App resources overview

Resources are the additional files and static content that your code uses, such as bitmaps, user interface strings, animation instructions, and more.

Always externalize app resources such as images and strings from your code, so that you can maintain them independently. Also, provide alternative resources for specific device configurations by grouping them in specially named resource directories. At runtime, Android uses the appropriate resource based on the current configuration. For example, you might want to provide different strings depending on the language setting.

Once you externalize your app resources, you can access them using resource IDs that are generated in your project's R class. This document shows you how to group the resources in your Android project. It also shows you how to provide alternative resources for specific device configurations and then access them from your app code or other XML files.

Group resource types

Place each type of resource in a specific subdirectory of your project's res/ directory. For example, here's the file hierarchy for a simple project:

MyProject/
    src/
        MyActivity.kt
    res/
        drawable/
            graphic.png
        mipmap/
            icon.png
        values/
            strings.xml

The res/ directory contains all the resources in its subdirectories: an image resource, a mipmap/ directory for launcher icons, and a string resource file. The resource directory names are important and are described in Table 1.

Note: For more information about using the mipmap folders, see Put app icons in mipmap directories.

Table 1. Resource directories supported inside project res/ directory.

Caution: Never save resource files directly inside the res/ directory. It causes a compiler error.

The resources that you save in the subdirectories defined in Table 1 are your default resources. That is, these resources define the default design and content for your app. However, different types of Android-powered devices might call for different types of resources.

For example, you can provide different string resources that translate the text in your user interface based on the device's language setting.

Note: In Compose, UIs, animations, and state-driven colors are declared in Kotlin, so the layout/, menu/, anim/, animator/, and color/ directories are obsolete for modern apps. For more information, see Animations in Compose and Anatomy of a theme in Compose.

Provide alternative resources

Most apps provide alternative resources to support specific device configurations. For instance, include alternative drawable resources for different screen densities and alternative string resources for different languages. At runtime, Android detects the current device configuration and loads the appropriate resources for your app.

To specify configuration-specific alternatives for a set of resources, do the following:

1. Create a new directory in res/ named in the form <resources_name>-<qualifier>.
   • <resources_name> is the directory name of the corresponding default resources (defined in Table 1).
   • <qualifier> is a name that specifies an individual configuration for which these resources are to be used (defined in Table 2).

   You can append more than one <qualifier>. Separate each one with a dash.

   Caution: When appending multiple qualifiers, you must place them in the same order in which they are listed in Table 2. If the qualifiers are ordered incorrectly, the resources are ignored.

2. Save the appropriate alternative resources in this new directory. The resource files must be named exactly the same as the default resource files.

For example, here are some default and alternative resources:

res/
    drawable/
        icon.png
        background.png
    drawable-hdpi/
        icon.png
        background.png

The hdpi qualifier indicates that the resources in that directory are for devices with a high-density screen. The images in these drawable directories are sized for specific screen densities, but the filenames are exactly the same. This way, the resource ID that you use to reference the icon.png or background.png image is always the same. Android selects the version of each resource that best matches the current device by comparing the device configuration information with the qualifiers in the resource directory name.

Caution: When defining an alternative resource, make sure you also define the resource in a default configuration. Otherwise, your app might encounter runtime exceptions when the device changes a configuration. For example, if you add a string to only values-en and not values, your app might encounter a Resource Not Found exception when the user changes the default system language.

Table 2 lists configuration qualifiers in order of precedence. You can add multiple qualifiers to one directory name by separating each qualifier with a dash. If you use multiple qualifiers for a resource directory, you must add them to the directory name in the order they are listed in the table.

Table 2. Configuration qualifier names.

Note: Not all versions of Android support all the qualifiers. Using a new qualifier implicitly adds the platform version qualifier so that older devices can ignore it. To avoid any issues, always include a set of default resources (a set of resources with no qualifiers). For more information, see the section about providing the best device compatibility with resources.

In Compose apps, layout- and dimension-related configuration qualifiers aren't needed. While they still exist, they are excluded from Table 2. These qualifiers include: layout direction, smallest width, available width, available height, screen size, screen aspect, round screen, and screen orientation. For the full table of configuration qualifiers in order of precedence, see App resources overview (Views).

Qualifier name rules

Here are some rules about using configuration qualifier names:

• You can specify multiple qualifiers for a single set of resources, separated by dashes. For example, drawable-en-rUS-night applies to US-English devices in night mode.
• The qualifiers must be in the order listed in Table 2.
  • Wrong: drawable-hdpi-night/
  • Correct: drawable-night-hdpi/
• Alternative resource directories can't be nested. For example, you can't have res/drawable/drawable-en/.
• Values are case-insensitive. The resource compiler converts directory names to lowercase before processing to avoid problems on case-insensitive file systems. Any capitalization in the names is only to benefit readability.
• Only one value for each qualifier type is supported. For example, if you want to use the same drawable files for Spain and France, you can't have a directory named drawable-es-fr/. Instead, you need two resource directories, such as drawable-es/ and drawable-fr/, which contain the appropriate files.

