Revert changes to androidx.media3.session.legacy

libraries/session/src/main/java/androidx/media3/session/legacy/AudioAttributesCompat.java

@@ -308,9 +308,10 @@ public class AudioAttributesCompat {
     }
     if (Build.VERSION.SDK_INT >= 26) {
       return new AudioAttributesCompat(new AudioAttributesImplApi26((AudioAttributes) aa));
-    } else {
+    } else if (Build.VERSION.SDK_INT >= 21) {
       return new AudioAttributesCompat(new AudioAttributesImplApi21((AudioAttributes) aa));
     }
+    return null;
   }
 
   // The rest of this file implements an approximation to AudioAttributes using old stream types
@@ -376,8 +377,10 @@ public class AudioAttributesCompat {
         mBuilderImpl = new AudioAttributesImplBase.Builder();
       } else if (Build.VERSION.SDK_INT >= 26) {
         mBuilderImpl = new AudioAttributesImplApi26.Builder();
-      } else {
+      } else if (Build.VERSION.SDK_INT >= 21) {
         mBuilderImpl = new AudioAttributesImplApi21.Builder();
+      } else {
+        mBuilderImpl = new AudioAttributesImplBase.Builder();
       }
     }
 
@@ -391,8 +394,10 @@ public class AudioAttributesCompat {
         mBuilderImpl = new AudioAttributesImplBase.Builder(aa);
       } else if (Build.VERSION.SDK_INT >= 26) {
         mBuilderImpl = new AudioAttributesImplApi26.Builder(checkNotNull(aa.unwrap()));
-      } else {
+      } else if (Build.VERSION.SDK_INT >= 21) {
         mBuilderImpl = new AudioAttributesImplApi21.Builder(checkNotNull(aa.unwrap()));
+      } else {
+        mBuilderImpl = new AudioAttributesImplBase.Builder(aa);
       }
     }
 
@@ -938,6 +943,7 @@ public class AudioAttributesCompat {
     }
   }
 
+  @RequiresApi(21)
   public static class AudioAttributesImplApi21 implements AudioAttributesImpl {
 
     @Nullable public AudioAttributes mAudioAttributes;
@@ -1014,6 +1020,7 @@ public class AudioAttributesCompat {
       return "AudioAttributesCompat: audioattributes=" + mAudioAttributes;
     }
 
+    @RequiresApi(21)
     static class Builder implements AudioAttributesImpl.Builder {
       final AudioAttributes.Builder mFwkBuilder;
 

libraries/session/src/main/java/androidx/media3/session/legacy/LegacyParcelableUtil.java

@@ -88,7 +88,7 @@ public final class LegacyParcelableUtil {
   // TODO: b/335804969 - Remove this workaround once the bug fix is in the androidx.media dependency
   @SuppressWarnings("unchecked")
   private static <T> T maybeApplyMediaDescriptionParcelableBugWorkaround(T value) {
-    if (Util.SDK_INT >= 23) {
+    if (Util.SDK_INT < 21 || Util.SDK_INT >= 23) {
       return value;
     }
     if (value instanceof android.support.v4.media.MediaBrowserCompat.MediaItem) {

libraries/session/src/main/java/androidx/media3/session/legacy/MediaBrowserCompat.java

@@ -56,6 +56,7 @@ import android.content.ComponentName;
 import android.content.Context;
 import android.content.Intent;
 import android.content.ServiceConnection;
+import android.media.MediaDescription;
 import android.media.browse.MediaBrowser;
 import android.os.BadParcelableException;
 import android.os.Binder;
@@ -72,8 +73,8 @@ import android.os.RemoteException;
 import android.support.v4.os.ResultReceiver;
 import android.text.TextUtils;
 import android.util.Log;
+import androidx.annotation.DoNotInline;
 import androidx.annotation.IntDef;
-import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.annotation.RequiresApi;
 import androidx.annotation.RestrictTo;
@@ -204,8 +205,10 @@ public final class MediaBrowserCompat {
       mImpl = new MediaBrowserImplApi26(context, serviceComponent, callback, rootHints);
     } else if (Build.VERSION.SDK_INT >= 23) {
       mImpl = new MediaBrowserImplApi23(context, serviceComponent, callback, rootHints);
-    } else {
+    } else if (Build.VERSION.SDK_INT >= 21) {
       mImpl = new MediaBrowserImplApi21(context, serviceComponent, callback, rootHints);
+    } else {
+      mImpl = new MediaBrowserImplBase(context, serviceComponent, callback, rootHints);
     }
   }
 
@@ -455,18 +458,20 @@ public final class MediaBrowserCompat {
      * Creates an instance from a framework {@link android.media.browse.MediaBrowser.MediaItem}
      * object.
      *
+     * <p>This method is only supported on API 21+. On API 20 and below, it returns null.
+     *
      * @param itemObj A {@link android.media.browse.MediaBrowser.MediaItem} object.
      * @return An equivalent {@link MediaItem} object, or null if none.
      */
     @Nullable
     public static MediaItem fromMediaItem(@Nullable Object itemObj) {
-      if (itemObj == null) {
+      if (itemObj == null || Build.VERSION.SDK_INT < 21) {
         return null;
       }
       MediaBrowser.MediaItem itemFwk = (MediaBrowser.MediaItem) itemObj;
-      int flags = itemFwk.getFlags();
+      int flags = Api21Impl.getFlags(itemFwk);
       MediaDescriptionCompat descriptionCompat =
-          MediaDescriptionCompat.fromMediaDescription(itemFwk.getDescription());
+          MediaDescriptionCompat.fromMediaDescription(Api21Impl.getDescription(itemFwk));
       return new MediaItem(descriptionCompat, flags);
     }
 
