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Pavel Zlotarenchuk
Asked:2020-01-11 03:51:31 +0000 UTC2020-01-11 03:51:31 +0000 UTC

MySql 无法连接到 Java 中的 WEB 应用程序

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我正在用 Java 为学期论文编写一个 Web 应用程序。启动时出现错误

SEVERE: Servlet.service() for servlet [tanat.servlet.IceFloeList] in context with path [/SalvageService] threw exception
java.lang.NullPointerException
    at tanat.database.DBUtils.queryIceFloe(DBUtils.java:19)
    at tanat.servlet.IceFloeList.doGet(IceFloeList.java:47)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)

发誓，因为我了解这门课

package tanat.database;

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

import tanat.beans.IceFloe;

public class DBUtils {

    public static List<IceFloe> queryIceFloe(Connection conn) throws SQLException {
        String sql = "SELECT `id`, `name_ice_floe`, ` number_of_fishermen` "
                + "FROM ice_floe ORDER BY 'number_of_fishermen' DESC;";

        PreparedStatement pstm = conn.prepareStatement(sql);  //на эту строку ругается
        ResultSet rs = pstm.executeQuery();
        List<IceFloe> list = new ArrayList<IceFloe>();

//пробывал через статмент ничего не меняеться
//      Statement statement = conn.createStatement();
//      ResultSet rs = statement.executeQuery(sql);

        while (rs.next()) {
            int id = rs.getInt("ID");
            String name = rs.getString("Name");
            int numberOfFishermen = rs.getInt("NumberOfFishermen");
            IceFloe iceFloe = new IceFloe();
            iceFloe.setId(id);
            iceFloe.setNameIceFloe(name);
            iceFloe.setNumberOfFishermen(numberOfFishermen);
            list.add(iceFloe);
        }
        return list;
    }

    public static IceFloe findIceFloe(Connection conn, String code) throws SQLException {
        String sql = "Select a.Code, a.Name, a.Price from Product a where a.Code=?";

        PreparedStatement pstm = conn.prepareStatement(sql);
        pstm.setString(1, code);

        ResultSet rs = pstm.executeQuery();

        while (rs.next()) {
            String name = rs.getString("Name");
            int numberOfFishermen = rs.getInt("NumberOfFishermen");
            IceFloe iceFloe = new IceFloe();
            iceFloe.setNameIceFloe(name);
            iceFloe.setNumberOfFishermen(numberOfFishermen);
            return iceFloe;
        }
        return null;
    }

    public static void updateIceFloe(Connection conn, IceFloe iceFloe) throws SQLException {
        String sql = "Update Product set Name =?, Price=? where Code=? ";

        PreparedStatement pstm = conn.prepareStatement(sql);

        pstm.setInt(1, iceFloe.getId());
        pstm.setString(2, iceFloe.getnameIceFloe());
        pstm.setInt(3, iceFloe.getNumberOfFishermen());

        pstm.executeUpdate();
    }

    public static void insertIceFloe(Connection conn, IceFloe iceFloe) throws SQLException {
        String sql = "Insert into Product(Code, Name,Price) values (?,?,?)";

        PreparedStatement pstm = conn.prepareStatement(sql);

        pstm.setInt(1, iceFloe.getId());
        pstm.setString(2, iceFloe.getnameIceFloe());
        pstm.setInt(3, iceFloe.getNumberOfFishermen());

        pstm.executeUpdate();
    }

    //удаление льдины
    //по идее нужно реализовать автоматическое удаление если количество рыбаков становиться равным 0
    public static void deleteIceFloe(Connection conn, String code) throws SQLException {
        String sql = "Delete From Product where Code= ?";

        PreparedStatement pstm = conn.prepareStatement(sql);

        pstm.setString(1, code);

        pstm.executeUpdate();
    }

}

请帮帮我，我已经摆弄了3天，剩下的时间不多了）如果您需要查看其他课程：https ://bitbucket.org/pahan-titan/salvageservice/src 提前谢谢

javascript

1 个回答

  1. Best Answer

    由于您在 Connection 类型的变量中的指定方法的参数中得到 null，因此值得深入研究分发连接的类的方向。

    因此，这样做...首先，连接这个 https://mvnrepository.com/artifact/mysql/mysql-connector-java/5.1.45作为jdbc驱动

    据我了解，分发 MySQLConnUtils 连接的类

    像这样写

    import com.mysql.fabric.jdbc.FabricMySQLDriver;
import java.sql.Connection;
import java.sql.Driver;
import java.sql.DriverManager;
import java.sql.SQLException;

public class MySQLConnUtils {

    public static Connection getMySQLConnection() throws SQLException {
        String hostName = "localhost";
        String dbName = "salvage_service";
        String dbPort = "3306";
        String userName = "root";
        String password = "qweqwe123123";
        String connectionURL = "jdbc:mysql://" + hostName + ":" + dbPort + "/" + dbName;

        Driver driver = new FabricMySQLDriver();
        DriverManager.registerDriver(driver);
        return DriverManager.getConnection(connectionURL, userName, password);

    }

}

    如果它不起作用，那么写出什么错误

    • 1

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