using jsp to check username and password -

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hello i'm learning bit jsp , have problem code below. create 1 index.jsp file , try run eclipse or directly tomcat 6.0. error line "if (name.equals("user") && password.equals("1234"))". error java.lang.nullpointerexception. glad advise.

<?xml version="1.0" encoding="iso-8859-1"?> <!doctype html public "-//w3c//dtd xhtml 1.1//en"           "http://www.w3.org/tr/xhtml11/dtd/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="de">  <head>   <title>practice</title> </head>  <body>  <h2>practice</h2> <h3>please enter name , password.</h3> <form method="post" action="">   <table> <tr><td style="text-align:center">name</td> <td><input type="text" name="name" size="80" /></td> </tr> <tr><td style="text-align:center">password</td> <td><input type="text" name="password" size="80" /></td> </tr> <tr><td><input type="submit" value="send" /></td> <td><input type="reset" value="reset" /></td> </tr> </table> </form>  <%-- testing name , password. --%> <% string name = request.getparameter("name");    string password = request.getparameter("password");    if (name.equals("user") && password.equals("1234"))      { %>        <p>your input correct!</p> <%   }    else if (!name.equals(""))      { %>        <p>please enter name!</p> <%   }     else if (!password.equals(""))      { %>        <p>please enter password!</p> <%   }     else if (!name.equals("") && !password.equals(""))      { %>        <p>please enter name , password!</p>  <%   }     else      { %>        <p>your input not correct!</p> <%   } %>  </body>   </html>

this wrong approach.

the jsp scriptlet code (the code in <% %> things) executed whenever page been requested, regardless of if it's been requested normal request (link, bookmark, etc) or post form submit.

on initial request should display empty form, code executed , in attempt grab request parameters, gets null. since cannot invoke methods on null, nullpointerexception.

the easiest workaround let execute on form submit check if request method post.

<%     if (request.getmethod().equalsignorecase("post")) {         // form been submitted. put code here.     } %>

normally, when form submitted, input fields not filled in end empty strings, not null. however, make code more robust, it's recommended check null anyway.

the correct solution prevent nullpointerexception check if reference not null before attempting access it.

    if (name != null && !name.equals("user")) {         // ...     }

alternatively, can wrap references since "user" guaranteed not null.

    if (!"user".equals(name)) {         // ...     }

however, scriptlets (raw java code inside jsp files) bad practice. normal practice let form submit servlet. can override dopost() method , write code there. can learn servlets in [servlets] tag info page here.


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