lint.md

Lint

Android's lint is a static analysis tool that Chromium uses to catch possible issues in Java code.

This is a list of checks that you might encounter.

[TOC]

How Chromium uses lint

Chromium only runs lint on apk or bundle targets that explicitly set enable_lint = true. Some example targets that have this set are:

• //chrome/android:monochrome_public_bundle
• //android_webview/support_library/boundary_interfaces:boundary_interface_example_apk
• //remoting/android:remoting_apk

My code has a lint error

If lint reports an issue in your code, there are several possible remedies. In descending order of preference:

Fix it

While this isn't always the right response, fixing the lint error or warning should be the default.

Suppress it locally

Java provides an annotation, @SuppressWarnings, that tells lint to ignore the annotated element. It can be used on classes, constructors, methods, parameters, fields, or local variables, though usage in Chromium is typically limited to the first three. You do not need to import it since it is in the java.lang package.

Like many suppression annotations, @SuppressWarnings takes a value that tells lint what to ignore. It can be a single String:

@SuppressWarnings("NewApi")
public void foo() {
    a.methodThatRequiresHighSdkLevel();
}

It can also be a list of Strings:

@SuppressWarnings({
        "NewApi",
        "UseSparseArrays"
        })
public Map<Integer, FakeObject> bar() {
    Map<Integer, FakeObject> shouldBeASparseArray = new HashMap<Integer, FakeObject>();
    another.methodThatRequiresHighSdkLevel(shouldBeASparseArray);
    return shouldBeASparseArray;
}

For resource xml files you can use tools:ignore:

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- TODO(crbug/###): remove tools:ignore once these colors are used -->
    <color name="hi" tools:ignore="NewApi,UnusedResources">@color/unused</color>
</resources>

The examples above are the recommended ways of suppressing lint warnings.

Suppress it in a lint-suppressions.xml file

lint can be given a per-target XML configuration file containing warnings or errors that should be ignored. Each target defines its own configuration file via the lint_suppressions_file gn variable. It is usually defined near its enable_lint gn variable.

These suppressions files should only be used for temporarily ignoring warnings that are too hard (or not possible) to suppress locally, and permanently ignoring warnings only for this target. To permanently ignore a warning for all targets, add the warning to the _DISABLED_ALWAYS list in build/android/gyp/lint.py. Disabling globally makes lint a bit faster.

The exception to the above rule is for warnings that affect multiple languages. Feel free to suppress those in lint-suppressions.xml files since it is not practical to suppress them in each language file and it is a lot of extra bloat to list out every language for every violation in lint-baseline.xml files.

Here is an example of how to structure a suppressions XML file:

<?xml version="1.0" encoding="utf-8" ?>
<lint>
  <!-- Chrome is a system app. -->
  <issue id="ProtectedPermissions" severity="ignore"/>
  <issue id="UnusedResources">
    <!-- 1 raw resources are accessed by URL in various places. -->
    <ignore regexp="gen/remoting/android/.*/res/raw/credits.*"/>
    <!-- TODO(crbug.com/###): Remove the following line.  -->
    <ignore regexp="The resource `R.string.soon_to_be_used` appears to be unused"/>
  </issue>
</lint>

What are lint-baseline.xml files for?

Baseline files are to help us introduce new lint warnings and errors without blocking on fixing all our existing code that violate these new errors. Since they are generated files, they should not be used to suppress lint warnings. One of the approaches above should be used instead. Eventually all the errors in baseline files should be either fixed or ignored permanently.

The following are some common scenarios where you may need to update baseline files.

I updated cmdline-tools and now there are tons of new errors!

This happens every time lint is updated, since lint is provided by cmdline-tools.

Baseline files are defined via the lint_baseline_file gn variable. It is usually defined near a target's enable_lint gn variable. To regenerate the baseline file, delete it and re-run the lint target. The command will fail, but the baseline file will have been generated.

This may need to be repeated for all targets that have set enable_lint = true, including downstream targets. Downstream baseline files should be updated and first to avoid build breakages. Each target has its own lint_baseline_file defined and so all these files can be removed and regenerated as needed.

I updated library X and now there are tons of new errors!

This is usually because library X's aar contains custom lint checks and/or custom annotation definition. Follow the same procedure as updates to cmdline-tools.