# Android Shorts 🩳: Custom lint rules

8 Apr 2023  
2 min read  
android, yapping  
https://www.costafotiadis.com/android-shorts-custom-lint-rules/  
human written · 100% human · [Pangram analysis](https://www.pangram.com/history/99545515-0d5a-4f85-ba6b-99bb40bae070)

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

Of course, I played around with [https://github.com/googlesamples/android-custom-lint-rules](https://github.com/googlesamples/android-custom-lint-rules) (_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](https://github.com/googlesamples/android-custom-lint-rules/blob/main/checks/src/main/java/com/example/lint/checks/SampleCodeDetector.kt).

As evidenced, no highlighting:

```kotlin
class Test {
    // We have a custom lint check bundled with :library
    // that this module depends on. The lint check looks
    // for mentions of "lint", which should trigger an
    // error
    val s = "lint"

    fun lint() { }
}
```

Run the lint check in Android Studio:

![](https://www.costafotiadis.com/images/2023/04/1-nH5v18_s6RM3bAQRY6nQ6g.png)

Voila!

![](https://www.costafotiadis.com/images/2023/04/1-JoIXpT9kHglyml5758o96A.png)

#### Editing rules

Alter this rule to check for `hello` .

<!-- SampleCodeDetector.kt, https://github.com/googlesamples/android-custom-lint-rules/blob/main/checks/src/main/java/com/example/lint/checks/SampleCodeDetector.kt -->
```kotlin
return object : UElementHandler() {
    override fun visitLiteralExpression(node: ULiteralExpression) {
        val string = node.evaluateString() ?: return
        if (string.contains("hello") && string.matches(Regex(".*\\bhello\\b.*"))) {
            context.report(
                ISSUE, node, context.getLocation(node),
                "This code mentions `hello`: **Congratulations**"
            )
        }
    }
}
```

*[SampleCodeDetector](https://github.com/googlesamples/android-custom-lint-rules/blob/main/checks/src/main/java/com/example/lint/checks/SampleCodeDetector.kt)*

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

#### Run lint again

This pops up 🫠:

```text
C:\code\android-custom-lint-rules\library\build\intermediates\lint_publish_jar\global\lint.jar: The process cannot access the file because it is being used by another process.
```

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

![](https://www.costafotiadis.com/images/2023/04/1-KNUfxnaBmcmpLd_EOxqQDA.png)

#### ..okay?

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:

![](https://www.costafotiadis.com/images/2023/04/1-MPVwr-zakwRFLKjNF1B_TA.png)

---

#### Considerations

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? (I couldn’t find any way to make this work outside the `build` folder)
-   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](https://github.com/JakeWharton/timber/tree/trunk/timber-lint/src/main/java/timber/lint) 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](https://repo1.maven.org/maven2/com/jakewharton/timber/timber/5.0.1/) and have a look)
-   The consumers don’t have to take any action just to see highlighting in the editor.

But!

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

---

#### Anyways

Hope you found this somewhat useful.

All credit goes to [https://github.com/googlesamples/android-custom-lint-rules](https://github.com/googlesamples/android-custom-lint-rules).