MySQLNonTransientConnectionException：為什麼我無法連接到我的 MySQL 資料庫伺服器？

JDBC Exception: Unable to Connect to MySQL Database Server

When attempting to connect to a MySQL database server using JDBC, you may encounter the following exception:

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Could not create connection to database server.

This error indicates that an unexpected error occurred while establishing a connection to the database.

Causes:

The most common cause of this exception is incompatible versions between MySQL Connector/J and the MySQL database server. Specifically, older versions of MySQL Connector/J may be incompatible with MySQL 8.0 or later.

Solution:

To resolve this issue, follow these steps:

1. Upgrade MySQL Connector/J: Ensure that you are using a version of MySQL Connector/J that is compatible with your MySQL server version. For MySQL 8.0, you should use MySQL Connector/J 5.1.46 or higher, or - preferably - 8.0.11 or higher.
2. Restart the database server: Once you have upgraded MySQL Connector/J, restart the MySQL database server to ensure that the changes take effect.
3. Verify the connection parameters: Double-check the connection parameters specified in your code, including the database URL, username, and password. Ensure that they are correct and that you have the necessary permissions to access the database.
4. Check the database firewall: If you have a firewall enabled on your system, verify that the port used by the MySQL server (usually 3306) is open for connections.

Additional Troubleshooting:

If the above steps do not resolve the issue, you can try the following:

1. Enable verbose logging: Add the following parameter to your database URL to enable verbose logging and obtain more detailed error messages: &log=true.
2. Examine the stack trace: The exception's stack trace may provide additional insights into the cause of the error.
3. Contact MySQL support: If you are still unable to resolve the issue, you may need to contact MySQL support for further assistance.

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