diff --git a/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/DefaultCastOptionsProvider.java b/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/DefaultCastOptionsProvider.java
index 8948173f60..ebadb0a08a 100644
--- a/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/DefaultCastOptionsProvider.java
+++ b/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/DefaultCastOptionsProvider.java
@@ -20,6 +20,7 @@ import com.google.android.gms.cast.CastMediaControlIntent;
 import com.google.android.gms.cast.framework.CastOptions;
 import com.google.android.gms.cast.framework.OptionsProvider;
 import com.google.android.gms.cast.framework.SessionProvider;
+import java.util.Collections;
 import java.util.List;
 
 /**
@@ -58,7 +59,7 @@ public final class DefaultCastOptionsProvider implements OptionsProvider {
 
   @Override
   public List<SessionProvider> getAdditionalSessionProviders(Context context) {
-    return null;
+    return Collections.emptyList();
   }
 
 }
diff --git a/extensions/flac/src/main/java/com/google/android/exoplayer2/ext/flac/FlacDecoder.java b/extensions/flac/src/main/java/com/google/android/exoplayer2/ext/flac/FlacDecoder.java
index 2d74bce5f1..9b15aff846 100644
--- a/extensions/flac/src/main/java/com/google/android/exoplayer2/ext/flac/FlacDecoder.java
+++ b/extensions/flac/src/main/java/com/google/android/exoplayer2/ext/flac/FlacDecoder.java
@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.ext.flac;
 
+import androidx.annotation.Nullable;
 import com.google.android.exoplayer2.Format;
 import com.google.android.exoplayer2.decoder.DecoderInputBuffer;
 import com.google.android.exoplayer2.decoder.SimpleDecoder;
@@ -94,6 +95,7 @@ import java.util.List;
   }
 
   @Override
+  @Nullable
   protected FlacDecoderException decode(
       DecoderInputBuffer inputBuffer, SimpleOutputBuffer outputBuffer, boolean reset) {
     if (reset) {
diff --git a/extensions/flac/src/main/java/com/google/android/exoplayer2/ext/flac/FlacDecoderJni.java b/extensions/flac/src/main/java/com/google/android/exoplayer2/ext/flac/FlacDecoderJni.java
index de038921aa..a97d99fa54 100644
--- a/extensions/flac/src/main/java/com/google/android/exoplayer2/ext/flac/FlacDecoderJni.java
+++ b/extensions/flac/src/main/java/com/google/android/exoplayer2/ext/flac/FlacDecoderJni.java
@@ -15,9 +15,11 @@
  */
 package com.google.android.exoplayer2.ext.flac;
 
+import androidx.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.extractor.ExtractorInput;
 import com.google.android.exoplayer2.util.FlacStreamInfo;
+import com.google.android.exoplayer2.util.Util;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 
@@ -37,15 +39,16 @@ import java.nio.ByteBuffer;
     }
   }
 
-  private static final int TEMP_BUFFER_SIZE = 8192; // The same buffer size which libflac has
+  private static final int TEMP_BUFFER_SIZE = 8192; // The same buffer size as libflac.
 
   private final long nativeDecoderContext;
 
-  private ByteBuffer byteBufferData;
-  private ExtractorInput extractorInput;
+  @Nullable private ByteBuffer byteBufferData;
+  @Nullable private ExtractorInput extractorInput;
+  @Nullable private byte[] tempBuffer;
   private boolean endOfExtractorInput;
-  private byte[] tempBuffer;
 
+  @SuppressWarnings("nullness:method.invocation.invalid")
   public FlacDecoderJni() throws FlacDecoderException {
     if (!FlacLibrary.isAvailable()) {
       throw new FlacDecoderException("Failed to load decoder native libraries.");
@@ -58,7 +61,8 @@ import java.nio.ByteBuffer;
 
   /**
    * Sets data to be parsed by libflac.
-   * @param byteBufferData Source {@link ByteBuffer}
+   *
+   * @param byteBufferData Source {@link ByteBuffer}.
    */
   public void setData(ByteBuffer byteBufferData) {
     this.byteBufferData = byteBufferData;
@@ -68,7 +72,8 @@ import java.nio.ByteBuffer;
 
   /**
    * Sets data to be parsed by libflac.
-   * @param extractorInput Source {@link ExtractorInput}
+   *
+   * @param extractorInput Source {@link ExtractorInput}.
    */
   public void setData(ExtractorInput extractorInput) {
     this.byteBufferData = null;
@@ -90,15 +95,15 @@ import java.nio.ByteBuffer;
 
