from html to servlet_问答_开发者_运维开发者技术经验分享

i'm learning html and sevlets,i wrote small aplication but i'm not getting any output when i click submit button from from.html page: below is my code

<html>
<body>
<h1 align="center>Color Selection Page</h1>
<form method="POST" action="/SelectColor.do" >
Select Color Charecterstics<p>
Color:
<select name="color" size="1">
<option>light
<option>amber
<option>brown
<option>dark
</select>

<br><br>

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

</form>
</body>
</html>

web.xml file

<?xml version="1.0" encoding="ISO-8859-1"?>

<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_2_5.xsd"
   version="2.5">

   <servlet>
    <servlet-name>ColorServlet</servlet-name>
    <servlet-class>com.example.web.ColorServlet</servlet-class>
    </servlet>

    <servlet-mapping>
    <servlet-name>ColorServlet</servlet-name>
    <url-pattern>/SelectColor.do</url-pattern>
    </servlet-mapping>

    <welcome-file-list>
    <welcome-file>form.html</welcome-file>
    </welcome-file-list>

</web-app>

servlet

package com.example.web;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class ColorServlet extends HttpServlet {
    public void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException,IOException{
        res.setContentType("text/html");
        try {
            PrintWriter out = r开发者_StackOverflowes.getWriter();
            out.println("Beer Selection ADvice<br>");
            String c=req.getParameter("color");
            out.println("<br>Got beer color "+c);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

I've tested your code and the problem comes from this line:

<h1 align="center>Color Selection Page</h1>

The align attribute is not closed, you need to add a double quote to close it.

<h1 align="center">Color Selection Page</h1>

This won't make your HTML valid - Eclipse still complains about "Invalid location of tag (center)" - but, at least, you'll be able to submit the form.

Actually, I'd recommend to write valid HTML or XHTML even if your code is working fine (note that you may have to use action="SelectColor.do" instead of action="/SelectColor.do" depending on the context path of your webapp but this is another story). Writing "bad" HTML will lead you to weird rendering issues and unexpected errors. You should learn to write HTML the right and good way.

The relative form action URL is likely wrong. Remove the leading slash / so that it becomes:

<form method="POST" action="SelectColor.do">

Otherwise it will become relative to the domain root. Starters often create web projects with a context name, e.g. http://localhost:8080/contextname/page.jsp. The servlet would be available by http://localhost:8080/contextname/servleturlpattern. Thus, in perspective to the JSP the form action should be action="servleturlpattern". But if you add a leading /, then it would in fact point to http://localhost:8080/servleturlpattern, which doesn't exist at all.

That said, I strongly recommend you to go through a decent HTML book/tutorial first. Your HTML is cluttered of syntax errors and you're also using a <center> element which is deprecated since 1998. There's a basic HTML tutorial/reference at w3schools.com. There's a HTML validator at w3.org. Further on there are many articles about writing semantic HTML.

With regard to JSP/Servlets, I know that you're learning, but you should in fact not use Servlets to output HTML. The JSP is meant for that. You need to forward the request to some result JSP page and access the data with help of EL.

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // Do some business logic here? Then forward to some result JSP page.
    request.getRequestDispatcher("/WEB-INF/result.jsp").forward(request, response);
}

The JSP should look like:

<!doctype html>
<html lang="en">
    <head><title>Result</title></head>
    <body>
        Beer Selection ADvice<br>
        <br>Got beer color ${param.color}
    </body>
</html>

The ${param.color} is Expression Language and this particular line does in fact sort of out.print(request.getParameter("color")) behind the scenes.

If you aren't doing any postprocessing logic in servlet at all (storing in DB, doing some business logic/validation, etcetera), then you in fact also don't need a servlet at all ;)

To learn more about JSP/Servlets, I can recommend you Marty Hall's Coreservlets tutorials.