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Android Shorts 🩳: Custom lint rules


Was trying to create a silly custom lint rule the other day.

Of course, I played around with (monkey-see-monkey-do).

I was under the impression that for editor highlighting to start working, I only had to build the app. (oh boy)

When I did manage to make it show up, I had issues editing, deleting or even adding more rules.

So, without further ado..

It’s not enough just writing a custom lint rule

Rule here.

As evidenced, no highlighting:

Run the lint check in Android Studio:


Editing rules

Alter this rule to check for hello:


The string literal that contains the word "lint" is still highlighted even after building because we need to…

Run lint again

This pops up 🫠:

Let’s have a look what is this even and why would it complain:


I am not a smart man but I am going to assume something like this happens:

  • lint command does stuff on first run.

  • At the end, this lint.jar file is placed into the build folder.

  • Once in there, this file is actively “watched” by Android Studio/some java process. That’s how we get highlighting in editor.

  • Trying to replace the lint.jar by re-running lint will meet resistance. It’s busy being actively used.

Exit Android Studio

Open the build folder in explorer and delete all files in it.

Now re-open AS. The highlighting should be gone.

Run lint again.

hello is highlighted:



Few things that are nagging me after messing with this for about an hour:

  • Should this lint.jar file be included in a project’s repository?

  • Do we have to do the delete/replace dance everytime there is a change? (how many times is a lint rule going to change anyway 😅)

  • Hook lint in the normal build task via gradle? (not a good idea)

But also positives:

  • Everyone can see the rules in the project. Easily accessible.

  • Anything CI related.

Bundling lint rules in a separate library

Timber is a classic example of shipping lint rules with a library.

  • The rules are built and packaged in the library as part of the publishing process. (you can explore inside the timber aar file and have a look)

  • The consumers don’t have to take any action just to see highlighting in the editor.


  • Flexibility is lost due to using an external library that is not in the main project.



Hope you found this somewhat useful.


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