Edupoly · Spring Foundation

Stage 10: Put data access behind a repository

Controller → Service → ProductRepository interface → FileProductRepository → Reader

  1. Define a small contract

    ProductRepository is our interface. It declares findAll() and findById(int id), without deciding how data is obtained. The interface itself cannot be instantiated with new.

  2. Implement the contract for this file example

    FileProductRepository implements ProductRepository and delegates to ProductFileReader. This concrete class supplies the method bodies. @Repository is a specialized component annotation for the data-access role. It does not automatically implement an arbitrary interface.

  3. Inject by interface

    ProductService accepts ProductRepository in its constructor. Spring finds the single implementation bean, FileProductRepository, and supplies that object. The field’s declared type is an interface; the actual referenced object is the implementation.

  4. Keep the service independent of file-reading details

    The service still applies the stock rule. The controller still supplies Model and view names. Only the data-access dependency has changed. This prepares us to replace file storage in Stage 11.

  5. Be precise about storage

    This stage reads the file into memory once at startup. There are no create, update or delete operations and nothing writes back to products.txt. A product repository here means a data-access object; it is not a Git source-code repository.

Run this stage

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

Start the application
mvn spring-boot:run

Open http://localhost:8088/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.

src/main/resources/application.properties
# Choose another available port if this port is busy.
server.port=8088

Expected output and checks

The pages behave just like Stage 9: 30 total rows, 29 in-stock rows and a 404 for an unknown ID. Follow the constructor types to identify the interface and the object Spring supplies.

Complete project files

Every project file is reproduced below. Create each file at the indicated path if building manually.

.gitignore
.gitignore
/target/
pom.xml
pom.xml
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>4.1.1</version>
    <relativePath/>
  </parent>
  <groupId>com.edupoly</groupId>
  <artifactId>spring-products-file-repository</artifactId>
  <version>1.0.0</version>
  <properties>
    <java.version>21</java.version>
  </properties>
  <dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-webmvc</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-thymeleaf</artifactId>
    </dependency>
  </dependencies>
  <build>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>
    </plugins>
  </build>
</project>
src/main/java/com/edupoly/products/FileProductRepository.java
src/main/java/com/edupoly/products/FileProductRepository.java
package com.edupoly.products;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import org.springframework.stereotype.Repository;

@Repository
public class FileProductRepository implements ProductRepository {
    private final ProductFileReader reader;

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

    @Override
    public List<Product> findAll() {
        return Arrays.asList(reader.getProducts());
    }

    @Override
    public Optional<Product> findById(int id) {
        for (Product product : reader.getProducts()) {
            if (product.getId() == id) {
                return Optional.of(product);
            }
        }
        return Optional.empty();
    }
}
src/main/java/com/edupoly/products/Product.java
src/main/java/com/edupoly/products/Product.java
package com.edupoly.products;

import java.math.BigDecimal;

public class Product {
    private int id;
    private String name;
    private String category;
    private BigDecimal price;
    private String brand;
    private int stock;
    private String description;
    private int warrantyMonths;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCategory() {
        return category;
    }

    public void setCategory(String category) {
        this.category = category;
    }

    public BigDecimal getPrice() {
        return price;
    }

    public void setPrice(BigDecimal price) {
        this.price = price;
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public int getStock() {
        return stock;
    }

    public void setStock(int stock) {
        this.stock = stock;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public int getWarrantyMonths() {
        return warrantyMonths;
    }

    public void setWarrantyMonths(int warrantyMonths) {
        this.warrantyMonths = warrantyMonths;
    }
}
src/main/java/com/edupoly/products/ProductApplication.java
src/main/java/com/edupoly/products/ProductApplication.java
package com.edupoly.products;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class ProductApplication {
    public static void main(String[] args) {
        SpringApplication.run(ProductApplication.class, args);
    }
}
src/main/java/com/edupoly/products/ProductController.java
src/main/java/com/edupoly/products/ProductController.java
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";
    }
}
src/main/java/com/edupoly/products/ProductFileReader.java
src/main/java/com/edupoly/products/ProductFileReader.java
package com.edupoly.products;

import java.io.IOException;
import org.springframework.core.io.ClassPathResource;
import org.springframework.stereotype.Component;
import tools.jackson.databind.json.JsonMapper;

