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書名： Spring MVC Beginner’s Guide
作者名： Amuthan G
本章字數： 568字
更新時間： 2021-07-16 11:25:43

Time for action – examining request mapping

Let's observe what will happen when you change the value attribute of the @RequestMapping annotation by executing the following steps:

Open your STS and run the webstore project; just right-click on your project and choose Run As | Run on Server. You will be able to view the same welcome message on the browser.
Now, go to the address bar of the browser and enter the URL, http://localhost:8080/webstore/welcome.
You will see the HTTP Status 404 error page on the browser, and you will also see the following warning in the console:
```
WARNING: No mapping found for HTTP request with URI [/webstore/welcome] in DispatcherServlet with name ' DefaultServlet'
```
An error log displaying the "No mapping found" warning message
Now, open the HomeController class, change the @RequestMapping annotation's value attribute to /welcome, and save it. Basically, your new request mapping annotation will look like @RequestMapping("/welcome").
Again, run the application and enter the same URL that you entered in step 2; now you will be able to see the same welcome message on the browser, without any request mapping error.
Finally, open the HomeController class and revert the changes that were made to the @RequestMapping annotation's value; just make it @RequestMapping("/") again and save it.

What just happened?

After starting our application, when we enter the URL http://localhost:8080/webstore/welcome on the browser, the dispatcher servlet (org.springframework.web.servlet.DispatcherServlet) immediately tries to find a matching controller method for the request path, /welcome.

Tip

In a Spring MVC application, the URL can logically be divided into five parts (see the following figure); the @RequestMapping annotation only matches against the URL request path. It omits the scheme, hostname, application name, and so on.

The @RequestMapping annotation has one more attribute called method to further narrow down the mapping based on the HTTP request method types (GET, POST, HEAD, OPTIONS, PUT, DELETE, and TRACE). If we do not specify the method attribute in the @RequestMapping annotation, the default method will be GET. We will learn more about the method attribute of the @RequestMapping annotation in Chapter 4, Working with Spring Tag Libraries, under the section on form processing.

The logical parts of a typical Spring MVC application URL

Since we don't have a corresponding request mapping for the given URL path, /welcome, we get the HTTP Status 404 error on the browser and the following error log on the console:

WARNING: No mapping found for HTTP request with URI [/webstore/welcome] in DispatcherServlet with name 'DefaultServlet'

From the error log, we can clearly understand that there is no request mapping for the URL path, /webstore/welcome. So, we try to map this URL path to the existing controller method; that's why, in step 4, we put only the request path value, /welcome, in the @RequestMapping annotation as the value attribute. Now everything works perfectly fine.

Finally, we reverted our @RequestMapping annotation's value to / again in step 6. Why did we do this? Because we want it to show the welcome page under the web request URL http://localhost:8080/webstore/ again. Observe carefully that here the last single character / is the request path. We will see more about request mapping in upcoming chapters.

Pop quiz – request mapping

Q1. If we have a Spring MVC application for library management called BookPedia and want to map a web request URL, http://localhost:8080/BookPedia/category/fiction, to a controller method, how will we form the @RequestMapping annotation?

@RequestMapping("/fiction").
@RequestMapping("/category/fiction").
@RequestMapping("/BookPedia/category/fiction").

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Spring MVC Beginner’s Guide

Time for action – examining request mapping

What just happened?

Tip

Pop quiz – request mapping