MVC fixes this by giving every concern its own place:

• Data lives in the Model — and nowhere else
• Display lives in the View — and nowhere else
• Logic lives in the Controller — and nowhere else

Implicit — return a filename

// Returns to "dashboard.xhtml"
public String save() {
    productService.persist(product);
    return "dashboard";
}

// Returns to "error.xhtml"
public String save() {
    return "error";
}

Explicit — faces-config.xml

<navigation-rule>
  <from-view-id>/login.xhtml</from-view-id>
  <navigation-case>
    <from-outcome>success</from-outcome>
    <to-view-id>/home.xhtml</to-view-id>
  </navigation-case>
  <navigation-case>
    <from-outcome>failure</from-outcome>
    <to-view-id>/login.xhtml</to-view-id>
  </navigation-case>
</navigation-rule>

Server-Side Forward (default)

If the user presses Refresh, the form is re-submitted. This can create duplicate records.

Redirect (best practice for POST actions)

// Add ?faces-redirect=true to trigger redirect
return "products?faces-redirect=true";

Built-in converters (f:)

Handle common data type conversions automatically:

Built-in validators (f:)

Enforce common range and length rules:

When you need a custom solution

• Custom Converter: when the data type is unique — e.g., your application has a special "ProductCode" format (PRD-####)
• Custom Validator: when the rule cannot be expressed as a range — e.g., "the username must not already exist in the database"

@FacesConverter("productCodeConverter")
public class ProductCodeConverter
         implements Converter<String> {

  /**
   * Called during Phase 2-3:
   * Browser sends "prd-0042" →
   * we return clean "PRD-0042"
   */
  @Override
  public String getAsObject(
        FacesContext ctx,
        UIComponent comp,
        String value) {

    if (value == null || value.isEmpty())
      return null;

    // Uppercase and validate format
    String upper = value.toUpperCase().trim();
    if (!upper.matches("PRD-\\d{4}")) {
      throw new ConverterException(
        new FacesMessage(
          "Format must be PRD-####"));
    }
    return upper;
  }

  /**
   * Called during Phase 6:
   * Java → display string for browser
   */
  @Override
  public String getAsString(
        FacesContext ctx,
        UIComponent comp,
        String value) {
    return (value != null) ? value : "";
  }
}

Use it in your Facelet

<h:inputText
  value="#{bean.productCode}">
  <f:converter
    converterId="productCodeConverter"/>
</h:inputText>
<h:message for="code"/>

@FacesValidator("uniqueUsernameValidator")
public class UniqueUsernameValidator
       implements Validator<String> {

  @Override
  public void validate(
        FacesContext context,
        UIComponent component,
        String username) {

    // Simulate checking the database
    if (usernameAlreadyExists(username)) {
      FacesMessage msg = new FacesMessage(
        FacesMessage.SEVERITY_ERROR,
        "Username already taken.",
        "Please choose another.");
      throw new ValidatorException(msg);
    }
  }

  private boolean usernameAlreadyExists(
        String u) {
    // Check against UserService / DB
    return List.of("admin","root")
               .contains(u);
  }
}

Use it in your Facelet

<h:inputText id="uname"
  value="#{regBean.username}">
  <f:validator
    validatorId="uniqueUsernameValidator"/>
</h:inputText>
<h:message for="uname"
            styleClass="error"/>