@@ -474,12 +479,14 @@ public final class MediaBrowserCompat {
      * Creates a list of {@link MediaItem} objects from a framework {@link
      * android.media.browse.MediaBrowser.MediaItem} object list.
      *
+     * <p>This method is only supported on API 21+. On API 20 and below, it returns null.
+     *
      * @param itemList A list of {@link android.media.browse.MediaBrowser.MediaItem} objects.
      * @return An equivalent list of {@link MediaItem} objects, or null if none.
      */
     @Nullable
     public static List<MediaItem> fromMediaItemList(@Nullable List<?> itemList) {
-      if (itemList == null) {
+      if (itemList == null || Build.VERSION.SDK_INT < 21) {
         return null;
       }
       List<MediaItem> items = new ArrayList<>(itemList.size());
@@ -594,7 +601,11 @@ public final class MediaBrowserCompat {
     @Nullable ConnectionCallbackInternal mConnectionCallbackInternal;
 
     public ConnectionCallback() {
+      if (Build.VERSION.SDK_INT >= 21) {
         mConnectionCallbackFwk = new ConnectionCallbackApi21();
+      } else {
+        mConnectionCallbackFwk = null;
+      }
     }
 
     /**
@@ -636,6 +647,7 @@ public final class MediaBrowserCompat {
       void onConnectionFailed();
     }
 
+    @RequiresApi(21)
     private class ConnectionCallbackApi21 extends MediaBrowser.ConnectionCallback {
       ConnectionCallbackApi21() {}
 
@@ -667,7 +679,7 @@ public final class MediaBrowserCompat {
 
   /** Callbacks for subscription related events. */
   public abstract static class SubscriptionCallback {
-    @NonNull final MediaBrowser.SubscriptionCallback mSubscriptionCallbackFwk;
+    @Nullable final MediaBrowser.SubscriptionCallback mSubscriptionCallbackFwk;
     final IBinder mToken;
     @Nullable WeakReference<Subscription> mSubscriptionRef;
 
@@ -675,8 +687,10 @@ public final class MediaBrowserCompat {
       mToken = new Binder();
       if (Build.VERSION.SDK_INT >= 26) {
         mSubscriptionCallbackFwk = new SubscriptionCallbackApi26();
-      } else {
+      } else if (Build.VERSION.SDK_INT >= 21) {
         mSubscriptionCallbackFwk = new SubscriptionCallbackApi21();
+      } else {
+        mSubscriptionCallbackFwk = null;
       }
     }
 
@@ -724,6 +738,7 @@ public final class MediaBrowserCompat {
       mSubscriptionRef = new WeakReference<>(subscription);
     }
 
+    @RequiresApi(21)
     private class SubscriptionCallbackApi21 extends MediaBrowser.SubscriptionCallback {
       SubscriptionCallbackApi21() {}
 
@@ -1628,6 +1643,7 @@ public final class MediaBrowserCompat {
     }
   }
 
+  @RequiresApi(21)
   static class MediaBrowserImplApi21
       implements MediaBrowserImpl,
           MediaBrowserServiceCallbackImpl,
@@ -1728,7 +1744,7 @@ public final class MediaBrowserCompat {
       if (mServiceBinderWrapper == null) {
         // TODO: When MediaBrowser is connected to framework's MediaBrowserService,
         // subscribe with options won't work properly.
-        mBrowserFwk.subscribe(parentId, callback.mSubscriptionCallbackFwk);
+        mBrowserFwk.subscribe(parentId, checkNotNull(callback.mSubscriptionCallbackFwk));
       } else {
         try {
           mServiceBinderWrapper.addSubscription(
@@ -2076,9 +2092,9 @@ public final class MediaBrowserCompat {
       // This is to prevent ClassNotFoundException when options has Parcelable in it.
       if (mServiceBinderWrapper == null || mServiceVersion < SERVICE_VERSION_2) {
         if (options == null) {
-          mBrowserFwk.subscribe(parentId, callback.mSubscriptionCallbackFwk);
+          mBrowserFwk.subscribe(parentId, checkNotNull(callback.mSubscriptionCallbackFwk));
         } else {
-          mBrowserFwk.subscribe(parentId, options, callback.mSubscriptionCallbackFwk);
+          mBrowserFwk.subscribe(parentId, options, checkNotNull(callback.mSubscriptionCallbackFwk));
         }
       } else {
         super.subscribe(parentId, options, callback);
@@ -2093,7 +2109,7 @@ public final class MediaBrowserCompat {
         if (callback == null) {
           mBrowserFwk.unsubscribe(parentId);
         } else {
-          mBrowserFwk.unsubscribe(parentId, callback.mSubscriptionCallbackFwk);
+          mBrowserFwk.unsubscribe(parentId, checkNotNull(callback.mSubscriptionCallbackFwk));
         }
       } else {
         super.unsubscribe(parentId, callback);
@@ -2450,4 +2466,19 @@ public final class MediaBrowserCompat {
       }
     }
   }
+
+  @RequiresApi(21)
+  private static class Api21Impl {
+    private Api21Impl() {}
+
+    @DoNotInline
+    static MediaDescription getDescription(MediaBrowser.MediaItem item) {
+      return item.getDescription();
+    }
+
+    @DoNotInline
+    static int getFlags(MediaBrowser.MediaItem item) {
+      return item.getFlags();
+    }
+  }
 }

libraries/session/src/main/java/androidx/media3/session/legacy/MediaBrowserServiceCompat.java

@@ -307,6 +307,7 @@ public abstract class MediaBrowserServiceCompat extends Service {
     }
   }
 
+  @RequiresApi(21)
   class MediaBrowserServiceImplApi21 implements MediaBrowserServiceImpl {
     final List<Bundle> mRootExtrasList = new ArrayList<>();
     @MonotonicNonNull MediaBrowserService mServiceFwk;
@@ -519,6 +520,7 @@ public abstract class MediaBrowserServiceCompat extends Service {
       return mCurConnection.browserInfo;
     }
 
+    @RequiresApi(21)
     class MediaBrowserServiceApi21 extends MediaBrowserService {
       @SuppressWarnings("method.invocation.invalid") // Calling base method from constructor
       MediaBrowserServiceApi21(Context context) {
@@ -1266,6 +1268,7 @@ public abstract class MediaBrowserServiceCompat extends Service {
     }
   }
 
