Preguntas de la entrevista de Spring MVC

Programación

1. Introducción

Spring MVC es el marco web original de Spring construido sobre la API de Servlet. Proporciona una arquitectura Modelo-Vista-Controlador que se puede utilizar para desarrollar aplicaciones web flexibles.

En este tutorial, nos centraremos en las preguntas relacionadas con él, ya que a menudo es un tema en una entrevista de trabajo de desarrollador de Spring.

Para obtener más preguntas sobre Spring Framework, puede consultar otro artículo relacionado con Spring de nuestra serie de preguntas para entrevistas.

2. Preguntas básicas de Spring MVC

Q1. ¿Por qué deberíamos usar Spring MVC?

Spring MVC implementa una clara separación de preocupaciones que nos permite desarrollar y probar nuestras aplicaciones fácilmente .

Los conceptos como:

  • Servlet de despachador
  • Controladores
  • Ver resolutores
  • Vistas, modelos
  • ModelAndView
  • Atributos de modelo y sesión

son completamente independientes entre sí y son responsables de una sola cosa.

Por tanto, MVC nos da una gran flexibilidad . Se basa en interfaces (con clases de implementación proporcionadas) y podemos configurar cada parte del marco mediante el uso de interfaces personalizadas.

Otra cosa importante es que no estamos atados a una tecnología de vista específica (por ejemplo, JSP), pero tenemos la opción de elegir entre las que más nos gustan .

Además, no usamos Spring MVC solo en el desarrollo de aplicaciones web, sino también en la creación de servicios web RESTful .

Q2. ¿Cuál es la función de la anotación @Autowired ?

La anotación @Autowired se puede utilizar con campos o métodos para inyectar un bean por tipo . Esta anotación le permite a Spring resolver e inyectar beans colaborativos en su bean.

Para obtener más detalles, consulte el tutorial sobre @Autowired en Spring.

Q3. Explicar un atributo de modelo

La anotación @ModelAttribute es una de las anotaciones más importantes en Spring MVC. Vincula un parámetro de método o un valor de retorno de método a un atributo de modelo con nombre y luego lo expone a una vista web .

Si lo usamos a nivel de método, indica que el propósito de ese método es agregar uno o más atributos del modelo.

Por otro lado, cuando se utiliza como argumento de método, indica que el argumento debe recuperarse del modelo. Cuando no esté presente, primero debemos crear una instancia y luego agregarlo al modelo. Una vez presente en el modelo, debemos completar los campos de argumentos de todos los parámetros de solicitud que tienen nombres coincidentes.

Puede encontrar más información sobre esta anotación en nuestro artículo relacionado con la anotación @ModelAttribute .

Q4. Explicar la diferencia entre @Controller y @RestController ?

La principal diferencia entre las anotaciones @Controller y @RestController es que la anotación @ResponseBody se incluye automáticamente en @RestController . Esto significa que no necesitamos anotar nuestros métodos de controlador con @ResponseBody . Necesitamos hacer esto en una clase @Controller si queremos escribir el tipo de respuesta directamente en el cuerpo de la respuesta HTTP.

Q5. Describir una variable de ruta

Podemos usar la anotación @PathVariable como un parámetro de método de manejo para extraer el valor de una variable de plantilla de URI .

Por ejemplo, si queremos obtener un usuario por id de www.mysite.com/user/123 , debemos asignar nuestro método en el controlador como / user / {id} :

@RequestMapping("/user/{id}") public String handleRequest(@PathVariable("id") String userId, Model map) {}

El @PathVariable sólo tiene un elemento llamado valor . Es opcional y lo usamos para definir el nombre de la variable de la plantilla de URI . Si omitimos el elemento de valor, entonces el nombre de la variable de la plantilla de URI debe coincidir con el nombre del parámetro del método.

También se permite tener varias anotaciones @PathVariable , ya sea declarándolas una tras otra:

@RequestMapping("/user/{userId}/name/{userName}") public String handleRequest(@PathVariable String userId, @PathVariable String userName, Model map) {}

o poniéndolos todos en un mapa o MultiValueMap :

@RequestMapping("/user/{userId}/name/{userName}") public String handleRequest(@PathVariable Map varsMap, Model map) {}

Q6. Validación con Spring MVC

Spring MVC admite especificaciones JSR-303 de forma predeterminada. Necesitamos agregar JSR-303 y sus dependencias de implementación a nuestra aplicación Spring MVC . Hibernate Validator, por ejemplo, es una de las implementaciones JSR-303 a nuestra disposición.

JSR-303 es una especificación de la API de Java para la validación de beans, parte de Jakarta EE y JavaSE, que garantiza que las propiedades de un bean cumplan con criterios específicos, utilizando anotaciones como @NotNull , @Min y @Max . Más información sobre la validación está disponible en el artículo Conceptos básicos de validación de Java Bean.

