When calling Servlet I get a 500 - Internal Server Error

Question

I am trying to do an exercise about AJAX using Servlets but I'm getting a internal server error. The program is to create a list of words and save them to a XML file.

My code is as follows:

HTML that calls save()

    <button onclick="save()">Save keywords</button>

save() method:

    function save(){

    var queryString = "";

    for (var i = 0; i < list.length; i++){
        if (queryString.length > 0)
        {
            queryString += "&";
        }
        queryString += "palavra=";
        queryString += list[i];         

    }

    xmlhttp = createRequest(); //creates a XMLHttpRequest
    xmlhttp.onreadystatechange = foo;
    xmlhttp.open("POST", "SaveXML", true);
    xmlhttp.send(queryString);

}

My servlet (Eclipse-generated):

    package br.com.gabriel;

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

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class SaveXML
 */
@WebServlet(description = "pega a hora do servidor", urlPatterns = { "/SaveXML" })

public class SaveXML extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public SaveXML() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        String[] words = request.getParameterValues("palavra");

        response.setContentType("text/xml");
        PrintWriter out = response.getWriter();

        out.println("<words>");

        for (int i = 0; i < words.length; i++)
        {
            out.println("<word>");
            out.println(words[i]);
            out.println("</word>");
        }

        out.println("</words>");

    }

}

My server log:

    GRAVE: Servlet.service() for servlet [br.com.gabriel.SaveXML] in context with path [/Palavras-Chave] threw exception
java.lang.NullPointerException
    at br.com.gabriel.SaveXML.doPost(SaveXML.java:40)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:315)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Thanks in advance, Gabriel

Seems you aren't sending the request parameters you think you are. Open your browser's network console and check what your request contains. — Sotirios Delimanolis, Jun 06 '14 at 00:59
POST request with query string ? Does not sound right. POST should contain data in body not in query string. — Juned Ahsan, Jun 06 '14 at 01:04
My queryString, according to the log... palavra=red&palavra=blue&palavra=green — Gabriel Sotero, Jun 06 '14 at 01:04
With the use of a queryString I would suggest that you use a GET method. — Scary Wombat, Jun 06 '14 at 01:08
@ScaryWombat Isn't there a limit of characters of a query string in URL (as the GET query String)? — Gabriel Sotero, Jun 06 '14 at 01:11
@GabrielSotero see this answer http://stackoverflow.com/questions/812925/what-is-the-maximum-possible-length-of-a-query-string — Scary Wombat, Jun 06 '14 at 01:44
Your method request.getParameterValues() must be returning null. See the API this method will return null value if no parameter is found. — Bilbo Baggins, Jun 06 '14 at 04:29

score 1 · Accepted Answer · answered Jun 06 '14 at 10:23

1

By default the Content-type is "text/plain" So change the Content-type to "application/x-www-form-urlencoded"

xmlhttp.open("POST", "SaveXML", true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");    
console.log(xmlhttp);

answered Jun 06 '14 at 10:23

Tusar

759
5
21

Thanks! It still doesn't work, but it was a important step, that I forgot, through the solution. – Gabriel Sotero Jun 06 '14 at 13:14

score 0 · Answer 2 · answered Oct 29 '14 at 04:17

0

is this on Tomcat 7 or 8 ? I had a similar issue on Tomcat 7 but it worked on Tomcat 8.

Think on Tomcat 7, we need to update the web.xml to use servlet 3.0

answered Oct 29 '14 at 04:17

Samant

92
1
3
13

When calling Servlet I get a 500 - Internal Server Error

2 Answers2