+  @RequiresApi(21)
   @SuppressWarnings({"rawtypes", "unchecked"})
   static class ResultWrapper<T> {
     MediaBrowserService.Result mResultFwk;
@@ -1328,8 +1331,10 @@ public abstract class MediaBrowserServiceCompat extends Service {
       mImpl = new MediaBrowserServiceImplApi26();
     } else if (Build.VERSION.SDK_INT >= 23) {
       mImpl = new MediaBrowserServiceImplApi23();
-    } else {
+    } else if (Build.VERSION.SDK_INT >= 21) {
       mImpl = new MediaBrowserServiceImplApi21();
+    } else {
+      mImpl = new MediaBrowserServiceImplBase();
     }
     mImpl.onCreate();
   }

libraries/session/src/main/java/androidx/media3/session/legacy/MediaControllerCompat.java

@@ -40,7 +40,6 @@ import android.text.TextUtils;
 import android.util.Log;
 import android.view.KeyEvent;
 import androidx.annotation.GuardedBy;
-import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.annotation.RequiresApi;
 import androidx.annotation.RestrictTo;
@@ -143,12 +142,9 @@ public final class MediaControllerCompat {
         .getWindow()
         .getDecorView()
         .setTag(R.id.media_controller_compat_view_tag, mediaController);
-    MediaController controllerFwk = null;
+    if (android.os.Build.VERSION.SDK_INT >= 21) {
-    if (mediaController != null) {
+      MediaControllerImplApi21.setMediaController(activity, mediaController);
-      Object sessionTokenObj = mediaController.getSessionToken().getToken();
-      controllerFwk = new MediaController(activity, (MediaSession.Token) sessionTokenObj);
     }
-    activity.setMediaController(controllerFwk);
   }
 
   /**
@@ -166,16 +162,11 @@ public final class MediaControllerCompat {
     Object tag = activity.getWindow().getDecorView().getTag(R.id.media_controller_compat_view_tag);
     if (tag instanceof MediaControllerCompat) {
       return (MediaControllerCompat) tag;
-    } else {
+    } else if (android.os.Build.VERSION.SDK_INT >= 21) {
-      MediaController controllerFwk = activity.getMediaController();
+      return MediaControllerImplApi21.getMediaController(activity);
-      if (controllerFwk == null) {
+    }
     return null;
   }
-      MediaSession.Token sessionTokenFwk = controllerFwk.getSessionToken();
-      return new MediaControllerCompat(
-          activity, MediaSessionCompat.Token.fromToken(sessionTokenFwk));
-    }
-  }
 
   @SuppressWarnings("WeakerAccess") /* synthetic access */
   static void validateCustomAction(@Nullable String action, @Nullable Bundle args) {
@@ -230,8 +221,10 @@ public final class MediaControllerCompat {
 
     if (Build.VERSION.SDK_INT >= 29) {
       mImpl = new MediaControllerImplApi29(context, sessionToken);
-    } else {
+    } else if (Build.VERSION.SDK_INT >= 21) {
       mImpl = new MediaControllerImplApi21(context, sessionToken);
+    } else {
+      mImpl = new MediaControllerImplBase(sessionToken);
     }
   }
 
@@ -642,6 +635,8 @@ public final class MediaControllerCompat {
   /**
    * Gets the underlying framework {@link android.media.session.MediaController} object.
    *
+   * <p>This method is only supported on API 21+.
+   *
    * @return The underlying {@link android.media.session.MediaController} object, or null if none.
    */
   @Nullable
@@ -654,14 +649,19 @@ public final class MediaControllerCompat {
    * #registerCallback}
    */
   public abstract static class Callback implements IBinder.DeathRecipient {
-    @NonNull final MediaController.Callback mCallbackFwk;
+    @Nullable final MediaController.Callback mCallbackFwk;
     @Nullable MessageHandler mHandler;
     @Nullable IMediaControllerCallback mIControllerCallback;
 
     // Sharing this in constructor
     @SuppressWarnings({"assignment.type.incompatible", "argument.type.incompatible"})
     public Callback() {
+      if (android.os.Build.VERSION.SDK_INT >= 21) {
         mCallbackFwk = new MediaControllerCallbackApi21(this);
+      } else {
+        mCallbackFwk = null;
+        mIControllerCallback = new StubCompat(this);
+      }
     }
 
     /**
@@ -789,6 +789,7 @@ public final class MediaControllerCompat {
     }
 
     // Callback methods in this class are run on handler which was given to registerCallback().
+    @RequiresApi(21)
     private static class MediaControllerCallbackApi21 extends MediaController.Callback {
       private final WeakReference<MediaControllerCompat.Callback> mCallback;
 
@@ -1973,6 +1974,7 @@ public final class MediaControllerCompat {
     }
   }
 
+  @RequiresApi(21)
   static class MediaControllerImplApi21 implements MediaControllerImpl {
     protected final MediaController mControllerFwk;
 
@@ -2000,7 +2002,7 @@ public final class MediaControllerCompat {
 
     @Override
     public final void registerCallback(Callback callback, Handler handler) {
-      mControllerFwk.registerCallback(callback.mCallbackFwk, handler);
+      mControllerFwk.registerCallback(checkNotNull(callback.mCallbackFwk), handler);
       synchronized (mLock) {
         IMediaSession extraBinder = mSessionToken.getExtraBinder();
         if (extraBinder != null) {
@@ -2022,7 +2024,7 @@ public final class MediaControllerCompat {
 
     @Override
     public final void unregisterCallback(Callback callback) {
-      mControllerFwk.unregisterCallback(callback.mCallbackFwk);
+      mControllerFwk.unregisterCallback(checkNotNull(callback.mCallbackFwk));
       synchronized (mLock) {
         IMediaSession extraBinder = mSessionToken.getExtraBinder();
         if (extraBinder != null) {
@@ -2149,7 +2151,7 @@ public final class MediaControllerCompat {
 
     @Override
     public int getRatingType() {
-      if (android.os.Build.VERSION.SDK_INT == 21) {
+      if (android.os.Build.VERSION.SDK_INT < 22) {
         try {
           IMediaSession extraBinder = mSessionToken.getExtraBinder();
           if (extraBinder != null) {
@@ -2302,6 +2304,27 @@ public final class MediaControllerCompat {
       mPendingCallbacks.clear();
     }
 
