Java 错误500：通过servlet将数据插入数据库_Java_Servlets

Java 错误500：通过servlet将数据插入数据库

java servlets

Java 错误500：通过servlet将数据插入数据库,java,servlets,Java,Servlets,我是Java中servlet等概念的初学者，最近我一直在尝试这些概念。我试图在customer（customerid、username、password）表中插入一行，customerid是自动生成的，但不幸的是，我总是遇到一个错误500。这是我的servlet、HTML文件和控制台上显示的错误消息 newUserLoginServelet.java package loginPackage; import java.io.IOException; import java.io.PrintWr

我是Java中servlet等概念的初学者，最近我一直在尝试这些概念。我试图在customer（customerid、username、password）表中插入一行，customerid是自动生成的，但不幸的是，我总是遇到一个错误500。这是我的servlet、HTML文件和控制台上显示的错误消息

newUserLoginServelet.java

package loginPackage;

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.Statement;

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 resources.MyUtil;

/**
 * Servlet implementation class NewUserLoginServlet
 */
@WebServlet(description = "Here a new/first time user sets up his/her login credentials.", urlPatterns = { "/NewUserLoginServlet" })
public class NewUserLoginServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

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

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

        String username = request.getParameter("username");
        String password = request.getParameter("password");

        String SQL = "insert into table customer (username, password) values (" + "'" + username + "','" + password + "');" ;

        Connection connection = MyUtil.getConnection(); /*Import resources.MyUtil */

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

        try {
             /*Throws SQLException*/
            Statement statement = connection.createStatement();
            statement.executeQuery(SQL);
            out.println("New user credentials injected into the database.");
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    /**
     * @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);
    }

}

package resources;

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

public class MyUtil {
    private static Connection connection = null;
    private static String url = "jdbc:mysql://localhost:3306/mysql";
    private static String password = "*****";
    private static String user = "root";

    public static Connection getConnection()  {
        if (connection == null) {
            try { 
                Class.forName("com.mysql.jdbc.Driver"); // ClassNotFoundException
                connection = DriverManager.getConnection(url, user, password);
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        }

        return (connection);
    }
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Welcome to the SG Supermarket.</title>
</head>
<body>

<h1>Welcome to the SG supermarket.</h1>

<a href="signIn.html">In case you are not a new user.</a></br>
<a href="signUp.html">In case you are a new user.</a>
<
/body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>User sign up (First time user)</title>
</head>
<body>
    <form action="NewUserLoginServlet">
    Please enter the credentials you wish to be.                </br> 
    Username : <input type="text" value="username">         </br>
    Password : <input type="password" value="password">     </br>
    <input type="submit" value="Submit your credentials.">  </br> 
    </form>
</body>
</html>

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>SGCart2</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>    
  </welcome-file-list>

   <servlet>
    <description>NewUserLoginServlet</description>
    <display-name>NewUserLoginServlet</display-name>
    <servlet-name>NewUserLoginServlet</servlet-name>
    <servlet-class>loginPackage.NewUserLoginServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>NewUserLoginServlet</servlet-name>
    <url-pattern>/NewUserLoginServlet</url-pattern>
  </servlet-mapping>

</web-app>

错误消息：

Type Exception Report

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

java.lang.NullPointerException
    loginPackage.NewUserLoginServlet.doGet(NewUserLoginServlet.java:51)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
Note The full stack trace of the root cause is available in the server logs.

 public class MyUtil {
        private static Connection conn = null;
        private static String url = "jdbc:mysql://localhost:3306/mysql";
        public static Connection getConnection()  {
                try { 
                    Class.forName("com.mysql.jdbc.Driver"); 
                    conn = DriverManager.getConnection(url, "root", "*****");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            return conn;
        }

  <servlet>
    <description>NewUserLoginServlet</description>
    <display-name>NewUserLoginServlet</display-name>
    <servlet-name>NewUserLoginServlet</servlet-name>
    <servlet-class>loginPackage.NewUserLoginServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>NewUserLoginServlet</servlet-name>
    <url-pattern>/NewUserLoginServlet</url-pattern>
  </servlet-mapping>

在数据库中插入数据是否有问题，或者在HTML和servlet之间创建连接失败？任何帮助都将不胜感激

我的数据库URL的屏幕截图
web.xml

<!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <title>Welcome to the SG Supermarket.</title> </head> <body> <h1>Welcome to the SG supermarket.</h1> <a href="signIn.html">In case you are not a new user.</a></br> <a href="signUp.html">In case you are a new user.</a> < /body> </html>

<!DOCTYPE html> <html> <head> <meta charset="UTF-8"> <title>User sign up (First time user)</title> </head> <body> <form action="NewUserLoginServlet"> Please enter the credentials you wish to be. </br> Username : <input type="text" value="username"> </br> Password : <input type="password" value="password"> </br> <input type="submit" value="Submit your credentials."> </br> </form> </body> </html>

