Stage 9: Move a product rule into a service

Download spring-products-service.zip · Complete file-by-file guide

1. Keep request handling in the controller

   The controller reads inStockOnly from the query string and supplies products to Model. It no longer decides which stock values qualify.

2. Put the rule in ProductService

   findAll(false) returns every product. findAll(true) adds only products whose stock is greater than zero to the result list. That if condition is our business rule. @Service is a specialized component annotation that tells readers this class performs application operations.

3. Inject the required objects

   Spring supplies ProductFileReader to ProductService and ProductService to ProductController. The browser never creates these Java objects. They are connected during application startup.

4. See an actual difference

   This project deliberately changes product 5 in products.txt to stock 0. All products therefore shows 30 rows; In-stock products shows 29. The original Stage 6 project is unchanged. The query ?inStockOnly=true selects the filtered operation.

5. Read the lookup method

   findById returns Optional<Product>. Optional is a Java type for a value that may be absent, not a Spring annotation. The controller turns an absent product into a 404 response.

Run this stage

Use JDK 21+ and Maven 3.9+. Extract spring-products-service.zip and open a terminal in the spring-products-service folder containing pom.xml. Keep the source files in their supplied paths.

mvn spring-boot:run

Open http://localhost:8087/products. Keep the terminal open. Stop with Ctrl+C. If the port is occupied, change server.port in src/main/resources/application.properties, restart and use the new port in the browser.

# Choose another available port if this port is busy.
server.port=8087

Expected output and checks

Compare /products (30 rows) and /products?inStockOnly=true (29 rows). Product 5 still has a details page with stock 0. An unknown ID returns 404.

Read the changed Java files

The downloadable project includes every file, the 30-product text file, templates, CSS, pom.xml and a standalone README.html with all source files. These are the key changes from the preceding example.

package com.edupoly.products;

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

@Service
public class ProductService {
    private final ProductFileReader reader;

    public ProductService(ProductFileReader reader) {
        this.reader = reader;
    }

    public List<Product> findAll(boolean inStockOnly) {
        List<Product> selected = new ArrayList<>();
        for (Product product : reader.getProducts()) {
            if (!inStockOnly || product.getStock() > 0) {
                selected.add(product);
            }
        }
        return selected;
    }

    public Optional<Product> findById(int id) {
        for (Product product : reader.getProducts()) {
            if (product.getId() == id) {
                return Optional.of(product);
            }
        }
        return Optional.empty();
    }
}

package com.edupoly.products;

import org.springframework.http.HttpStatus;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.server.ResponseStatusException;

@Controller
public class ProductController {
    private final ProductService service;

    public ProductController(ProductService service) {
        this.service = service;
    }

    @GetMapping({"/", "/products"})
    public String listProducts(
            @RequestParam(name = "inStockOnly", defaultValue = "false") boolean inStockOnly,
            Model model) {
        model.addAttribute("products", service.findAll(inStockOnly));
        return "products";
    }

    @GetMapping("/products/{id}")
    public String productDetails(@PathVariable("id") int id, Model model) {
        Product product = service.findById(id).orElseThrow(() ->
                new ResponseStatusException(HttpStatus.NOT_FOUND, "Product not found"));
        model.addAttribute("product", product);
        return "product-details";
    }
}

Practice

Change the stock rule from greater than zero to at least 10. Predict the filtered count from products.txt before running.

References: Spring constructor injection, Spring Data MongoDB repositories.

Why use different component annotations?

@Component marks a general helper such as ProductFileReader. @Service identifies ProductService as the home of application operations and business rules. @Repository identifies the data-access responsibility introduced in Stage 10. @Controller identifies web request handlers. These stereotypes help discovery and communicate responsibility; choosing a name does not automatically write the method logic. A repository stereotype can also participate in persistence exception translation when the relevant Spring infrastructure is configured.