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Sean Wade

Sean Wade

4 years ago

14 Tips for Writing Spring MVC Controllers

In this article, I’m going to share with you some of the fundamental techniques and best practices for writing a controller class with the Spring MVC framework. Typically, in Spring MVC, we write a controller class to handle requests coming from the client.

Then, the controller invokes a business class to process business-related tasks, and then redirects the client to a logical view name, which is resolved by Spring’s dispatcher servlet in order to render results or output. That completes a round trip of a typical request-response cycle.

Here are summary of the tips you will learn throughout this article:

  1. Using the @Controller stereotype
  2. Implementing the Controller Interface
  3. Extending the AbstractController Class
  4. Specifying URL Mapping for Handler Method
  5. Specifying HTTP Request Methods for Handler Method
  6. Mapping Request Parameters to Handler Method
  7. Returning Model And View
  8. Putting Objects into the Model
  9. Redirection in Handler Method
  10. Handling Form Submission and Form Validation
  11. Handling File Upload
  12. Autowiring Business Classes in the Controller
  13. Accessing HttpServletRequest and HttpServletResponse
  14. Following the Single Responsibility Principle

1. Using the @Controller Stereotype

This is the simplest way to create a controller class that can handle one or multiple requests. Just by annotating a class with the @Controllerstereotype, for example:

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

@Controller
public class HomeController {

    @RequestMapping("/")
    public String visitHome() {

        // do something before returning view name

        return "home";
    }
}

As you can see, the visitHome() method handles requests coming to the application’s context path (/) by redirecting to the view named home.

NOTE: the **@Controller **stereotype can only be used when annotation-driven is enabled in Spring’s configuration file:

<annotation-driven />

When annotation-driven is enabled, the Spring container automatically scans for classes under the package specified in the following statement:

<context:component-scan base-package="net.codejava.spring" />

The classes annotated by the @Controllerannotation are configured as controllers. This is most preferable because of its simplicity: There’s no need to declare beans for controllers in the configuration file.

NOTE: By using the @Controllerannotation, you can have a multi-actions controller class that is able to serve multiple different requests. For example:

@Controller
public class MultiActionController {

    @RequestMapping("/listUsers")
    public ModelAndView listUsers() {

    }

    @RequestMapping("/saveUser")
    public ModelAndView saveUser(User user) {

    }

    @RequestMapping("/deleteUser")
    public ModelAndView deleteUser(User user) {

    }
}

As you can see in the above controller class, there are three handler methods that process three different requests /listUsers, /saveUser, and /deleteUser, respectively.

2. Implementing the Controller Interface

Another (and maybe classic) way of creating a controller in Spring MVC is having a class implement the Controller interface. For example:

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.Controller;

public class MainController implements Controller {

    @Override
    public ModelAndView handleRequest(HttpServletRequest request,
            HttpServletResponse response) throws Exception {
        System.out.println("Welcome main");
        return new ModelAndView("main");
    }
}

The implementing class must override the handleRequest() method, which will be invoked by the Spring dispatcher servlet when a matching request comes in. The request URL pattern handled by this controller is defined in the Spring’s context configuration file as follows:

<bean name="/main" class="net.codejava.spring.MainController" />

However, a drawback of this approach is that the controller class cannot handle multiple request URLs.

3. Extending the AbstractController Class

If you want to easily control the supported HTTP methods, session, and content caching. having your controller class extended the AbstractController class is ideal. Consider the following example:

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.AbstractController;

public class BigController extends AbstractController {

    @Override
    protected ModelAndView handleRequestInternal(HttpServletRequest request,
            HttpServletResponse response) throws Exception {
        System.out.println("You're big!");
        return new ModelAndView("big");
    }
}

This creates a single-action controller with configurations regarding the supported methods, session, and caching, which can then be specified in the bean declaration of the controller. For example:

<bean name="/big" class="net.codejava.spring.BigController">
    <property name="supportedMethods" value="POST"/>
</bean>

This configuration indicates that only the POST method is supported by this controller’s hander method. For other configuration (session, caching), see AbstractController.

Spring MVC also offers several controller classes designed for specific purposes, including:

4. Specifying URL Mapping for the Handler Method

This is the mandatory task you must do when coding a controller class, which is designed for handling one or more specific requests. Spring MVC provides the @RequestMapping annotation, which is used for specifying URL mapping. For example:

@RequestMapping("/login")

That maps the URL pattern /login to be handled by the annotated method or class. When this annotation is used at the class level, the class becomes a single-action controller. For example:

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

@Controller
@RequestMapping("/hello")
public class SingleActionController {

    @RequestMapping(method = RequestMethod.GET)
    public String sayHello() {
        return "hello";
    }
}

When the @RequestMapping annotation is used at the method level, you can have a multi-action controller. For example:

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

@Controller
public class UserController {

    @RequestMapping("/listUsers")
    public String listUsers() {
        return "ListUsers";
    }

    @RequestMapping("/saveUser")
    public String saveUser() {
        return "EditUser";
    }

    @RequestMapping("/deleteUser")
    public String deleteUser() {
        return "DeleteUser";
    }
}

The @RequestMapping annotation can be also used for specifying multiple URL patterns to be handled by a single method. For example:

@RequestMapping({"/hello", "/hi", "/greetings"})

In addition, this annotation has other properties that may be useful in some cases, e.g. the method property which is covered in the next section.

