Improve code inspection with annotations

Using code inspections tools, such as lint, can help you find problems and improve your code, but inspection tools can only infer so much. Android resource IDs, for example, use an int to identify strings, graphics, colors, and other resource types, so inspection tools can't tell when you have specified a string resource where you should have specified a color. This situation means that your app may render incorrectly or fail to run at all, even if you use code inspection.

Annotations let you provide hints to code inspections tools, such as lint, to help detect these more subtle code problems. Annotations are added as metadata tags that you attach to variables, parameters, and return values to inspect method return values, passed parameters, local variables, and fields. When used with code inspection tools, annotations can help you detect problems such as null pointer exceptions and resource type conflicts.

Android supports a variety of annotations through the Jetpack Annotations Library. You can access the library through the androidx.annotation package.

Note: If a module has a dependency on an annotation processor, you must use either the kapt or ksp dependency configuration for Kotlin or the annotationProcessor dependency configuration for Java to add that dependency.

Add annotations to your project

To enable annotations in your project, add the androidx.annotation:annotation dependency to your library or app. Any annotations you add are checked when you run a code inspection or lint task.

Add the Jetpack Annotations library dependency

The Jetpack Annotations library is published on Google's Maven Repository. To add the Jetpack Anotations library to your project, include the following line in the dependencies block of your build.gradle or build.gradle.kts file:

dependencies {
    implementation("androidx.annotation:annotation:1.9.1")
}

dependencies {
    implementation 'androidx.annotation:annotation:1.9.1'
}

If you use annotations in your own library module, the annotations are included as part of the Android Archive (AAR) artifact in XML format in the annotations.zip file. Adding the androidx.annotation dependency doesn't introduce a dependency for any downstream users of your library.

Note: If you're using other Jetpack libraries, you might not need to add the androidx.annotation dependency. Because many other Jetpack libraries depend on the Annotations Library, you might already have access to the annotations.

For a complete list of annotations included in the Jetpack repository, either see the Jetpack Annotations library reference or use the autocomplete feature to display the available options for the import androidx.annotation. statement.

Run code inspections

To start a code inspection from Android Studio, which includes validating annotations and automatic lint checking, select Analyze > Inspect Code from the menu. Android Studio displays conflict messages to flag potential problems where your code conflicts with annotations and to suggest possible resolutions.

You can also enforce annotations by running the lint task using the command line. Although this might be useful for flagging problems with a continuous integration server, the lint task doesn't enforce nullness annotations (described in the following section); only Android Studio does this. For more information on enabling and running lint inspections, see Improving your code with lint checks.

Although annotation conflicts generate warnings, these warnings don't prevent your app from compiling.

Nullness annotations

Add @Nullable and @NonNull annotations to check the nullness of a given variable, parameter, or return value. The @Nullable annotation indicates a variable, parameter, or return value that can be null. @NonNull indicates a variable, parameter, or return value that can't be null.

For example, if a local variable that contains a null value is passed as a parameter to a method with the @NonNull annotation attached to that parameter, building the code generates a warning indicating a non-null conflict. Also, attempting to reference the result of a method marked by @Nullable without first checking whether the result is null generates a nullness warning. Only use @Nullable on a method's return value if every use of the method must be explicitly null-checked.

The following example demonstrates nullability in action. The Kotlin example code doesn't leverage the @NonNull annotation because it's automatically added to the generated bytecode when a non-nullable type is specified. The Java example leverages the @NonNull annotation on the context and attrs parameters to check that the passed parameter values aren't null. It also checks that the onCreateView() method itself doesn't return null:

...
    /** Annotation not used because of the safe-call operator(?)**/
    override fun onCreateView(
            name: String?,
            context: Context,
            attrs: AttributeSet
    ): View? {
        ...
    }
...

import androidx.annotation.NonNull;
...
    /** Add support for inflating the <fragment> tag. **/
    @NonNull
    @Override
    public View onCreateView(String name, @NonNull Context context,
      @NonNull AttributeSet attrs) {
      ...
      }
...