   /**
    * Reads up to {@code length} bytes from the data source.
-   * <p>
-   * This method blocks until at least one byte of data can be read, the end of the input is
+   *
+   * <p>This method blocks until at least one byte of data can be read, the end of the input is
    * detected or an exception is thrown.
-   * <p>
-   * This method is called from the native code.
+   *
+   * <p>This method is called from the native code.
    *
    * @param target A target {@link ByteBuffer} into which data should be written.
-   * @return Returns the number of bytes read, or -1 on failure. It's not an error if this returns
-   * zero; it just means all the data read from the source.
+   * @return Returns the number of bytes read, or -1 on failure. If all of the data has already been
+   *     read from the source, then 0 is returned.
    */
   public int read(ByteBuffer target) throws IOException, InterruptedException {
     int byteCount = target.remaining();
@@ -106,18 +111,20 @@ import java.nio.ByteBuffer;
       byteCount = Math.min(byteCount, byteBufferData.remaining());
       int originalLimit = byteBufferData.limit();
       byteBufferData.limit(byteBufferData.position() + byteCount);
-
       target.put(byteBufferData);
-
       byteBufferData.limit(originalLimit);
     } else if (extractorInput != null) {
+      ExtractorInput extractorInput = this.extractorInput;
+      byte[] tempBuffer = Util.castNonNull(this.tempBuffer);
       byteCount = Math.min(byteCount, TEMP_BUFFER_SIZE);
-      int read = readFromExtractorInput(0, byteCount);
+      int read = readFromExtractorInput(extractorInput, tempBuffer, /* offset= */ 0, byteCount);
       if (read < 4) {
         // Reading less than 4 bytes, most of the time, happens because of getting the bytes left in
         // the buffer of the input. Do another read to reduce the number of calls to this method
         // from the native code.
-        read += readFromExtractorInput(read, byteCount - read);
+        read +=
+            readFromExtractorInput(
+                extractorInput, tempBuffer, read, /* length= */ byteCount - read);
       }
       byteCount = read;
       target.put(tempBuffer, 0, byteCount);
@@ -234,7 +241,8 @@ import java.nio.ByteBuffer;
     flacRelease(nativeDecoderContext);
   }
 
-  private int readFromExtractorInput(int offset, int length)
+  private int readFromExtractorInput(
+      ExtractorInput extractorInput, byte[] tempBuffer, int offset, int length)
       throws IOException, InterruptedException {
     int read = extractorInput.read(tempBuffer, offset, length);
     if (read == C.RESULT_END_OF_INPUT) {
diff --git a/extensions/opus/build.gradle b/extensions/opus/build.gradle
index 56acbdb7d3..0795079c6b 100644
--- a/extensions/opus/build.gradle
+++ b/extensions/opus/build.gradle
@@ -39,6 +39,7 @@ android {
 
 dependencies {
     implementation project(modulePrefix + 'library-core')
+    implementation 'androidx.annotation:annotation:1.0.2'
     testImplementation project(modulePrefix + 'testutils-robolectric')
     androidTestImplementation 'androidx.test:runner:' + androidXTestVersion
     androidTestImplementation 'androidx.test.ext:junit:' + androidXTestVersion
diff --git a/extensions/opus/src/main/java/com/google/android/exoplayer2/ext/opus/OpusDecoder.java b/extensions/opus/src/main/java/com/google/android/exoplayer2/ext/opus/OpusDecoder.java
index f8ec477b88..dbce33b923 100644
--- a/extensions/opus/src/main/java/com/google/android/exoplayer2/ext/opus/OpusDecoder.java
+++ b/extensions/opus/src/main/java/com/google/android/exoplayer2/ext/opus/OpusDecoder.java
@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.ext.opus;
 
+import androidx.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.decoder.CryptoInfo;
 import com.google.android.exoplayer2.decoder.DecoderInputBuffer;
@@ -150,6 +151,7 @@ import java.util.List;
   }
 
   @Override
+  @Nullable
   protected OpusDecoderException decode(
       DecoderInputBuffer inputBuffer, SimpleOutputBuffer outputBuffer, boolean reset) {
     if (reset) {
diff --git a/extensions/opus/src/main/java/com/google/android/exoplayer2/ext/opus/OpusLibrary.java b/extensions/opus/src/main/java/com/google/android/exoplayer2/ext/opus/OpusLibrary.java
index cd5d67f686..5529701c06 100644
--- a/extensions/opus/src/main/java/com/google/android/exoplayer2/ext/opus/OpusLibrary.java
+++ b/extensions/opus/src/main/java/com/google/android/exoplayer2/ext/opus/OpusLibrary.java
@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.ext.opus;
 
