Service layer: use cases and transactions in Spring

Introduction

Controllers should be thin. The service (or “application” / “use case”) layer holds the rules: create an item, validate, apply one database transaction when multiple writes must commit together. This post uses @Service and @Transactional.

Real-world explanation

• @Service is a stereotype for @Component with a name that signals intent in reviews
• @Transactional on a class (or public methods of a class proxied) opens a transaction around the method, commit on success, rollback on unchecked exceptions (default)
• Constructor injection of repositories—immutable final fields, easy to test with mocks

Step-by-step: ItemService

package com.example.demoservice.service;
 
import com.example.demoservice.model.Item;
import com.example.demoservice.repository.ItemRepository;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;
 
import java.util.List;
 
@Service
public class ItemService {
 
  private final ItemRepository itemRepository;
 
  public ItemService(ItemRepository itemRepository) {
    this.itemRepository = itemRepository;
  }
 
  @Transactional
  public Item create(String name) {
    if (itemRepository.existsByName(name.trim())) {
      throw new IllegalStateException("name taken");
    }
    return itemRepository.save(new Item(name.trim()));
  }
 
  @Transactional(readOnly = true)
  public List<Item> list() {
    return itemRepository.findAll();
  }
}

When to add @Transactional on the controller?

Usually not. It hides use case boundaries and is easy to misuse. Keep it on the service.

Common mistakes

• Calling private this.foo() with @Transactional on foo - self-invocation bypasses the proxy in default Spring. Move foo to another bean, or use interface-based AOP, or restructure
• Swallowing exceptions and expecting rollback to still happen—must rethrow
• Fat service class with 2000 lines—split by feature or by read/write models

Best practices

• One use case method, one name that reads like a sentence (create, deactivateItem)
• For read APIs, readOnly = true on read service methods to help Hibernate and datasource tuning
• Return domain objects to the web layer, then map to DTO in controller (or a mapper class) as we do next

Final summary

The service coordinates repositories and rules. Transactions belong here. Next: HTTP in the controller and status codes for the client.