+    @SuppressWarnings("argument.type.incompatible") // Activity.setMediaController is not annotated
+    static void setMediaController(Activity activity, MediaControllerCompat mediaControllerCompat) {
+      MediaController controllerFwk = null;
+      if (mediaControllerCompat != null) {
+        Object sessionTokenObj = mediaControllerCompat.getSessionToken().getToken();
+        controllerFwk = new MediaController(activity, (MediaSession.Token) sessionTokenObj);
+      }
+      activity.setMediaController(controllerFwk);
+    }
+
+    @Nullable
+    static MediaControllerCompat getMediaController(Activity activity) {
+      MediaController controllerFwk = activity.getMediaController();
+      if (controllerFwk == null) {
+        return null;
+      }
+      MediaSession.Token sessionTokenFwk = controllerFwk.getSessionToken();
+      return new MediaControllerCompat(
+          activity, MediaSessionCompat.Token.fromToken(sessionTokenFwk));
+    }
+
     private static class ExtraBinderRequestResultReceiver extends ResultReceiver {
       private WeakReference<MediaControllerImplApi21> mMediaControllerImpl;
 
@@ -2388,6 +2411,7 @@ public final class MediaControllerCompat {
     }
   }
 
+  @RequiresApi(21)
   static class TransportControlsApi21 extends TransportControls {
     protected final MediaController.TransportControls mControlsFwk;
 

libraries/session/src/main/java/androidx/media3/session/legacy/MediaDescriptionCompat.java

@@ -294,8 +294,19 @@ public final class MediaDescriptionCompat implements Parcelable {
 
   @Override
   public void writeToParcel(Parcel dest, int flags) {
+    if (Build.VERSION.SDK_INT < 21) {
+      dest.writeString(mMediaId);
+      TextUtils.writeToParcel(mTitle, dest, flags);
+      TextUtils.writeToParcel(mSubtitle, dest, flags);
+      TextUtils.writeToParcel(mDescription, dest, flags);
+      dest.writeParcelable(mIcon, flags);
+      dest.writeParcelable(mIconUri, flags);
+      dest.writeBundle(mExtras);
+      dest.writeParcelable(mMediaUri, flags);
+    } else {
       ((MediaDescription) getMediaDescription()).writeToParcel(dest, flags);
     }
+  }
 
   @Override
   public String toString() {
@@ -305,19 +316,22 @@ public final class MediaDescriptionCompat implements Parcelable {
   /**
    * Gets the underlying framework {@link android.media.MediaDescription} object.
    *
+   * <p>This method is only supported on {@link android.os.Build.VERSION_CODES#LOLLIPOP} and later.
+   *
    * @return An equivalent {@link android.media.MediaDescription} object, or null if none.
    */
+  @RequiresApi(21)
   public Object getMediaDescription() {
     if (mDescriptionFwk != null) {
       return mDescriptionFwk;
     }
-    MediaDescription.Builder bob = new MediaDescription.Builder()
+    MediaDescription.Builder bob = Api21Impl.createBuilder();
-        .setMediaId(mMediaId)
+    Api21Impl.setMediaId(bob, mMediaId);
-        .setTitle(mTitle)
+    Api21Impl.setTitle(bob, mTitle);
-        .setSubtitle(mSubtitle)
+    Api21Impl.setSubtitle(bob, mSubtitle);
-        .setDescription(mDescription)
+    Api21Impl.setDescription(bob, mDescription);
-        .setIconBitmap(mIcon)
+    Api21Impl.setIconBitmap(bob, mIcon);
-        .setIconUri(mIconUri);
+    Api21Impl.setIconUri(bob, mIconUri);
     // Media URI was not added until API 23, so add it to the Bundle of extras to
     // ensure the data is not lost - this ensures that
     // fromMediaDescription(getMediaDescription(mediaDescriptionCompat)) returns
@@ -331,14 +345,14 @@ public final class MediaDescriptionCompat implements Parcelable {
         extras = new Bundle(mExtras);
       }
       extras.putParcelable(DESCRIPTION_KEY_MEDIA_URI, mMediaUri);
-      bob.setExtras(extras);
+      Api21Impl.setExtras(bob, extras);
     } else {
-      bob.setExtras(mExtras);
+      Api21Impl.setExtras(bob, mExtras);
     }
     if (Build.VERSION.SDK_INT >= 23) {
       Api23Impl.setMediaUri(bob, mMediaUri);
     }
-    mDescriptionFwk = bob.build();
+    mDescriptionFwk = Api21Impl.build(bob);
 
     return mDescriptionFwk;
   }
@@ -346,22 +360,24 @@ public final class MediaDescriptionCompat implements Parcelable {
   /**
    * Creates an instance from a framework {@link android.media.MediaDescription} object.
    *
+   * <p>This method is only supported on API 21+.
+   *
    * @param descriptionObj A {@link android.media.MediaDescription} object, or null if none.
    * @return An equivalent {@link MediaMetadataCompat} object, or null if none.
    */
   @Nullable
   @SuppressWarnings("deprecation")
   public static MediaDescriptionCompat fromMediaDescription(Object descriptionObj) {
-    if (descriptionObj != null) {
+    if (descriptionObj != null && Build.VERSION.SDK_INT >= 21) {
       Builder bob = new Builder();
       MediaDescription description = (MediaDescription) descriptionObj;
-      bob.setMediaId(description.getMediaId());
+      bob.setMediaId(Api21Impl.getMediaId(description));
-      bob.setTitle(description.getTitle());
+      bob.setTitle(Api21Impl.getTitle(description));
-      bob.setSubtitle(description.getSubtitle());
+      bob.setSubtitle(Api21Impl.getSubtitle(description));
-      bob.setDescription(description.getDescription());
+      bob.setDescription(Api21Impl.getDescription(description));
-      bob.setIconBitmap(description.getIconBitmap());
+      bob.setIconBitmap(Api21Impl.getIconBitmap(description));
-      bob.setIconUri(description.getIconUri());
+      bob.setIconUri(Api21Impl.getIconUri(description));
-      Bundle extras = description.getExtras();
+      Bundle extras = Api21Impl.getExtras(description);
       extras = MediaSessionCompat.unparcelWithClassLoader(extras);
       if (extras != null) {
         extras = new Bundle(extras);
@@ -403,9 +419,13 @@ public final class MediaDescriptionCompat implements Parcelable {
       new Parcelable.Creator<MediaDescriptionCompat>() {
         @Override
         public MediaDescriptionCompat createFromParcel(Parcel in) {
+          if (Build.VERSION.SDK_INT < 21) {
+            return new MediaDescriptionCompat(in);
+          } else {
             return checkNotNull(
                 fromMediaDescription(MediaDescription.CREATOR.createFromParcel(in)));
           }
+        }
 