5. Specifying HTTP Request Methods for the Handler Method

You can specify which HTTP method (GET, POST, PUT, …) is supported by a handler method using the method property of the @RequestMapping annotation. Here’s an example:

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

@Controller
public class LoginController {

    @RequestMapping(value = "/login", method = RequestMethod.GET)
    public String viewLogin() {
        return "LoginForm";
    }

    @RequestMapping(value = "/login", method = RequestMethod.POST)
    public String doLogin() {
        return "Home";
    }
}

As you can see, this controller has two methods that handle the same URL pattern /login, but the former is for the GET method and the latter is for the POST method.

For more information about using the @RequestMapping annotation, see @RequestMapping annotation.

6. Mapping Request Parameters to Handler Method

One of the cool features of Spring MVC is that you can retrieve request parameters as regular parameters of the handler method by using the @RequestParam annotation. This is a good way to decouple the controller from the HttpServletRequest interface of the Servlet API.

Let’s look at various examples. Consider the following method:

@RequestMapping(value = "/login", method = RequestMethod.POST)
public String doLogin(@RequestParam String username,
                      @RequestParam String password) {

}

Spring binds the method parameters username and password to the HTTP request parameters with the same names. That means you can invoke a URL as follows (if the request method is GET):

http://localhost:8080/spring/login?username=scott&password=tiger

Type conversion is also done automatically. For example, if you declare a parameter of type integer as follows:

@RequestParam int securityNumber

Then, Spring will automatically convert value of the request parameter (String) to the specified type (integer) in the handler method.

In case the parameter name is different than the variable name, you can specify the actual name of the parameter as follows:

@RequestParam("SSN") int securityNumber

The @RequestParam annotation also has two additional attributes, which might be useful in some cases. Therequiredattribute specifies whether the parameter is mandatory or not. For example:

@RequestParam(required = false) String country

That means the parameter country is optional; hence, it can be missing from the request. In the above example, the variable country will be null if there is no such parameter present in the request.

Another attribute is defaultValue, which can be used as a fallback value when the request parameter is empty. For example:

@RequestParam(defaultValue = "18") int age

Spring also allows us to access all parameters as a Map object if the method parameter is of type Map<String, String>. For example:

doLogin(@RequestParam Map<String, String> params)

Then the map params contains all request parameters in the form of key-value pairs. For more information about using the @RequestParam annotation, see @RequestParam annotation.

7. Returning Model and View

After business logic is processed, a handler method should return a view, which is then resolved by the Spring’s dispatcher servlet. Spring allows us to return either a String or a ModelAndView object from the handler method. In the following example, the handler method returns a String and represents a view named LoginForm:

@RequestMapping(value = "/login", method = RequestMethod.GET)
public String viewLogin() {
    return "LoginForm";
}

That’s the simplest way of returning a view name. But if you want to send additional data to the view, you must return a ModelAndView object. Consider the following handler method:

@RequestMapping("/listUsers")
public ModelAndView listUsers() {

    List<User> listUser = new ArrayList<>();
    // get user list from DAO...

    ModelAndView modelView = new ModelAndView("UserList");
    modelView.addObject("listUser", listUser);

    return modelView;
}

As you can see, this handler method returns a ModelAndView object that holds the view name UserList and a collection of User objects, which can be used in the view.

Spring is also very flexible, as you can declare the ModelAndView object as a parameter of the handler method instead of creating a new one. Thus, the above example can be re-written as follows:

@RequestMapping("/listUsers")
public ModelAndView listUsers(ModelAndView modelView) {

    List<User> listUser = new ArrayList<>();
    // get user list from DAO...

    modelView.setViewName("UserList");
    modelView.addObject("listUser", listUser);

    return modelView;
}

You can learn more about the ModelAndView class by visiting: ModelAndView class.

8. Putting Objects Into the Model

In an application that follows the MVC architecture, the controller © should pass data into the model (M), which is then used in the view (V). As we see in the previous example, the addObject() method of the ModelAndView class is for putting an object to the model, in form of name-value pair:

modelView.addObject("listUser", listUser);
modelView.addObject("siteName", new String("CodeJava.net"));
modelView.addObject("users", 1200000);

Again, Spring is very flexible. You can declare a parameter of type Map in the handler method; Spring uses this map to store objects for the model. Let’s see another example:

@RequestMapping(method = RequestMethod.GET)
public String viewStats(Map<String, Object> model) {
    model.put("siteName", "CodeJava.net");
    model.put("pageviews", 320000);

    return "Stats";
}

This is even simpler than using the ModelAndView object. Depending on your taste, you can use either Map or ModelAndView object. Thanks for the flexibility of Spring.