After you save alternative resources into directories named with these qualifiers, Android automatically applies the resources in your app based on the current device configuration. Each time a resource is requested, Android checks for alternative resource directories that contain the requested resource file, then finds the best-matching resource.

If there are no alternative resources that match a particular device configuration, then Android uses the corresponding default resources—the set of resources for a particular resource type that doesn't include a configuration qualifier.

Create alias resources

When you have a resource that you'd like to use for more than one device configuration but you don't want to provide it as a default resource, you don't need to put the same resource in more than one alternative resource directory. Instead, you can create an alternative resource that acts as an alias for a resource saved in your default resource directory.

Drawables

For example, imagine you have an app icon, icon.png, and need a unique version of it for different locales. However, two locales, English-Canadian and French-Canadian, need to use the same version. You don't need to copy the same image into the resource directory for both English-Canadian and French-Canadian. Instead, you can save the image that's used for both using any name other than icon.png, such as icon_ca.png, and put it in the default res/drawable/ directory. Then create an icon.xml file in res/drawable-en-rCA/ and res/drawable-fr-rCA/ that refers to the icon_ca.png resource using the <bitmap> element.

<?xml version="1.0" encoding="utf-8"?>
<bitmap xmlns:android="http://schemas.android.com/apk/res/android" android:src="@drawable/icon_ca" />

This lets you store just one version of the PNG file and two small XML files that point to it. Then you can use painterResource(R.drawable.icon) and the system will pick the appropriate file once it detects the locale.

Strings and other simple values

To create an alias to an existing string, use the resource ID of the desired string as the value for the new string:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="hello">Hello</string>
    <string name="hi">@string/hello</string>
</resources>

The R.string.hi resource is now an alias for the R.string.hello.

Other simple values work the same way, such as colors:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="red">#f00</color>
    <color name="highlight">@color/red</color>
</resources>

Access your app resources

Once you provide a resource in your application, you can apply it by referencing its resource ID. All resource IDs are defined in your project's R class, which the aapt tool automatically generates.

When your application is compiled, aapt generates the R class, which contains resource IDs for all the resources in your res/ directory. For each type of resource, there is an R subclass, such as R.drawable for all drawable resources. And for each resource of that type, there is a static integer, for example, R.drawable.icon. This integer is the resource ID that you can use to retrieve your resource.

Although the R class is where resource IDs are specified, you don't need to look there to discover a resource ID. A resource ID is always composed of the following:

• The resource type: each resource is grouped into a "type," such as string or drawable.
• The resource name, which is the filename excluding the extension.

Access resources in Compose

Jetpack Compose provides built-in, composable-aware functions to access resources securely.

• Strings:

  stringResource(id = R.string.hello)

• Drawables:

  painterResource(id = R.drawable.my_icon)

Access resources in non-UI code

If you need to access resources outside of your UI hierarchy—such as in a ViewModel, a Repository, or a system Service—you can resolve them using the Context.

// Retrieve a localized string resource
val greeting = context.getString(R.string.hello_world)

You can also retrieve individual resources using methods in Resources, which you can get an instance of with getResources.

Syntax

Here's the syntax to reference a resource in code:

[<package_name>.]R.<resource_type>.<resource_name>

• <package_name> is the name of the package in which the resource is located (not required when referencing resources from your own package).
• <resource_type> is the R subclass for the resource type.
• <resource_name> is either the resource filename without the extension or the android:name attribute value in the XML element, for simple values.

For more information about each resource type and how to reference them, see Resources in Compose.

Access original files

While uncommon, you might need to access your original files and directories. If you do, then saving your files in res/ won't work for you, because the only way to read a resource from res/ is with the resource ID. Instead, you can save your resources in the assets/ directory.

Files saved in the assets/ directory are not given a resource ID, so you can't reference them through the R class or from XML resources. Instead, you can query files in the assets/ directory like a normal file system and read raw data using AssetManager.

However, if all you require is the ability to read raw data (such as a video or audio file), then save the file in the res/raw/ directory and read a stream of bytes using openRawResource.

Access platform resources

Android contains a number of standard resources, such as system styles and themes. To access these, qualify your resource reference with the android package class. For example: painterResource(android.R.drawable.ic_menu_info_details).

Provide the best device compatibility with resources

For your app to support multiple device configurations, it's very important that you always provide default resources for each type of resource that your app uses.

For example, if your app supports several languages, always include a values/ directory (in which your strings are saved) without a language and region qualifier. If you instead put all your string files in directories that have a language and region qualifier, then your app crashes when run on a device set to a language that your strings don't support.

As long as you provide default values/ resources, then your app runs properly, even if the user doesn't understand the language it presents. It's better than crashing.

