Validating User Input

import javax.validation.constraints.Email;
import javax.validation.constraints.NotBlank;

public class Employee {
    @NotBlank
    private String username;

    private String title;

    @Email(message = "Please enter a valid e-mail address")
    private String email;

    // + other fields, constructors, setters and getters
}

import { Binder, field } from '@vaadin/form';
import EmployeeModel from 'Frontend/generated/com/example/application/EmployeeModel';

...

private binder = new Binder(this, EmployeeModel);

render() {
  const { model } = this.binder;

  return html`
    <vaadin-text-field label="Username"
      ...="${field(model.username)}"></vaadin-text-field>
    <vaadin-text-field label="Title"
      ...="${field(model.title)}"></vaadin-text-field>
    <vaadin-email-field label="Email"
      ...="${field(model.email)}"></vaadin-email-field>
  `;
}

@Retention(RUNTIME)
@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER, TYPE_USE })
@Constraint(validatedBy = { StrongPasswordValidator.class })
public @interface StrongPassword {

    // min required password strength on the scale from 1 to 5
    int minStrength() default 4;

    String message() default "Please enter a strong password";
    Class<?>[] groups() default { };
    Class<? extends Payload>[] payload() default { };
}

public class StrongPasswordValidator implements ConstraintValidator<StrongPassword, String> {

    @Override
    public boolean isValid(String object, ConstraintValidatorContext constraintContext) {
        // Use the zxcvbn library to measure the password strength
        Strength strength = zxcvbn.measure(object);

        // fail the validation if the measured strength is insufficient
        if (strength.getScore() < minStrength) {
            constraintContext
                .buildConstraintViolationWithTemplate(
                        strength.getFeedback().getWarning())
                .addConstraintViolation();
            return false;
        }

        return true;
    }
}

<dependency>
    <groupId>com.nulab-inc</groupId>
    <artifactId>zxcvbn</artifactId>
    <version>1.3.0</version>
</dependency>

public class Employee {

    @StrongPassword
    private String password;
}

private binder = new Binder(this, EmployeeModel);

render() {
  const { model } = this.binder;

  return html`
    <vaadin-password-field label="Password"
      ...="${field(model.password)}"></vaadin-password-field>

    <vaadin-button @click="${this.save}">Save</vaadin-button>
  `;
}

import { Binder, field, NotEmpty, Min, Size, Email } from '@vaadin/form';

@customElement('my-demo-view')
export class MyDemoView extends LitElement {
  private binder = new Binder(this, PersonModel);

  protected firstUpdated(_changedProperties: any) {
    super.firstUpdated(args);

    const model = this.binder.model;

    this.binder.for(model.name).addValidator(
      new NotEmpty({
        message: 'Please enter a name'
      }));

    this.binder.for(model.username).addValidator(
      new Size({
        message: 'Please pick a username 3 to 15 symbols long',
        min: 3,
        max: 15
      }));

    this.binder.for(model.age).addValidator(
      new Min({
        message: 'Please enter an age of 18 or above',
        value: 18
      }));

    this.binder.for(model.email).addValidator(new Email());
  }

  render() {
    const model = this.binder.model;
    return html`
      <vaadin-text-field label="Name"
        ...="${field(model.name)}"></vaadin-text-field>
      <vaadin-text-field label="Username"
        ...="${field(model.username)}"></vaadin-text-field>
      <vaadin-integer-field label="Age"
        ...="${field(model.age)}"></vaadin-integer-field>
      <vaadin-email-field label="Email"
        ...="${field(model.email)}"></vaadin-email-field>
    `;
  }
}