+import androidx.annotation.Nullable;
 import com.google.android.exoplayer2.ExoPlayerLibraryInfo;
 import com.google.android.exoplayer2.drm.ExoMediaCrypto;
 import com.google.android.exoplayer2.util.LibraryLoader;
@@ -30,7 +31,7 @@ public final class OpusLibrary {
   }
 
   private static final LibraryLoader LOADER = new LibraryLoader("opusV2JNI");
-  private static Class<? extends ExoMediaCrypto> exoMediaCryptoType;
+  @Nullable private static Class<? extends ExoMediaCrypto> exoMediaCryptoType;
 
   private OpusLibrary() {}
 
@@ -56,9 +57,8 @@ public final class OpusLibrary {
     return LOADER.isAvailable();
   }
 
-  /**
-   * Returns the version of the underlying library if available, or null otherwise.
-   */
+  /** Returns the version of the underlying library if available, or null otherwise. */
+  @Nullable
   public static String getVersion() {
     return isAvailable() ? opusGetVersion() : null;
   }
diff --git a/extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/VpxDecoder.java b/extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/VpxDecoder.java
index 57e5481b55..0e13e82630 100644
--- a/extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/VpxDecoder.java
+++ b/extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/VpxDecoder.java
@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.ext.vp9;
 
+import androidx.annotation.Nullable;
 import android.view.Surface;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.decoder.CryptoInfo;
@@ -120,8 +121,9 @@ import java.nio.ByteBuffer;
   }
 
   @Override
-  protected VpxDecoderException decode(VpxInputBuffer inputBuffer, VpxOutputBuffer outputBuffer,
-      boolean reset) {
+  @Nullable
+  protected VpxDecoderException decode(
+      VpxInputBuffer inputBuffer, VpxOutputBuffer outputBuffer, boolean reset) {
     ByteBuffer inputData = inputBuffer.data;
     int inputSize = inputData.limit();
     CryptoInfo cryptoInfo = inputBuffer.cryptoInfo;
diff --git a/extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/VpxLibrary.java b/extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/VpxLibrary.java
index 2c25f674d6..5106ab67ad 100644
--- a/extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/VpxLibrary.java
+++ b/extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/VpxLibrary.java
@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.ext.vp9;
 
+import androidx.annotation.Nullable;
 import com.google.android.exoplayer2.ExoPlayerLibraryInfo;
 import com.google.android.exoplayer2.drm.ExoMediaCrypto;
 import com.google.android.exoplayer2.util.LibraryLoader;
@@ -30,7 +31,7 @@ public final class VpxLibrary {
   }
 
   private static final LibraryLoader LOADER = new LibraryLoader("vpx", "vpxV2JNI");
-  private static Class<? extends ExoMediaCrypto> exoMediaCryptoType;
+  @Nullable private static Class<? extends ExoMediaCrypto> exoMediaCryptoType;
 
   private VpxLibrary() {}
 
@@ -56,9 +57,8 @@ public final class VpxLibrary {
     return LOADER.isAvailable();
   }
 
-  /**
-   * Returns the version of the underlying library if available, or null otherwise.
-   */
+  /** Returns the version of the underlying library if available, or null otherwise. */
+  @Nullable
   public static String getVersion() {
     return isAvailable() ? vpxGetVersion() : null;
   }
@@ -67,6 +67,7 @@ public final class VpxLibrary {
    * Returns the configuration string with which the underlying library was built if available, or
    * null otherwise.
    */
+  @Nullable
   public static String getBuildConfig() {
     return isAvailable() ? vpxGetBuildConfig() : null;
   }
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/decoder/SimpleDecoder.java b/library/core/src/main/java/com/google/android/exoplayer2/decoder/SimpleDecoder.java
index f8204f6be3..b5650860e9 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/decoder/SimpleDecoder.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/decoder/SimpleDecoder.java
@@ -301,5 +301,6 @@ public abstract class SimpleDecoder<
    * @param reset Whether the decoder must be reset before decoding.
    * @return A decoder exception if an error occurred, or null if decoding was successful.
    */
-  protected abstract @Nullable E decode(I inputBuffer, O outputBuffer, boolean reset);
+  @Nullable
+  protected abstract E decode(I inputBuffer, O outputBuffer, boolean reset);
 }