Resolving Freemarker.core.InvalidReferenceException During User Input Validation in Java

Handling InvalidReferenceException in Apache FreeMarker

Java with Spring Boot - Backend Validation Approach

// Backend Controller for Registration Form Handling
@PostMapping("/registration")
public String registration(@ModelAttribute @Valid UserForm userForm,
                               BindingResult result, Model model) {
    // Validate user form using a custom validator
    userValidator.validate(userForm, result);
    // Attach validation errors to the model
    model.addAttribute("errors", result);
    // Check if there are errors in form input
    if (result.hasErrors()) {
        return "registration"; // Return to the registration page
    }
    return "redirect:/"; // Redirect to home page upon success
}

Optimized Template for Error Handling in FreeMarker

Freemarker Template (.ftlh) Approach for Dynamic Error Handling

<form action="/registration" method="POST">
<label for="name">Name:</label>
<input type="text" id="name" name="name" value="${userForm.name!}" required>
<#if errors?? && errors.hasFieldErrors("name")>
    <div style="color:red;">${errors.getFieldError('name')!['defaultMessage']}</div>
</#if>

<label for="email">Email:</label>
<input type="email" id="email" name="email" value="${userForm.email!}" required>
<#if errors?? && errors.hasFieldErrors("email")>
    <div style="color:red;">${errors.getFieldError('email')!['defaultMessage']}</div>
</#if>

<button type="submit">Register</button>
</form>

Unit Testing the Controller and Validation Process

JUnit 5 and MockMVC for Backend Testing

@WebMvcTest(RegistrationController.class)
public class RegistrationControllerTest {
    @Autowired
    private MockMvc mockMvc;

    @Test
    public void shouldReturnErrorMessagesForInvalidInput() throws Exception {
        mockMvc.perform(post("/registration")
                .param("name", "")
                .param("email", "invalid-email"))
                .andExpect(status().isOk())
                .andExpect(model().attributeHasFieldErrors("userForm", "name", "email"))
                .andExpect(view().name("registration"));
    }
}

Common Questions and Solutions for FreeMarker InvalidReferenceException

1. What is InvalidReferenceException in FreeMarker?
2. InvalidReferenceException occurs when FreeMarker attempts to access a missing or null variable. Using !myDefault in expressions helps handle null values.
3. How can I avoid the null errors in FreeMarker templates?
4. Incorporate the ?? operator to check if a value exists and apply a default fallback using the !myDefault operator.
5. Why does my error handling code fail in FreeMarker?
6. If you use the getFieldError() method in FreeMarker, make sure that the BindingResult object is passed to the model in the backend for proper validation handling.
7. How does the BindingResult object work in Spring Boot?
8. BindingResult holds the result of form validation. It captures errors, which can be displayed in the FreeMarker template for each field.
9. How can I implement a custom validator in Spring Boot?
10. To create a custom validator, define a class that implements the ConstraintValidator interface, and apply it to fields that require custom validation logic.

Wrapping Up the Key Insights

Handling errors like InvalidReferenceException in FreeMarker requires attention to both backend validation and frontend template handling. Ensuring the BindingResult object is properly populated and passed to the view is key in avoiding null references during form validation.

By implementing safe checks for null values and providing fallback defaults, you can prevent crashes and deliver a better user experience. Understanding how to synchronize form data validation with template rendering is essential for building robust Java web applications using FreeMarker.