如何从index.html获取servlet动作（请求参数）

Question

I have in index.html:

<li><a href="list?action=list">List</a></li>

and in servlet class

public class Servlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String action = request.getParameter("action");
    if (action.equalsIgnoreCase("List")){
          // do something.....
         }
    }
}

web.xml

<servlet>
        <servlet-name>Servlet</servlet-name>
        <servlet-class>ru.proj.top.web.Servlet</servlet-class>
        <load-on-startup>0</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>Servlet</servlet-name>
        <url-pattern>/list</url-pattern>
    </servlet-mapping>

action in servlet is null.

How can I send this parameter from index.html to servlet?

Answer 1

I wouldn't name a servlet "Servlet" for test purposes. May be, somewhere in the big world there exists a base class with the same name...

So I call it "SimpleServlet".

Then you have this web.xml (be aware of the fully qualified class name):

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
          http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
         version="3.0">

    <servlet>
        <servlet-name>simpleServlet</servlet-name>
        <servlet-class>de.so.SimpleServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>simpleServlet</servlet-name>
        <url-pattern>/list</url-pattern>
    </servlet-mapping>
</web-app>

Now, if you call http://localhost:8080/simpleServlet/list?action=demo , the variable action in doGet() will contain "demo"

Also, I suggest to check action for null before calling a method on it:

    String action = req.getParameter("action");
    if (action != null)
    {
        // do something
    }
    else
    {
        // do something else
    }

Answer 2

From your question it looks like that the url mapped to the Servlet with <url-pattern> in web.xml is different from what you are using in the href attribute of <a/> element in the index.html. After changing that url mapping to the consistent one, the code should work.

You can add this value to the href attribute to work seamlessly if you use a JSP page instead of simple html page. This will dynamically add the context path of the application to the url and will prevent you from statically typing your context path name.

<%= request.getContextPath() %>/list?action=list