How do you implement form validation in Thymeleaf?

Table of Contents

Introduction

Form validation is an essential part of any web application to ensure that user input is correct and meets the required criteria. When working with Thymeleaf and Spring Boot, form validation can be integrated seamlessly using Spring’s validation annotations. These annotations help define rules for user input, and with Thymeleaf, you can display appropriate error messages to the user in real-time.

In this guide, we will walk through how to implement form validation in a Spring Boot application using Thymeleaf. We will cover the use of validation annotations, binding form data, and displaying error messages in Thymeleaf templates.

Steps to Implement Form Validation in Thymeleaf

Step 1: Add Dependencies

First, ensure that you have the necessary dependencies for Spring Boot, Thymeleaf, and Spring Validation in your pom.xml or build.gradle.

For Maven:

For Gradle:

Step 2: Create a Form-Backing Object with Validation Annotations

In Spring Boot, validation is typically performed using annotations from javax.validation.constraints. You can use annotations like @NotNull, @Size, @Email, etc., to validate fields in your form backing object.

Example: Form-Backing Object

In this example:

  • @NotBlank: Ensures that the field is not empty.
  • @Size: Validates the length of the string.
  • @Email: Ensures that the email follows a valid format.

Step 3: Create a Controller with Validation Logic

Next, we need to create a controller method to handle form submissions, bind the form data, and perform validation.

Example: Controller

In this controller:

  • The @Valid annotation triggers validation on the User object.
  • BindingResult is used to capture validation errors.
  • If there are errors (bindingResult.hasErrors()), the form is returned with the error messages.
  • If validation passes, the user object is processed, and the success page is displayed.

Step 4: Create Thymeleaf Templates for the Form and Error Display

Example: userForm.html (Form Template)

In the userForm.html template:

  • th:object="${user}" binds the form to the User object in the model.
  • th:field="*{name}" and th:field="*{email}" automatically bind the input fields to the respective properties of the User object.
  • th:errors="*{name}" and th:errors="*{email}" display the error messages for the respective fields if validation fails.
  • th:if="${#fields.hasErrors('fieldName')}" checks if there are validation errors for a specific field.

Example: userSuccess.html (Success Template)

Step 5: Running the Application

When you run the application and navigate to /user, the form will be displayed. If the user submits the form with invalid data (e.g., an empty name or invalid email), the error messages will appear next to the form fields. Upon successful form submission, the success page (userSuccess.html) will be displayed.

Step 6: Customizing Error Messages (Optional)

You can customize error messages globally by defining messages in a messages.properties file in the src/main/resources directory.

Example: messages.properties

In this file:

  • The NotBlank.user.name and Size.user.name properties correspond to the validation annotations in the User class.
  • The Email.user.email property customizes the email validation message.

Conclusion

By integrating form validation in Thymeleaf with Spring Boot, you can ensure that user input meets the necessary requirements and provide real-time feedback to the user. Using validation annotations such as @NotBlank, @Size, and @Email, you can define the validation rules in your form backing objects, and Thymeleaf templates can display validation error messages accordingly. This integration enhances the user experience by guiding users to correct input errors efficiently.

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