Gentleman Programming Gentleman Skills Spring Boot 3

When to Use

Load this skill when:

• Building a Spring Boot 3.3+ service or API
• Wiring beans with dependency injection
• Defining configuration properties and validation
• Implementing REST controllers and service layers

Critical Patterns

Pattern 1: Constructor injection only

Always use constructor injection; avoid field injection.

Pattern 2: Typed configuration properties

Use @ConfigurationProperties with validation, not scattered @Value.

Pattern 3: Transaction boundaries at service layer

Apply @Transactional on application services, not controllers.

Code Examples

Example 1: Configuration properties with validation

package com.acme.config;

import jakarta.validation.constraints.NotBlank;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.validation.annotation.Validated;

@Validated
@ConfigurationProperties(prefix = "payment")
public record PaymentProperties(
  @NotBlank String provider,
  @NotBlank String apiKey
) { }

package com.acme;

import org.springframework.boot.context.properties.ConfigurationPropertiesScan;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
@ConfigurationPropertiesScan
public class Application { }

Example 2: Service with constructor injection + transaction

package com.acme.order.application;

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

@Service
public final class OrderService {
  private final OrderRepository repository;

  public OrderService(OrderRepository repository) {
    this.repository = repository;
  }

  @Transactional
  public void placeOrder(OrderCommand command) {
    repository.save(command.toEntity());
  }
}

Example 3: REST controller with DTO records

package com.acme.order.api;

import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

@RestController
@RequestMapping("/orders")
public final class OrderController {
  private final OrderService service;

  public OrderController(OrderService service) {
    this.service = service;
  }

  @PostMapping
  public ResponseEntity<OrderResponse> place(@RequestBody OrderRequest request) {
    service.placeOrder(request.toCommand());
    return ResponseEntity.ok(new OrderResponse("ok"));
  }

  public record OrderRequest(String sku, int qty) {
    public OrderCommand toCommand() { return new OrderCommand(sku, qty); }
  }

  public record OrderResponse(String status) { }
}

Anti-Patterns

Don't: Use field injection

// BAD: field injection
@Service
public class OrderService {
  @org.springframework.beans.factory.annotation.Autowired
  private OrderRepository repository;
}

Don't: Scatter configuration with @Value

// BAD: hard to validate and test
@Service
public class PaymentService {
  @org.springframework.beans.factory.annotation.Value("${payment.apiKey}")
  private String apiKey;
}

Quick Reference

Resources

• Spring Boot Reference: https://docs.spring.io/spring-boot/docs/current/reference/html/
• Spring Framework Reference: https://docs.spring.io/spring-framework/reference/

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