com.mysql.jdbc.exceptions.jdbc4.CommunicationsException:通信链接失败 [英] com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

问题描述

我正在努力让我的数据库与我的 Java 程序对话.

I'm working on getting my database to talk to my Java programs.

谁能给我一个使用 JDBC 的快速而肮脏的示例程序?

Can someone give me a quick and dirty sample program using the JDBC?

我遇到了一个相当大的错误:

I'm getting a rather stupendous error:

Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure 
    The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:409)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1122)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2260)
    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:787)
    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:49)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:409)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:357)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:285)
    at java.sql.DriverManager.getConnection(DriverManager.java:582)
    at java.sql.DriverManager.getConnection(DriverManager.java:207)
    at SqlTest.main(SqlTest.java:22)
Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
    The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:409)
    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1122)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:344)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2181)
    ... 12 more
Caused by: java.net.ConnectException: Connection refused
    at java.net.PlainSocketImpl.socketConnect(Native Method)
    at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333)
    at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:195)
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:182)
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:432)
    at java.net.Socket.connect(Socket.java:529)
    at java.net.Socket.connect(Socket.java:478)
    at java.net.Socket.<init>(Socket.java:375)
    at java.net.Socket.<init>(Socket.java:218)
    at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:256)
    at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:293)
    ... 13 more

测试文件的内容:

import com.mysql.jdbc.*;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class SqlTest {

    public static void main(String [] args) throws Exception {
        // Class.forName( "com.mysql.jdbc.Driver" ); // do this in init
        // // edit the jdbc url 
        Connection conn = DriverManager.getConnection( 
            "jdbc:mysql://localhost:3306/projects?user=user1&password=123");
        // Statement st = conn.createStatement();
        // ResultSet rs = st.executeQuery( "select * from table" );

        System.out.println("Connected?");
    }
}

推荐答案

所以，你有一个

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException:通信链接失败
java.net.ConnectException: 连接被拒绝

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
java.net.ConnectException: Connection refused

我引用了 这个答案，其中还包含一个步骤-MySQL+JDBC 分步教程:

I'm quoting from this answer which also contains a step-by-step MySQL+JDBC tutorial:

如果您收到 SQLException: Connection denied 或 Connection timed out 或 MySQL 特定的 CommunicationsException:通信链路失败，则表示数据库根本无法访问.这可能有以下一种或多种原因:

If you get a SQLException: Connection refused or Connection timed out or a MySQL specific CommunicationsException: Communications link failure, then it means that the DB isn't reachable at all. This can have one or more of the following causes:

1. JDBC URL 中的 IP 地址或主机名错误.
2. 本地 DNS 服务器无法识别 JDBC URL 中的主机名.
3. JDBC URL 中的端口号丢失或错误.
4. 数据库服务器已关闭.
5. 数据库服务器不接受 TCP/IP 连接.
6. 数据库服务器的连接用完了.
7. Java 和 DB 之间的某些东西正在阻止连接，例如防火墙或代理.
   
   

要解决其中一个，请遵循以下建议:

To solve the one or the other, follow the following advices:

1. 使用 ping 验证和测试它们.
2. 刷新 DNS 或改用 JDBC URL 中的 IP 地址.
3. 根据MySQL DB的my.cnf进行验证.
4. 启动数据库.
5. 验证是否在没有 --skip-networking 选项 的情况下启动了 mysqld.
6. 重新启动数据库并相应地修复您的代码，使其在finally 中关闭连接.
7. 禁用防火墙和/或配置防火墙/代理以允许/转发端口.
   
   

1. Verify and test them with ping.
2. Refresh DNS or use IP address in JDBC URL instead.
3. Verify it based on my.cnf of MySQL DB.
4. Start the DB.
5. Verify if mysqld is started without the --skip-networking option.
6. Restart the DB and fix your code accordingly that it closes connections in finally.
7. Disable firewall and/or configure firewall/proxy to allow/forward the port.
   
   

另见:

• 我该怎么做在基于 servlet 的应用程序中连接到 JDBC 数据库/数据源?
• 在多线程系统中使用静态 java.sql.Connection 实例是否安全?

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