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This guide walks you through the process of configuring a web application form to support validation.
You’ll build a simple Spring MVC application that take user input and checks the input using standard validation annotations. You’ll also see how to display the error message on the screen so the user can re-enter a valid input.
The application involves validating a user’s name and age, so first you need to create a class to back the form to create a person.
src/main/java/hello/PersonForm.java
link:complete/src/main/java/hello/PersonForm.java[role=include]The PersonForm class has two attributes: name and age. It is flagged with several standard validation annotations:
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@Size(min=2, max=30)will only allow names between 2 and 30 characters long -
@NotNullwon’t allow a null value, which is what Spring MVC generates if the entry is empty -
@Min(18)won’t allow if the age is less than 18
In addition to that, you can also see getters/setters for name and age as well as a convenient toString() method.
Now that you have defined a form backing object, it’s time to create a simple web controller.
src/main/java/hello/WebController.java
link:complete/src/main/java/hello/WebController.java[role=include]This controller has a GET and a POST method, both mapped to /.
The showForm method returns the form template. It includes a PersonForm in its method signature so the template can associate form attributes with a PersonForm.
The checkPersonFormInfo method accepts two arguments:
-
A
personFormobject marked up with@Validto gather the attributes filled out in the form you’re about to build. -
A
bindingResultobject so you can test for and retrieve validation errors.
You can retrieve all the attributes from the form bound to the PersonForm object. In the code, you test for errors, and if so, send the user back to the original form template. In that situation, all the error attributes are displayed.
If all of the person’s attribute are valid, then it redirects the browser to the final results template.
Now you build the "main" page.
src/main/resources/templates/form.html
link:complete/src/main/resources/templates/form.html[role=include]The page contains a simple form with each field in a separate slot of a table. The form is geared to post towards /. It is marked as being backed up by the person object that you saw in the GET method in the web controller. This is known as a bean-backed form. There are two fields in the PersonForm bean, and you can see them tagged th:field="{name}" and th:field="{age}". Next to each field is a secondary element used to show any validation errors.
Finally, you have a button to submit. In general, if the user enters a name or age that violates the @Valid constraints, it will bounce back to this page with the error message on display. If a valid name and age is entered, the user is routed to the next web page.
src/main/resources/templates/results.html
link:complete/src/main/resources/templates/results.html[role=include]|
Note
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In this simple example, these web pages don’t have any sophisticated CSS or JavaScript. But for any production web site, it’s valuable to learn how to style your web pages. |
For this application, you are using the template language of Thymeleaf. This application needs more than raw HTML.
src/main/java/hello/Application.java
link:complete/src/main/java/hello/Application.java[role=include]To activate Spring MVC, you would normally add @EnableWebMvc to the Application class. But Spring Boot’s @SpringBootApplication already adds this annotation when it detects spring-webmvc on your classpath. This same annotation allows it to find the annotated @Controller class and its methods.
The Thymeleaf configuration is also taken care of by @SpringBootApplication: by default templates are located in the classpath under templates/ and are resolved as views by stripping the '.html' suffix off the file name. Thymeleaf settings can be changed and overridden in a variety of ways depending on what you need to achieve, but the details are not relevant to this guide.
The application should be up and running within a few seconds.
If you visit http://localhost:8080/, you should see something like this:
What happens if you enter A for the name and 15 for your age and click on Submit?
Here you can see that because it violated the constraints in the PersonForm class, you get bounced back to the "main" page. If you click on Submit with nothing in the entry box, you get a different error.
If you enter a valid name and age, you end up on the results page!
Congratulations! You have coded a simple web application with validation built into a domain object. This way you can ensure the data meets certain criteria and that the user inputs it correctly.