Nullability analysis

Android Studio supports running a nullability analysis to automatically infer and insert nullness annotations in your code. A nullability analysis scans the contracts throughout the method hierarchies in your code to detect:

• Calling methods that can return null.
• Methods that should not return null.
• Variables, such as fields, local variables, and parameters, that can be null.
• Variables, such as fields, local variables, and parameters, that can't hold a null value.

The analysis then automatically inserts the appropriate null annotations in the detected locations.

To run a nullability analysis in Android Studio, select Analyze > Infer Nullity. Android Studio inserts the Android @Nullable and @NonNull annotations in detected locations in your code. After running a null analysis, it's a good practice to verify the injected annotations.

Note: When adding nullness annotations, autocomplete may suggest the IntelliJ @Nullable and @NotNull annotations instead of the Android null annotations and may auto-import the corresponding library. However, the Android Studio lint checker only looks for the Android null annotations. When verifying your annotations, confirm that your project uses the Android null annotations so the lint checker can properly notify you during code inspection.

Resource annotations

Validating resource types can be useful because Android references to resources, such as drawable and string resources, are passed as integers.

Code that expects a parameter to reference a specific type of resource, such as a String, can be passed to the expected reference type of int, but actually reference a different type of resource, such as an R.string resource.

For example, add @StringRes annotations to check whether a resource parameter contains an R.string reference, as shown here:

abstract fun setTitle(@StringRes resId: Int)

public abstract void setTitle(@StringRes int resId)

During code inspection, the annotation generates a warning if an R.string reference isn't passed in the parameter.

Annotations for other resource types, such as @DrawableRes, @DimenRes, @ColorRes, and @InterpolatorRes, can be added using the same annotation format and run during the code inspection.

If your parameter supports multiple resource types, you can put more than one resource type annotation on a given parameter. Use @AnyRes to indicate that the annotated parameter can be any type of R resource.

Although you can use @ColorRes to specify that a parameter should be a color resource, a color integer (in the RRGGBB or AARRGGBB format) isn't recognized as a color resource. Instead, use the @ColorInt annotation to indicate that a parameter must be a color integer. The build tools will flag incorrect code that passes a color resource ID such as android.R.color.black, rather than a color integer, to annotated methods.

Thread annotations

Thread annotations check whether a method is called from a specific type of thread. The following thread annotations are supported:

• @MainThread
• @UiThread
• @WorkerThread
• @BinderThread
• @AnyThread

The build tools treat the @MainThread and @UiThread annotations as interchangeable, so you can call @UiThread methods from @MainThread methods and vice versa. However, it's possible for a UI thread to be different from the main thread, in the case of system apps with multiple views on different threads. Therefore, you should annotate methods associated with an app's view hierarchy with @UiThread and annotate only methods associated with an app's lifecycle with @MainThread.

If all methods in a class share the same threading requirement, you can add a single thread annotation to the class to verify that all methods in the class are called from the same type of thread.

A common use of thread annotations is to validate that methods or classes annotated with @WorkerThread are only called from an appropriate background thread.

Value constraint annotations

Use the @IntRange, @FloatRange, and @Size annotations to validate the values of passed parameters. Both @IntRange and @FloatRange are most useful when applied to parameters where users are likely to get the range wrong.

The @IntRange annotation validates that an integer or long parameter value is within a specified range. The following example indicates that the alpha parameter must contain an integer value from 0 to 255:

fun setAlpha(@IntRange(from = 0, to = 255) alpha: Int) { ... }

public void setAlpha(@IntRange(from=0,to=255) int alpha) { ... }

The @FloatRange annotation checks whether a float or double parameter value is within a specified range of floating point values. The following example indicates that the alpha parameter must contain a float value from 0.0 to 1.0:

fun setAlpha(@FloatRange(from = 0.0, to = 1.0) alpha: Float) {...}

public void setAlpha(@FloatRange(from=0.0, to=1.0) float alpha) {...}

The @Size annotation checks the size of a collection or array or the length of a string. The @Size annotation can be used to verify the following qualities:

• Minimum size, such as @Size(min=2)
• Maximum size, such as @Size(max=2)
• Exact size, such as @Size(2)
• A number that the size must be a multiple of, such as @Size(multiple=2)

For example, @Size(min=1) checks whether a collection is not empty, and @Size(3) validates that an array contains exactly three values.

