Service layer

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Goal

Introduce a service class between the controller and the repository so that business logic is not mixed into HTTP-handling code. After this page ItemService wraps the three operations the controller will expose, and ItemNotFoundException provides a typed signal for missing records.

Prerequisites

• Entity and repository

Why a service layer

Putting repository calls directly in the controller works for small examples, but it conflates two concerns:

• HTTP concern — what status code to return, how to read the request body, what to write to the response.
• Business concern — what it means to create an item, what happens when an item is not found.

A service class holds the second concern. The controller delegates to it. This makes both classes easier to test in isolation: page 07 shows how to test the controller without a real database and how to test the repository without an HTTP layer.

ItemNotFoundException

Create src/main/java/com/example/itemsapi/ItemNotFoundException.java:

package com.example.itemsapi;

public class ItemNotFoundException extends RuntimeException {
    public ItemNotFoundException(Long id) {
        super("item " + id + " not found");
    }
}

This is a plain RuntimeException subclass. Spring does not know about it yet — the exception handler on page 06 will map it to an HTTP 404 response.

ItemService

Create src/main/java/com/example/itemsapi/ItemService.java:

package com.example.itemsapi;

import org.springframework.stereotype.Service;
import java.util.List;

@Service
public class ItemService {

    private final ItemRepository repo;

    public ItemService(ItemRepository repo) {
        this.repo = repo;
    }

    public List<Item> list() {
        return repo.findAll();
    }

    public Item get(Long id) {
        return repo.findById(id)
            .orElseThrow(() -> new ItemNotFoundException(id));
    }

    public Item create(Item item) {
        return repo.save(item);
    }
}

Key points:

• @Service registers the class as a Spring component. Spring will inject it wherever ItemService is declared as a constructor parameter.
• Constructor injection — public ItemService(ItemRepository repo) — is the preferred style in Spring Boot 3.x. No @Autowired annotation is needed when there is exactly one constructor.
• get(Long id) uses Optional.orElseThrow to convert an absent value into the typed exception. The controller does not need an if-check.
• create(Item item) calls repo.save(item) and returns the saved entity, which now has the database-generated id populated.

Verify: project compiles

./mvnw compile

Expected output:

BUILD SUCCESS

Next → Controller CRUD