@Component
public class ProductFileReader {
    private final Product[] products;

    public ProductFileReader() throws IOException {
        var resource = new ClassPathResource("data/products.txt");
        try (var input = resource.getInputStream()) {
            JsonMapper mapper = JsonMapper.builder().build();
            products = mapper.readValue(input, Product[].class);
        }
        System.out.println("Created ProductFileReader");
    }

    public Product[] getProducts() {
        return products;
    }
}
src/main/java/com/edupoly/products/ProductRepository.java
src/main/java/com/edupoly/products/ProductRepository.java
package com.edupoly.products;

import java.util.List;
import java.util.Optional;

public interface ProductRepository {
    List<Product> findAll();
    Optional<Product> findById(int id);
}
src/main/java/com/edupoly/products/ProductService.java
src/main/java/com/edupoly/products/ProductService.java
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 ProductRepository repository;

    public ProductService(ProductRepository repository) {
        this.repository = repository;
    }

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

    public Optional<Product> findById(int id) {
        return repository.findById(id);
    }
}
src/main/resources/application.properties
src/main/resources/application.properties
# Choose another available port if this port is busy.
server.port=8088
src/main/resources/data/products.txt
src/main/resources/data/products.txt
[
  {
    "id": 1,
    "name": "Wireless Mouse",
    "category": "Electronics",
    "price": 699,
    "brand": "EduTech",
    "stock": 12,
    "description": "Wireless Mouse for everyday learning, study and work. Product 1 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 2,
    "name": "Mechanical Keyboard",
    "category": "Electronics",
    "price": 3499,
    "brand": "EduTech",
    "stock": 19,
    "description": "Mechanical Keyboard for everyday learning, study and work. Product 2 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 3,
    "name": "USB-C Hub",
    "category": "Electronics",
    "price": 1599,
    "brand": "EduTech",
    "stock": 26,
    "description": "USB-C Hub for everyday learning, study and work. Product 3 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 4,
    "name": "Laptop Stand",
    "category": "Electronics",
    "price": 1299,
    "brand": "EduTech",
    "stock": 33,
    "description": "Laptop Stand for everyday learning, study and work. Product 4 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 5,
    "name": "Webcam",
    "category": "Electronics",
    "price": 2199,
    "brand": "EduTech",
    "stock": 0,
    "description": "Webcam for everyday learning, study and work. Product 5 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 6,
    "name": "Bluetooth Speaker",
    "category": "Electronics",
    "price": 2499,
    "brand": "EduTech",
    "stock": 47,
    "description": "Bluetooth Speaker for everyday learning, study and work. Product 6 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 7,
    "name": "Noise-Cancelling Headphones",
    "category": "Electronics",
    "price": 4999,
    "brand": "EduTech",
    "stock": 54,
    "description": "Noise-Cancelling Headphones for everyday learning, study and work. Product 7 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 8,
    "name": "Portable SSD",
    "category": "Electronics",
    "price": 6499,
    "brand": "EduTech",
    "stock": 61,
    "description": "Portable SSD for everyday learning, study and work. Product 8 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 9,
    "name": "USB Flash Drive",
    "category": "Electronics",
    "price": 599,
    "brand": "EduTech",
    "stock": 68,
    "description": "USB Flash Drive for everyday learning, study and work. Product 9 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 10,
    "name": "Monitor",
    "category": "Electronics",
    "price": 12999,
    "brand": "EduTech",
    "stock": 15,
    "description": "Monitor for everyday learning, study and work. Product 10 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 11,
    "name": "Desk Lamp",
    "category": "Electronics",
    "price": 899,
    "brand": "EduTech",
    "stock": 22,
    "description": "Desk Lamp for everyday learning, study and work. Product 11 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 12,
    "name": "Power Bank",
    "category": "Electronics",
    "price": 1799,
    "brand": "EduTech",
    "stock": 29,
    "description": "Power Bank for everyday learning, study and work. Product 12 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 13,
    "name": "Smartphone Stand",
    "category": "Electronics",
    "price": 399,
    "brand": "EduTech",
    "stock": 36,
    "description": "Smartphone Stand for everyday learning, study and work. Product 13 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 14,
    "name": "HDMI Cable",
    "category": "Electronics",
    "price": 499,
    "brand": "EduTech",
    "stock": 43,
    "description": "HDMI Cable for everyday learning, study and work. Product 14 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 15,
    "name": "Ethernet Adapter",
