java - doPost() 没有看到参数

Question

我敢肯定，可能已经有人问过类似的问题，而且我已经为此工作了几个小时，直到我想……这越来越荒谬了。

看到我突然有了足够的时间，而且我假设你们中的很多人也有，我想我可以问问 StackOverflow 看看他们是否可以提供帮助。另外，对于这个问题中的所有细节，我提前道歉，我只想向您提供我从这令人沮丧的几个小时中收集到的所有信息。

好的，所以我正在尝试在 Eclipse 中使用 Java Servlet 构建一个非常基本的 Web 论坛。这是一个大约一周前到期的家庭作业，我已经提交了它，但没有 100% 工作，但我现在重新审视它，试图找出我哪里出错了。我相信你在想，“但是 Karla，有一个更简单的方法可以做到这一点”。我确信有，但这是在引入 JSP 之前关于 MVC 的一个项目，所以它必须以这种方式完成。

我的问题：似乎问题出在 servlet CreateTopic 中doPost()，它无法提取参数 'id' 以便它可以重定向到显示主题。我将发布我目前拥有的所有 servlet。所以它应该做什么的简短回答：

显示所有论坛
如果您单击论坛名称，您将获得该论坛下的主题列表
如果您单击创建主题，您应该能够添加到您点击添加的那个论坛下的主题列表中，这一切都会下降。

这是我所知道的：

该参数是一个字符串，我尝试使用它来拉它Integer.valueOf(…)并且Integer.parseInt(…)它不起作用。
我得到的错误是一个null指针，我已经通过手动插入一个 ID 号对其进行了测试，它重定向得很好，所以我认为doPost()它似乎无法提取参数 ID 存在问题。
我知道doGet()能够拉出这个参数，因为它可以通过类 Forum 的 getter 和 setter 拉取对象的名称和论坛下列出的主题数量的计数器。
我也尝试过将getForum()字符串作为参数，我觉得程序唯一的问题是通过整数解析，但我现在意识到这主要是由于指针为空，这令人困惑，因为doGet()是可以访问此参数。

我对所有建议和/或解决方案持开放态度，非常感谢！

服务器：汤姆猫 v8.5

IDE：Eclipse 版本：2019-12 (4.14.0) 构建 ID：20191212-1212

Java 版本：13

显示论坛：

package web.WebForum;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletConfig;
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 DisplayForum
*/
@WebServlet("/DisplayForum")
public class DisplayForum extends HttpServlet {
   private static final long serialVersionUID = 1L;

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

   public void init(ServletConfig config) throws ServletException {

       super.init(config);

       Topic topic = new Topic("Eclipse Problem","Karla Valencia");

       List<Forum> forums = new ArrayList<Forum>();
       forums.add(new Forum("General Discussion",topic));
       forums.add(new Forum("CS3220 Web Programming"));

       getServletContext().setAttribute("forums", forums);

   }




   /**
    * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
    */
   @SuppressWarnings("unchecked")
   protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
       // TODO Auto-generated method stub
       //response.getWriter().append("Served at: ").append(request.getContextPath());




       List<Forum> forums = (List<Forum>) getServletContext().getAttribute("forums");

       response.setContentType("text/html");
       PrintWriter out = response.getWriter();
       out.println("<html><head><title>Forums</title></head><body>");

       out.println("<table border = '1'>");
       out.println("<tr><th>Forum</th><th>Topics</th></tr>");

       for(Forum forum : forums) {
           out.println("<tr><td><a href='DisplayTopics?id=" + forum.getForumId()+"'>" + forum.getForumName() + "</a></td><td style = 'text-align: center'>" + forum.getNumberOfTopics() + "</td><td>" + forum.getForumId() +"</td></tr>");
       }


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

       out.println("</body></html>");


   }

   /**
    * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
    */
   protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
       // TODO Auto-generated method stub
       doGet(request, response);
   }

}

显示主题：

package web.WebForum;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;

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 DisplayTopics
 */
@WebServlet("/DisplayTopics")
public class DisplayTopics extends HttpServlet {
    private static final long serialVersionUID = 1L;

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

    public Forum getForum (int id) {

        @SuppressWarnings("unchecked")
        List<Forum> forums = (List<Forum>) getServletContext().getAttribute("forums");

        for(Forum forum: forums) {
            if(forum.getForumId() == id) {

                return forum;

            }
        }
        return null;

    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        //response.getWriter().append("Served at: ").append(request.getContextPath());

        int id = Integer.valueOf(request.getParameter("id"));
        Forum forum = getForum(id);
        List<Topic> topics = forum.getTopics();



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

        out.println("<html><head><title>Forums</title></head><body>");
        out.println("<h2>"+ forum.forumName +"</h2><table border = '1' style = 'text-align: center'>");
        out.println("<tr><th>Topic</th><th>Author</th><th>Replies</th><th>Last Post</th></tr>");

        for(Topic topic : topics) {
            out.println("<tr><td><a href='#'>" + topic.getTopicName() + "</a></td><td style = 'text-align: center'>" + topic.getAuthor() + "</td><td>" + topic.getNumComments() +"</td><td>"+ topic.getLastPost() +"</td></tr>");
        }

