Solve the problem that the value of the action attribute of the form is null when transferring parameters

One . Abnormal recurrence

lately Yige There is a student studying Servlet Conduct Web When developing , Try using the... In the form action Pass parameters , And he found out that Servlet The parameter value passed from the front end cannot be received in .

Let's take a look at his code first , The specific code is as follows ：

1. front end Code Parameters

Here we use action Configure the back-end interface and passed parameters 1001.

<form action="/user?id=1001">

    <input type="submit" value=" Submit " />

</form>

2.  backstage Servlet Code

Here is the corresponding backend Servlet Code , You can get... From the request parameters id The value of the parameter .

@WebServlet("/user")
public class MyServlet extends HttpServlet {
    
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String id  = request.getParameter("id");
        System.out.println(id);
    }
}

After the above code is executed , The final print result is ： null

This student is very curious , Why? id The value of is null？ Should not be 1001 Do you ？ What's going on here ？

Two . abnormal reason

For such a result , The student expressed his doubts , Then why is the result null Well ？ He never understood the reason , So I came to Yige Solve it for him . therefore Yige Just take his code , He carefully ruled it out .

adopt debug debugging , I find , If from url Direct access in http://localhost:8080/user?id=1001 when , You can receive the data from the front end . But when using form When transferring parameters from a form , The obtained value is null. thus it can be seen , The problem should be action Parameters above , It's not that our back-end code has a problem ！

The reason why the above phenomenon occurs , This is because when action Reference time , The default is get Request mode ,get The request will action The parameter in is cleared , That is to say url The parameter in is cleared , When passing, it becomes http://localhost:8080/user? This form , therefore Servlet The corresponding request parameters cannot be obtained in the code .

3、 ... and . Solution

So how to solve this problem ？ here Yige The following are given. 2 Specific solutions .

1. First option , We can Change the request mode in the form to post. because post A request passes parameters in the form of a request body ,url The parameters in will not be cleaned up .

2. Second option , We can Use hidden controls to pass parameters , At this time, whether it is get/post Requests can be .

here Yige Let's show you the code example of using hidden controls to pass parameters ：

<form action="/user">

    <input type="hidden" name="id" value="1001"/>

    <input type="submit" value=" Submit " />

</form>

Everyone should remember that , If we were to get Ask for a reference , Will take url Means of transmission ; If so post Ask for a reference , Will be id Parameters are passed to the background in the form of request body Servlet in .

Now you know how this problem arises 、 How to solve it ？ If you have other questions , You can post the questions in the comment area , Yige I will give you a detailed answer when I see it .