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
orConnection timed out
or a MySQL specificCommunicationsException: Communications link failure
, then it means that the DB isn't reachable at all. This can have one or more of the following causes:
- JDBC URL 中的 IP 地址或主机名错误.
- 本地 DNS 服务器无法识别 JDBC URL 中的主机名.
- JDBC URL 中的端口号丢失或错误.
- 数据库服务器已关闭.
- 数据库服务器不接受 TCP/IP 连接.
- 数据库服务器的连接用完了.
- 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:
- 使用
ping
验证和测试它们. - 刷新 DNS 或改用 JDBC URL 中的 IP 地址.
- 根据MySQL DB的
my.cnf
进行验证. - 启动数据库.
- 验证是否在没有
--skip-networking 选项
的情况下启动了 mysqld. - 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. - 禁用防火墙和/或配置防火墙/代理以允许/转发端口.
- Verify and test them with
ping
. - Refresh DNS or use IP address in JDBC URL instead.
- Verify it based on
my.cnf
of MySQL DB. - Start the DB.
- Verify if mysqld is started without the
--skip-networking option
. - Restart the DB and fix your code accordingly that it closes connections in
finally
. - 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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