<?xml version="1.0" encoding="UTF-8"?> <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0"> <display-name>SGCart2</display-name> <welcome-file-list> <welcome-file>index.html</welcome-file> </welcome-file-list> <servlet> <description>NewUserLoginServlet</description> <display-name>NewUserLoginServlet</display-name> <servlet-name>NewUserLoginServlet</servlet-name> <servlet-class>loginPackage.NewUserLoginServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>NewUserLoginServlet</servlet-name> <url-pattern>/NewUserLoginServlet</url-pattern> </servlet-mapping> </web-app>

SGCart2 index.html NewUserLoginServlet NewUserLoginServlet NewUserLoginServlet loginPackage.NewUserLoginServlet NewUserLoginServlet /NewUserLoginServlet
此sql语句不正确：

String SQL = "insert into table customer (username, password) values (" + "'" + username + "','" + password + "');" ;
应该是：

String SQL = "insert into customer (username, password) values ('" + username + "','" + password + "');" ;

你也应该考虑做一个PraveReDATA语句，它要简单得多，你不必在使用双+单引号的情况下混在一起，如果你的INSERT语句大得多，这可能会非常混乱：

String username = request.getParameter("username"); String password = request.getParameter("password"); try(Connection conn= MyUtil.getConnection()){ PreparedStatement pst = conn.prepareStatement("insert into customer (username,password) values (?,?);"); pst.setString(1, username); pst.setString(2, password); pst.executeUpdate(); }catch (SQLException e) { e.printStackTrace(); }
无论何时插入、更新或删除，都必须使用.executeUpdate（）而不是。executeQuery（）
此外，我还建议查看MVC体系结构。（模型视图控制器）。。。在servlet中执行数据库命令不是很好的做法。出于安全和组织的原因，您应该将此逻辑分开
您得到的错误是NewUserLoginServlet中第51行的NullPointerExection。这意味着这一行是错误的：

Statement statement = connection.createStatement();
在DBConnection类（MyUtil）中删除此括号：
应该是：

return connection;
编辑：尝试将其作为您的数据库连接类：

Type Exception Report Description The server encountered an unexpected condition that prevented it from fulfilling the request. Exception java.lang.NullPointerException loginPackage.NewUserLoginServlet.doGet(NewUserLoginServlet.java:51) javax.servlet.http.HttpServlet.service(HttpServlet.java:635) javax.servlet.http.HttpServlet.service(HttpServlet.java:742) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) Note The full stack trace of the root cause is available in the server logs.

public class MyUtil { private static Connection conn = null; private static String url = "jdbc:mysql://localhost:3306/mysql"; public static Connection getConnection() { try { Class.forName("com.mysql.jdbc.Driver"); conn = DriverManager.getConnection(url, "root", "*****"); } catch (Exception e) { e.printStackTrace(); } return conn; }

<servlet> <description>NewUserLoginServlet</description> <display-name>NewUserLoginServlet</display-name> <servlet-name>NewUserLoginServlet</servlet-name> <servlet-class>loginPackage.NewUserLoginServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>NewUserLoginServlet</servlet-name> <url-pattern>/NewUserLoginServlet</url-pattern> </servlet-mapping>
编辑2：在web.xml中，应该有如下内容：

Type Exception Report Description The server encountered an unexpected condition that prevented it from fulfilling the request. Exception java.lang.NullPointerException loginPackage.NewUserLoginServlet.doGet(NewUserLoginServlet.java:51) javax.servlet.http.HttpServlet.service(HttpServlet.java:635) javax.servlet.http.HttpServlet.service(HttpServlet.java:742) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) Note The full stack trace of the root cause is available in the server logs.

public class MyUtil { private static Connection conn = null; private static String url = "jdbc:mysql://localhost:3306/mysql"; public static Connection getConnection() { try { Class.forName("com.mysql.jdbc.Driver"); conn = DriverManager.getConnection(url, "root", "*****"); } catch (Exception e) { e.printStackTrace(); } return conn; }

<servlet> <description>NewUserLoginServlet</description> <display-name>NewUserLoginServlet</display-name> <servlet-name>NewUserLoginServlet</servlet-name> <servlet-class>loginPackage.NewUserLoginServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>NewUserLoginServlet</servlet-name> <url-pattern>/NewUserLoginServlet</url-pattern> </servlet-mapping>

NewUserLoginServlet NewUserLoginServlet NewUserLoginServlet loginPackage.NewUserLoginServlet NewUserLoginServlet /NewUserLoginServlet
在表单上尝试添加method=“GET”@Austin不起作用。不知道出了什么问题。您是否已将servlet添加到web.xmlNo。我的印象是web.xml已经引用了index.html，这反过来又导致了我的servlet。看看这篇文章，它在解释如何设置servlet方面做得很好。我按照您的要求做了更改，但是我得到了一个空白屏幕，我的数据库也没有更新。我再也没有收到任何错误了。@abhijeet在您的web.xml文件中，您的newuserlogin servlet映射到了什么？实际上什么都没有。它从index.html@JonathanAgain开始，你不应该把它放在。。。。该部分仅适用于用户在访问您的站点时首先查看的jsp/html。。。如果该文件中没有servlet映射，那么servlet将无法工作。就这么简单。请记住重新启动服务器并清理项目/确保所有信息正确无误，否则会出错。是的，我做了@乔纳森