Question

线程中的异常“main”java.lang.Error：未解决的编译问题：类型不匹配：无法从java.sql.Statement转换为com.mysql.jdbc.Statement

我是java的初学者我正在尝试使用mysql数据库我从mysql.com下载了mysql-connector-java-5.1.23-bin.jar文件，我已将此jar文件添加到我的构建路径中我的项目，但以下代码中有错误 线程中的异常“main”java.lang.Error：未解决的编译问题：类型不匹配：无法从java.sql.Statement转换为com.mysql.jdbc.Statement

package com.example.demo;

import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;

import com.mysql.jdbc.Connection;
import com.mysql.jdbc.Statement;

public class DBConnect
{
private static final String userName = "root";
private static final String userpwd = "sverma";
private static final String CONN_STR = "jdbc:mysql://localhost:3306/phpweb_db";

public static void main(String[] args) throws SQLException
{

    Connection conn = null;
    Statement st = null;
    ResultSet rs = null;

    try
    {
        DriverManager.getConnection(CONN_STR, userName, userpwd);
        st=conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
        rs = st.executeQuery("select * from user");
        rs.last();
        System.out.println("No of rows: " + rs.getRow());

        // System.out.println("Connected Successfully...");
    }
    catch (SQLException e)
    {
        System.err.println(e);

    }
    finally
    {
        if (rs != null)
        {
            rs.close();
        }
        if (st != null)
        {
            st.close();
        }
        if (conn != null)
        {
            conn.close();
        }
    }
}

}

Answer 1

错误的课程

import com.mysql.jdbc.Connection;
import com.mysql.jdbc.Statement;

应该是

import java.sql.Connection;
import java.sql.Statement;

实际上，java将特定数据库引擎的所有内容分离。从来不需要导入MySQL（或ProgressSQL或......）类。

要在运行时使这些类可用，try之后的第一件事是在获得连接之前：

Class.forName("com.mysql.jdbc.Driver");

这种技术允许从配置文件中读取所有字符串，并编写与数据库无关的代码。

缺少：conn = ...

conn = DriverManager.getConnection(CONN_STR, userName, userpwd);

Answer 2

package com.example.demo;

import java.sql.*;


public class DBConnect
{
private static final String userName = "root";
private static final String userpwd = "sverma";
private static final String CONN_STR = "jdbc:mysql://localhost:3306/phpweb_db";

public static void main(String[] args) throws SQLException
{

    Connection conn;
    Statement st;
    ResultSet rs;

    try
    {
        Class.forName("com.mysql.jdbc.Driver");
        DriverManager.getConnection(CONN_STR, userName, userpwd);
        st=conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
        rs = st.executeQuery("select * from user");
        rs.last();
        System.out.println("No of rows: " + rs.getRow());

        // System.out.println("Connected Successfully...");
    }
    catch (SQLException e)
    {
        System.err.println(e);

    }
    finally
    {
        if (rs != null)
        {
            rs.close();
        }
        if (st != null)
        {
            st.close();
        }
        if (conn != null)
        {
            conn.close();
        }
    }
}

Mysql数据库异常

2 个答案: