponseStatusException(NOT_FOUND)).

Testing (Ch 4)

15. Match test slice to the layer being tested (Ch 4):

    • Web layer only → @WebMvcTest(MyController.class) + MockMvc
    • Repository only → @DataJpaTest
    • Full app → @SpringBootTest
    • External service → @MockBean to replace

16. Use MockMvc for controller assertions without starting a server (Ch 4):

    mockMvc.perform(get("/books/1"))
           .andExpect(status().isOk())
           .andExpect(jsonPath("$.title").value("Spring Boot in Action"));
    

17. Use @MockBean to isolate the unit under test (Ch 4). Replaces the real bean in the Spring context with a Mockito mock — cleaner than manual wiring.

18. Test security explicitly (Ch 4). Use .with(user("admin").roles("ADMIN")) or @WithMockUser to assert secured endpoints reject unauthenticated requests.

Actuator (Ch 7)

19. Enable Actuator in every production app (Ch 7). Add spring-boot-starter-actuator. At minimum expose health and info.

20. Write custom HealthIndicator for critical dependencies (Ch 7):

    @Component
    public class DatabaseHealthIndicator implements HealthIndicator {
        @Override
        public Health health() {
            return canConnect() ? Health.up().build()
                                : Health.down().withDetail("reason", "timeout").build();
        }
    }
    

21. Add custom metrics via MeterRegistry (Ch 7). Counter, gauge, timer — gives Prometheus/Grafana visibility into business events.

22. Restrict Actuator exposure in production (Ch 7):

    management.endpoints.web.exposure.include=health,info
    management.endpoint.health.show-details=when-authorized
    

Deployment (Ch 8)

23. Package as an executable JAR by default (Ch 8). mvn package produces a fat JAR with embedded Tomcat. Run with java -jar app.jar. No application server needed.

24. Create a production profile (Ch 8). application-production.properties sets spring.datasource.url, disables dev tools, sets log levels to WARN.

25. Use Flyway or Liquibase for database migrations (Ch 8). Add spring-boot-starter-flyway; place scripts in classpath:db/migration/V1__init.sql. Never use spring.jpa.hibernate.ddl-auto=create in production.

---

Starter Cheat Sheet (Ch 2, Appendix B)

Need	Starter
REST API	spring-boot-starter-web
JPA / Hibernate	spring-boot-starter-data-jpa
Security	spring-boot-starter-security
Observability	spring-boot-starter-actuator
Testing	spring-boot-starter-test
Thymeleaf views	spring-boot-starter-thymeleaf
Redis cache	spring-boot-starter-data-redis
Messaging	spring-boot-starter-amqp
DB migration	flyway-core

---

Code Structure Template

// Main class (Ch 2)
@SpringBootApplication
public class LibraryApp {
    public static void main(String[] args) {
        SpringApplication.run(LibraryApp.class, args);
    }
}

// Entity (Ch 2)
@Entity
public class Book {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String title;
    private String isbn;
    // constructors, getters, setters
}

// Repository (Ch 2)
public interface BookRepository extends JpaRepository<Book, Long> {
    List<Book> findByTitleContainingIgnoreCase(String title);
}

// Service (Ch 2) — constructor injection
@Service
public class BookService {
    private final BookRepository repo;
    public BookService(BookRepository repo) { this.repo = repo; }

    public Book findById(Long id) {
        return repo.findById(id)
                   .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND));
    }
}

// Controller (Ch 2)
@RestController
@RequestMapping("/api/books")
public class BookController {
    private final BookService service;
    public BookController(BookService service) { this.service = service; }

    @GetMapping("/{id}")
    public ResponseEntity<Book> getBook(@PathVariable Long id) {
        return ResponseEntity.ok(service.findById(id));
    }

    @PostMapping
    public ResponseEntity<Book> createBook(@RequestBody Book book) {
        Book saved = service.save(book);
        URI location = URI.create("/api/books/" + saved.getId());
        return ResponseEntity.created(location).body(saved);
    }
}

// application.properties (Ch 3)
// spring.datasource.url=jdbc:postgresql://localhost/library
// spring.datasource.username=${DB_USER}
// spring.datasource.password=${DB_PASS}
// spring.jpa.hibernate.ddl-auto=validate
// management.endpoints.web.exposure.include=health,info

// application-dev.properties (Ch 3)
// spring.datasource.url=jdbc:h2:mem:library
// spring.jpa.hibernate.ddl-auto=create-drop
// logging.level.org.springframework=DEBUG

---

Priority of Practices by Impact

Critical (Security & Correctness)

• Ch 3: Never hardcode credentials — use ${ENV_VAR} in properties
• Ch 3: Secure Actuator endpoints — env, beans, shutdown must require auth
• Ch 4: Test secured endpoints explicitly — assert 401/403 on unauthenticated requests
• Ch 8: Never use ddl-auto=create in production — use Flyway/Liquibase

Important (Idiom & Maintainability)

• Ch 2: Constructor injection over @Autowired field injection
• Ch 2: @RestController over @Controller + @ResponseBody for APIs
• Ch 2: Optional from repository, never null
• Ch 3: @ConfigurationProperties over scattered @Value for grouped config
• Ch 3: Profiles for environment differences — not if statements
• Ch 4: @WebMvcTest for controller tests — not full @SpringBootTest
• Ch 7: Custom HealthIndicator for each critical dependency

Suggestions (Polish)

• Ch 3: Custom error pages in templates/error/ — no code needed
• Ch 7: Custom metrics via MeterRegistry for business events
• Ch 8: Production profile disables dev tools, sets WARN log level
• Ch 2: Use spring-boot-devtools in dev for live reload