Is a trivial “Hello, World” web site with SpringMVC.
This application is an outcome of [this](https://spring.io/guides/gs/serving-web-content/) guide, which walks you through the process of creating a “Hello, World” web site with Spring. It serves a static home page and that will also accept HTTP GET requests at: http://localhost:8080/greeting. It will respond with a web page that displays HTML. The body of the HTML will contain a greeting: “Hello, World!”
You can customize the greeting with an optional name parameter in the query string. The URL might then be http://localhost:8080/greeting?name=User. The `name` parameter value overrides the default value of `World` and is reflected in the response by the content changing to “Hello, User!”
# Spring Initializr
In VsCode press `cmd+shif+p` and type `Spring Initilizr`. Choose next dependencies:
In Spring’s approach to building web sites, HTTP requests are handled by a controller. You can easily identify the controller by the [`@Controller`](http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/stereotype/Controller.html) annotation. In the following example, `GreetingController` handles GET requests for `/greeting` by returning the name of a [`View`](http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/servlet/View.html) (in this case, `greeting`). A `View` is responsible for rendering the HTML content. The following listing (from `src/main/java/djmil/hellomvc/GreetingController.java`) shows the controller:
public String greeting(@RequestParam(name="name", required=false, defaultValue="World") String name, Model model) {
// NOTE: Here we can request some data from our RESTful backend as well
model.addAttribute("name", name);
return "greeting"; // <<--template
}
}
```
This controller is concise and simple, but there is plenty going on. We break it down step by step.
The`@GetMapping`annotation ensures that HTTP GET requests to`/greeting`are mapped to the`greeting()` method.
[`@RequestParam`](http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/bind/annotation/RequestParam.html)binds the value of the query string parameter`name`into the`name`parameter of the`greeting()`method. This query string parameter is not`required`. If it is absent in the request, the`defaultValue`of`World`is used. The value of the`name`parameter is added to a[`Model`](http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/ui/Model.html)object, ultimately making it accessible to the view template.
The implementation of the `greeting()` method body relies on a view technology (in this case,[Thymeleaf](http://www.thymeleaf.org/doc/tutorials/2.1/thymeleafspring.html)) to perform server-side rendering of the HTML.
> [!note] Make sure you have Thymeleaf on your classpath. Artifact co-ordinates: `org.springframework.boot:spring-boot-starter-thymeleaf`
Thymeleaf parses the`greeting.html`template and evaluates the`th:text` expression to render the value of the`${name}`parameter that was set in the controller. The following listing`src/main/resources/templates/greeting.html` shows thetemplate:
```html
<!DOCTYPE HTML>
<htmlxmlns:th="http://www.thymeleaf.org">
<head>
<title>Getting Started: Serving Web Content</title>
Now that the web site is running, visit http://localhost:8080/greeting, where you should see “Hello, World!”
Provide a `name` query string parameter by visiting http://localhost:8080/greeting?name=User). Notice how the message changes from “Hello, World!” to “Hello, User!”:
This change demonstrates that the [`@RequestParam`](http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/bind/annotation/RequestParam.html) arrangement in `GreetingController` is working as expected. The `name` parameter has been given a default value of `World`, but it can be explicitly overridden through the query string.
# Add a Home Page
Static resources, including HTML and JavaScript and CSS, can be served from your Spring Boot application by dropping them into the right place in the source code. By default, Spring Boot serves static content from resources in the classpath at `/static` (or `/public`). The `index.html` resource is special because, if it exists, it is used as a "welcome page" for serving web-content. Which means it is served up as the root resource (that is, at `http://localhost:8080/`). For this, you need to create the following file `src/main/resources/static/index.html`:
```html
<!DOCTYPE HTML>
<html>
<head>
<title>Getting Started: Serving Web Content</title>
A common feature of developing web applications is coding a change, restarting your application, and refreshing the browser to view the change. This entire process can eat up a lot of time. To speed up this refresh cycle, Spring Boot offers with a handy module known as [spring-boot-devtools](http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#using-boot-devtools):
- Enables LiveReload to automatically refresh the browser.
- Other reasonable defaults based on development instead of production.
## Interactivity
Let's add minimal interactivity to the page by introducing an input field and a button. On the button press, a simple JavaScript will reload the page (by calling [[README#Web Controller]] 's `Get` endpoint) providing a value from the input filed as an URL parameter. Update `greeting.htm` with next lines:
Let's test our simple app with JUnit. We will concentrate on using Spring Test and Spring Boot features to test the interactions between Spring and your code.
## Sanity check
The first thing you can do is write a simple sanity check test that will fail if the application context cannot start.
From `src/test/java/djmil/hellomvc/SmokeTest.java`
The `@SpringBootTest` annotation tells Spring Boot to look for a main configuration class (one with `@SpringBootApplication`, for instance) and use that to start a Spring application context. You can run this test in your IDE or on the command line (by running `./mvnw test` or `./gradlew test`), and it should pass.
Spring interprets the `@Autowired` annotation, and the controller is injected before the test methods are run. We use [AssertJ](http://joel-costigliola.github.io/assertj/) (which provides `assertThat()` and other methods) to express the test assertions.
> A nice feature of the Spring Test support is that the application context is cached between tests. That way, if you have multiple methods in a test case or multiple test cases with the same configuration, they incur the cost of starting the application only once. You can control the cache by using the @DirtiesContext annotation.
You can as SpringBoot to test your app at the different depth. Trading test coverage for speed.
### HTTP requests: application
Here, we are starting our application and listen for a connection (as it would do in production) and then send an HTTP request and assert the response. The following listing (from `src/test/java/djmil/hellomvc/HttpRequestTest.java`) shows how to do so:
public void greetingShouldReturnDefaultMessage() throws Exception {
assertThat(this.restTemplate.getForObject("http://localhost:" + port + "/greeting",
String.class)).contains("Hello, World");
}
}
```
Note the use of `webEnvironment=RANDOM_PORT` to start the server with a random port (useful to avoid conflicts in test environments) and the injection of the port with `@LocalServerPort`. Also, note that Spring Boot has automatically provided a `TestRestTemplate` for you. All you have to do is add `@Autowired` to it.
Another useful approach is **to not start the server at all** but to test only the layer below that, where Spring handles the incoming HTTP request and hands it off to your controller. That way, almost of the full stack is used, and your code will be called in exactly the same way as if it were processing a real HTTP request but without the cost of starting the server. To do that, use Spring’s `MockMvc` and ask for that to be injected for you by using the `@AutoConfigureMockMvc` annotation on the test case. The following listing (from `src/test/java/djmil/hellomvc/WebApplicationTest.java`) shows how to do so:
In this test, the full Spring application context is started but without the server.
### Controller: web layer only
We can narrow the tests to only the web layer by using `@WebMvcTest`, as the following listing (from `src/test/java/djmil/hellomvc/WebLayerTest.java`) shows:
```java
@WebMvcTest
public class WebLayerTest {
@Autowired
private MockMvc mockMvc;
@Test
public void shouldReturnDefaultMessage() throws Exception {
The test assertion is the same as in the previous case. However, in this test, Spring Boot instantiates only the web layer rather than the whole context. In an application with multiple controllers, you can even ask for only one to be instantiated by using, for example, `@WebMvcTest(HomeController.class)`.
