How is used @RequestAttribute and @ModelAttribute annotation in this Spring MVC showcase example?

Question

I am pretty new in Spring MVC.

In this period I am studying the Spring MVC showcase example downlodable from STS dashboard.

I am having some problems understanding how Custom Resolvable Web Arguments are handled in this example.

In practice I have the following situation:

In my home.jsp view I have the following link:

<a id="customArg" class="textLink" href="<c:url value="/data/custom" />">Custom</a> 

This link generate an HTTP Request towards the URL: "/data/custom"

The controller class that contains the method that handles this request has the following code:

@Controller
public class CustomArgumentController {

@ModelAttribute
void beforeInvokingHandlerMethod(HttpServletRequest request) {
    request.setAttribute("foo", "bar");
}


@RequestMapping(value="/data/custom", method=RequestMethod.GET)
public @ResponseBody String custom(@RequestAttribute("foo") String foo) {
    return "Got 'foo' request attribute value '" + foo + "'";
}

 }

The method that handles this HTTP Request is custom()

So when the previous link is clicked the HTTP Request is handled by the custom method...

I am having problems understanding what the @RequestAttribute annotation.

I think that, in this case, it binds the request attribute named foo to a new String foo variable.

But... where is this attribute taken from? Is this variable taken by Spring?

Ok...my idea is that this request attribute is taken from a HttpServletRequest object...

I think this because, in this class, I have also have the beforeInvokingHandlerMethod() method that have a speacking name...so it seems that this method seta an attribute, that have name=foo and value=bar, inside an HttpServletRequest object...and then so the custom() method can use this value...

In fact my output is:

Got 'foo' request attribute value 'bar'

Why is the beforeInvokingHandlerMethod() called before the custom() method?

And why is the beforeInvokingHandlerMethod() annoted by @ModelAttribute annotation? What does this case mean?

Solution

You are correct in assumption of @RequestAttribute, it need not be set in beforeInvokingHandlerMethod. Assume you have a method mapped to /data/init which forwards request to /data/custom. In this case request attribute can be set in init method also.

Why the beforeInvokingHandlerMethod() is called before the custom() method?

And why the beforeInvokingHandlerMethod() is annoted by @ModelAttribute annotation? what means in this case?

you will get the reason here http://static.springsource.org/spring/docs/3.1.x/spring-framework-reference/html/mvc.html#mvc-ann-modelattrib-methods

An @ModelAttribute on a method indicates the purpose of that method is to add one or more model attributes. Such methods support the same argument types as @RequestMapping methods but cannot be mapped directly to requests. Instead @ModelAttribute methods in a controller are invoked before @RequestMapping methods, within the same controller.