How to require tab-indentation with CheckStyle?

Question

In CheckStyle there is a module (File Tab Character) that checks that there are not tab-characters in the source code. Their rationale is:

• Developers should not need to configure the tab width of their text editors in order to be able to read source code.
• From the Apache jakarta coding standards: In a distributed development environment, when the commit messages get sent to a mailing list, they are almost impossible to read if you use tabs.

To ensure there is the correct number of spaces, there is an additional module (Indentation).

I prefer using tabs for indentation and want to add this requirement to my CheckStyle-file. My rationale:

• Developers should have the opportunity to configure the space used for indentation
• Tabs are a logical and configurable unit for indentation, n spaces is just an arbitrary number of spaces.

Unfortunately I could not find a way to do this with CheckStyle.

Answer 1

There is no ready-made check which does this, but you can configure the RegexpSinglelineJava check accordingly. The following configuration goes into your checkstyle.xml under TreeWalker:

<module name="RegexpSinglelineJava">
    <property name="format" value="^\t* "/>
    <property name="message" value="Indent must use tab characters"/>
    <property name="ignoreComments" value="true"/>
</module>

It checks if there are spaces in the indent. Setting ignoreComments eliminates the problem that Javadoc comments often have at least one space before the asterisk.