(#) Handler reference leaks !!! WARNING: Handler reference leaks This is a warning. Id : `HandlerLeak` Summary : Handler reference leaks Severity : Warning Category : Performance Platform : Android Vendor : Android Open Source Project Feedback : https://issuetracker.google.com/issues/new?component=192708 Since : Initial Affects : Kotlin and Java files Editing : This check runs on the fly in the IDE editor Implementation : [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/HandlerDetector.java) Tests : [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/HandlerDetectorTest.java) Copyright Year : 2012 Since this Handler is declared as an inner class, it may prevent the outer class from being garbage collected. If the Handler is using a `Looper` or `MessageQueue` for a thread other than the main thread, then there is no issue. If the `Handler` is using the `Looper` or `MessageQueue` of the main thread, you need to fix your `Handler` declaration, as follows: Declare the `Handler` as a static class; In the outer class, instantiate a `WeakReference` to the outer class and pass this object to your `Handler` when you instantiate the `Handler`; Make all references to members of the outer class using the `WeakReference` object. (##) Example Here is an example of lint warnings produced by this check: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text src/test/pkg/HandlerTest.java:12:Warning: This Handler class should be static or leaks might occur (test.pkg.HandlerTest.Inner) [HandlerLeak] public class Inner extends Handler { // ERROR ----- src/test/pkg/HandlerTest.java:18:Warning: This Handler class should be static or leaks might occur (anonymous android.os.Handler) [HandlerLeak] Handler anonymous = new Handler() { // ERROR ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is the source file referenced above: `src/test/pkg/HandlerTest.java`: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers package test.pkg; import android.os.Looper; import android.os.Handler; import android.os.Message; public class HandlerTest extends Handler { // OK public static class StaticInner extends Handler { // OK public void dispatchMessage(Message msg) { super.dispatchMessage(msg); } } public class Inner extends Handler { // ERROR public void dispatchMessage(Message msg) { super.dispatchMessage(msg); } } void method() { Handler anonymous = new Handler() { // ERROR public void dispatchMessage(Message msg) { super.dispatchMessage(msg); } }; Looper looper = null; Handler anonymous2 = new Handler(looper) { // OK public void dispatchMessage(Message msg) { super.dispatchMessage(msg); } }; } public class WithArbitraryLooper extends Handler { public WithArbitraryLooper(String unused, Looper looper) { // OK super(looper, null); } public void dispatchMessage(Message msg) { super.dispatchMessage(msg); } } } ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You can also visit the [source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/HandlerDetectorTest.java) for the unit tests for this check to see additional scenarios. The above example was automatically extracted from the first unit test found for this lint check, `HandlerDetector.testRegistered`. To report a problem with this extracted sample, visit https://issuetracker.google.com/issues/new?component=192708. (##) Suppressing You can suppress false positives using one of the following mechanisms: * Using a suppression annotation like this on the enclosing element: ```kt // Kotlin @Suppress("HandlerLeak") fun method() { problematicStatement() } ``` or ```java // Java @SuppressWarnings("HandlerLeak") void method() { problematicStatement(); } ``` * Using a suppression comment like this on the line above: ```kt //noinspection HandlerLeak problematicStatement() ``` * Using a special `lint.xml` file in the source tree which turns off the check in that folder and any sub folder. A simple file might look like this: ```xml <?xml version="1.0" encoding="UTF-8"?> <lint> <issue id="HandlerLeak" severity="ignore" /> </lint> ``` Instead of `ignore` you can also change the severity here, for example from `error` to `warning`. You can find additional documentation on how to filter issues by path, regular expression and so on [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html). * In Gradle projects, using the DSL syntax to configure lint. For example, you can use something like ```gradle lintOptions { disable 'HandlerLeak' } ``` In Android projects this should be nested inside an `android { }` block. * For manual invocations of `lint`, using the `--ignore` flag: ``` $ lint --ignore HandlerLeak ...` ``` * Last, but not least, using baselines, as discussed [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).