Java：Null-POINTER异常 - 用服务器中的数据库值替换HTML中的值

Question

我正在完成一个学校项目 - 用户提交表单后，将创建一个对象，并将其添加到MySQL数据库中。然后，将显示一个由时间戳分类的标题的“列表”。它还必须具有缩略图图片和删除按钮。我正在尝试在HTML页面中替换输入文本值，并以数组的形式使用数据库中的值。 ApplicationDao正在与数据库进行通信，并应该将这些值放在数组中。

我一直在关注教程https://www.linkedin.com/learning/java-ee-servlets-and-javaserver-pages-jsp/search-building-a-servlet-响应？autoplay = true＆amp;简历= false＆amp; u = 2199673 但是这些值没有被替换，我得到以下例外： ***更新：将日志log = new TextLog更改为log = new TextLog之后，我现在正在收回以下错误！感谢您的帮助！

**我也知道重复一些代码，应该在一个页面上，我只是想找出解决方案！

   Jun. 16, 2022 1:41:41 P.M. org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [com.algonquin.loggy.servlets.deleteServlet] in context with path [/Loggy-Lab3] threw exception
java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0
    at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
    at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
    at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:248)
    at java.base/java.util.Objects.checkIndex(Objects.java:372)
    at java.base/java.util.ArrayList.get(ArrayList.java:459)
    at com.algonquin.loggy.servlets.deleteServlet.getHTMLString(deleteServlet.java:65)
    at com.algonquin.loggy.servlets.deleteServlet.doGet(deleteServlet.java:39)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:655)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:540)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:687)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:382)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:895)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1722)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
    at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.base/java.lang.Thread.run(Thread.java:834)



Here is my code:

public  class Log {
    private String uuid;
    private String title;
    private String content;
    private String createTimestamp;
    
  
    
    
    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
       public String getUUID() {
            return uuid;
        }

        public void setUUID(String uuid) {  
            this.uuid = uuid;
        }

        public String getContent() {
            return content;
        }

        public void setContent(String content) {
            this.content = content;
        }

        public String getCreateTimestamp() {
            return createTimestamp;
        }

        public void setCreateTimestamp(String createTimestamp) {
            this.createTimestamp=createTimestamp;
            
        }
}


import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.text.MessageFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.List;
import java.util.UUID;

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

import com.algonquin.loggy.beans.Log;
import com.algonquin.loggy.beans.TextLog;
import com.algonquin.loggy.dao.ApplicationDao;
import com.algonquin.loggy.dao.DBConnection;

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


    /**
     * @see HttpServlet#HttpServlet()
     */
    public LogsServlet() {
        super();
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
     *      response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String htmlResponse = "<html><head><link rel=\"stylesheet\" href=\"style.css\"><title>Loggy Servlet Form</title></head>"
                + "<body><form id=\"logForm\" action=\"LogsServlet\" method=\"post\" >\n"
                + "        <h3>Create A New Log!</h3>\n"
                + "       \n"
                + "        <label id=\"logTitleLabel\" for=\"title\">Title :</label>\n"
                + "        <br>\n"
                + "        <input type=\"text\" id=\"logTitle\" name=\"title\" maxLength=\"60\">\n"
                + "        <br>\n"
                
                + "        <label id=\"logContentLabel\" for=\"content\">Content :</label>\n"
                + "        <br>\n"
                + "        <input type=\"text\" id=\"logContent\" name=\"content\">\n"
                + "        <br>\n"
                + "        <button type=\"submit\" id=\"submitLog\">Submit Log</button>\n"
                + "        \n"
                + "        </form></body></html>";;
        PrintWriter writer = response.getWriter();
        writer.write(htmlResponse);
    }


protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException{
    
    String title = request.getParameter("title");
    String content = request.getParameter("content");
    String output;
    String uuid =UUID.randomUUID().toString();
    String createTimestamp=new SimpleDateFormat("yyyy.MM.dd.HH.mm.ss").format(new java.util.Date());
    
    Log log= new TextLog();
    //ApplicationDao dao = new ApplicationDao();
    //List<Log> logs= dao.listLogs();
    

    
    PrintWriter writer = response.getWriter();
    try {
        
        log.setTitle(title);
        log.setContent(content);
        log.setCreateTimestamp(createTimestamp);
        log.setUUID(uuid);
        
