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This guide walks you through the process of creating a "hello world" web site with Spring.
You’ll build a service that will accept HTTP GET requests at:
http://localhost:8080/greeting
and respond with a web page displaying a greeting:
"Hello, World!"
You can customize the greeting with an optional name parameter in the query string:
http://localhost:8080/greeting?name=User
The name parameter value overrides the default value of "World" and is reflected in the response:
"Hello, User!"
build.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 these requests by the @Controller annotation. In the following example, the 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:
src/main/java/hello/GreetingController.java
link:complete/src/main/java/hello/GreetingController.java[role=include]This controller is concise and simple, but there’s plenty going on. Let’s break it down step by step.
The @RequestMapping annotation ensures that HTTP requests to /greeting are mapped to the greeting() method.
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Note
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The above example does not specify GET vs. PUT, POST, and so forth, because @RequestMapping maps all HTTP operations by default. Use @RequestMapping(method=GET) to narrow this mapping.
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@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 below and evaluates the th:text expression to render the value of the ${name} parameter that was set in the controller.
src/main/resources/templates/greeting.html
link:complete/src/main/resources/templates/greeting.html[role=include]Although it is possible to package this service as a traditional WAR file for deployment to an external application server, the simpler approach demonstrated below creates a standalone application. You package everything in a single, executable JAR file, driven by a good old Java main() method. Along the way, you use Spring’s support for embedding the Tomcat servlet container as the HTTP runtime, instead of deploying to an external instance.
src/main/java/hello/Application.java
link:complete/src/main/java/hello/Application.java[role=include]The main() method defers to the SpringApplication helper class, providing Application.class as an argument to its run() method. This tells Spring to read the annotation metadata from Application and to manage it as a component in the Spring application context.
The @ComponentScan annotation tells Spring to search recursively through the hello package and its children for classes marked directly or indirectly with Spring’s @Component annotation. This directive ensures that Spring finds and registers the GreetingController, because it is marked with @Controller, which in turn is a kind of @Component annotation.
The @EnableAutoConfiguration annotation switches on reasonable default behaviors based on the content of your classpath. For example, because the application depends on the embeddable version of Tomcat (tomcat-embed-core.jar), a Tomcat server is set up and configured with reasonable defaults on your behalf. And because the application also depends on Spring MVC (spring-webmvc.jar), a Spring MVC DispatcherServlet is configured and registered for you — no web.xml necessary! Auto-configuration is a powerful, flexible mechanism. See the API documentation for further details.
Logging output is displayed. The service should be up and running within a few seconds.
Now that the web site is running, visit http://localhost:8080/greeting, where you see:
"Hello, World!"
Provide a name query string parameter with http://localhost:8080/greeting?name=User. Notice how the message changes from "Hello, World!" to "Hello, User!":
"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 can always be explicitly overridden through the query string.