This guide walks you through the process of creating a "hello world" web site with Spring.
You’ll build an application that has a static home page, and also will accept HTTP GET requests at:
http://localhost:8080/greeting
and respond with a web page displaying HTML. The body of the HTML contains 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!"
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 @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 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]A common feature of developing web apps is coding a change, restarting your app, and refreshing the browser to view the change. This entire process can eat up a lot of time. To speed up the cycle of things, Spring Boot comes with a handy module known as spring-boot-devtools.
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Enable hot swapping
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Switches template engines to disable caching
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Enables LiveReload to refresh browser automatically
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Other reasonable defaults based on development instead of production
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]Logging output is displayed. The app 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 body 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.
Static resources, like HTML or JavaScript or CSS, can easily be served from your Spring Boot application just 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 it is used as a "welcome page" if it exists, which means it will be served up as the root resource, i.e. at http://localhost:8080/ in our example. So create this file:
src/main/resources/static/index.html
link:complete/src/main/resources/static/index.html[role=include]and when you restart the app you will see the HTML at http://localhost:8080/.
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