Stage 27 · trace a request to a Thymeleaf page

Stage 27: trace a request to a Thymeleaf page

Follow one familiar product request and observe the framework step introduced in this lesson.

Open this stage in the online learning path

Before you start

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.

Browser → Tomcat → DispatcherServlet → HandlerMapping / adapter → Controller → Thymeleaf view → Response

What we change and why

Reuse a tiny product catalog
ProductService returns two Product records. There is no database in this lesson, so the request-handling mechanism is the only changing concern.
Follow the HTML request
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.
Resolve and render the view
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.
See framework infrastructure without implementing it
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.

Run this example

Open a terminal in the extracted folder containing pom.xml. Run one command at a time.

Terminal · Windows, macOS and Linux

mvn test
mvn spring-boot:run

Open http://localhost:8127/products. Keep the terminal running. Stop with Ctrl+C. If the port is occupied, edit server.port in application.properties before restarting.

Expected result

Expected output · ignore framework startup messages

Browser: a table with Notebook / 80 and Pen / 20
Terminal: Controller: products

Object identifiers and elapsed times can vary. Compare the relationship between values rather than copying an identifier.

Understand each complete file

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.

pom.xml

pom.xml · complete file

<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-27-mvc-flow</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>

src/main/java/com/edupoly/lesson/Product.java

src/main/java/com/edupoly/lesson/Product.java · complete file

package com.edupoly.lesson;

public record Product(int id, String name, int price) { }

src/main/java/com/edupoly/lesson/ProductService.java

src/main/java/com/edupoly/lesson/ProductService.java · complete file

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)); }
}

src/main/java/com/edupoly/lesson/ProductController.java

src/main/java/com/edupoly/lesson/ProductController.java · complete file

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";
    }
}

src/main/resources/templates/products.html

src/main/resources/templates/products.html · complete file

<!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>

src/main/java/com/edupoly/lesson/Application.java

src/main/java/com/edupoly/lesson/Application.java · complete file

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); }
}

src/main/resources/application.properties

src/main/resources/application.properties · complete file

server.port=8127
spring.application.name=spring-stage-27-mvc-flow

src/test/java/com/edupoly/lesson/LessonTest.java

src/test/java/com/edupoly/lesson/LessonTest.java · complete file

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 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());
        }
    }
}

Try a failure, then correct it

Rename the returned view to missing-template and request /products. The template cannot be resolved. Restore products.

Words you can now explain

Word	Meaning in this example
HandlerMapping	Finds the handler corresponding to /products.
View resolution	Connects the returned view name to a renderable HTML view.

Practice assignment

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.

Read the official documentation →