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

        out.println("</br><a href='CreateTopic?id="+ forum.getForumId() +"' > Create Topic</a>");
        out.println("</body></html>");
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

创建主题 Servlet：（我认为问题出在哪里）

package web.WebForum;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;

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


@WebServlet("/CreateTopic")
public class CreateTopic extends HttpServlet {
    private static final long serialVersionUID = 1L;


    public CreateTopic() {
        super();
        // TODO Auto-generated constructor stub
    }


    public Forum getForum (int id) {

        @SuppressWarnings("unchecked")
        List<Forum> forums = (List<Forum>) getServletContext().getAttribute("forums");

        for(Forum forum: forums) {
            if(forum.getForumId() == id) {

                return forum;
            }
        }
        return null;
    }

    public Forum getForum(String id) {

        @SuppressWarnings("unchecked")
        List<Forum> forums = (List<Forum>) getServletContext().getAttribute("forums");

        for(Forum forum: forums) {

            if(String.valueOf(forum.getForumId()) == id) {
                return forum;
            }
        }
        return null;
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub

        Integer id = Integer.valueOf(request.getParameter("id"));
        Forum forum = getForum(id);

        request.setAttribute("forumName", forum.getForumId());

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<html><head><title>Create Topic</title></head><body>");
        out.println("<h2><a href='DisplayTopics?id=" + forum.getForumId()+"'>" +forum.forumName +"</a> > Create Topic</h2>");
        out.println("<form action='CreateTopic' method='post'>");
        out.println("<table border = '1'><tr><th>Your Name: </th><td><input type ='text' name='author'></td></tr>");
        out.println("<tr><th>Subject: </th><td><input type ='text' name='name'></td></tr>");
        out.println("<tr><th>Content</th><td><textarea name='comment'cols='40' rows='4'></textarea></td></tr>");
        out.println("<tr><td col='4' row ='1'><input type='submit' name='add' value='Add'/></td></tr>");
        out.println("</table></form></body></html>");

        out.println(id);

    }

    @SuppressWarnings("unchecked")
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub

        String author = request.getParameter("author");
        String name = request.getParameter("name");
        String comment = request.getParameter("comment");

        List<Forum> forums = (List<Forum>) getServletContext().getAttribute("forums");



        Topic topic = new Topic(author,name,comment);   



        response.sendRedirect("DisplayTopics?id=#");

    }

}

论坛类：


package web.WebForum;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

public class Forum {
    static AtomicInteger count = new AtomicInteger(0);
    String forumName;
    int numberOfTopics;
    private final int forumId;

    List<Topic> topics = new ArrayList<Topic>();

    public Forum(String forumName) {
        this.forumName = forumName;
        this.numberOfTopics = 0;
        this.forumId = count.incrementAndGet();

    }

    public Forum(String forumName, Topic topic) {
        this.forumName = forumName;
        this.numberOfTopics = 0;
        this.forumId = count.incrementAndGet();
        addTopic(topic);

    }

    public Forum(String forumName, int numberOfTopics) {
        this.forumName = forumName;
        this.numberOfTopics = numberOfTopics;
        this.forumId = count.incrementAndGet();
    }

    public void addTopic(Topic topic){
        this.topics.add(topic);
        this.numberOfTopics++;
    }

    public String getForumName() {
        return forumName;
    }

    public void setForumName(String forumName) {
        this.forumName = forumName;
    }

    public int getNumberOfTopics() {
        return numberOfTopics;
    }

    public void setNumberOfTopics(int numberOfTopics) {
        this.numberOfTopics = numberOfTopics;
    }

    public int getForumId() {
        return forumId;
    }

    public List<Topic> getTopics() {
        return topics;
    }

    public void setTopics(List<Topic> topics) {
        this.topics = topics;
    }


}

话题类：

package web.WebForum;

import java.util.ArrayList;
import java.util.List;

public class Topic {

    String topicName;
    String author;
    int numComments;
    String lastPost = "no posts";
    String comment;

    List<String> comments = new ArrayList<String>();

    public Topic ( String topicName, String author,String comment) {

        this.topicName = topicName;
        this.author = author;
        this.comment = comment;

    }

public Topic ( String topicName, String author) {

        this.topicName = topicName;
        this.author = author;

    }
    public void addComment(String comment) {

        comments.add(comment);
        //this.lastPost = comment.postedOn;
        this.numComments++;
    }

    public String getTopicName() {
        return topicName;
    }

    public void setTopicName(String topicName) {
        this.topicName = topicName;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public int getNumComments() {
        return numComments;
    }

    public void setNumComments(int numComments) {
        this.numComments = numComments;
    }

    public String getLastPost() {
        return lastPost;
    }

    public void setLastPost(String lastPost) {
        this.lastPost = lastPost;
    }

    public List<String> getComments() {
        return (List<String>) comments;
    }

    public void setComments(List<String> comments) {
        this.comments = (List<String>) comments;
    }
}

java - doPost() 没有看到参数

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