java - How does the Spring @ResponseBody annotation work?

Question

I have a method that is annotated in the following way:

/**
* Provide a list of all accounts.
*/
//  TODO 02: Complete this method.  Add annotations to respond
//  to GET /accounts and return a List<Account> to be converted.
//  Save your work and restart the server.  You should get JSON results when accessing 
//  http://localhost:8080/rest-ws/app/accounts
@RequestMapping(value="/orders", method=RequestMethod.GET)
public @ResponseBody List<Account> accountSummary() {
    return accountManager.getAllAccounts();
}

So I know that by this annotation:

@RequestMapping(value="/orders", method=RequestMethod.GET)

this method handle GET HTTP requests made to the resource represented by the URL /orders.

This method calls a DAO object that returns a List.

where Account represents a user on the system and has some fields that represent this user, something like:

public class Account {

    @Id
    @Column(name = "ID")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long entityId;

    @Column(name = "NUMBER")
    private String number;

    @Column(name = "NAME")
    private String name;

    @OneToMany(cascade=CascadeType.ALL)
    @JoinColumn(name = "ACCOUNT_ID")
    private Set<Beneficiary> beneficiaries = new HashSet<Beneficiary>();

    ...............................
    ...............................
    ...............................
}

My question is: How exactly the does the @ResponseBody annotation work?

It is situated before the returned List<Account> object so I think that it refers to this List. The course documentation states that this annotation serves the function to:

ensure that the result will be written to the HTTP response by an HTTP Message Converter (instead of an MVC View).

And also reading on the official Spring documentation: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/bind/annotation/ResponseBody.html

it seems that it takes the List<Account> object and puts it into the Http Response. Is this correct or am I misunderstanding?

Written into the comment of the previous accountSummary() method there is:

You should get JSON results when accessing http://localhost:8080/rest-ws/app/accounts

So what exactly does this mean? Does it mean that the List<Account> object returned by the accountSummary() method is automatically converted into JSON format and then put into the Http Response? Or what?

If this assertion is true, where is it specified that the object will be automatically converted into JSON format? Is the standard format adopted when the @ResponseBody annotation is used or is it specified elsewhere?

Answer 1

First of all, the annotation doesn't annotate List. It annotates the method, just as RequestMapping does. Your code is equivalent to

@RequestMapping(value="/orders", method=RequestMethod.GET)
@ResponseBody
public List<Account> accountSummary() {
    return accountManager.getAllAccounts();
}

Now what the annotation means is that the returned value of the method will constitute the body of the HTTP response. Of course, an HTTP response can't contain Java objects. So this list of accounts is transformed to a format suitable for REST applications, typically JSON or XML.

The choice of the format depends on the installed message converters, on the values of the produces attribute of the @RequestMapping annotation, and on the content type that the client accepts (that is available in the HTTP request headers). For example, if the request says it accepts XML, but not JSON, and there is a message converter installed that can transform the list to XML, then XML will be returned.