        //logsList.add(log);
        
        output="<html><body><h3>Log successfully created</h3></body></html>";
        writer.write(output);
        }
        catch(Exception e) {
            output="<html><body><h3>Log could not be created</h3></body></html>";
            writer.write(output);
        }
    
            

    String indexForm = "<html><head><link rel=\"stylesheet\" href=\"style.css\"><title>Loggy Servlet Form</title></head>"
            + "<body><form id=\"logForm\" action=\"LogsServlet\" method=\"post\" >\n"
            + "        <h3>Create A New Log!</h3>\n"
            + "       \n"
            + "        <label id=\"logTitleLabel\" for=\"title\">Title :</label>\n"
            + "        <br>\n"
            + "        <input type=\"text\" id=\"logTitle\" name=\"title\" maxLength=\"60\">\n"
            + "        <br>\n"
        
            + "        <label id=\"logContentLabel\" for=\"content\">Content :</label>\n"
            + "        <br>\n"
            + "        <input type=\"text\" id=\"logContent\" name=\"content\">\n"
            + "        <br>\n"
            + "        <button type=\"submit\" id=\"submitLog\">Submit Log</button>\n"
            + "        \n"
            + "        </form></body></html>";
    
    writer.write(indexForm);
    
    String insertQuery ="INSERT INTO `logs`(`uuid`, `title`, `content`, `createTimestamp`) VALUES (?,?,?,?)";
    Connection connection =DBConnection.getConnectionToDatabase();
    
    
    java.sql.PreparedStatement statement;
    try {
        statement = connection.prepareStatement(insertQuery);
        statement.setString(1,log.getUUID());
        statement.setString(2,log.getTitle());
        statement.setString(3,log.getContent());
        statement.setString(4,log.getCreateTimestamp());
        
        statement.executeUpdate();
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    
    String list ="<div class=\"logContainer\">\n"
            + "        <form>\n"
            + "        \n"
            + "        <div class=\"logContainerItem\">\n"
            + "        <img src=\"images/journal1.png\" height=\"60px\" width=\"60px\">\n"
            + "        <input type=\"text\" name=\"log\" value=\"{0}\">\n"
            + "        <button>Delete Log</button>\n"
            + "        </div>\n"
            + "        \n"
            + "         <div class=\"logContainerItem\">\n"
            + "        <img src=\"images/journal1.png\" height=\"60px\" width=\"60px\">\n"
            + "        <input type=\"text\" name=\"log\" value=\"{1}\">\n"
            + "        <button>Delete Log</button>\n"
            + "        </div>\n"
            + "        \n"
            + "         <div class=\"logContainerItem\">\n"
            + "        <img src=\"images/journal1.png\" height=\"60px\" width=\"60px\">\n"
            + "        <input type=\"text\" name=\"log\" value=\"{2}\">\n"
            + "        <button>Delete Log</button>\n"
            + "        </div>\n"
            + "        \n"
            + "         <div class=\"logContainerItem\">\n"
            + "        <img src=\"images/journal1.png\" height=\"60px\" width=\"60px\">\n"
            + "        <input type=\"text\" name=\"log\" value=\"{3}\">\n"
            + "        <button>Delete Log</button>\n"
            + "        </div>\n"
            + "        \n"
            + "        </form>\n"
            + "        \n"
            + "        \n"
            + "        \n"
            + "        </div>\n"
            + "";
    
    writer.write(list);
    
    
    
    
    //String page = getHTMLString(request.getServletContext().getRealPath("/html/ListLogs.html"), logs);
    //response.getWriter().write(page);

}

//public String getHTMLString(String filePath, List<Log> logs) throws IOException{
//  BufferedReader reader = new BufferedReader(new FileReader(filePath));
//  String line="";
//  StringBuffer buffer = new StringBuffer();
//  while((line=reader.readLine())!=null){
//      buffer.append(line);
//  }
    
//  reader.close();
//  String page = buffer.toString();
    
//  page = MessageFormat.format(page,logs.get(0).getTitle(),
            //logs.get(1).getTitle(),logs.get(2).getTitle(),0);
    
    //return page;
    
