Follow one familiar product request and observe the framework step introduced in this lesson.
Open this stage in the online learning path
Use JDK 21 or newer and Maven 3.9 or newer. Verify with java -version and mvn -version. Extract this project into its own folder; do not merge its files with another stage.
This lesson reuses the product catalog and Todo responsibilities from Stages 8–14. Only the feature described below is new. Read the ordered explanation before opening the complete files.
ProductService returns two Product records. There is no database in this lesson, so the request-handling mechanism is the only changing concern.
Tomcat receives /products and invokes the registered DispatcherServlet. A HandlerMapping identifies ProductController.products. A HandlerAdapter invokes that method. The controller adds products to Model and returns the view name products.
Spring MVC’s view-resolution infrastructure locates the Thymeleaf view. The Thymeleaf template resolver finds templates/products.html, and the template engine renders its rows. Returning products is a view name, not the returned product data.
Boot configures the DispatcherServlet and MVC infrastructure because this application has the MVC starter. You write the controller, service and template. Use a debugger breakpoint in ProductController.products, then step through the call stack to see the framework invoking it.
Open /api/products or send GET in Postman. @RestController applies response-body semantics. A suitable HttpMessageConverter serializes the returned List<Product> to JSON; it does not select products.html. Both endpoints use the same service. The same annotation does not mean every value uses JSON: converter selection also considers the returned type and negotiated media type.
Open a terminal in the extracted folder containing pom.xml. Run one command at a time.
mvn test
mvn spring-boot:runOpen http://localhost:8128/products. Keep the terminal running. Stop with Ctrl+C. If the port is occupied, edit server.port in application.properties before restarting.
Browser: a table with Notebook / 80 and Pen / 20
Terminal: Controller: products
GET /api/products: [{"id":1,"name":"Notebook","price":80},{"id":2,"name":"Pen","price":20}]
HTML Content-Type: text/html
API Content-Type: application/jsonObject identifiers and elapsed times can vary. Compare the relationship between values rather than copying an identifier.
Create the following files at exactly these paths, or use the ZIP. Source files include their imports. The test checks the behaviour described in the lesson.
<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-stage-28-json-converter</artifactId><version>1.0.0</version>
<properties><java.version>21</java.version></properties>
<dependencies>
<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter</artifactId></dependency>
<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope></dependency>
<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>
package com.edupoly.lesson;
public record Product(int id, String name, int price) { }
package com.edupoly.lesson;
import java.util.List;
import org.springframework.stereotype.Service;
@Service public class ProductService {
public List<Product> findAll() { return List.of(new Product(1, "Notebook", 80), new Product(2, "Pen", 20)); }
}
package com.edupoly.lesson;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
@Controller public class ProductController {
private final ProductService service;
public ProductController(ProductService service) { this.service = service; }
@GetMapping("/products") public String products(Model model) {
System.out.println("Controller: products");
model.addAttribute("products", service.findAll()); return "products";
}
}
package com.edupoly.lesson;
import java.util.List;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController public class ProductApi {
private final ProductService service;
public ProductApi(ProductService service) { this.service = service; }
@GetMapping("/api/products") public List<Product> products() { return service.findAll(); }
}
<!doctype html>
<html lang="en" xmlns:th="http://www.thymeleaf.org"><head><meta charset="utf-8"><title>Products</title></head>
<body><h1>Products</h1><table><thead><tr><th>ID</th><th>Name</th><th>Price</th></tr></thead>
<tbody><tr th:each="product : ${products}"><td th:text="${product.id}"></td><td th:text="${product.name}"></td><td th:text="${product.price}"></td></tr></tbody></table></body></html>
package com.edupoly.lesson;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class Application {
public static void main(String[] args) { SpringApplication.run(Application.class, args); }
}
server.port=8128
spring.application.name=spring-stage-28-json-converter
package com.edupoly.lesson;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
class LessonTest {
@Test void realHttpRequestRendersProducts() throws Exception {
try (var context = org.springframework.boot.SpringApplication.run(Application.class, "--server.port=0")) {
int port = context.getEnvironment().getProperty("local.server.port", Integer.class);
var client = java.net.http.HttpClient.newHttpClient();
String base = "http://127.0.0.1:" + port;
var response = client.send(java.net.http.HttpRequest.newBuilder(java.net.URI.create(base + "/products")).build(), java.net.http.HttpResponse.BodyHandlers.ofString());
assertEquals(200, response.statusCode()); assertTrue(response.body().contains("Notebook"));
assertFalse(response.body().contains("th:each"));
assertTrue(response.headers().firstValue("Content-Type").orElse("").contains("text/html"));
var api = client.send(java.net.http.HttpRequest.newBuilder(java.net.URI.create(base + "/api/products")).build(), java.net.http.HttpResponse.BodyHandlers.ofString());
assertEquals(200, api.statusCode()); assertTrue(api.body().contains("\"Notebook\""));
assertTrue(api.headers().firstValue("Content-Type").orElse("").contains("application/json"));
var missing = client.send(java.net.http.HttpRequest.newBuilder(java.net.URI.create(base + "/missing")).build(), java.net.http.HttpResponse.BodyHandlers.ofString());
assertEquals(404, missing.statusCode());
}
}
}
Return a plain view name from a @RestController method and observe that the text becomes response data. Use @Controller for a view-producing handler.
| Word | Meaning in this example |
|---|---|
| Message converter | Writes returned Product objects as an HTTP response body. |
| Response body | The actual JSON data returned instead of a template name. |
Add one product and verify both the rendered table and, where present, JSON output. Explain the part of the request flow changed in this stage.