Spring ofrece la anotación @Validator y la clase BindingResult . La implementación del Validator generará errores en el método del controlador de solicitudes del controlador cuando tengamos datos no válidos. Entonces podemos usar la clase BindingResult para obtener esos errores.

Además de usar las implementaciones existentes, podemos hacer las nuestras. Para hacerlo, primero creamos una anotación que cumpla con las especificaciones JSR-303. Luego, implementamos la clase Validator . Otra forma sería implementar la interfaz Validator de Spring y configurarla como validador a través de la anotación @InitBinder en la clase Controller .

Para ver cómo implementar y usar sus propias validaciones, consulte el tutorial sobre Validación personalizada en Spring MVC.

Q7. ¿Qué son @RequestBody y @ResponseBody ?

La anotación @RequestBody , que se utiliza como parámetro del método del controlador, vincula el cuerpo de la solicitud HTTP a una transferencia o un objeto de dominio . Spring deserializa automáticamente la solicitud HTTP entrante al objeto Java utilizando Http Message Converters.

Cuando usamos la anotación @ResponseBody en un método de controlador en el controlador Spring MVC, indica que escribiremos el tipo de retorno del método directamente en el cuerpo de la respuesta HTTP . No lo pondremos en un modelo y Spring no se interpretará como un nombre de vista.

Please check out the article on @RequestBody and @ResponseBody to see more details about these annotations.

Q8. Explain Model, ModelMap and ModelAndView?

The Model interface defines a holder for model attributes. The ModelMap has a similar purpose, with the ability to pass a collection of values. It then treats those values as if they were within a Map. We should note that in Model (ModelMap) we can only store data. We put data in and return a view name.

On the other hand, with the ModelAndView, we return the object itself. We set all the required information, like the data and the view name, in the object we're returning.

You can find more details in the article on Model, ModelMap, and ModelView.

Q9. Explain SessionAttributes and SessionAttribute

The @SessionAttributes annotation is used for storing the model attribute in the user’s session. We use it at the controller class level, as shown in our article about the Session Attributes in Spring MVC:

@Controller @RequestMapping("/sessionattributes") @SessionAttributes("todos") public class TodoControllerWithSessionAttributes { @GetMapping("/form") public String showForm(Model model, @ModelAttribute("todos") TodoList todos) { // method body return "sessionattributesform"; } // other methods }

In the previous example, the model attribute ‘todos‘ will be added to the session if the @ModelAttribute and the @SessionAttributes have the same name attribute.

If we want to retrieve the existing attribute from a session that is managed globally, we'll use @SessionAttribute annotation as a method parameter:

@GetMapping public String getTodos(@SessionAttribute("todos") TodoList todos) { // method body return "todoView"; }

Q10. What is the Purpose of @EnableWebMVC?

The @EnableWebMvc annotation's purpose is to enable Spring MVC via Java configuration. It's equivalent to in an XML configuration. This annotation imports Spring MVC Configuration from WebMvcConfigurationSupport. It enables support for @Controller-annotated classes that use @RequestMapping to map incoming requests to a handler method.

You can learn more about this and similar annotations in our Guide to the Spring @Enable Annotations.

Q11. What Is ViewResolver in Spring?

The ViewResolver enables an application to render models in the browser – without tying the implementation to a specific view technology – by mapping view names to actual views.

For more details about the ViewResolver, have a look at our Guide to the ViewResolver in Spring MVC.

Q12. What is the BindingResult?

BindingResult is an interface from org.springframework.validation package that represents binding results. We can use it to detect and report errors in the submitted form. It's easy to invoke — we just need to ensure that we put it as a parameter right after the form object we're validating. The optional Model parameter should come after the BindingResult, as it can be seen in the custom validator tutorial:

@PostMapping("/user") public String submitForm(@Valid NewUserForm newUserForm, BindingResult result, Model model) { if (result.hasErrors()) { return "userHome"; } model.addAttribute("message", "Valid form"); return "userHome"; }

When Spring sees the @Valid annotation, it'll first try to find the validator for the object being validated. Then it'll pick up the validation annotations and invoke the validator. Finally, it'll put found errors in the BindingResult and add the latter to the view model.

Q13. What is a Form Backing Object?

The form backing object or a Command Object is just a POJO that collects data from the form we're submitting.

We should keep in mind that it doesn't contain any logic, only data.

To learn how to use form backing object with the forms in Spring MVC, please take a look at our article about Forms in Spring MVC.

Q14. What Is the Role of the @Qualifier Annotation?

It is used simultaneously with the @Autowired annotation to avoid confusion when multiple instances of a bean type are present.