    
//}




}



import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.Timestamp;
import java.util.ArrayList;
import java.util.List;

import com.algonquin.loggy.beans.Log;
import com.algonquin.loggy.beans.TextLog;
import com.algonquin.loggy.services.ApplicationService;

public class ApplicationDao implements ApplicationService {


        @SuppressWarnings("null")
        public List<Log> listLogs() {
                
            Log log=null;
            List<Log> logs = new ArrayList<>();
            
            
            try{
                Connection connection = DBConnection.getConnectionToDatabase();
                
                String sql = "SELECT title FROM `logs` ORDER BY createTimestamp";
                
                Statement statement = connection.createStatement();
                
                ResultSet set = statement.executeQuery(sql);
                
                while(set.next()){
                    
                    log = new Log();
                    log.setTitle(set.getString("title"));
                    log.setContent(set.getString("content"));
                    log.setCreateTimestamp(set.getString("createTimestamp"));
                    log.setUUID(set.getString("UUID"));
                    
            
                    logs.add(log);
                    
                }

            }
            catch(SQLException exception){
                exception.printStackTrace();
            }
            return logs;

            }
    }


package com.algonquin.loggy.servlets;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.text.MessageFormat;
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;

import com.algonquin.loggy.beans.Log;
import com.algonquin.loggy.dao.ApplicationDao;




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

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        
        
        
        //call DAO layer and get all products for search criteria
        ApplicationDao dao = new ApplicationDao();
        List<Log> logs = dao.listLogs();
        
        //write the products data back to the client browser
        String page = getHTMLString(req.getServletContext().getRealPath("/ListLogs.html"), logs);
        resp.getWriter().write(page);
        
        
    }
    
    /**
     * this methods reads the HTML template as a String, replaces placeholders
     * with the values and returns the entire page as a String
     * @param filePath
     * @param products
     * @return
     * @throws IOException
     */
    
    public String getHTMLString(String filePath, List<Log> logs) throws IOException{
        BufferedReader reader = new BufferedReader(new FileReader(filePath));
        String line="";
        StringBuffer buffer = new StringBuffer();
        while((line=reader.readLine())!=null){
            buffer.append(line);
        }
        
        reader.close();
        String page = buffer.toString();
        
        page = MessageFormat.format(page, logs.get(0).getTitle(),
                logs.get(1).getTitle(),logs.get(2).getTitle(),logs.get(3).getTitle());
        
        return page;
        
        
    }
}


<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>List Of Submitted Logs</title>
</head>
<body>
   <div class="logContainer">
        <form method="get" action="deleteServlet">
        
        <div class="logContainerItem">
        <img src="images/journal1.png" height="60px" width="60px">
        <input type="text" name="log" value="{0}">
        <button>Delete Log</button>
        </div>
        
         <div class="logContainerItem">
        <img src="images/journal1.png" height="60px" width="60px">
        <input type="text" name="log" value="{1}">
        <button>Delete Log</button>
        </div>
        
         <div class="logContainerItem">
        <img src="images/journal1.png" height="60px" width="60px">
        <input type="text" name="log" value="{2}">
        <button>Delete Log</button>
        </div>
        
         <div class="logContainerItem">
        <img src="images/journal1.png" height="60px" width="60px">
        <input type="text" name="log" value="{3}">
        <button>Delete Log</button>
        </div>
        
        </form>
        
        
        
        </div>

</body>
</html>


import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class DBConnection {

    // Database Schema
    // CREATE DATABASE loggy DEFAULT CHARACTER SET utf8 COLLATE utf8_unicode_ci;
    // CREATE TABLE logs (uuid CHAR(40) NOT NULL PRIMARY KEY, title CHAR(128),
    // content TEXT, createTimestamp Date);

     private static final String dbUser = "user";
        private static final String dbPassword = "password";
        private static Connection connection = null;
        

        public static Connection getConnectionToDatabase() {
            

            try {

                // load the driver class
                Class.forName("com.mysql.jdbc.Driver");
                System.out.println("MySQL JDBC Driver Registered!");

                // get hold of the DriverManager
                connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/loggy",dbUser,dbPassword);
            } catch (ClassNotFoundException e) {
                System.out.println("Where is your MySQL JDBC Driver?");
                e.printStackTrace();

            }

            catch (SQLException e) {
                System.out.println("Connection Failed! Check output console");
                e.printStackTrace();

            }

            if (connection != null) {
                System.out.println("Connection made to DB!");
            }
            return connection;
            
        }
        

          
    }

谢谢您，希望我的问题有意义！