You can use Bean Validation to constrain your data in many different ways. In this recipe, we are going to use it to validate a JSF form so that we can validate it as soon as the user tries to submit it and avoid any invalid data right away.

First, we need to add our dependencies:

<dependency>
    <groupId>javax</groupId>
    <artifactId>javaee-api</artifactId>
    <version>8.0</version>
    <scope>provided</scope>
</dependency>

You need to perform the following steps to complete this recipe:

1. Let's create a User object that will be attached to our JSF page:

@Named
@RequestScoped
public class User {
    
    @NotBlank (message = "Name should not be blank")
    @Size (min = 4, max = 10,message = "Name should be between 
    4 and 10 characters")
    private String name;
    
    @Email (message = "Invalid e-mail format")
    @NotBlank (message = "E-mail should not be blank")
    private String email;
    
    @PastOrPresent (message = "Created date should be 
    past or present")
    @NotNull (message = "Create date should not be null")
    private LocalDate created;
    
    @Future (message = "Expires should be a future date")
    @NotNull (message = "Expires should not be null")
    private LocalDate expires;


    //DO NOT FORGET TO IMPLEMENT THE GETTERS AND SETTERS

    ...

2. Now, we need to define the method that will be fired once all the data is valid:

    public void valid(){
        FacesContext
                .getCurrentInstance()
                .addMessage(
                    null, 
                    new FacesMessage(FacesMessage.SEVERITY_INFO,
                    "Your data is valid", ""));
    }

3. Now, our JSF page references each User class field that's been declared, as follows:

<h:body>
 <h:form>
 <h:outputLabel for="name" value="Name" />
 <h:inputText id="name" value="#{user.name}" />
 <br/>
 <h:outputLabel for="email" value="E-mail" />
 <h:inputText id="email" value="#{user.email}" />
 <br/>
 <h:outputLabel for="created" value="Created" />
 <h:inputText id="created" value="#{user.created}">
     <f:convertDateTime type="localDate" pattern="dd/MM/uuuu" /> 
 </h:inputText>
 <br/>
 <h:outputLabel for="expire" value="Expire" />
 <h:inputText id="expire" value="#{user.expires}">
     <f:convertDateTime type="localDate" pattern="dd/MM/uuuu" /> 
 </h:inputText>
 <br/>
 <h:commandButton value="submit" type="submit" action="#{user.valid()}" />
 </h:form>
</h:body>

If you run this code, all the fields will be validated once you click the Submit button. Try it for yourself!

Let's check each declared constraint:

    @NotBlank (message = "Name should not be blank")
    @Size (min = 4, max = 10,message = "Name should be between 
           4 and 10 characters")
    private String name;

The @NotBlank annotation will deny not only null values, but also whitespace values, and @Size speaks for itself:

    @Email (message = "Invalid e-mail format")
    @NotBlank (message = "E-mail shoud not be blank")
    private String email;

The @Email constraint will check the email string format:

    @PastOrPresent (message = "Created date should be past or present")
    @NotNull (message = "Create date should not be null")
    private LocalDate created;

@PastOrPresent will constrain LocalDate to be in the past or until the present date. It can't be in the future.

Here, we can't use @NotBlank as there is no blank date, only null, so we avoid this by using @NotNull:

    @Future (message = "Expires should be a future date")
    @NotNull (message = "Expires should not be null")
    private LocalDate expires;

This is the same as the preceding constraint, but constraints for a future date.

In our UI, there are two places we should take a careful look at:

 <h:inputText id="created" value="#{user.created}">
     <f:convertDateTime type="localDate" pattern="dd/MM/uuuu" /> 
 </h:inputText>

 ...

 <h:inputText id="expire" value="#{user.expires}">
     <f:convertDateTime type="localDate" pattern="dd/MM/uuuu" /> 
 </h:inputText>

Here, we are using convertDateTime to automatically convert the data that's inputted into inputText according to the dd/MM/uuuu pattern.

You can view the full source code for this recipe at https://github.com/eldermoraes/javaee8-cookbook/tree/master/chapter02/ch02-beanvalidation.