Hot questions for Spring MVC

Question:

I use Ubuntu and installed cURL on it. I want to test my Spring REST application with cURL. I wrote my POST code at the Java side. However, I want to test it with cURL. I am trying to post a JSON data. Example data is like this:

{"value":"30","type":"Tip 3","targetModule":"Target 3","configurationGroup":null,"name":"Configuration Deneme 3","description":null,"identity":"Configuration Deneme 3","version":0,"systemId":3,"active":true}

I use this command:

curl -i \
    -H "Accept: application/json" \
    -H "X-HTTP-Method-Override: PUT" \
    -X POST -d "value":"30","type":"Tip 3","targetModule":"Target 3","configurationGroup":null,"name":"Configuration Deneme 3","description":null,"identity":"Configuration Deneme 3","version":0,"systemId":3,"active":true \
    http://localhost:8080/xx/xxx/xxxx

It returns this error:

HTTP/1.1 415 Unsupported Media Type
Server: Apache-Coyote/1.1
Content-Type: text/html;charset=utf-8
Content-Length: 1051
Date: Wed, 24 Aug 2011 08:50:17 GMT

The error description is this:

The server refused this request because the request entity is in a format not supported by the requested resource for the requested method ().

Tomcat log: "POST /ui/webapp/conf/clear HTTP/1.1" 415 1051

What is the right format of the cURL command?

This is my Java side PUT code (I have tested GET and DELETE and they work):

@RequestMapping(method = RequestMethod.PUT)
public Configuration updateConfiguration(HttpServletResponse response, @RequestBody Configuration configuration) { //consider @Valid tag
    configuration.setName("PUT worked");
    //todo If error occurs response.sendError(HttpServletResponse.SC_NOT_FOUND);
    return configuration;
}

Answer:

You need to set your content-type to application/json. But -d sends the Content-Type application/x-www-form-urlencoded, which is not accepted on Spring's side.

Looking at the curl man page, I think you can use -H:

-H "Content-Type: application/json"

Full example:

curl --header "Content-Type: application/json" \
  --request POST \
  --data '{"username":"xyz","password":"xyz"}' \
  http://localhost:3000/api/login

(-H is short for --header, -d for --data)

Note that -request POST is optional if you use -d, as the -d flag implies a POST request.


On Windows, things are slightly different. See the comment thread.

Question:

I'm a little confused as to how the inversion of control (IoC) works in Spring.

Say I have a service class called UserServiceImpl that implements UserService interface.

How would this be @Autowired?

And in my Controllers, how would I instantiate an instance of this service?

Would I just do the following?

UserService userService = new UserServiceImpl();

Answer:

First, and most important - all Spring beans are managed - they "live" inside a container, called "application context".

Second, each application has an entry point to that context. Web applications have a Servlet, JSF uses a el-resolver, etc. Also, there is a place where the application context is bootstrapped and all beans - autowired. In web applications this can be a startup listener.

Autowiring happens by placing an instance of one bean into the desired field in an instance of another bean. Both classes should be beans, i.e. they should be defined to live in the application context.

What is "living" in the application context? This means that the context instantiates the objects, not you. I.e. - you never make new UserServiceImpl() - the container finds each injection point and sets an instance there.

In your controllers, you just have the following:

@Controller // Defines that this class is a spring bean
@RequestMapping("/users")
public class SomeController {

    // Tells the application context to inject an instance of UserService here
    @Autowired
    private UserService userService;

    @RequestMapping("/login")
    public void login(@RequestParam("username") String username,
           @RequestParam("password") String password) {

        // The UserServiceImpl is already injected and you can use it
        userService.login(username, password);

    }
}

A few notes:

  • In your applicationContext.xml you should enable the <context:component-scan> so that classes are scanned for the @Controller, @Service, etc. annotations.
  • The entry point for a Spring-MVC application is the DispatcherServlet, but it is hidden from you, and hence the direct interaction and bootstrapping of the application context happens behind the scene.
  • UserServiceImpl should also be defined as bean - either using <bean id=".." class=".."> or using the @Service annotation. Since it will be the only implementor of UserService, it will be injected.
  • Apart from the @Autowired annotation, Spring can use XML-configurable autowiring. In that case all fields that have a name or type that matches with an existing bean automatically get a bean injected. In fact, that was the initial idea of autowiring - to have fields injected with dependencies without any configuration. Other annotations like @Inject, @Resource can also be used.

Question:

I'm using Spring MVC for a simple JSON API, with @ResponseBody based approach like the following. (I already have a service layer producing JSON directly.)