    "category": "Electronics",
    "price": 999,
    "brand": "EduTech",
    "stock": 50,
    "description": "Ethernet Adapter for everyday learning, study and work. Product 15 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 16,
    "name": "Graphics Tablet",
    "category": "Electronics",
    "price": 3999,
    "brand": "EduTech",
    "stock": 57,
    "description": "Graphics Tablet for everyday learning, study and work. Product 16 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 17,
    "name": "Microphone",
    "category": "Electronics",
    "price": 2999,
    "brand": "EduTech",
    "stock": 64,
    "description": "Microphone for everyday learning, study and work. Product 17 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 18,
    "name": "Laptop Backpack",
    "category": "Electronics",
    "price": 1499,
    "brand": "EduTech",
    "stock": 71,
    "description": "Laptop Backpack for everyday learning, study and work. Product 18 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 19,
    "name": "Screen Cleaner",
    "category": "Electronics",
    "price": 249,
    "brand": "EduTech",
    "stock": 18,
    "description": "Screen Cleaner for everyday learning, study and work. Product 19 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 20,
    "name": "Mouse Pad",
    "category": "Electronics",
    "price": 299,
    "brand": "EduTech",
    "stock": 25,
    "description": "Mouse Pad for everyday learning, study and work. Product 20 includes a simple user guide and protective packaging.",
    "warrantyMonths": 12
  },
  {
    "id": 21,
    "name": "Notebook",
    "category": "Study Essentials",
    "price": 149,
    "brand": "StudyWorks",
    "stock": 32,
    "description": "Notebook for everyday learning, study and work. Product 21 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  },
  {
    "id": 22,
    "name": "Ballpoint Pen Set",
    "category": "Study Essentials",
    "price": 199,
    "brand": "StudyWorks",
    "stock": 39,
    "description": "Ballpoint Pen Set for everyday learning, study and work. Product 22 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  },
  {
    "id": 23,
    "name": "Water Bottle",
    "category": "Study Essentials",
    "price": 599,
    "brand": "StudyWorks",
    "stock": 46,
    "description": "Water Bottle for everyday learning, study and work. Product 23 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  },
  {
    "id": 24,
    "name": "Desk Organizer",
    "category": "Study Essentials",
    "price": 449,
    "brand": "StudyWorks",
    "stock": 53,
    "description": "Desk Organizer for everyday learning, study and work. Product 24 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  },
  {
    "id": 25,
    "name": "Reading Light",
    "category": "Study Essentials",
    "price": 699,
    "brand": "StudyWorks",
    "stock": 60,
    "description": "Reading Light for everyday learning, study and work. Product 25 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  },
  {
    "id": 26,
    "name": "Calculator",
    "category": "Study Essentials",
    "price": 499,
    "brand": "StudyWorks",
    "stock": 67,
    "description": "Calculator for everyday learning, study and work. Product 26 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  },
  {
    "id": 27,
    "name": "Whiteboard",
    "category": "Study Essentials",
    "price": 1299,
    "brand": "StudyWorks",
    "stock": 14,
    "description": "Whiteboard for everyday learning, study and work. Product 27 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  },
  {
    "id": 28,
    "name": "Marker Set",
    "category": "Study Essentials",
    "price": 299,
    "brand": "StudyWorks",
    "stock": 21,
    "description": "Marker Set for everyday learning, study and work. Product 28 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  },
  {
    "id": 29,
    "name": "Book Stand",
    "category": "Study Essentials",
    "price": 799,
    "brand": "StudyWorks",
    "stock": 28,
    "description": "Book Stand for everyday learning, study and work. Product 29 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  },
  {
    "id": 30,
    "name": "Travel Mug",
    "category": "Study Essentials",
    "price": 649,
    "brand": "StudyWorks",
    "stock": 35,
    "description": "Travel Mug for everyday learning, study and work. Product 30 includes a simple user guide and protective packaging.",
    "warrantyMonths": 6
  }
]
src/main/resources/static/style.css
src/main/resources/static/style.css
body{margin:0;padding:28px 16px;background:#f5f7fb;color:#17253d;font:16px/1.6 system-ui,sans-serif}main{max-width:1100px;margin:20px auto;padding:28px;background:#fff;border:1px solid #dbe2ec;border-radius:12px}h1{margin-top:0}a{color:#2457a6;text-underline-offset:3px}.table-scroll{overflow-x:auto}table{width:100%;border-collapse:collapse}th,td{text-align:left;padding:12px;border-bottom:1px solid #dbe2ec}th{background:#edf3fc}tbody tr:hover{background:#f8faff}.detail{max-width:760px}dl{display:grid;grid-template-columns:160px minmax(0,1fr);gap:12px 20px;margin:24px 0}dt{font-weight:650}dd{margin:0;overflow-wrap:anywhere}@media(max-width:500px){main{padding:16px}dl{grid-template-columns:1fr;gap:4px}dd{margin-bottom:12px}}

