1. Overview
One of the most important Spring MVC annotations is the @ModelAttribute annotation.
@ModelAttribute is an annotation that binds a method parameter or method return value to a named model attribute, and then exposes it to a web view.
In this tutorial, we’ll demonstrate the usability and functionality of this annotation through a common concept, a form submitted from a company’s employee.
2. @ModelAttribute in Depth
As the introductory paragraph revealed, we can use @ModelAttribute either as a method parameter or at the method level.
2.1. At the Method Level
When we use the annotation at the method level, it indicates the purpose of the method is to add one or more model attributes. Such methods support the same argument types as @RequestMapping methods, but they can’t be mapped directly to requests.
Let’s look at a quick example here to understand how this works:
@ModelAttribute
public void addAttributes(Model model) {
model.addAttribute("msg", "Welcome to the Netherlands!");
}
In the above example, we see a method that adds an attribute named msg to all models defined in the controller class.
Of course, we’ll see this in action later in the article.
In general, Spring MVC will always make a call to that method first, before it calls any request handler methods. Basically, @ModelAttribute methods are invoked before the controller methods annotated with @RequestMapping are invoked. This is because the model object has to be created before any processing starts inside the controller methods.
It’s also important that we annotate the respective class as @ControllerAdvice. Thus, we can add values in Model that’ll be identified as global. This actually means that for every request, a default value exists for every method in the response.
2.2. As a Method Argument
When we use the annotation as a method argument, it indicates to retrieve the argument from the model. When the annotation isn’t present, it should first be instantiated, and then added to the model. Once present in the model, the arguments fields should populate from all request parameters that have matching names.
In the following code snippet, we’ll populate the employee model attribute with data from a form submitted to the addEmployee endpoint. Spring MVC does this behind the scenes before invoking the submit method:
@RequestMapping(value = "/addEmployee", method = RequestMethod.POST)
public String submit(@ModelAttribute("employee") Employee employee) {
// Code that uses the employee object
return "employeeView";
}
Later in this article, we’ll see a complete example of how to use the employee object to populate the employeeView template.
It binds the form data with a bean. The controller annotated with @RequestMapping can have custom class argument(s) annotated with @ModelAttribute.
In Spring MVC, we refer to this as data binding, a common mechanism that saves us from having to parse each form field individually.
3. Form Example
In this section, we’ll look at the example outlined in the overview section, a very basic form that prompts a user (specifically a company employee) to enter some personal information (specifically name and id). After the submission is complete, and without any errors, the user expects to see the previously submitted data displayed on another screen.
3.1. The View
Let’s first create a simple form with id and name fields:
<form:form method="POST" action="/spring-mvc-basics/addEmployee"
modelAttribute="employee">
<form:label path="name">Name</form:label>
<form:input path="name" />
<form:label path="id">Id</form:label>
<form:input path="id" />
<input type="submit" value="Submit" />
</form:form>
3.2. The Controller
Here’s the controller class, where we’ll implement the logic for the aforementioned view:
@Controller
@ControllerAdvice
public class EmployeeController {
private Map<Long, Employee> employeeMap = new HashMap<>();
@RequestMapping(value = "/addEmployee", method = RequestMethod.POST)
public String submit(
@ModelAttribute("employee") Employee employee,
BindingResult result, ModelMap model) {
if (result.hasErrors()) {
return "error";
}
model.addAttribute("name", employee.getName());
model.addAttribute("id", employee.getId());
employeeMap.put(employee.getId(), employee);
return "employeeView";
}
@ModelAttribute
public void addAttributes(Model model) {
model.addAttribute("msg", "Welcome to the Netherlands!");
}
}
In the submit() method, we have an Employee object bound to our View. We can map our form fields to an object model as simply as that. In the method, we’re fetching values from the form and setting them to ModelMap.
In the end, we return employeeView, which means that we call the respective JSP file as a View representative.
Furthermore, there’s also an addAttributes() method. Its purpose is to add values in the Model that’ll be identified globally. That is, every request to every controller method will return a default value as a response. We also have to annotate the specific class as @ControllerAdvice.
3.3. The Model
As previously mentioned, the Model object is very simple and contains all that the “front-end” attributes require. Now let’s have a look at an example:
@XmlRootElement
public class Employee {
private long id;
private String name;
public Employee(long id, String name) {
this.id = id;
this.name = name;
}
// standard getters and setters removed
}
3.4. Wrap Up
@ControllerAdvice assists a controller, and in particular, @ModelAttribute methods that apply to all @RequestMapping methods. Of course, our addAttributes() method will be the very first to run, prior to the rest of the @RequestMapping methods.
Keeping that in mind, and after both submit() and addAttributes() are run, we can refer to them in the View returned from the Controller class by mentioning their given name inside a dollarized curly-braces duo, like ${name}.
3.5. Results View
Now let’s print what we received from the form:
<h3>${msg}</h3>
Name : ${name}
ID : ${id}
4. Conclusion
In this article, we investigated the use of the @ModelAttribute annotation for both method arguments and method level use cases*.*
The implementation of this article can be found in the github project.