Using @ModelAttribute annotation in Spring MVC

Background

I developed a chat application with Spring MVC, and here are the codes in the ChatController Class and the chat.html page:

ChatController

chat.html

Problem

Notice that I used the newChatForm identifier in th:object present in the chat.html page to name the ChatForm parameter both in the getChatPage and addChat method.

But when I removed the @ModelAttribute("newChatForm") in one of those methods, I got the error below:

What’s wrong

Since I included th:object in the chat.html page, and so for the mapping to be done, a ChatForm object needs to be included in the Spring Container to be used by Spring MVC to map the form data.

Spring MVC will check in the Spring Container if there is any instance with the name similar to th:object. If there is one it ends up using that instance in the mapping.

But when I only put ChatForm newChatForm in the method parameter, I am telling Spring that this parameter must be instantiated (with the default name chatForm, which is the camelCase pattern of ChatForm), because it will be used in the method as well as the Model. The ChatForm is only included in the Spring Container not being included by the Model in the chat.html page.

Since the default name chatForm is different from the newChatForm identifier in th:object present in the chat.html, Spring MVC can’t find any instance to map the form data, which causes the error I received.

However, when I use @ModelAttribute("newChatForm"), I am indicating to Spring that besides instantiating the ChatForm, this ChatForm must be included in the page through the newChatForm identifier, since this inclusion is made through the Model. So no error is thrown.

So, the inclusion of the @ModelAttribute depends on the identifiers being used, if both are equal, its inclusion is not necessary, otherwise, we need it so that a mapping error does not occur.

Solution

There are two ways to solve this problem:

1. Use chatForm identifier both in chat.html and in the ChatController method parameter

   chat.html

   <form th:action="@{'/chat'}" th:object="${chatForm}" action="#" method="POST">
     // nothing changes
   </form>

   ChatController.java

   @GetMapping
   public String getChatPage(ChatForm chatForm, Model model) {
       // nothing changes
   }
   
   @PostMapping
   public String addChat(ChatForm chatForm, Model model) {
       // change variable name from "newChatForm" to "chatForm"
   }

   This way Spring MVC directly uses the ChatForm instance in the Spring Container for mapping, so we don’t need to use @ModelAttribute.

   However, this approach requires that other developers have some knowledge of how Spring works under the hood, otherwise they will probably delete the ChatForm thinking that this parameter is not doing anything.

2. Create the getChatForm method to return the instance

   Every time the "/chat" endpoint is triggered, Spring will automatically assign a ChatForm instance to the chat.html page.

   For improving the readability of the code by showing more clearly the process of assigning the instance by Spring,

   We can create the getChatForm method which will be responsible for including the object in the Spring Container and which can be used by Spring MVC for mapping:

   ChatController.java

   @ModelAttribute("newChatForm")
   public ChatForm getChatForm {
       return new ChatForm();
   }

   This way we can remove the @ModelAttribute("newChatForm") from both methods and the ChatForm from the getChatPage method:

   ChatController.java

   @GetMapping
   public String getChatPage(Model model) {
       // nothing changes
   }
   
   @PostMapping
   public String addChat(ChatForm newChatForm, Model model) {
       // nothing changes
   }

References

Difference between using and not using @modelAttribute in method parameter

What is the meaning of @ModelAttribute annotation at method argument level?

Using @ModelAttribute annotation in Spring MVC

Remove @ModelAttribute(“newChatForm”) and get error

Why we should add @ModelAttribute in GET method?