@Detailed introduction of requestparam annotation

@RequestParam （org.springframework.web.bind.annotation.RequestParam） Used to assign a specified request parameter to a formal parameter in a method .

There are three properties ：

（1）value： Request parameter name （ You have to configure ）

（2）required： Is it necessary to , The default is true, namely This parameter must be included in the request , If not included , An exception will be thrown （ Optional configuration ）

（3）defaultValue： The default value is , If the value is set ,required Set automatically to false, Whether you have configured it or not required, What values are configured , All are false（ Optional configuration ）

1 For demonstration effect , First you need to create a test control class , Then create a test method ,_ As shown in the figure ,_____ Method to add a parameter ,____ At this time, the method parameter is not annotated

1. 

2. adopt idea Tool launch web project , Then access the test results through the browser , Without parameters, the obtained parameters are null, But the page can display normally , The request can be executed normally .

   

   With parameters , Parameters can be obtained and printed normally

   

3. Let's compare the effect of adding annotations , But just add @RequestParam, Setting parameters without any annotations .

   

4. Visit the browser again to see the effect , Bring it now userName The parameter page can display and print the result value normally , But as the picture shows , Without parameters , Page error reporting , return 400, And prompt userName non-existent

   

   

5. Of course, such restrictions are optional ,@RequestParam Add in required=false To turn off the limit of required parameters , This is the same as the first effect without annotation

   

   

6. @RequestParam It can also be used when the parameter is empty , Set the default value of a parameter for us , As shown in the figure , You can see it , Without parameters , What you get is the default value

   

7. Of default parameters key Name is the name of the parameter in the method , But through @RequestParam You can customize another , Because under certain conditions , The method name is fixed and cannot be modified , But the request parameter needs to pass another parameter key Name to pass on , This can be achieved in such a way