The following example indicates that the location array must contain at least one element:

fun getLocation(button: View, @Size(min=1) location: IntArray) {
    button.getLocationOnScreen(location)
}

void getLocation(View button, @Size(min=1) int[] location) {
    button.getLocationOnScreen(location);
}

Permission annotations

Use the @RequiresPermission annotation to validate the permissions of the caller of a method. To check for a single permission from a list of valid permissions, use the anyOf attribute. To check for a set of permissions, use the allOf attribute. The following example annotates the setWallpaper() method to indicate that the caller of the method must have the permission.SET_WALLPAPERS permission:

@RequiresPermission(Manifest.permission.SET_WALLPAPER)
@Throws(IOException::class)
abstract fun setWallpaper(bitmap: Bitmap)

@RequiresPermission(Manifest.permission.SET_WALLPAPER)
public abstract void setWallpaper(Bitmap bitmap) throws IOException;

The following example requires the caller of the copyImageFile() method to have both read access to external storage and read access to location metadata in the copied image:

@RequiresPermission(allOf = [
    Manifest.permission.READ_EXTERNAL_STORAGE,
    Manifest.permission.ACCESS_MEDIA_LOCATION
])
fun copyImageFile(dest: String, source: String) {
    ...
}

@RequiresPermission(allOf = {
    Manifest.permission.READ_EXTERNAL_STORAGE,
    Manifest.permission.ACCESS_MEDIA_LOCATION})
public static final void copyImageFile(String dest, String source) {
    //...
}

For permissions on intents, place the permission requirement on the string field that defines the intent action name:

@RequiresPermission(android.Manifest.permission.BLUETOOTH)
const val ACTION_REQUEST_DISCOVERABLE = "android.bluetooth.adapter.action.REQUEST_DISCOVERABLE"

@RequiresPermission(android.Manifest.permission.BLUETOOTH)
public static final String ACTION_REQUEST_DISCOVERABLE =
            "android.bluetooth.adapter.action.REQUEST_DISCOVERABLE";

For permissions on content providers that need separate permissions for read and write access, wrap each permission requirement in an @RequiresPermission.Read or @RequiresPermission.Write annotation:

@RequiresPermission.Read(RequiresPermission(READ_HISTORY_BOOKMARKS))
@RequiresPermission.Write(RequiresPermission(WRITE_HISTORY_BOOKMARKS))
val BOOKMARKS_URI = Uri.parse("content://browser/bookmarks")

@RequiresPermission.Read(@RequiresPermission(READ_HISTORY_BOOKMARKS))
@RequiresPermission.Write(@RequiresPermission(WRITE_HISTORY_BOOKMARKS))
public static final Uri BOOKMARKS_URI = Uri.parse("content://browser/bookmarks");

Indirect permissions

When a permission depends on the specific value supplied to a method's parameter, use @RequiresPermission on the parameter itself without listing the specific permissions. For example, the startActivity(Intent) method uses an indirect permission on the intent passed to the method:

abstract fun startActivity(@RequiresPermission intent: Intent, bundle: Bundle?)

public abstract void startActivity(@RequiresPermission Intent intent, @Nullable Bundle)

When you use indirect permissions, the build tools perform data flow analysis to check whether the argument passed into the method has any @RequiresPermission annotations. They then enforce any existing annotations from the parameter on the method itself. In the startActivity(Intent) example, annotations in the Intent class cause the resulting warnings on invalid uses of startActivity(Intent) when an intent without the appropriate permissions is passed to the method, as shown in figure 1.

Figure 1. The warning generated from an indirect permissions annotation on the startActivity(Intent) method.