9. Redirection in Handler Method

In case you want to redirect the user to another URL if a condition is met, just append redirect:/ before the URL. The following code snippet gives an example:

// check login status....

if (!isLogin) {
    return new ModelAndView("redirect:/login");
}

// return a list of Users

In the above code, the user will be redirected to the /login URL if it is not logged in.

10. Handling Form Submission and Form Validation

Spring makes it easy to handle form submission by providing the @ModelAttribute annotation for binding form fields to a form backing object, and the BindingResult interface for validating form fields. The following code snippet shows a typical handler method that is responsible for handling and validating form data:

@Controller
public class RegistrationController {

    @RequestMapping(value = "/doRegister", method = RequestMethod.POST)
    public String doRegister(
        @ModelAttribute("userForm") User user, BindingResult bindingResult) {

        if (bindingResult.hasErrors()) {
            // form validation error

        } else {
            // form input is OK
        }

        // process registration...

        return "Success";
    }
}

Learn more about the @ModelAttribute annotation and the BindingResultinterface from Spring’s official documentation:

11. Handling File Upload

Spring also makes it easy to handle file upload within a handler method, by automatically binding upload data to an array of CommonsMultipartFile objects. Spring uses Apache Commons FileUpload as the underlying multipart resolver.

The following code snippet shows how easy it is to get files uploaded from the client:

@RequestMapping(value = "/uploadFiles", method = RequestMethod.POST)
public String handleFileUpload(
        @RequestParam CommonsMultipartFile[] fileUpload) throws Exception {

    for (CommonsMultipartFile aFile : fileUpload){

        // stores the uploaded file
        aFile.transferTo(new File(aFile.getOriginalFilename()));

    }

    return "Success";
}

You can learn the complete solution for handling file upload with Spring MVC by following this Spring MVC File Upload Tutorial.

12. Autowiring Business Classes in the Controller

A controller should delegate the processing of business logic to relevant business classes. For this purpose, you can use the @Autowired annotation to let Spring automatically inject the actual implementation of a business class to the controller. Consider the following code snippet of a controller class:

@Controller
public class UserController {

    @Autowired
    private UserDAO userDAO;

    public String listUser() {
        // handler method to list all users
        userDAO.list();
    }

    public String saveUser(User user) {
        // handler method to save/update a user
        userDAO.save(user);
    }

    public String deleteUser(User user) {
        // handler method to delete a user
        userDAO.delete(user);
    }

    public String getUser(int userId) {
        // handler method to get a user
        userDAO.get(userId);
    }
}

Here, all business logic related to User management is provided by an implementation of the UserDAO interface. For example:

interface UserDAO {

    List<User> list();

    void save(User user);

    void checkLogin(User user);
}

By using the @Autowired annotation, the handler methods can delegate tasks to the business class, as we can see in the above example:

List<User> listUser = userDAO.list();

For more information about the @Autowired annotation, see Annotation Type Autowired.

13. Accessing HttpServletRequest and HttpServletResponse

In some cases, you need to directly access the HttpServletRequest or HttpServletResponse objects within a handler method. By the flexibility of Spring, just add a relevant parameter to the handler method. For example:

@RequestMapping("/download")
public String doDownloadFile(
        HttpServletRequest request, HttpServletResponse response) {

    // access the request

    // access the response

    return "DownloadPage";
}

Spring detects and automatically injects the HttpServletRequest and HttpServletResponse objects into the method. Then, you can access the request and response such as getting InputStream, OutputStream, or returning a specific HTTP code.

14. Following the Single Responsibility Principle

Finally, there are two good practices you should follow when designing and coding controllers in Spring MVC:

  • A controller class should not execute business logic. Instead, it should delegate business processing to relevant business classes. This keeps the controller focusing on its designed responsibility is to control workflows of the application. For example:
@Controller
public class UserController {

    @Autowired
    private UserDAO userDAO;

    public String listUser() {
        // handler method to list all users
        userDAO.list();
    }

    public String saveUser(User user) {
        // handler method to save/update a user
        userDAO.save(user);
    }

    public String deleteUser(User user) {
        // handler method to delete a user
        userDAO.delete(user);
    }

    public String getUser(int userId) {
        // handler method to get a user
        userDAO.get(userId);
    }
}
  • Create each separate controller for each business domain. For example, UserController for controlling workflows of the user management, OrderController for controlling workflows of order processing, etc. For example:
@Controller
public class UserController {

}

@Controller
public class ProductController {

}

@Controller
public class OrderController {

}

@Controller
public class PaymentController {

}

There you have it! I have shared 14 tips that will help you write controller classes in Spring MVC properly and efficiently. However, that’s not the end. If you have other tips or suggestions, feel free to share your thoughts in the comments.

#spring-boot #java

14 Tips for Writing Spring MVC Controllers
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