我已经写了一个Java Servlet代码来下载文件，但出现错误

我如何找出此错误？

HTTP状态500-类型异常报告

消息

描述服务器遇到的内部错误阻止了它从满足此请求。

例外

java.lang.nullpointerexception p1.download.doget(downlation.java:76( javax.servlet.http.httpservlet.service(httpservlet.java:622( javax.servlet.http.httpservlet.service(httpservlet.java:729( org.apache.tomcat.websocket.server.wsfilter.dofilter(wsfilter.java:52( org.netbeans.modules.web.monitor.server.monitorfilter.dofilter(Monitorfilter.java:393( 请注意，根本原因的完整堆栈轨迹可在Apache中使用 tomcat/8.0.27 logs。

apache tomcat/8.0.27

代码：

package p1;
/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 *
 * @author Admin
 */
public class Download extends HttpServlet
{
    /**
     * Processes requests for both HTTP <code>GET</code> and <code>POST</code>
     * methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException
    {
        response.setContentType("text/html;charset=UTF-8");
        try (PrintWriter out = response.getWriter())
        {
            /* TODO output your page here. You may use following sample code. */
            out.println("<!DOCTYPE html>");
            out.println("<html>");
            out.println("<head>");
            out.println("<title>Servlet Download</title>");            
            out.println("</head>");
            out.println("<body>");
            out.println("<h1>Servlet Download at " + request.getContextPath() + "</h1>");
            out.println("</body>");
            out.println("</html>");
        }
    }
    // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException
    {
        response.setContentType("application/jar");
        ServletContext ctx=getServletContext();
        InputStream is=ctx.getResourceAsStream("/filefroup1.jar");
        int read=0;
        byte[] bytes=new byte[1024];
        OutputStream os=response.getOutputStream();
        while((read=is.read(bytes))!=-1)
        {
            os.write(bytes, 0, read);
        }
        os.flush();
        os.close();
    }
    /**
     * Handles the HTTP <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException
    {
        processRequest(request, response);
    }
    /**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo()
    {
        return "Short description";
    }// </editor-fold>
}

<html>
    <head>
        <title>TODO supply a title</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
    </head>
    <body>
        <form action="/Download">
            <input type="submit" value="download">
        </form>
    </body>
</html>

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" 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">
    <servlet>
        <servlet-name>Download</servlet-name>
        <servlet-class>p1.Download</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>Download</servlet-name>
        <url-pattern>/Download</url-pattern>
    </servlet-mapping>
    <session-config>
        <session-timeout>
            30
        </session-timeout>
    </session-config>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
</web-app>

plirkee是正确的，

我运行了您的代码，并创建了一个名为filefroup.jar的文件，然后将其粘贴在WebContent中，您的代码对我有用。

添加了此行，否则文件命名为"下载"

response.setHeader("Content-disposition", "attachment; filename="+ "filefroup1.jar");

因此，您似乎对文件的存储位置有问题，因为您没有指定其结构中的位置，我想它不在WebContent中，并且您想将其放置在项目的其他位置？

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