         @Override
         public MediaDescriptionCompat[] newArray(int size) {
@@ -526,6 +546,99 @@ public final class MediaDescriptionCompat implements Parcelable {
     }
   }
 
+  @RequiresApi(21)
+  private static class Api21Impl {
+    private Api21Impl() {}
+
+    @DoNotInline
+    static MediaDescription.Builder createBuilder() {
+      return new MediaDescription.Builder();
+    }
+
+    @DoNotInline
+    static void setMediaId(MediaDescription.Builder builder, @Nullable String mediaId) {
+      builder.setMediaId(mediaId);
+    }
+
+    @DoNotInline
+    static void setTitle(MediaDescription.Builder builder, @Nullable CharSequence title) {
+      builder.setTitle(title);
+    }
+
+    @DoNotInline
+    static void setSubtitle(MediaDescription.Builder builder, @Nullable CharSequence subtitle) {
+      builder.setSubtitle(subtitle);
+    }
+
+    @DoNotInline
+    static void setDescription(
+        MediaDescription.Builder builder, @Nullable CharSequence description) {
+      builder.setDescription(description);
+    }
+
+    @DoNotInline
+    static void setIconBitmap(MediaDescription.Builder builder, @Nullable Bitmap icon) {
+      builder.setIconBitmap(icon);
+    }
+
+    @DoNotInline
+    static void setIconUri(MediaDescription.Builder builder, @Nullable Uri iconUri) {
+      builder.setIconUri(iconUri);
+    }
+
+    @DoNotInline
+    static void setExtras(MediaDescription.Builder builder, @Nullable Bundle extras) {
+      builder.setExtras(extras);
+    }
+
+    @DoNotInline
+    static MediaDescription build(MediaDescription.Builder builder) {
+      return builder.build();
+    }
+
+    @DoNotInline
+    @Nullable
+    static String getMediaId(MediaDescription description) {
+      return description.getMediaId();
+    }
+
+    @DoNotInline
+    @Nullable
+    static CharSequence getTitle(MediaDescription description) {
+      return description.getTitle();
+    }
+
+    @DoNotInline
+    @Nullable
+    static CharSequence getSubtitle(MediaDescription description) {
+      return description.getSubtitle();
+    }
+
+    @DoNotInline
+    @Nullable
+    static CharSequence getDescription(MediaDescription description) {
+      return description.getDescription();
+    }
+
+    @DoNotInline
+    @Nullable
+    static Bitmap getIconBitmap(MediaDescription description) {
+      return description.getIconBitmap();
+    }
+
+    @DoNotInline
+    @Nullable
+    static Uri getIconUri(MediaDescription description) {
+      return description.getIconUri();
+    }
+
+    @DoNotInline
+    @Nullable
+    static Bundle getExtras(MediaDescription description) {
+      return description.getExtras();
+    }
+  }
+
   @RequiresApi(23)
   private static class Api23Impl {
     private Api23Impl() {}

libraries/session/src/main/java/androidx/media3/session/legacy/MediaMetadataCompat.java

@@ -22,12 +22,14 @@ import android.annotation.SuppressLint;
 import android.graphics.Bitmap;
 import android.media.MediaMetadata;
 import android.net.Uri;
+import android.os.Build;
 import android.os.Bundle;
 import android.os.Parcel;
 import android.os.Parcelable;
 import android.text.TextUtils;
 import android.util.Log;
 import androidx.annotation.Nullable;
+import androidx.annotation.RequiresApi;
 import androidx.annotation.RestrictTo;
 import androidx.annotation.StringDef;
 import androidx.collection.ArrayMap;
@@ -555,12 +557,14 @@ public final class MediaMetadataCompat implements Parcelable {
   /**
    * Creates an instance from a framework {@link android.media.MediaMetadata} object.
    *
+   * <p>This method is only supported on {@link android.os.Build.VERSION_CODES#LOLLIPOP} and later.
+   *
    * @param metadataObj A {@link android.media.MediaMetadata} object, or null if none.
    * @return An equivalent {@link MediaMetadataCompat} object, or null if none.
    */
   @Nullable
   public static MediaMetadataCompat fromMediaMetadata(@Nullable Object metadataObj) {
-    if (metadataObj != null) {
+    if (metadataObj != null && Build.VERSION.SDK_INT >= 21) {
       Parcel p = Parcel.obtain();
       ((MediaMetadata) metadataObj).writeToParcel(p, 0);
       p.setDataPosition(0);
@@ -576,8 +580,11 @@ public final class MediaMetadataCompat implements Parcelable {
   /**
    * Gets the underlying framework {@link android.media.MediaMetadata} object.
    *
+   * <p>This method is only supported on {@link android.os.Build.VERSION_CODES#LOLLIPOP} and later.
+   *
    * @return An equivalent {@link android.media.MediaMetadata} object, or null if none.
    */
+  @RequiresApi(21)
   public Object getMediaMetadata() {
     if (mMetadataFwk == null) {
       Parcel p = Parcel.obtain();

libraries/session/src/main/java/androidx/media3/session/legacy/MediaSessionManager.java

@@ -71,8 +71,10 @@ public final class MediaSessionManager {
   private MediaSessionManager(Context context) {
     if (Build.VERSION.SDK_INT >= 28) {
       mImpl = new MediaSessionManagerImplApi28(context);
-    } else {
+    } else if (Build.VERSION.SDK_INT >= 21) {
       mImpl = new MediaSessionManagerImplApi21(context);
+    } else {
+      mImpl = new MediaSessionManagerImplBase(context);
     }
   }
 