The build tools generate the warning on startActivity(Intent) from the annotation on the corresponding intent action name in the Intent class:

@RequiresPermission(Manifest.permission.CALL_PHONE)
const val ACTION_CALL = "android.intent.action.CALL"

@RequiresPermission(Manifest.permission.CALL_PHONE)
public static final String ACTION_CALL = "android.intent.action.CALL";

If necessary, you can substitute @RequiresPermission for @RequiresPermission.Read or @RequiresPermission.Write when annotating a method's parameter. However, for indirect permissions @RequiresPermission should not be used in conjunction with either the read or the write permissions annotations.

Return value annotations

Use the @CheckResult annotation to validate that a method's result or return value is actually used. Instead of annotating every non-void method with @CheckResult, add the annotation to clarify the results of potentially confusing methods.

For example, new Java developers often mistakenly think that <String>.trim() removes whitespace from the original string. Annotating the method with @CheckResult flags uses of <String>.trim() where the caller doesn't do anything with the method's return value.

The following example annotates the checkPermissions() method to check whether the return value of the method is actually referenced. It also names the enforcePermission() method as a method to be suggested to the developer as a replacement:

@CheckResult(suggest = "#enforcePermission(String,int,int,String)")
abstract fun checkPermission(permission: String, pid: Int, uid: Int): Int

@CheckResult(suggest="#enforcePermission(String,int,int,String)")
public abstract int checkPermission(@NonNull String permission, int pid, int uid);

CallSuper annotations

Use the @CallSuper annotation to validate that an overriding method calls the super implementation of the method.

The following example annotates the onCreate() method to ensure that any overriding method implementations call super.onCreate():

@CallSuper
override fun onCreate(savedInstanceState: Bundle?) {
}

@CallSuper
protected void onCreate(Bundle savedInstanceState) {
}

Typedef annotations

Typedef annotations check whether a particular parameter, return value, or field references a specific set of constants. They also enable code completion to automatically offer the allowed constants.

Use the @IntDef and @StringDef annotations to create enumerated annotations of integer and string sets to validate other types of code references.

Typedef annotations use @interface to declare the new enumerated annotation type. The @IntDef and @StringDef annotations, along with @Retention, annotate the new annotation and are necessary to define the enumerated type. The @Retention(RetentionPolicy.SOURCE) annotation tells the compiler not to store the enumerated annotation data in the .class file.

The following example shows the steps to create an annotation that checks whether a value passed as a method parameter references one of the defined constants:

import androidx.annotation.IntDef
//...
// Define the list of accepted constants and declare the NavigationMode annotation.
@Retention(AnnotationRetention.SOURCE)
@IntDef(NAVIGATION_MODE_STANDARD, NAVIGATION_MODE_LIST, NAVIGATION_MODE_TABS)
annotation class NavigationMode

// Declare the constants.
const val NAVIGATION_MODE_STANDARD = 0
const val NAVIGATION_MODE_LIST = 1
const val NAVIGATION_MODE_TABS = 2

abstract class ActionBar {

    // Decorate the target methods with the annotation.
    // Attach the annotation.
    @get:NavigationMode
    @setparam:NavigationMode
    abstract var navigationMode: Int

}

import androidx.annotation.IntDef;
//...
public abstract class ActionBar {
    //...
    // Define the list of accepted constants and declare the NavigationMode annotation.
    @Retention(RetentionPolicy.SOURCE)
    @IntDef({NAVIGATION_MODE_STANDARD, NAVIGATION_MODE_LIST, NAVIGATION_MODE_TABS})
    public @interface NavigationMode {}

    // Declare the constants.
    public static final int NAVIGATION_MODE_STANDARD = 0;
    public static final int NAVIGATION_MODE_LIST = 1;
    public static final int NAVIGATION_MODE_TABS = 2;

    // Decorate the target methods with the annotation.
    @NavigationMode
    public abstract int getNavigationMode();

    // Attach the annotation.
    public abstract void setNavigationMode(@NavigationMode int mode);
}

When you build this code, a warning is generated if the mode parameter doesn't reference one of the defined constants (NAVIGATION_MODE_STANDARD, NAVIGATION_MODE_LIST, or NAVIGATION_MODE_TABS).

