Error Cause & Solution:
1. Configuration is wrong: driver=com.mysql.jdbc.Driver.
2. Wrong database connection address: url=jdbc:mysql://localhost:3306/test (test is user-defined creation, variable).
3. Wrong password or account: username=root (default is root).
4. Wrong account: password=root (password is set by the user).
5. The database is not started or not authorized to access.
6. The corresponding driver jar package mysql-connector-java-5.1.6-bin.jar has not been introduced, or the mysql-connector-java dependency has not been injected in the Maven project.