@@ -371,6 +373,7 @@ public final class MediaSessionManager {
     }
   }
 
+  @RequiresApi(21)
   private static class MediaSessionManagerImplApi21 extends MediaSessionManagerImplBase {
     MediaSessionManagerImplApi21(Context context) {
       super(context);

libraries/session/src/main/java/androidx/media3/session/legacy/PlaybackStateCompat.java

@@ -840,14 +840,16 @@ public final class PlaybackStateCompat implements Parcelable {
   /**
    * Creates an instance from a framework {@link android.media.session.PlaybackState} object.
    *
+   * <p>This method is only supported on API 21+.
+   *
    * @param stateObj A {@link android.media.session.PlaybackState} object, or null if none.
    * @return An equivalent {@link PlaybackStateCompat} object, or null if none.
    */
   @Nullable
   public static PlaybackStateCompat fromPlaybackState(@Nullable Object stateObj) {
-    if (stateObj != null) {
+    if (stateObj != null && Build.VERSION.SDK_INT >= 21) {
       PlaybackState stateFwk = (PlaybackState) stateObj;
-      List<PlaybackState.CustomAction> customActionFwks = stateFwk.getCustomActions();
+      List<PlaybackState.CustomAction> customActionFwks = Api21Impl.getCustomActions(stateFwk);
       List<PlaybackStateCompat.CustomAction> customActions = null;
       if (customActionFwks != null) {
         customActions = new ArrayList<>(customActionFwks.size());
@@ -867,16 +869,16 @@ public final class PlaybackStateCompat implements Parcelable {
       }
       PlaybackStateCompat stateCompat =
           new PlaybackStateCompat(
-              stateFwk.getState(),
+              Api21Impl.getState(stateFwk),
-              stateFwk.getPosition(),
+              Api21Impl.getPosition(stateFwk),
-              stateFwk.getBufferedPosition(),
+              Api21Impl.getBufferedPosition(stateFwk),
-              stateFwk.getPlaybackSpeed(),
+              Api21Impl.getPlaybackSpeed(stateFwk),
-              stateFwk.getActions(),
+              Api21Impl.getActions(stateFwk),
               ERROR_CODE_UNKNOWN_ERROR,
-              stateFwk.getErrorMessage(),
+              Api21Impl.getErrorMessage(stateFwk),
-              stateFwk.getLastPositionUpdateTime(),
+              Api21Impl.getLastPositionUpdateTime(stateFwk),
               customActions,
-              stateFwk.getActiveQueueItemId(),
+              Api21Impl.getActiveQueueItemId(stateFwk),
               extras);
       stateCompat.mStateFwk = stateFwk;
       return stateCompat;
@@ -888,28 +890,30 @@ public final class PlaybackStateCompat implements Parcelable {
   /**
    * Gets the underlying framework {@link android.media.session.PlaybackState} object.
    *
+   * <p>This method is only supported on API 21+.
+   *
    * @return An equivalent {@link android.media.session.PlaybackState} object, or null if none.
    */
   @Nullable
   public Object getPlaybackState() {
-    if (mStateFwk == null) {
+    if (mStateFwk == null && Build.VERSION.SDK_INT >= 21) {
-      PlaybackState.Builder builder = new PlaybackState.Builder()
+      PlaybackState.Builder builder = Api21Impl.createBuilder();
-          .setState(mState, mPosition, mSpeed, mUpdateTime)
+      Api21Impl.setState(builder, mState, mPosition, mSpeed, mUpdateTime);
-          .setBufferedPosition(mBufferedPosition)
+      Api21Impl.setBufferedPosition(builder, mBufferedPosition);
-          .setActions(mActions)
+      Api21Impl.setActions(builder, mActions);
-          .setErrorMessage(mErrorMessage);
+      Api21Impl.setErrorMessage(builder, mErrorMessage);
       for (PlaybackStateCompat.CustomAction customAction : mCustomActions) {
         PlaybackState.CustomAction action =
             (PlaybackState.CustomAction) customAction.getCustomAction();
         if (action != null) {
-          builder.addCustomAction(action);
+          Api21Impl.addCustomAction(builder, action);
         }
       }
-      builder.setActiveQueueItemId(mActiveItemId);
+      Api21Impl.setActiveQueueItemId(builder, mActiveItemId);
       if (Build.VERSION.SDK_INT >= 22) {
         Api22Impl.setExtras(builder, mExtras);
       }
-      mStateFwk = builder.build();
+      mStateFwk = Api21Impl.build(builder);
     }
     return mStateFwk;
   }
@@ -972,19 +976,22 @@ public final class PlaybackStateCompat implements Parcelable {
      * Creates an instance from a framework {@link android.media.session.PlaybackState.CustomAction}
      * object.
      *
+     * <p>This method is only supported on API 21+.
+     *
      * @param customActionObj A {@link android.media.session.PlaybackState.CustomAction} object, or
      *     null if none.
      * @return An equivalent {@link PlaybackStateCompat.CustomAction} object, or null if none.
      */
+    @RequiresApi(21)
     public static PlaybackStateCompat.CustomAction fromCustomAction(Object customActionObj) {
       PlaybackState.CustomAction customActionFwk = (PlaybackState.CustomAction) customActionObj;
-      Bundle extras = customActionFwk.getExtras();
+      Bundle extras = Api21Impl.getExtras(customActionFwk);
       MediaSessionCompat.ensureClassLoader(extras);
       PlaybackStateCompat.CustomAction customActionCompat =
           new PlaybackStateCompat.CustomAction(
-              customActionFwk.getAction(),
+              Api21Impl.getAction(customActionFwk),
-              customActionFwk.getName(),
+              Api21Impl.getName(customActionFwk),
-              customActionFwk.getIcon(),
+              Api21Impl.getIcon(customActionFwk),
               extras);
       customActionCompat.mCustomActionFwk = customActionFwk;
       return customActionCompat;
@@ -994,18 +1001,21 @@ public final class PlaybackStateCompat implements Parcelable {
      * Gets the underlying framework {@link android.media.session.PlaybackState.CustomAction}
      * object.
      *
+     * <p>This method is only supported on API 21+.
+     *
      * @return An equivalent {@link android.media.session.PlaybackState.CustomAction} object, or
      *     null if none.
      */
     @Nullable
     public Object getCustomAction() {
-      if (mCustomActionFwk != null) {
+      if (mCustomActionFwk != null || Build.VERSION.SDK_INT < 21) {
         return mCustomActionFwk;
       }
 
