This guide walks you through the process of creating a “Hello, World” web site with Spring.
You will build an application that has 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!”
For all Spring applications, you should start with the Spring Initializr. The Initializr offers a fast way to pull in all the dependencies you need for an application and does a lot of the setup for you. This example needs the Spring Web, Thymeleaf, and Spring Boot DevTools dependencies. The following image shows the Initializr set up for this sample project:
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Note
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The preceding image shows the Initializr with Maven chosen as the build tool. You
can also use Gradle. It also shows values of com.example and serving-web-content as
the Group and Artifact, respectively. You will use those values throughout the rest of
this sample.
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The following listing shows the pom.xml file that is created when you choose Maven:
link:initial/pom.xml[role=include]The following listing shows the build.gradle file that is created when you choose Gradle:
link:initial/build.gradle[role=include]In Spring’s approach to building web sites, HTTP requests are handled by a controller. You
can easily identify the controller by the @Controller annotation. In the
following example, GreetingController handles GET requests for /greeting by returning
the name of a View (in this case, greeting). A View is responsible for
rendering the HTML content. The following listing (from
src/main/java/com/example/servingwebcontent/GreetingController.java) shows the
controller:
link:complete/src/main/java/com/example/servingwebcontent/GreetingController.java[role=include]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 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 object, ultimately making it
accessible to the view template.
The implementation of the method body relies on a view technology (in this case,
Thymeleaf) to perform
server-side rendering of the HTML. 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 (from
src/main/resources/templates/greeting.html) shows the greeting.html template:
link:complete/src/main/resources/templates/greeting.html[role=include]|
Tip
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Make sure you have Thymeleaf on your classpath (artifact co-ordinates: org.springframework.boot:spring-boot-starter-thymeleaf). It is already there in the "initial" and "complete" samples in Github.
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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. Spring Boot Devtools:
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Enables hot swapping.
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Switches template engines to disable caching.
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Enables LiveReload to automatically refresh the browser.
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Other reasonable defaults based on development instead of production.
The Spring Initializr creates an application class for you. In this case, you need not
further modify the class provided by the Spring Initializr. The following listing (from
src/main/java/com/example/servingwebcontent/ServingWebContentApplication.java) shows the
application class:
link:complete/src/main/java/com/example/servingwebcontent/ServingWebContentApplication.java[role=include]Logging output is displayed. The application should be up and running within a few seconds.
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 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.
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,"serving-web-content/ which means it is served up as the root resource (that is, at
`http://localhost:8080/). As a result, you need to create the following file (which you
can find in src/main/resources/static/index.html):
link:complete/src/main/resources/static/index.html[role=include]When you restart the application, you will see the HTML at http://localhost:8080/.
The following guides may also be helpful: