在 jdbc 中使用 localhost 时发现网络连接错误

Posted 2023-03-27

【中文标题】在 jdbc 中使用 localhost 时发现网络连接错误

【英文标题】：I found a net connection error while using localhost in jdbc

【发布时间】：2014-08-01 07:34:03

【问题描述】：

我编写了这个程序来连接我在同一台计算机上的数据库，但我每次都发现网络连接错误。 com.mysql.jdbc.CommunicationsException：由于底层异常导致通信链接失败：

import java.sql.*;
public class Demo 

        public static void main(String[] args) 

        try    
            Class.forName("com.mysql.jdbc.Driver");   
            Connection connection = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/db","username","pwd");   
            Statement stmt = connection.createStatement();
            String str="SHOW TABLES";
            ResultSet resultSet=stmt.executeQuery(str);
            while (resultSet.next()) 
                System.out.println(resultSet.getString(1));
            
            catch(Exception e)
                e.printStackTrace();
            
    

我的 Stacktrace 是：-

com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception: 

** BEGIN NESTED EXCEPTION ** 

java.net.ConnectException
MESSAGE: Connection refused: connect

STACKTRACE:

java.net.ConnectException: Connection refused: connect
    at java.net.PlainSocketImpl.socketConnect(Native Method)
    at java.net.PlainSocketImpl.doConnect(Unknown Source)
    at java.net.PlainSocketImpl.connectToAddress(Unknown Source)
    at java.net.PlainSocketImpl.connect(Unknown Source)
    at java.net.SocksSocketImpl.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at java.net.Socket.<init>(Unknown Source)
    at java.net.Socket.<init>(Unknown Source)
    at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:256)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:271)
    at com.mysql.jdbc.Connection.createNewIO(Connection.java:2771)
    at com.mysql.jdbc.Connection.<init>(Connection.java:1555)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:285)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at Demo.main(Demo.java:8)


** END NESTED EXCEPTION **



Last packet sent to the server was 0 ms ago.
    at com.mysql.jdbc.Connection.createNewIO(Connection.java:2847)
    at com.mysql.jdbc.Connection.<init>(Connection.java:1555)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:285)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at Demo.main(Demo.java:8)

【问题讨论】：

检查您的数据库配置。可能是防火墙问题或端口错误。

你的mysql服务运行了吗？

您的数据库服务器是否已启动并正在侦听端口 3306 ？

【参考方案1】：

试试这个：

DriverManager.getConnection("jdbc:mysql://localhost:3306/dbname","username","password");

如果没有解决，请转到您安装 Mysql 的位置并打开 MySQL Notifier 并检查服务器是否正在运行，如果没有启动它，然后尝试执行您的程序。

【参考方案2】：

这个错误主要是因为Mysql server 无法访问或者没有运行。检查以下几点：

1. Check your DB Connection whether MYSQL is up and running. 
2. Check if you are able to connect from command prompt
3. Restart the mysql server and try again.