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程序交谈。

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

有人可以使用JDBC给我一个快速而脏的示例程序吗？

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

我收到了一个相当惊人的错误：

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：拒绝连接

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

我引用这个答案还包含一步一步的MySQL + JDBC教程：

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

如果你得到 SQLException：连接被拒绝或连接超时或特定于MySQL的 CommunicationsException：
通讯链接失败，则表示根本无法访问数据库。这可能有以下一个或多个原因：

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中缺少或错误端口号。


4. 数据库服务器


5. 数据库服务器不接受TCP / IP连接。


6. 数据库服务器连接已用完。


7. Java和DB之间的某些东西阻塞了连接，例如防火墙或代理。
   
   
   

1. IP address or hostname in JDBC URL is wrong.
2. Hostname in JDBC URL is not recognized by local DNS server.
3. Port number is missing or wrong in JDBC URL.
4. DB server is down.
5. DB server doesn't accept TCP/IP connections.
6. DB server has run out of connections.
7. Something in between Java and DB is blocking connections, e.g. a firewall or proxy.
   
   

要解决其中一个问题，请遵循以下建议：

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

1. 使用 ping 验证并测试它们。


2. 刷新DNS或使用JDBC URL中的IP地址。


3. 根据MySQL DB的 my.cnf 验证它。


4. 启动数据库。


5. 验证mysqld是否在没有 - skip-networking选项的情况下启动。


6. 重新启动数据库并修改代码，以便它关闭中的连接。


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.
   
   

参见：

• 如何在基于servlet的应用程序中连接到JDBC数据库/数据源？


• 使用静态java.sql.Connection是否安全多线程系统中的实例？



See also:

• How should I connect to JDBC database / datasource in a servlet based application?
• Is it safe to use a static java.sql.Connection instance in a multithreaded system?

这篇关于com.mysql.jdbc.exceptions.jdbc4.CommunicationsException：通信链接失败的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！