Custom Lint Rules

The lint source code contains a lot of documentation on how to write custom checks; this git repository contains a snapshot of this documentation which you can read here:

• Full API Guide
• Other docs

Lint

The Android lint tool is a static code analysis tool that checks your project source files for potential bugs and optimization improvements for correctness, security, performance, usability, accessibility, and internationalization. Lint comes with around 400 built-in checks, but it can be extended with additional custom checks. This sample project shows how those sample checks can be built and packaged.

Note that while Android Lint has the name "Android" in it, it is no longer an Android-specific static analysis tool; it's a general static analysis tool, and inside Google for example it is run to analyze server-side Java and Kotlin code.

NOTE: The lint API is not a final API; if you rely on this be prepared to adjust your code for the next tools release.

Introduction

The Android Lint API allows users to create custom lint checks. For example, if you are the author of an Android library project, and your library project has certain usage requirements, you can write additional lint rules to check that your library is used correctly, and then you can distribute those extra lint rules for users of the library. Similarly, you may have company-local rules you'd like to enforce.

This sample demonstrates how to create a custom lint checks and corresponding tests for those rules.

Sample Lint Checks

This project shows how Android Studio as well as the Android Gradle plugin handles packaging of lint rules.

Lint Check Jar Library

First, there's the lint check implementation itself. That's done in the "checks" project, which just applies the Gradle "java" or "kotlin" plugins, and that project produces a jar. Note that the dependencies for the lint check project (other than its testing dependencies) must all be "compileOnly":

dependencies {
    compileOnly "com.android.tools.lint:lint-api:$lintVersion"
    compileOnly "com.android.tools.lint:lint-checks:$lintVersion"
	...

Lint Check AAR Library

Next, there's a separate Android library project, called "library". This library doesn't have any code on its own (though it could). However, in its build.gradle, it specifies this:

dependencies {
    lintPublish project(':checks')
}

This tells the Gradle plugin to take the output from the "checks" project and package that as a "lint.jar" payload inside this library's AAR file. When that's done, any other projects that depends on this library will automatically be using the lint checks.

App Modules

Note that you don't have to go through the extra "library indirection" if you have a lint check that you only want to apply to one or more app modules. You can simply include the lintChecks dependency as shown above there as well, and then lint will include these rules when analyzing the project.

Lint Version

The lint version of the libraries (specified in this project as the lintVersion variable in build.gradle) should be the same version that is used by the Gradle plugin.

If the Gradle plugin version is X.Y.Z, then the Lint library version is X+23.Y.Z.

For example, for AGP 7.0.0-alpha08, the lint API versions are 30.0.0-alpha08.

Getting Started

Fetch code

git clone https://github.com/googlesamples/android-custom-lint-rules.git
cd android-custom-lint-rules

Run The Sample

Run the :app:lint target to have first the custom lint checks in checks/ compiled, then wrapped into the library, and finally run lint on a sample app module which has violations of the check enforced by sample check in this project:

$ ./gradlew :app:lint

> Task :app:lintDebug

Scanning app: ...
Wrote HTML report to file:///demo/android-custom-lint-rules/app/build/reports/lint-results-debug.html
Wrote SARIF report to file:///demo/android-custom-lint-rules/app/build/reports/lint-results-debug.sarif

/demo/android-custom-lint-rules/app/src/main/java/com/android/example/Test.kt:8: Warning: This code mentions lint: Congratulations [SampleId]
    val s = "lint"
             ~~~~

   Explanation for issues of type "SampleId":
   This check highlights string literals in code which mentions the word lint.
   Blah blah blah.

   Another paragraph here.

   Vendor: Android Open Source Project
   Contact: https://github.com/googlesamples/android-custom-lint-rules
   Feedback: https://github.com/googlesamples/android-custom-lint-rules/issues

0 errors, 1 warnings

BUILD SUCCESSFUL in 1s

Lint Dependencies

When building your own rules, you will likely want to know which dependencies you should bring into your own project. The below descriptions of the dependencies included within this project serve to help you make that decision:

Source Dependencies

• com.android.tools.lint:lint-api: The most important one; it contains things like LintClient, the Detector base class, the Issue class, and everything else that Lint checks rely on in the Lint framework.
• com.android.tools.lint:lint-checks: Contains the built-in checks that are developed internally. Also contains utilities that are sometimes useful for other lint checks, such as the VersionChecks class (which figures out whether a given UAST element is known to only be called at a given API level, either by surrounding if >= SDK-version checks or if < SDK-version early returns in the method).

Test Dependencies

• com.android.tools.lint:lint-tests: Contains useful utilities for writing unit tests for Lint checks, including the LintDetectorTest base class.
• com.android.tools.lint:lint: Lint checks don't need to depend on this. It's a separate artifact used by tools that want to integrate lint with the command line, such as the Gradle integration of lint. This is where things like terminal output, HTML reporting, command line parsing etc is handled.

The APIs in all but the lint-api artifact are more likely to change incompatibly than the lint-api artifact.

Support

• The "lint-dev" Google group: https://groups.google.com/forum/#!forum/lint-dev
• Stack Overflow: http://stackoverflow.com/questions/tagged/android

If you've found an error in this sample, please file an issue: https://github.com/googlesamples/android-custom-lint-rules/issues

Patches are encouraged, and may be submitted by forking this project and submitting a pull request through GitHub.

License

Licensed under the Apache 2.0 license. See the LICENSE file for details.

How to make contributions?

Please read and follow the steps in the CONTRIBUTING