App resources overview

Resources are the additional files and static content that your code uses, such as bitmaps, layout definitions, 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 a different UI layout depending on the screen size or 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.java
    res/
        drawable/
            graphic.png
        layout/
            main.xml
            info.xml
        mipmap/
            icon.png
        values/
            strings.xml

The res/ directory contains all the resources in its subdirectories: an image resource, two layout resources, 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.

For more information about the individual types of resources, see the Resource types overview.

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 layout resources for devices that have larger than normal screens to take advantage of the extra screen space. You can also provide different string resources that translate the text in your user interface based on the device's language setting. To provide these different resources for different device configurations, you need to provide alternative resources in addition to your default resources.

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.

Figure 1. Two devices using different layout resources based on screen size.

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 the valid 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. For example, using a w600dp qualifier automatically includes the v13 qualifier, because the available-width qualifier was new in API level 13. 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.

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-land applies to US-English devices in landscape orientation.
• The qualifiers must be in the order listed in table 2.
  • Wrong: drawable-hdpi-port/
  • Correct: drawable-port-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. However, you aren't required to actually duplicate the files in both locations. Instead, you can create an alias to a resource, as described in the Create alias resources section.

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.

Note: Not all resources offer a mechanism by which you can create an alias to another resource. In particular, animation, menu, raw, and other unspecified resources in the xml/ directory don't offer this feature.

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. This lets you store just one version of the PNG file and two small XML files that point to it. See the examples in the following sections for details.

Drawable

To create an alias to an existing drawable, use the <drawable> element:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <drawable name="icon">@drawable/icon_ca</drawable>
</resources>

If you save this file as icon.xml in an alternative resource directory, such as res/values-en-rCA/, it is compiled into a resource that you can reference as R.drawable.icon, but is actually an alias for the R.drawable.icon_ca resource, which is saved in res/drawable/.

Layout

To create an alias to an existing layout, use the <include> element, wrapped in a <merge>:

<?xml version="1.0" encoding="utf-8"?>
<merge>
    <include layout="@layout/main_ltr"/>
</merge>

If you save this file as main.xml, it is compiled into a resource you can reference as R.layout.main, but is actually an alias for the R.layout.main_ltr resource.

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, drawable, and layout. For more information about the different types, see Resource types overview.
• The resource name, which is either the filename excluding the extension or the value in the XML android:name attribute, if the resource is a simple value, such as a string.

There are two ways you can access a resource:

• In code: using a static integer from a subclass of your R class, such as:

  R.string.hello

  string is the resource type and hello is the resource name. There are many Android APIs that can access your resources when you provide a resource ID in this format. For more information, see the Access resources in code section.

• In XML: using a special XML syntax that corresponds to the resource ID defined in your R class, such as:

  @string/hello

  string is the resource type and hello is the resource name. You can use this syntax in an XML resource any place where a value is expected that you provide in a resource. For more information, see the Access resources from XML section.

Access resources in code

You can use a resource in code by passing the resource ID as a method parameter. For example, you can set an ImageView to use the res/drawable/myimage.png resource using setImageResource():

val imageView = findViewById(R.id.myimageview) as ImageView
imageView.setImageResource(R.drawable.myimage)

ImageView imageView = (ImageView) findViewById(R.id.myimageview);
imageView.setImageResource(R.drawable.myimage);

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 Resource types overview.

Use cases

There are many methods that accept a resource ID parameter, and you can retrieve resources using methods in Resources. You can get an instance of Resources using Context.getResources().

Here are some examples of accessing resources in code:

// Load a background for the current screen from a drawable resource.
window.setBackgroundDrawableResource(R.drawable.my_background_image)

// Set the Activity title by getting a string from the Resources object, because
//  this method requires a CharSequence rather than a resource ID.
window.setTitle(resources.getText(R.string.main_title))

// Load a custom layout for the current screen.
setContentView(R.layout.main_screen)

// Set a slide in animation by getting an Animation from the Resources object.
flipper.setInAnimation(AnimationUtils.loadAnimation(this,
        R.anim.hyperspace_in))

// Set the text on a TextView object using a resource ID.
val msgTextView = findViewById(R.id.msg) as TextView
msgTextView.setText(R.string.hello_message)

// Load a background for the current screen from a drawable resource.
getWindow().setBackgroundDrawableResource(R.drawable.my_background_image) ;

// Set the Activity title by getting a string from the Resources object, because
//  this method requires a CharSequence rather than a resource ID.
getWindow().setTitle(getResources().getText(R.string.main_title));

// Load a custom layout for the current screen.
setContentView(R.layout.main_screen);

// Set a slide in animation by getting an Animation from the Resources object.
flipper.setInAnimation(AnimationUtils.loadAnimation(this,
        R.anim.hyperspace_in));

// Set the text on a TextView object using a resource ID.
TextView msgTextView = (TextView) findViewById(R.id.msg);
msgTextView.setText(R.string.hello_message);

Caution: Don't modify the R.java file by hand. It is generated by the aapt tool when your project is compiled. Any changes are overridden next time you compile.

