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

我连接到MySQL数据库的程序运行正常。然后，在不更改用于设置连接的任何代码的情况下，我得到以下异常：

My program that connects to a MySQL database was working fine. Then, without changing any code used to set up the connection, I get this exception:

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.

发生了什么事？

使用的代码获取连接：

private static Connection getDBConnection() throws SQLException, InstantiationException, IllegalAccessException, ClassNotFoundException {
    String username = "user";
    String password = "pass";
    String url = "jdbc:mysql://www.domain.com:3306/dbName?connectTimeout=3000";

    Class.forName("com.mysql.jdbc.Driver");
    Connection conn = DriverManager.getConnection(url, username, password);
    return conn;
}

推荐答案

这是一个包装异常，不是很有趣。这个异常的根本原因实际上告诉我们一些根本原因。请在堆栈跟踪中进一步了解一下。很可能你面临 SQLException：连接被拒绝或 SQLException：连接超时。

This is a wrapped exception and not really interesting. It is the root cause of the exception which actually tells us something about the root cause. Please look a bit further in the stacktrace. The chance is big that you'll then face a SQLException: Connection refused or SQLException: Connection timed out.

如果在你的情况下也是如此，那么所有可能的原因是：

If this is true in your case as well, then all the possible causes are:

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. 缺少端口号或者JDBC URL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. 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. 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 either, follow the following advices:

1. 使用 ping 验证并测试它们。


2. 刷新DNS或使用IP地址而是在JDBC URL中。


3. 根据MySQL DB的 my.cnf 验证它。


4. 启动它。


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


6. 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 it.
5. Verify if mysqld is started without the --skip-networking option.
6. Disable firewall and/or configure firewall/proxy to allow/forward the port.

顺便说一句（并且与实际问题），您不一定需要在每个 getConnection（）调用上加载JDBC驱动程序。在启动期间只需一次即可。

By the way (and unrelated to the actual problem), you don't necessarily need to load the JDBC driver on every getConnection() call. Just only once during startup is enough.

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