This guide walks you through the process of configuring a web application form to support validation.

What you’ll build

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.

What you’ll need

How to complete this guide

Like most Spring Getting Started guides, you can start from scratch and complete each step or you can bypass basic setup steps that are already familiar to you. Either way, you end up with working code.

To start from scratch, move on to Build with Gradle.

To skip the basics, do the following:

When you finish, you can check your results against the code in gs-validating-form-input/complete.

Build with Gradle

Build with Gradle

First you set up a basic build script. You can use any build system you like when building apps with Spring, but the code you need to work with Gradle and Maven is included here. If you’re not familiar with either, refer to Building Java Projects with Gradle or Building Java Projects with Maven.

Create the directory structure

In a project directory of your choosing, create the following subdirectory structure; for example, with mkdir -p src/main/java/hello on *nix systems:

└── src
    └── main
        └── java
            └── hello

Create a Gradle build file

Below is the initial Gradle build file.

build.gradle

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:2.2.1.RELEASE")
    }
}

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'idea'
apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management'

bootJar {
    baseName = 'gs-validating-form-input'
    version =  '0.1.0'
}

repositories {
    mavenCentral()
}

sourceCompatibility = 1.8
targetCompatibility = 1.8

dependencies {
    compile("org.springframework.boot:spring-boot-starter-thymeleaf")
    compile("org.springframework.boot:spring-boot-starter-web")
    compile("org.hibernate.validator:hibernate-validator")
    compile("org.apache.tomcat.embed:tomcat-embed-el")
    testCompile("org.springframework.boot:spring-boot-starter-test")
}

The Spring Boot gradle plugin provides many convenient features:

  • It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.

  • It searches for the public static void main() method to flag as a runnable class.

  • It provides a built-in dependency resolver that sets the version number to match Spring Boot dependencies. You can override any version you wish, but it will default to Boot’s chosen set of versions.

Build with Maven

Build with Maven

First you set up a basic build script. You can use any build system you like when building apps with Spring, but the code you need to work with Maven is included here. If you’re not familiar with Maven, refer to Building Java Projects with Maven.

Create the directory structure

In a project directory of your choosing, create the following subdirectory structure; for example, with mkdir -p src/main/java/hello on *nix systems:

└── src
    └── main
        └── java
            └── hello

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.springframework</groupId>
    <artifactId>gs-validating-form-input</artifactId>
    <version>0.1.0</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.2.1.RELEASE</version>
    </parent>

    <properties>
        <java.version>1.8</java.version>
     </properties>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-thymeleaf</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.hibernate.validator</groupId>
            <artifactId>hibernate-validator</artifactId>
        </dependency>
        <dependency>
            <groupId>org.apache.tomcat.embed</groupId>
            <artifactId>tomcat-embed-el</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

</project>

The Spring Boot Maven plugin provides many convenient features:

  • It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.

  • It searches for the public static void main() method to flag as a runnable class.

  • It provides a built-in dependency resolver that sets the version number to match Spring Boot dependencies. You can override any version you wish, but it will default to Boot’s chosen set of versions.

Build with your IDE

Build with your IDE

Create a PersonForm object

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

package hello;

import javax.validation.constraints.Min;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;

public class PersonForm {

  @NotNull
  @Size(min=2, max=30)
  private String name;

  @NotNull
  @Min(18)
  private Integer age;

  public String getName() {
    return this.name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public Integer getAge() {
    return age;
  }

  public void setAge(Integer age) {
    this.age = age;
  }

  public String toString() {
    return "Person(Name: " + this.name + ", Age: " + this.age + ")";
  }
}

The PersonForm class has two attributes: name and age. It is flagged with several standard validation annotations:

  • @Size(min=2, max=30) will only allow names between 2 and 30 characters long

  • @NotNull won’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.

Create a web controller

Now that you have defined a form backing object, it’s time to create a simple web controller.

src/main/java/hello/WebController.java

package hello;

import javax.validation.Valid;

import org.springframework.stereotype.Controller;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.servlet.config.annotation.ViewControllerRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;


@Controller
public class WebController implements WebMvcConfigurer {

  @Override
  public void addViewControllers(ViewControllerRegistry registry) {
    registry.addViewController("/results").setViewName("results");
  }

  @GetMapping("/")
  public String showForm(PersonForm personForm) {
    return "form";
  }

  @PostMapping("/")
  public String checkPersonInfo(@Valid PersonForm personForm, BindingResult bindingResult) {

    if (bindingResult.hasErrors()) {
      return "form";
    }

    return "redirect:/results";
  }
}

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 personForm object marked up with @Valid to gather the attributes filled out in the form you’re about to build.

  • A bindingResult object 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.

Build an HTML front end

Now you build the "main" page.

src/main/resources/templates/form.html

<html>
    <body>
        <form action="#" th:action="@{/}" th:object="${personForm}" method="post">
            <table>
                <tr>
                    <td>Name:</td>
                    <td><input type="text" th:field="*{name}" /></td>
                    <td th:if="${#fields.hasErrors('name')}" th:errors="*{name}">Name Error</td>
                </tr>
                <tr>
                    <td>Age:</td>
                    <td><input type="text" th:field="*{age}" /></td>
                    <td th:if="${#fields.hasErrors('age')}" th:errors="*{age}">Age Error</td>
                </tr>
                <tr>
                    <td><button type="submit">Submit</button></td>
                </tr>
            </table>
        </form>
    </body>
</html>

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 personForm 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

<html>
	<body>
		Congratulations! You are old enough to sign up for this site.
	</body>
</html>
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.

Create an Application class

For this application, you are using the template language of Thymeleaf. This application needs more than raw HTML.

src/main/java/hello/Application.java

package hello;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {

  public static void main(String[] args) throws Exception {
    SpringApplication.run(Application.class, args);
  }

}

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.

Build an executable JAR

You can run the application from the command line with Gradle or Maven. You can also build a single executable JAR file that contains all the necessary dependencies, classes, and resources and run that. Building an executable jar so makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.

If you use Gradle, you can run the application by using ./gradlew bootRun. Alternatively, you can build the JAR file by using ./gradlew build and then run the JAR file, as follows:

java -jar build/libs/gs-validating-form-input-0.1.0.jar

If you use Maven, you can run the application by using ./mvnw spring-boot:run. Alternatively, you can build the JAR file with ./mvnw clean package and then run the JAR file, as follows:

java -jar target/gs-validating-form-input-0.1.0.jar
The steps described here create a runnable JAR. You can also build a classic WAR file.

The application should be up and running within a few seconds.

If you visit http://localhost:8080/, you should see something like this:

valid 01

What happens if you enter A for the name and 15 for your age and click on Submit?

valid 02
valid 03

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.

valid 04

If you enter a valid name and age, you end up on the results page!

valid 05

Summary

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.

See Also

The following guides may also be helpful:

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All guides are released with an ASLv2 license for the code, and an Attribution, NoDerivatives creative commons license for the writing.