Access resources from XML

You can define values for some XML attributes and elements using a reference to an existing resource. You often do this when creating layout files, to supply strings and images for your widgets.

For example, if you add a Button to your layout, use a string resource for the button text:

<Button
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/submit" />

Syntax

Here is the syntax to reference a resource in an XML resource:

@[<package_name>:]<resource_type>/<resource_name>

• <package_name> is the name of the package in which the resource is located (not required when referencing resources from the same 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 Resource types overview.

Use cases

In some cases, you must use a resource for a value in XML, such as to apply a drawable image to a widget, but you can also use a resource in XML any place that accepts a simple value. For example, if you have the following resource file that includes a color resource and a string resource:

<?xml version="1.0" encoding="utf-8"?>
<resources>
   <color name="opaque_red">#f00</color>
   <string name="hello">Hello!</string>
</resources>

You can use these resources in the following layout file to set the text color and text string:

<?xml version="1.0" encoding="utf-8"?>
<EditText xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:textColor="@color/opaque_red"
    android:text="@string/hello" />

In this case, you don't need to specify the package name in the resource reference, because the resources are from your own package. To reference a system resource, you need to include the package name, as shown in the following example:

<?xml version="1.0" encoding="utf-8"?>
<EditText xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:textColor="@android:color/secondary_text_dark"
    android:text="@string/hello" />

Note: Always use string resources, so that your application can be localized for other languages. For information about creating alternative resources (such as localized strings), see Provide alternative resources. For a complete guide to localizing your application for other languages, see Localize your app.

You can even use resources in XML to create aliases. For example, you can create a drawable resource that is an alias for another drawable resource:

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

This sounds redundant, but can be very useful when using alternative resource. For more information, see the section about creating alias resources.

Reference style attributes

A style attribute resource lets you reference the value of an attribute in the currently applied theme. Referencing a style attribute lets you customize the look of UI elements by styling them to match standard variations supplied by the current theme, instead of supplying a hardcoded value. Referencing a style attribute essentially says, "Use the style that is defined by this attribute in the current theme."

To reference a style attribute, the name syntax is almost identical to the normal resource format, but instead of the "at" symbol (@), use a question mark (?). The resource type portion is optional. So the reference syntax is as follows:

?[<package_name>:][<resource_type>/]<resource_name>

For example, here's how you can reference an attribute to set the text color to match the secondary text color of the system theme:

<EditText id="text"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:textColor="?android:textColorSecondary"
    android:text="@string/hello_world" />

Here, the android:textColor attribute specifies the name of a style attribute in the current theme. Android now uses the value applied to the android:textColorSecondary style attribute as the value for android:textColor in this widget. Because the system resource tool knows that an attribute resource is expected in this context, you do not need to explicitly state the type, which is ?android:attr/textColorSecondary. You can exclude the attr type.

Access original files

While uncommon, you might need 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 styles, themes, and layouts. To access these resources, qualify your resource reference with the android package name. For example, Android provides a layout resource you can use for list items in a ListAdapter:

listAdapter = ArrayAdapter(this, android.R.layout.simple_list_item_1, myarray)

setListAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, myarray));

In this example, simple_list_item_1 is a layout resource defined by the platform for items in a ListView. You can use this instead of creating your own layout for list items.

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.

Likewise, if you provide different layout resources based on the screen orientation, pick one orientation as your default. For example, instead of providing layout resources in layout-land/ for landscape and layout-port/ for portrait, leave one as the default, such as layout/ for landscape and layout-port/ for portrait.

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 older versions don't support. If you use a new resource qualifier, but maintain code compatibility with older versions of Android, then when an older 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-port/
drawable-en-notouch-12key/
drawable-port-ldpi/
drawable-port-notouch-12key/

And assume the following is the device configuration:

Locale = en-GB
Screen orientation = port
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-port.

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-port/
   drawable-en-notouch-12key/
   drawable-port-ldpi/
   drawable-port-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-port-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-port/
   drawable-en-notouch-12key/
   drawable-port-ldpi/
   drawable-port-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, screen orientation is the next qualifier for which there are any matches. So, resources that don't specify a screen orientation are eliminated:

   drawable-en/
   drawable-en-port/
   drawable-en-notouch-12key/
   

   The remaining directory is drawable-en-port.

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 (orientation, 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-port-notouch-12key is eliminated.