Let's see an example. We declared two similar beans in XML config:

When we try to wire the bean, we'll get an org.springframework.beans.factory.NoSuchBeanDefinitionException. To fix it, we need to use @Qualifier to tell Spring about which bean should be wired:

@Autowired @Qualifier("person1") private Person person;

Q15. What Is the Role of the @Required Annotation?

The @Required annotation is used on setter methods, and it indicates that the bean property that has this annotation must be populated at configuration time. Otherwise, the Spring container will throw a BeanInitializationException exception.

Also, @Required differs from @Autowired – as it is limited to a setter, whereas @Autowired is not. @Autowired can be used to wire with a constructor and a field as well, while @Required only checks if the property is set.

Let's see an example:

public class Person { private String name; @Required public void setName(String name) { this.name = name; } }

Now, the name of the Person bean needs to be set in XML config like this:

Please note that @Required doesn't work with Java based @Configuration classes by default. If you need to make sure that all your properties are set, you can do so when you create the bean in the @Bean annotated methods.

Q16. Describe the Front Controller Pattern

In the Front Controller pattern, all requests will first go to the front controller instead of the servlet. It'll make sure that the responses are ready and will send them back to the browser. This way we have one place where we control everything that comes from the outside world.

The front controller will identify the servlet that should handle the request first. Then, when it gets the data back from the servlet, it'll decide which view to render and, finally, it'll send the rendered view back as a response:

To see the implementation details, please check out our Guide to the Front Controller Pattern in Java.

Q17. What Are Model 1 and Model 2 Architectures?

Model 1 and Model 2 represent two frequently used design models when it comes to designing Java Web Applications.

In Model 1, a request comes to a servlet or JSP where it gets handled. The servlet or the JSP processes the request, handles business logic, retrieves and validates data, and generates the response:

Since this architecture is easy to implement, we usually use it in small and simple applications.

On the other hand, it isn't convenient for large-scale web applications. The functionalities are often duplicated in JSPs where business and presentation logic are coupled.

The Model 2 is based on the Model View Controller design pattern and it separates the view from the logic that manipulates the content.

Furthermore, we can distinguish three modules in the MVC pattern: the model, the view, and the controller. The model is representing the dynamic data structure of an application. It's responsible for the data and business logic manipulation. The view is in charge of displaying the data, while the controller serves as an interface between the previous two.

In Model 2, a request is passed to the controller, which handles the required logic in order to get the right content that should be displayed. The controller then puts the content back into the request, typically as a JavaBean or a POJO. It also decides which view should render the content and finally passes the request to it. Then, the view renders the data:

3. Advanced Spring MVC Questions

Q18. What’s the Difference Between @Controller, @Component, @Repository, and @Service Annotations in Spring?

According to the official Spring documentation, @Component is a generic stereotype for any Spring-managed component. @Repository, @Service, and @Controller are specializations of @Component for more specific use cases, for example, in the persistence, service, and presentation layers, respectively.

Let's take a look at specific use cases of last three:

  • @Controller – indicates that the class serves the role of a controller, and detects @RequestMapping annotations within the class
  • @Service – indicates that the class holds business logic and calls methods in the repository layer
  • @Repository – indicates that the class defines a data repository; its job is to catch platform-specific exceptions and re-throw them as one of Spring’s unified unchecked exceptions

Q19. What Are DispatcherServlet and ContextLoaderListener?

Simply put, in the Front Controller design pattern, a single controller is responsible for directing incoming HttpRequests to all of an application’s other controllers and handlers.

Spring’s DispatcherServlet implements this pattern and is, therefore, responsible for correctly coordinating the HttpRequests to the right handlers.

On the other hand, ContextLoaderListener starts up and shuts down Spring’s root WebApplicationContext. It ties the lifecycle of ApplicationContext to the lifecycle of the ServletContext. We can use it to define shared beans working across different Spring contexts.

For more details on DispatcherServlet, please refer to this tutorial.

Q20. What Is a MultipartResolver and When Should We Use It?

The MultipartResolver interface is used for uploading files. The Spring framework provides one MultipartResolver implementation for use with Commons FileUpload and another for use with Servlet 3.0 multipart request parsing.

Using these, we can support file uploads in our web applications.

Q21. What Is Spring MVC Interceptor and How to Use It?

Spring MVC Interceptors allow us to intercept a client request and process it at three places – before handling, after handling, or after completion (when the view is rendered) of a request.

The interceptor can be used for cross-cutting concerns and to avoid repetitive handler code like logging, changing globally used parameters in Spring model, etc.

For details and various implementations, take a look at Introduction to Spring MVC HandlerInterceptor article.

Q22. What is an Init Binder?