.catalog-nav{margin:20px 0}form{margin:18px 0}button{padding:9px 14px;cursor:pointer}input{padding:8px;max-width:140px}form button{margin-left:8px}
src/main/resources/templates/product-details.html
src/main/resources/templates/product-details.html
<!doctype html>
<html lang="en" xmlns:th="http://www.thymeleaf.org">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title th:text="${product.name}">Product details</title>
    <link rel="stylesheet" th:href="@{/style.css}">
</head>
<body>
<main class="detail">
    <h1>Product details</h1>
    <h2 th:text="${product.name}">Wireless Mouse</h2>
    <dl>
        <dt>ID</dt><dd th:text="${product.id}">1</dd>
        <dt>Name</dt><dd th:text="${product.name}">Wireless Mouse</dd>
        <dt>Category</dt><dd th:text="${product.category}">Electronics</dd>
        <dt>Price (INR)</dt><dd th:text="${product.price}">699</dd>
        <dt>Brand</dt><dd th:text="${product.brand}">EduTech</dd>
        <dt>Stock</dt><dd th:text="${product.stock}">12</dd>
        <dt>Description</dt><dd th:text="${product.description}">Product description</dd>
        <dt>Warranty (months)</dt><dd th:text="${product.warrantyMonths}">12</dd>
    </dl>
    <a th:href="@{/products}">Back to products</a>
</main>
</body>
</html>
src/main/resources/templates/products.html
src/main/resources/templates/products.html
<!doctype html>
<html lang="en" xmlns:th="http://www.thymeleaf.org">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>Products</title>
    <link rel="stylesheet" th:href="@{/style.css}">
</head>
<body>
<main>
    <h1>Products</h1>
    <p>Stage 10: File-based repository. Select More details for a product.</p>
    <nav class="catalog-nav"><a th:href="@{/products}">All products</a> · <a th:href="@{/products(inStockOnly=true)}">In-stock products</a></nav>
    <div class="table-scroll">
    <table>
        <thead>
        <tr><th>ID</th><th>Name</th><th>Category</th><th>Price (INR)</th><th>Action</th></tr>
        </thead>
        <tbody>
        <tr th:each="product : ${products}">
            <td th:text="${product.id}">1</td>
            <td th:text="${product.name}">Wireless Mouse</td>
            <td th:text="${product.category}">Electronics</td>
            <td th:text="${product.price}">699</td>
            <td><a th:href="@{/products/{id}(id=${product.id})}">More details</a></td>
        </tr>
        </tbody>
    </table>
    </div>
</main>
</body>
</html>

Practice

Trace a details request through controller, service, interface and implementation. Identify which class would change if the storage format changed.

References: Spring constructor injection, Spring Data MongoDB repositories.

After Stage 10: the Spring words to recognize

TermOur concrete exampleIts place in Spring
Repository patternProductRepositoryA boundary through which the service obtains stored products.
@RepositoryFileProductRepositoryMarks our concrete data-access class as a Spring component.
Interface and implementationProductRepository / FileProductRepositoryThe contract lists operations; the implementation contains their code.

One small check: Point to the class, constructor or annotation that demonstrates each term before continuing.

Continue learning

  1. Stage 8: Let Spring supply a product-file reader — complete project ZIP
  2. Stage 9: Move a product rule into a service — complete project ZIP
  3. Stage 10: Put data access behind a repository — complete project ZIP
  4. Stage 11: Store and change products in MongoDB — complete project ZIP
  5. Stage 12: Create beans with explicit factory methods — complete project ZIP
  6. Stage 13: MongoDB connectivity — complete project ZIP
  7. Stage 14: Todo list application — complete project ZIP

1. Connect the new concepts to familiar ones