The Struts 2 development community prefers code following a few conventions listed here. Please respect them, if you want to contribute to the project.
In general the Struts 2 project follows the official Java Code Conventions:
Generally there is nothing wrong with having tabs, except that there is a historic agreement that we want to favor space character over tabs for the Struts codebase. The official Java Code Conventions leaves it open whether to use tab or space characters. The WebWork project, which was merged into the Struts project as the base for Struts 2, already had a "no tab character" convention.
Another reasons is that commit messages are generally more readable with spaces.
Modern IDEs can support the code style with just a tick.