A method annotated with @InitBinder is used to customize a request parameter, URI template, and backing/command objects. We define it in a controller and it helps in controlling the request. In this method, we register and configure our custom PropertyEditors, a formatter, and validators.

The annotation has the ‘value‘ element. If we don't set it, the @InitBinder annotated methods will get called on each HTTP request. If we set the value, the methods will be applied only for particular command/form attributes and/or request parameters whose names correspond to the ‘value‘ element.

It's important to remember that one of the arguments must be WebDataBinder. Other arguments can be of any type that handler methods support except for command/form objects and corresponding validation result objects.

Q23. Explain a Controller Advice

The @ControllerAdvice annotation allows us to write global code applicable to a wide range of controllers. We can tie the range of controllers to a chosen package or a specific annotation.

By default, @ControllerAdvice applies to the classes annotated with @Controller (or @RestController). We also have a few properties that we use if we want to be more specific.

If we want to restrict applicable classes to a package, we should add the name of the package to the annotation:

@ControllerAdvice("my.package") @ControllerAdvice(value = "my.package") @ControllerAdvice(basePackages = "my.package")

It's also possible to use multiple packages, but this time we need to use an array instead of the String.

Besides restricting to the package by its name, we can do it by using one of the classes or interfaces from that package:

@ControllerAdvice(basePackageClasses = MyClass.class)

The ‘assignableTypes‘ element applies the @ControllerAdvice to the specific classes, while ‘annotations‘ does it for particular annotations.

It's noteworthy to remember that we should use it along with @ExceptionHandler. This combination will enable us to configure a global and more specific error handling mechanism without the need to implement it every time for every controller class.

Q24. What Does the @ExceptionHandler Annotation Do?

The @ExceptionHandler annotation allows us to define a method that will handle the exceptions. We may use the annotation independently, but it's a far better option to use it together with the @ControllerAdvice. Thus, we can set up a global error handling mechanism. In this way, we don't need to write the code for the exception handling within every controller.

Let's take a look at the example from our article about Error Handling for REST with Spring:

@ControllerAdvice public class RestResponseEntityExceptionHandler extends ResponseEntityExceptionHandler { @ExceptionHandler(value = { IllegalArgumentException.class, IllegalStateException.class }) protected ResponseEntity handleConflict(RuntimeException ex, WebRequest request) { String bodyOfResponse = "This should be application specific"; return handleExceptionInternal(ex, bodyOfResponse, new HttpHeaders(), HttpStatus.CONFLICT, request); } }

We should also note that this will provide @ExceptionHandler methods to all controllers that throw IllegalArgumentException or IllegalStateException. The exceptions declared with @ExceptionHandler should match the exception used as the argument of the method. Otherwise, the exception resolving mechanism will fail at runtime.

One thing to keep in mind here is that it's possible to define more than one @ExceptionHandler for the same exception. We can't do it in the same class though since Spring would complain by throwing an exception and failing on startup.

On the other hand, if we define those in two separate classes, the application will start, but it'll use the first handler it finds, possibly the wrong one.

Q25. Exception Handling in Web Applications

We have three options for exceptions handling in Spring MVC:

  • per exception
  • per controller
  • globally

If an unhandled exception is thrown during web request processing, the server will return an HTTP 500 response. To prevent this, we should annotate any of our custom exceptions with the @ResponseStatus annotation. This kind of exceptions is resolved by HandlerExceptionResolver.

This will cause the server to return an appropriate HTTP response with the specified status code when a controller method throws our exception. We should keep in mind that we shouldn't handle our exception somewhere else for this approach to work.

Another way to handle the exceptions is by using the @ExceptionHandler annotation. We add @ExceptionHandler methods to any controller and use them to handle the exceptions thrown from inside that controller. These methods can handle exceptions without the @ResponseStatus annotation, redirect the user to a dedicated error view, or build a totally custom error response.

We can also pass in the servlet-related objects (HttpServletRequest, HttpServletResponse, HttpSession, and Principal) as the parameters of the handler methods. But, we should remember that we can't put the Model object as the parameter directly.

The third option for handling errors is by @ControllerAdvice classes. It'll allow us to apply the same techniques, only this time at the application level and not only to the particular controller. To enable this, we need to use the @ControllerAdvice and the @ExceptionHandler together. This way exception handlers will handle exceptions thrown by any controller.

Para obtener información más detallada sobre este tema, consulte el artículo Manejo de errores para REST con Spring.

4. Conclusión

En este artículo, hemos explorado algunas de las preguntas relacionadas con Spring MVC que podrían surgir en la entrevista técnica para los desarrolladores de Spring. Debe tener en cuenta estas preguntas como punto de partida para futuras investigaciones, ya que esta no es una lista exhaustiva.

¡Le deseamos mucha suerte en las próximas entrevistas!