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 {@code int} to identify strings, graphics, colors and other resource types, so inspection tools cannot 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 allow you to provide hints to code inspections tools like {@code lint}, to help detect these, more subtle code problems. They are added as metadata tags that you attach to variables, parameters, and return values to inspect method return values, passed parameters, and local variables and fields. When used with code inspections tools, annotations can help you detect problems, such as null pointer exceptions and resource type conflicts.
For more information on enabling lint inspections and running lint, see Improving Your Code with lint.
Android supports a variety of annotations for insertion in the methods, parameters, and return values in your code, for example:
R.string
resource.Drawable
resource. Color
resource. Interpolator
resource. R.
resource. @UiThreadFor a complete list of the supported annotations, either examine the contents of the
{@link android.support.annotation Support-Annotations} library or use the
auto-complete feature to display the available options for the import
android.support.annotation. statement. The
SDK Manager packages the
{@link android.support.annotation Support-Annotations} library in the Android Support Repository
for use with Android Studio and in the Android
Support Library for use with other Android
development tools.
To add annotations to your code, first add a dependency to the
{@link android.support.annotation Support-Annotations} library. In Android Studio,
add the dependency using the File > Project Structure > Dependencies menu
option or your build.gradle file. The following example shows how to add the
{@link android.support.annotation Support-Annotations} library dependency in the
build.gradle file:
dependencies {
compile 'com.android.support:support-annotations:22.2.0'
}
The {@link android.support.annotation Support-Annotations} library is decorated with the supported annotations so using this library's methods and resources automatically checks the code for potential problems.
If you include annotations in a library and use the
Android Plugin for Gradle
to build an Android ARchive (AAR) artifact of that library, the annotations are included as part
of the artifact in XML format in the annotations.zip file.
To start a code inspection from Android Studio, which includes validating annotations and automatic lint checking, select Analyze > Inspect Code from the menu options. Android Studio displays conflict messages throughout the code to indication annotation conflicts and suggest possible resolutions.
Add {@link android.support.annotation.Nullable @Nullable} and {@link android.support.annotation.NonNull @NonNull} annotations to check the nullness of a given variable, parameter, or return value. For example, if a local variable that contains a null value is passed as a parameter to a method with the {@link android.support.annotation.NonNull @NonNull} annotation attached to that parameter, building the code generates a warning indicating a non-null conflict.
This example attaches the {@link android.support.annotation.NonNull @NonNull} annotation to
the context and attrs parameters to check that the passed parameter
values are not null.
import android.support.annotation.NonNull;
...
/** Add support for inflating the <fragment> tag. */
@NonNull
@Override
public View onCreateView(String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
...
}
...
Note: Android Studio supports running a nullability analysis to automatically infer and insert nullness annotations in your code. For more information about inferring nullability in Android Studio, see Annotations in Android Studio.
Validating resource types can be useful as Android references to resources, such as
Drawables and
R.string resources, are
passed as integers. Code that expects a parameter to reference a specific type of resource, for
example Drawables,
can be passed the expected reference type of int, but actually reference a different
type of resource, such as a R.string resource.
For example, add {@link android.support.annotation.StringRes @StringRes} annotations to check
that a resource parameter contains a
R.string
reference. During code inspection, the annotation generates a warning if a R.string
reference is not passed in the parameter.
This example attaches the {@link android.support.annotation.StringRes @StringRes}
annotation to the resId parameter to validate that it is really a string resource.
import android.support.annotation.StringRes;
...
public abstract void setTitle(@StringRes int resId);
...
Annotations for the other resource types, such as {@link android.support.annotation.DrawableRes @DrawableRes}, {@link android.support.annotation.DimenRes @DimenRes}, {@link android.support.annotation.ColorRes @ColorRes}, and {@link android.support.annotation.InterpolatorRes @InterpolatorRes} can be added using the same annotation format and run during the code inspection.
Thread annotations check if a method is called from a specific type of thread. The following thread annotations are supported:
@UiThread @MainThread @WorkerThread @BinderThread
Note: The @MainThread
and the @UiThread annotations are interchangeable so
methods calls from either thread type are allowed for these annotations.
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 the thread annotation is to validate method overrides in the AsyncTask class as this class performs background operations and publishes results only on the UI thread.
Use the @IntRange,
@FloatRange, and
@Size annotations to validate the values of passed
parameters.
The @IntRange annotation validates that the parameter
value is within a specified range. The following example ensures that the alpha
parameter contains an integer value from 0 to 255:
public void setAlpha(@IntRange(from=0,to=255) int alpha) { … }
The @FloatRange annotation checks that the parameter
value is within a specified range of floating point values. The following example ensures that the
alpha parameter contains a float value from 0.0 to 1.0:
public void setAlpha(@FloatRange(from=0.0, to=1.0) float alpha) {...}
The @Size annotation checks the size of a collection or
array, as well as the length of a string. For example, use the @Size(min=1)
annotation to check if a collection is not empty, and the @Size(2) annotation to
validate that an array contains exactly two values. The following example ensures that the
location array contains at least one element:
int[] location = new int[3]; button.getLocationOnScreen(@Size(min=1) location);
Use the @RequiresPermission annotation to
validate the permissions of the caller of a method. To check for a single permission from a
list the 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 ensure that the caller of the method has the
permission.SET_WALLPAPERS permission.
@RequiresPermission(Manifest.permission.SET_WALLPAPER) public abstract void setWallpaper(Bitmap bitmap) throws IOException;
This example requires the caller of the {@code copyFile()} method to have both read and write permissions to external storage:
@RequiresPermission(allOf = {
Manifest.permission.READ_EXTERNAL_STORAGE,
Manifest.permission.WRITE_EXTERNAL_STORAGE})
public static final void copyFile(String dest, String source) {
...
}
Use the @CheckResults annotation to
validate that a method's result or return value is actually used. The following example annotates
the checkPermissions method to ensure 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)") public abstract int checkPermission(@NonNull String permission, int pid, int uid);{@link android.support.annotation.StringDef @StringDef}
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
protected void onCreate(Bundle savedInstanceState) {
}
Use the {@link android.support.annotation.IntDef @IntDef} and {@link android.support.annotation.StringDef @StringDef} annotations so you can create enumerated annotations of integer and string sets to validate other types of code references, such as passing references to a set of constants.
The following example illustrates the steps to create an enumerated annotation that ensures a value passed as a method parameter references one of the defined constants.
import android.support.annotation.IntDef;
...
public abstract class ActionBar {
...
//Define the list of accepted constants
@IntDef({NAVIGATION_MODE_STANDARD, NAVIGATION_MODE_LIST, NAVIGATION_MODE_TABS})
//Tell the compiler not to store annotation data in the .class file
@Retention(RetentionPolicy.SOURCE)
//Declare the NavigationMode annotation
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 does
not reference one of the defined constants (NAVIGATION_MODE_STANDARD,
NAVIGATION_MODE_LIST, or NAVIGATION_MODE_TABS).
You can also define an annotation with a flag to check if a parameter
or return value references a valid pattern. This example creates the
DisplayOptions annotation with a list of valid DISPLAY_ constants.
import android.support.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 does not reference a valid pattern.