-      return new PlaybackState.CustomAction.Builder(mAction, mName, mIcon)
+      PlaybackState.CustomAction.Builder builder =
-          .setExtras(mExtras)
+          Api21Impl.createCustomActionBuilder(mAction, mName, mIcon);
-          .build();
+      Api21Impl.setExtras(builder, mExtras);
+      return Api21Impl.build(builder);
     }
 
     public static final Parcelable.Creator<PlaybackStateCompat.CustomAction> CREATOR =
@@ -1391,6 +1401,142 @@ public final class PlaybackStateCompat implements Parcelable {
     }
   }
 
+  @RequiresApi(21)
+  private static class Api21Impl {
+    private Api21Impl() {}
+
+    @DoNotInline
+    static PlaybackState.Builder createBuilder() {
+      return new PlaybackState.Builder();
+    }
+
+    @DoNotInline
+    static void setState(
+        PlaybackState.Builder builder,
+        int state,
+        long position,
+        float playbackSpeed,
+        long updateTime) {
+      builder.setState(state, position, playbackSpeed, updateTime);
+    }
+
+    @DoNotInline
+    static void setBufferedPosition(PlaybackState.Builder builder, long bufferedPosition) {
+      builder.setBufferedPosition(bufferedPosition);
+    }
+
+    @DoNotInline
+    static void setActions(PlaybackState.Builder builder, long actions) {
+      builder.setActions(actions);
+    }
+
+    @SuppressWarnings("argument.type.incompatible") // Platform class not annotated as nullable
+    @DoNotInline
+    static void setErrorMessage(PlaybackState.Builder builder, @Nullable CharSequence error) {
+      builder.setErrorMessage(error);
+    }
+
+    @DoNotInline
+    static void addCustomAction(
+        PlaybackState.Builder builder, PlaybackState.CustomAction customAction) {
+      builder.addCustomAction(customAction);
+    }
+
+    @DoNotInline
+    static void setActiveQueueItemId(PlaybackState.Builder builder, long id) {
+      builder.setActiveQueueItemId(id);
+    }
+
+    @DoNotInline
+    static List<PlaybackState.CustomAction> getCustomActions(PlaybackState state) {
+      return state.getCustomActions();
+    }
+
+    @DoNotInline
+    static PlaybackState build(PlaybackState.Builder builder) {
+      return builder.build();
+    }
+
+    @DoNotInline
+    static int getState(PlaybackState state) {
+      return state.getState();
+    }
+
+    @DoNotInline
+    static long getPosition(PlaybackState state) {
+      return state.getPosition();
+    }
+
+    @DoNotInline
+    static long getBufferedPosition(PlaybackState state) {
+      return state.getBufferedPosition();
+    }
+
+    @DoNotInline
+    static float getPlaybackSpeed(PlaybackState state) {
+      return state.getPlaybackSpeed();
+    }
+
+    @DoNotInline
+    static long getActions(PlaybackState state) {
+      return state.getActions();
+    }
+
+    @Nullable
+    @DoNotInline
+    static CharSequence getErrorMessage(PlaybackState state) {
+      return state.getErrorMessage();
+    }
+
+    @DoNotInline
+    static long getLastPositionUpdateTime(PlaybackState state) {
+      return state.getLastPositionUpdateTime();
+    }
+
+    @DoNotInline
+    static long getActiveQueueItemId(PlaybackState state) {
+      return state.getActiveQueueItemId();
+    }
+
+    @DoNotInline
+    static PlaybackState.CustomAction.Builder createCustomActionBuilder(
+        String action, CharSequence name, int icon) {
+      return new PlaybackState.CustomAction.Builder(action, name, icon);
+    }
+
+    @SuppressWarnings("argument.type.incompatible") // Platform class not annotated as nullable
+    @DoNotInline
+    static void setExtras(PlaybackState.CustomAction.Builder builder, @Nullable Bundle extras) {
+      builder.setExtras(extras);
+    }
+
+    @DoNotInline
+    static PlaybackState.CustomAction build(PlaybackState.CustomAction.Builder builder) {
+      return builder.build();
+    }
+
+    @Nullable
+    @DoNotInline
+    static Bundle getExtras(PlaybackState.CustomAction customAction) {
+      return customAction.getExtras();
+    }
+
+    @DoNotInline
+    static String getAction(PlaybackState.CustomAction customAction) {
+      return customAction.getAction();
+    }
+
+    @DoNotInline
+    static CharSequence getName(PlaybackState.CustomAction customAction) {
+      return customAction.getName();
+    }
+
+    @DoNotInline
+    static int getIcon(PlaybackState.CustomAction customAction) {
+      return customAction.getIcon();
+    }
+  }
+
   @RequiresApi(22)
   private static class Api22Impl {
     private Api22Impl() {}

libraries/session/src/main/java/androidx/media3/session/legacy/RatingCompat.java

@@ -323,6 +323,8 @@ public final class RatingCompat implements Parcelable {
   /**
    * Creates an instance from a framework {@link android.media.Rating} object.
    *
+   * <p>This method is only supported on API 19+.
+   *
    * @param ratingObj A {@link android.media.Rating} object, or null if none.
    * @return An equivalent {@link RatingCompat} object, or null if none.
    */
@@ -364,6 +366,8 @@ public final class RatingCompat implements Parcelable {
   /**
    * Gets the underlying framework {@link android.media.Rating} object.
    *
+   * <p>This method is only supported on API 19+.
+   *
    * @return An equivalent {@link android.media.Rating} object, or null if none.
    */
   @Nullable

libraries/session/src/main/java/androidx/media3/session/legacy/VolumeProviderCompat.java

