Struts 2 uses several configuration files to define the settings and behavior of an application. These files are typically stored in the WEB-INF/classes directory of a Struts 2 web application and can be modified to customize the application's behavior. Here are the main Struts 2 configuration files:

1. web.xml: The web.xml file is the standard deployment descriptor for a Java web application. It contains the configuration information that the web container needs to deploy the application, such as servlet mappings, security settings, and error pages.

2. struts.xml: The struts.xml file is the main configuration file for a Struts 2 application. It defines the application's behavior, including the action mappings, interceptors, and results. This file is usually divided into several sections, each of which is responsible for a specific feature of the application.

3. struts.properties: The struts.properties file is a configuration file that contains global settings for a Struts 2 application. It allows you to customize various aspects of the application, such as the default date format, the validation rules, and the resource bundle location.

4. struts.xml validation file: The struts.xml validation file is a configuration file that defines the validation rules for the application. It allows you to specify rules for validating user input and displaying error messages. The validation rules are applied automatically by Struts 2 when a user submits a form.

5. struts.xml localization file: The struts.xml localization file is a configuration file that contains the message resources for the application. It allows you to define the text messages that are used in the application and supports internationalization and localization.

6. struts-default.xml: The struts-default.xml file is a configuration file that contains the default settings for a Struts 2 application. It is included automatically by Struts 2 and provides a set of predefined interceptors, results, and other components that are commonly used in a Struts 2 application.

Overall, these configuration files provide a powerful and flexible way to define the behavior of a Struts 2 application. By modifying these files, you can customize the application to meet your specific requirements and create a robust and scalable web application.