Providing default resources is important not only because your app might run on a configuration you hadn't anticipated, but also because new versions of Android sometimes add configuration qualifiers that lower versions don't support. If you use a new resource qualifier, but maintain code compatibility with lower versions of Android, then when a lower version of Android runs your app, it crashes if you don't provide default resources, because it can't use the resources named with the new qualifier.

For example, if your minSdkVersion is set to 4, and you qualify all of your drawable resources using night mode (night or notnight, which were added in API level 8), then an API level 4 device can't access your drawable resources and crashes. In this case, you probably want notnight to be your default resources, so exclude that qualifier and put your drawable resources in either drawable/ or drawable-night/.

In short, to provide the best device compatibility, always provide default resources for the resources your app needs to perform properly. Then create alternative resources for specific device configurations using configuration qualifiers.

There is one exception to this rule: If your app's minSdkVersion is 4 or greater, you don't need default drawable resources when you provide alternative drawable resources with the screen density qualifier. Even without default drawable resources, Android can find the best match among the alternative screen densities and scale the bitmaps as necessary. However, for the best experience on all types of devices, provide alternative drawables for all three types of density.

How Android finds the best-matching resource

When you request a resource for which you provide alternatives, Android selects which alternative resource to use at runtime, depending on the current device configuration. To demonstrate how Android selects an alternative resource, assume the following drawable directories each contain different versions of the same images:

drawable/
drawable-en/
drawable-fr-rCA/
drawable-en-night/
drawable-en-notouch-12key/
drawable-night-ldpi/
drawable-night-notouch-12key/

And assume the following is the device configuration:

Locale = en-GB
Night mode = night
Screen pixel density = hdpi
Touchscreen type = notouch
Primary text input method = 12key

By comparing the device configuration to the available alternative resources, Android selects drawables from drawable-en-night.

The system arrives at its decision for which resources to use with the following logic:

Figure 2. Flowchart of how Android finds the best-matching resource.

1. Eliminate resource files that contradict the device configuration.

   The drawable-fr-rCA/ directory is eliminated, because it contradicts the en-GB locale.

   drawable/
   drawable-en/
   drawable-fr-rCA/
   drawable-en-night/
   drawable-en-notouch-12key/
   drawable-night-ldpi/
   drawable-night-notouch-12key/
   

   Exception: Screen pixel density is the one qualifier that is not eliminated due to a contradiction. Even though the screen density of the device is hdpi, drawable-night-ldpi/ isn't eliminated because every screen density is considered to be a match at this point. For information, see Screen compatibility overview.

2. Find the next-highest-precedence qualifier in the list (Table 2). (Start with MCC.)
3. Do any of the resource directories include this qualifier?
   • If no, return to step two and look at the next qualifier. In this example, the answer is "no" until the language qualifier is reached.
   • If yes, continue to step four.
4. Eliminate resource directories that don't include this qualifier. In this example, the system next eliminates all the directories that don't include a language qualifier:

   drawable/
   drawable-en/
   drawable-en-night/
   drawable-en-notouch-12key/
   drawable-night-ldpi/
   drawable-night-notouch-12key/
   

   Exception: If the qualifier in question is screen pixel density, Android selects the option that most closely matches the device screen density. In general, Android prefers scaling down a larger original image to scaling up a smaller original image. For more information, see Screen compatibility overview.

5. Repeat steps two, three, and four until only one directory remains. In this example, night mode is the next qualifier for which there are any matches. So, resources that don't specify night mode are eliminated:

   drawable-en/
   drawable-en-night/
   drawable-en-notouch-12key/
   

   The remaining directory is drawable-en-night.

Though this procedure is executed for each resource requested, the system optimizes some aspects of it. One such optimization is that once the device configuration is known, it might eliminate alternative resources that can never match. For example, if the configuration language is English, then any resource directory that has a language qualifier set to something other than English is never included in the pool of resources checked (though a resource directory without the language qualifier is still included).

When selecting resources based on the screen size qualifiers, the system uses resources designed for a screen smaller than the current screen if there are no resources that better match. For example, a large-size screen uses normal-size screen resources if necessary.

However, if the only available resources are larger than the current screen, the system doesn't use them and your app crashes if no other resources match the device configuration. This happens, for example, if all layout resources are tagged with the xlarge qualifier, but the device is a normal-size screen.

Note: The precedence of the qualifier (in Table 2) is more important than the number of qualifiers that exactly match the device. In the preceding example, at step four the last choice on the list includes three qualifiers that exactly match the device (night mode, touchscreen type, and input method), while drawable-en has only one parameter that matches (language). However, language has a higher precedence than these other qualifiers, so drawable-night-notouch-12key is eliminated.

Additional resources

To learn more about app resources, see the following additional resources:

Documentation

• Resources in Compose

Views content

• App resources overview (Views)