Spring 3 MVC: Using @ModelAttribute in Your JSPs

In this tutorial we will create a simple Spring 3 MVC application that uses JSPs for the presentation layer. We will create a simnple CRUD system using the @ModelAttribute

What is @ModelAttribute

@ModelAttribute has two usage scenarios in controllers. When you place it on a method parameter, @ModelAttribute maps a model attribute to the specific, annotated method parameter (see the processSubmit() method below). This is how the controller gets a reference to the object holding the data entered in the form.

You can also use @ModelAttribute at the method level to provide reference data for the model (see the populatePetTypes() method in the following example). For this usage the method signature can contain the same types as documented previously for the @RequestMapping annotation.

Note
@ModelAttribute annotated methods are executed before the chosen @RequestMapping annotated handler method. They effectively pre-populate the implicit model with specific attributes, often loaded from a database. Such an attribute can then already be accessed through @ModelAttribute annotated handler method parameters in the chosen handler method, potentially with binding and validation applied to it.

Source: Spring 3 Reference Documentation

The @ModelAttribute is a convenient annotation for exposing your objects to your JSP pages, and returning the object back to your Controllers.

If you're familiar with the classin Spring of doing things, this annotation combines functionalities of a formBackingObject (or commandObject) and SimpleFormController's referenceData. However, as of Spring 3.0, these have been deprecated in favor of annotated controllers (Source: Spring 3 API SimpleFormController)

Let's start by declaring the required Spring configurations.

To enable Spring MVC we need to add it in the web.xml

web.xml

Take note of the URL pattern. When accessing any pages in our MVC application, the host name must be appended with

/krams

In the web.xml we declared a servlet-name spring. By convention, we must declare a spring-servlet.xml as well.

spring-servlet.xml


This XML config declares a view resolver. All references to a JSP name in the controllers will map to a corresponding JSP in the /WEB-INF/jsp location.

By convention, we must declare an applicationContext.xml

applicationContext.xml

This XML config declares three beans to activate the Spring 3 MVC programming model.

We're done with the required Spring XML configurations. We now focus on the two usage patterns of the @ModelAttribute annotation.

Pattern 1: Method Level

You can also use @ModelAttribute at the method level to provide reference data for the model (see the populatePetTypes() method in the following example). For this usage the method signature can contain the same types as documented previously for the @RequestMapping annotation.
Source: Spring 3 Reference Documentation

This means we declare a method in our controller and annotate the method with @ModelAttribute.

We will examine this pattern by displaying a list of addresses. First let's declare our controller.

AddressController

The controller has a method named getAllAddresses() that's been annotated with @ModelAttribute. We also provide a model name addresses

The model attribute addresses is referenced in the JSP page.

addressespage.jsp

In this controller we have to mappings:

/address/list1
/address/list2

Both mappings will yield the same result. However the models are retrieved in different ways.

The getAllUsingModelAttribute() method doesn't pass any model but instead relies on the @ModelAttribute for its data

Here's the actual screenshot of the JSP page from /address/list1

The getAllUsingModel() method retrieves the model by manually adding it on the Model. It also adds an extra model data named greetings

Here's the actual screenshot of the JSP page from /address/list2

They both produce the same list of addresses. So what's the difference? I don't see any except for one major difference:

@ModelAttribute annotated methods are executed before the chosen @RequestMapping annotated handler method. They effectively pre-populate the implicit model with specific attributes, often loaded from a database.

Source: Spring 3 Reference Documentation

Using the @ModelAttribute automatically prepopulates your list. If you're doing some updates in your controller, you might still get the old list unless you specifically override the list from the @ModelAttribute.

This is better explained when we move to the next usage pattern.

Pattern 2: Method Parameter Level

When you place it on a method parameter, @ModelAttribute maps a model attribute to the specific, annotated method parameter (see the processSubmit() method below). This is how the controller gets a reference to the object holding the data entered in the form.rameters in the chosen handler method, potentially with binding and validation applied to it.

Source: Spring 3 Reference Documentation

This means we have a method with a parameter and we annotate that parameter with a @ModelAttribute. The purpose of this pattern is to pass the data from the JSP page back to your controller. The data is automatically converted to a Java object.

We will examine this pattern by displaying a list of persons that we can edit and update. This example will also showcase some of Spring 3's RESTful annotations.

We begin by defining our main controller.

MainController.

This controller declares two @ModelAttribute at the method level. We already know what that does. Then there's a @ModelAttribute at the method parameter level:

This basically means your JSP page is sending a model attribute named personAttribute. Your controller picks this model attribute and assign it to a Java object Person. In this way you're manipulating an object instead of HTTP request parameters.

In this controller we have three mappings:

/main - retrieve all persons
/main/edit/{id} - (GET) retrieve and edit a person by his id 
/main/edit/{id} - (POST) save a person based on his id

Notice we have two /main/edit/{id}. How does our controller know which one to call? The controller's @RequestMapping doesn't just rely on the mapping value but it also uses the method type. In our case, it's either POST or GET. The GET method is used when we retrieve a page; whereas, the POST method is used when we're submitting a form. For more info, please check the following blog from SpringSource Annotated Web MVC Controllers in Spring 2.5

Also, we're using a special identifier in the mappings. We have declared a {id} in the path, and referenced that as @PathVariable in the method parameter. This is a URI template, one of the RESTful features of Spring 3 MVC.

What is a URI template?

A URI template is a URI-like string, containing one or more variable names. When these variables are substituted for values, the template becomes a URI. For more information, see the proposed RFC.

Source: REST in Spring 3: @MVC

For a thorough description of this subject, please visit the blog from SpringSourceREST in Spring 3: @MVC

Let's examine the associate JSP view for each mappings.

personspage.jsp

This is referenced by the mapping /main. There's nothing special here. This is exactly similar with our Address example earlier.

editpage.jsp

This JSP page is returned by the following controller method:

This method searches a person based on his id. If found, a model attribute is added in the Model. The name of the attribute is personAttribute. This can be any name.

To access the edit page, we need to manually type the following URL in the browser:

http://localhost:8080/spring-jsp-model-attribute/krams/main/edit/2

Just make sure to change the number to match the id that you want to edit.

Here's the Edit page:

When we're done editing the person, we submit the form.

Notice we use POST to submit the data. We also use the same model attribute name. We also pass the id in the action URL.

Once the data is sent, it is picked by the controller and assigned to the saveEdit method.

I advise my readers to read the important comments within this method. To display again the list of persons we can either pass the model manually or rely on the @ModelAttribute on the Method Level. However, there's a problem if you follow the Method Level. It doesn't show the updated list. You have to refresh the browser again to see it. Why?

Note
@ModelAttribute annotated methods are executed before the chosen @RequestMapping annotated handler method. They effectively pre-populate the implicit model with specific attributes, often loaded from a database.

Source: Spring 3 Reference Documentation

Because the list is populated even before it's processed by the method! As a workaround, you must pass the model manually so that you don't need to refresh the browser.

Here's the final JSP page.

Our application is now finished. We've managed to build a simple Spring 3 MVC application that uses JSPs for the presentation layer. We've also discussed the usage patterns of the @ModelAttribute.

The best way to learn further is to try the actual application.

Download the project
You can access the project site at Google's Project Hosting at http://code.google.com/p/spring-mvc-jsp-model-attribute/

You can download the project as a Maven build. Look for the spring-jsp-model-attribute.zip in the Download sections.

You can run the project directly using an embedded server via Maven.
For Tomcat: mvn tomcat:run
For Jetty: mvn jetty:run

If you want to learn more about Spring MVC and Jasper, feel free to read my other tutorials in the Tutorials section.

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