Combine @IntDef and @IntRange to indicate that an integer can be either a given set of constants or a value within a range.

Enable combining constants with flags

If users can combine the allowed constants with a flag (such as |, &, ^, and so on), you can define an annotation with a flag attribute to check whether a parameter or return value references a valid pattern.

The following example creates the DisplayOptions annotation with a list of valid DISPLAY_ constants:

import androidx.annotation.IntDef
...

@IntDef(flag = true, value = [
    DISPLAY_USE_LOGO,
    DISPLAY_SHOW_HOME,
    DISPLAY_HOME_AS_UP,
    DISPLAY_SHOW_TITLE,
    DISPLAY_SHOW_CUSTOM
])
@Retention(AnnotationRetention.SOURCE)
annotation class DisplayOptions
...

import androidx.annotation.IntDef;
...

@IntDef(flag=true, value={
        DISPLAY_USE_LOGO,
        DISPLAY_SHOW_HOME,
        DISPLAY_HOME_AS_UP,
        DISPLAY_SHOW_TITLE,
        DISPLAY_SHOW_CUSTOM
})
@Retention(RetentionPolicy.SOURCE)
public @interface DisplayOptions {}

...

When you build code with an annotation flag, a warning is generated if the decorated parameter or return value doesn't reference a valid pattern.

Keep annotation

The @Keep annotation ensures that an annotated class or method is not removed when the code is minified at build time. This annotation is typically added to methods and classes that are accessed through reflection to prevent the compiler from treating the code as unused.

For more information about how to minify your code and specify which code is not to be removed, see Shrink, obfuscate, and optimize your app.

Code visibility annotations

Use the following annotations to denote the visibility of specific portions of code, such as methods, classes, fields, or packages.

Make code visible for testing

The @VisibleForTesting annotation indicates that an annotated method is more visible than normally necessary to make the method testable. This annotation has an optional otherwise argument that lets you designate what the visibility of the method would be if not for the need to make it visible for testing. Lint uses the otherwise argument to enforce the intended visibility.

In the following example, myMethod() is normally private, but it is package-private for tests. With the VisibleForTesting.PRIVATE designation, lint displays a message if this method is called from outside the context allowed by private access, such as from a different compilation unit.

@VisibleForTesting(otherwise = VisibleForTesting.PRIVATE)
fun myMethod() {
    ...
}

@VisibleForTesting(otherwise = VisibleForTesting.PRIVATE)
void myMethod() { ... }

You can also specify @VisibleForTesting(otherwise = VisibleForTesting.NONE) to indicate that a method exists only for testing. This form is the same as using @RestrictTo(TESTS). They both perform the same lint check.

Restrict an API

The @RestrictTo annotation indicates that access to the annotated API (package, class, or method) is limited, as follows:

Subclasses

Use the annotation form @RestrictTo(RestrictTo.Scope.SUBCLASSES) to restrict API access to subclasses only.

Only classes that extend the annotated class can access this API. The Java protected modifier is not restrictive enough, because it allows access from unrelated classes within the same package. Also, there are cases when you want to leave a method public for future flexibility, because you can never make a previously protected and overridden method public, but you want to provide a hint that the class is intended for usages within the class or from subclasses only.

Libraries

Use the annotation form @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX) to restrict API access to your libraries only.

Only your library code can access the annotated API. This lets you not only organize your code in whatever package hierarchy you want but also share the code among a group of related libraries. This option is already available to the Jetpack libraries that have a lot of implementation code that is not meant for external use, but that has to be public to share it across the various complementary Jetpack libraries.

Testing

Use the annotation form @RestrictTo(RestrictTo.Scope.TESTS) to prevent other developers from accessing your testing APIs.

Only testing code can access the annotated API. This prevents other developers from using APIs for development that you intend for testing purposes only.