@RequestMapping(value = "/matches/{matchId}", produces = "application/json")
@ResponseBody
public String match(@PathVariable String matchId) {
    String json = matchService.getMatchJson(matchId);
    if (json == null) {
        // TODO: how to respond with e.g. 400 "bad request"?
    }
    return json;
}

Question is, in the given scenario, what is the simplest, cleanest way to respond with a HTTP 400 error?

I did come across approaches like:

return new ResponseEntity(HttpStatus.BAD_REQUEST);

...but I can't use it here since my method's return type is String, not ResponseEntity.


Answer:

change your return type to ResponseEntity<>, then you can use below for 400

return new ResponseEntity<>(HttpStatus.BAD_REQUEST);

and for correct request

return new ResponseEntity<>(json,HttpStatus.OK);

UPDATE 1

after spring 4.1 there are helper methods in ResponseEntity could be used as

return ResponseEntity.status(HttpStatus.BAD_REQUEST).body(null);

and

return ResponseEntity.ok(json);

Question:

What is the purpose and usage of @ModelAttribute in Spring MVC?


Answer:

@ModelAttribute refers to a property of the Model object (the M in MVC ;) so let's say we have a form with a form backing object that is called "Person" Then you can have Spring MVC supply this object to a Controller method by using the @ModelAttribute annotation:

public String processForm(@ModelAttribute("person") Person person){
    person.getStuff();
}

On the other hand the annotation is used to define objects which should be part of a Model. So if you want to have a Person object referenced in the Model you can use the following method:

@ModelAttribute("person")
public Person getPerson(){
    return new Person();
}

This annotated method will allow access to the Person object in your View, since it gets automatically added to the Models by Spring.

See "Using @ModelAttribute".

Question:

I have this problem:

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: mvc3.model.Topic.comments, no session or session was closed

Here is the model:

@Entity
@Table(name = "T_TOPIC")
public class Topic {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int id;

    @ManyToOne
    @JoinColumn(name="USER_ID")
    private User author;

    @Enumerated(EnumType.STRING)    
    private Tag topicTag;

    private String name;
    private String text;

    @OneToMany(mappedBy = "topic", cascade = CascadeType.ALL)
    private Collection<Comment> comments = new LinkedHashSet<Comment>();

    ...

    public Collection<Comment> getComments() {
           return comments;
    }

}

The controller, which calls model looks like the following:

@Controller
@RequestMapping(value = "/topic")
public class TopicController {

    @Autowired
    private TopicService service;

    private static final Logger logger = LoggerFactory.getLogger(TopicController.class);


    @RequestMapping(value = "/details/{topicId}", method = RequestMethod.GET)
    public ModelAndView details(@PathVariable(value="topicId") int id)
    {

            Topic topicById = service.findTopicByID(id);
            Collection<Comment> commentList = topicById.getComments();

            Hashtable modelData = new Hashtable();
            modelData.put("topic", topicById);
            modelData.put("commentList", commentList);

            return new ModelAndView("/topic/details", modelData);

     }

}

The jsp-page looks li the following:

<%@page import="com.epam.mvc3.helpers.Utils"%>
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ page session="false" %>
<html>
<head>
      <title>View Topic</title>
</head>
<body>

<ul>
<c:forEach items="${commentList}" var="item">
<jsp:useBean id="item" type="mvc3.model.Comment"/>
<li>${item.getText()}</li>

</c:forEach>
</ul>
</body>
</html>

Exception is rised, when viewing jsp. In the line with c:forEach loop


Answer:

If you know that you'll want to see all Comments every time you retrieve a Topic then change your field mapping for comments to:

@OneToMany(fetch = FetchType.EAGER, mappedBy = "topic", cascade = CascadeType.ALL)
private Collection<Comment> comments = new LinkedHashSet<Comment>();

Collections are lazy-loaded by default, take a look at this if you want to know more.

Question:

Difference between spring @Controller and @RestController annotation.

Can @Controller annotation be used for both Web MVC and REST applications? If yes, how can we differentiate if it is Web MVC or REST application.


Answer:

  • @Controller is used to mark classes as Spring MVC Controller.
  • @RestController is a convenience annotation that does nothing more than adding the @Controller and @ResponseBody annotations (see: Javadoc)

So the following two controller definitions should do the same

@Controller
@ResponseBody
public class MyController { }

@RestController
public class MyRestController { }

Question:

Is it possible to use multiple @RequestMapping spring annotations in a method? Like:

@RequestMapping("/")
@RequestMapping("")
@RequestMapping("/welcome")
public String welcomeHandler(){
 return("welcome");
}

Answer:

@RequestMapping has a String[] value parameter, so you should be able to specify multiple values like this:

@RequestMapping(value={"", "/", "welcome"})