Edupoly · Spring Foundation

Stage 8: Let Spring supply a product-file reader

Browser → ProductController → ProductFileReader → Product[] → Model → Thymeleaf

  1. Start from Stage 6

    The earlier controller loaded products.txt itself. This stage moves that exact stream-based reading code into ProductFileReader. The file still contains the same 30 objects. The Product data objects remain ordinary Java objects.

  2. Mark the helper with @Component

    Spring discovers ProductFileReader because it is in the application package. Spring creates one reader object during startup and keeps its reference. The reader constructor loads the file once. @Component does not make every Product object a bean.

  3. Ask for the reader through the controller constructor

    ProductController(ProductFileReader reader) declares what the controller needs. Spring creates the reader first, then supplies that object when constructing the controller. this.reader = reader stores the supplied reference; it does not create another reader. A class with one constructor does not need @Autowired on that constructor.

  4. Observe creation and reuse

    Start the app and read the terminal. Created ProductFileReader appears once. Refresh /products three times: the reader identity printed for each request stays the same in this running application. The identity number is only a demonstration aid, not a permanent identifier. Restarting creates a new application context and new objects.

  5. Connect DI, IoC and singleton to those lines

    The reader is a dependency because the controller needs it. Supplying it through the constructor is dependency injection (DI). Spring taking responsibility for constructing and connecting these objects is inversion of control (IoC). By default each bean definition has one shared instance per application context: singleton scope. Do not store request-specific values in shared controller fields.

Run this stage

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

Start the application
mvn spring-boot:run

Open http://localhost:8086/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=8086

Expected output and checks

Open /products: 30 product rows. Open /products/1: full details. Repeated list requests print the same reader identity. /products/99999 returns 404.

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-di</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/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.server.ResponseStatusException;

@Controller
public class ProductController {
    private final ProductFileReader reader;

    public ProductController(ProductFileReader reader) {
        this.reader = reader;
        System.out.println("Created ProductController with its supplied reader");
    }

    @GetMapping({"/", "/products"})
    public String listProducts(Model model) {
        System.out.println("Reader used by controller: " + System.identityHashCode(reader));
        model.addAttribute("products", reader.getProducts());
        return "products";
    }

    @GetMapping("/products/{id}")
    public String productDetails(@PathVariable("id") int id, Model model) {
        for (Product product : reader.getProducts()) {
            if (product.getId() == id) {
                model.addAttribute("product", product);
                return "product-details";
            }
        }
        throw new ResponseStatusException(HttpStatus.NOT_FOUND, "Product not found");
    }
}
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/resources/application.properties
src/main/resources/application.properties
# Choose another available port if this port is busy.
server.port=8086
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": 40,
    "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 8: A reader component supplied to a controller. Select More details for a product.</p>
    <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

Add a diagnostic line in the reader constructor and refresh the page. Explain why the constructor does not run for every request.

References: Spring constructor injection, Spring Data MongoDB repositories.

After Stage 8: the Spring words to recognize

TermOur concrete exampleIts place in Spring
@ComponentProductFileReaderMarks this class for discovery as a Spring-managed object.
Bean and containerThe reader object; the application contextThe bean is the actual managed object. The container is Spring’s system that creates and keeps that reader and the controller.
Constructor injection / DI / IoCProductController(ProductFileReader reader)Spring supplies the required reader instead of the controller constructing it.
Singleton scopeThe shared reader instanceOne instance for this bean definition in this application context by default.

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