@@ -19,8 +19,10 @@ import static androidx.annotation.RestrictTo.Scope.LIBRARY;
 
 import android.media.VolumeProvider;
 import android.os.Build;
+import androidx.annotation.DoNotInline;
 import androidx.annotation.IntDef;
 import androidx.annotation.Nullable;
+import androidx.annotation.RequiresApi;
 import androidx.annotation.RestrictTo;
 import androidx.media3.common.util.UnstableApi;
 import java.lang.annotation.Retention;
@@ -59,6 +61,7 @@ public abstract class VolumeProviderCompat {
   private final int mMaxVolume;
   @Nullable private final String mControlId;
   private int mCurrentVolume;
+  @Nullable private Callback mCallback;
 
   @Nullable private VolumeProvider mVolumeProviderFwk;
 
@@ -129,7 +132,13 @@ public abstract class VolumeProviderCompat {
    */
   public final void setCurrentVolume(int currentVolume) {
     mCurrentVolume = currentVolume;
-    getVolumeProvider().setCurrentVolume(currentVolume);
+    if (Build.VERSION.SDK_INT >= 21) {
+      VolumeProvider volumeProviderFwk = (VolumeProvider) getVolumeProvider();
+      Api21Impl.setCurrentVolume(volumeProviderFwk, currentVolume);
+    }
+    if (mCallback != null) {
+      mCallback.onVolumeChanged(this);
+    }
   }
 
   /**
@@ -157,12 +166,24 @@ public abstract class VolumeProviderCompat {
    */
   public void onAdjustVolume(int direction) {}
 
+  /**
+   * Sets a callback to receive volume changes.
+   *
+   * <p>Used internally by the support library.
+   */
+  public void setCallback(@Nullable Callback callback) {
+    mCallback = callback;
+  }
+
   /**
    * Gets the underlying framework {@link android.media.VolumeProvider} object.
    *
+   * <p>This method is only supported on API 21+.
+   *
    * @return An equivalent {@link android.media.VolumeProvider} object, or null if none.
    */
-  public VolumeProvider getVolumeProvider() {
+  @RequiresApi(21)
+  public Object getVolumeProvider() {
     if (mVolumeProviderFwk == null) {
       if (Build.VERSION.SDK_INT >= 30) {
         mVolumeProviderFwk =
@@ -194,4 +215,19 @@ public abstract class VolumeProviderCompat {
     }
     return mVolumeProviderFwk;
   }
+
+  /** Listens for changes to the volume. */
+  public abstract static class Callback {
+    public abstract void onVolumeChanged(VolumeProviderCompat volumeProvider);
+  }
+
+  @RequiresApi(21)
+  private static class Api21Impl {
+    private Api21Impl() {}
+
+    @DoNotInline
+    static void setCurrentVolume(VolumeProvider volumeProvider, int currentVolume) {
+      volumeProvider.setCurrentVolume(currentVolume);
+    }
+  }
 }

libraries/test_session_current/src/androidTest/java/androidx/media3/session/legacy/AudioAttributesCompatTest.java

@@ -21,7 +21,9 @@ import static org.hamcrest.core.IsNot.not;
 
 import android.media.AudioAttributes;
 import android.media.AudioManager;
+import android.os.Build;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
+import androidx.test.filters.SdkSuppress;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
@@ -50,7 +52,11 @@ public class AudioAttributesCompatTest {
   AudioAttributesCompat mNotificationLegacyAAC;
 
   @Before
+  @SdkSuppress(minSdkVersion = 21)
   public void setUpApi21() {
+    if (Build.VERSION.SDK_INT < 21) {
+      return;
+    }
     mMediaAA =
         new AudioAttributes.Builder()
             .setContentType(AudioAttributes.CONTENT_TYPE_MUSIC)
@@ -74,6 +80,7 @@ public class AudioAttributesCompatTest {
   }
 
   @Test
+  @SdkSuppress(minSdkVersion = 21)
   public void testCreateWithAudioAttributesApi21() {
     assertThat(mMediaAACFromAA, not(equalTo(null)));
     assertThat((AudioAttributes) mMediaAACFromAA.unwrap(), equalTo(mMediaAA));
@@ -83,6 +90,7 @@ public class AudioAttributesCompatTest {
   }
 
   @Test
+  @SdkSuppress(minSdkVersion = 21)
   public void testEqualityApi21() {
     assertThat("self equality", mMediaAACFromAA, equalTo(mMediaAACFromAA));
     assertThat("different things", mMediaAACFromAA, not(equalTo(mNotificationAAC)));
@@ -118,6 +126,7 @@ public class AudioAttributesCompatTest {
   }
 
   @Test
+  @SdkSuppress(minSdkVersion = 21)
   public void testLegacyStreamTypeInferenceApi21() {
     assertThat(mMediaAACFromAA.getLegacyStreamType(), equalTo(AudioManager.STREAM_MUSIC));
   }

libraries/test_session_current/src/androidTest/java/androidx/media3/session/legacy/MediaDescriptionCompatTest.java

@@ -22,6 +22,7 @@ import android.net.Uri;
 import android.os.Bundle;
 import androidx.media3.test.session.common.TestUtils;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
+import androidx.test.filters.SdkSuppress;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
@@ -29,6 +30,7 @@ import org.junit.runner.RunWith;
 @RunWith(AndroidJUnit4.class)
 public class MediaDescriptionCompatTest {
 
+  @SdkSuppress(minSdkVersion = 21)
   @Test
   public void roundTripViaFrameworkObject_returnsEqualMediaUriAndExtras() {
     Uri mediaUri = Uri.parse("androidx://media/uri");
@@ -52,6 +54,7 @@ public class MediaDescriptionCompatTest {
     TestUtils.equals(createExtras(), restoredDescription2.getExtras());
   }
 
+  @SdkSuppress(minSdkVersion = 21)
   @Test
   public void getMediaDescription_withMediaUri_doesNotTouchExtras() {
     MediaDescriptionCompat originalDescription =