core, alts, cronet: fix ByteBuffer covariant method usages (#7349)

Java 9 introduces overridden methods with covariant return types for the following methods in java.nio.ByteBuffer:

- position​(int newPosition)
- limit​(int newLimit)
- flip​()
- clear​()
- mark​()
- reset​()
- rewind​()

In Java 9 they all now return ByteBuffer, whereas the methods they override return Buffer, resulting in exceptions like this when executing on Java 8 and lower:

java.lang.NoSuchMethodError: java.nio.ByteBuffer.limit(I)Ljava/nio/ByteBuffer

This is because the generated byte code includes the static return type of the method, which is not found on Java 8 and lower because the overloaded methods with covariant return types don't exist (the issue appears even with source and target 8 or lower in compilation parameters).
The solution is to cast ByteBuffer instances to Buffer before calling the method.

alts/src/main/java/io/grpc/alts/internal/AltsFraming.java

@@ -17,6 +17,7 @@
 package io.grpc.alts.internal;
 
 import com.google.common.base.Preconditions;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 import java.security.GeneralSecurityException;
@@ -63,10 +64,10 @@ public final class AltsFraming {
     }
     Producer producer = new Producer();
     ByteBuffer inputAlias = input.duplicate();
-    inputAlias.limit(input.position() + dataSize);
+    ((Buffer) inputAlias).limit(input.position() + dataSize);
     producer.readBytes(inputAlias);
     producer.flush();
-    input.position(inputAlias.position());
+    ((Buffer) input).position(inputAlias.position());
     ByteBuffer output = producer.getRawFrame();
     return output;
   }
@@ -166,10 +167,10 @@ public final class AltsFraming {
       int frameLength = buffer.position() + getFrameSuffixLength();
 
       // Set the limit and move to the start.
-      buffer.flip();
+      ((Buffer) buffer).flip();
 
       // Advance the limit to allow a crypto suffix.
-      buffer.limit(buffer.limit() + getFrameSuffixLength());
+      ((Buffer) buffer).limit(buffer.limit() + getFrameSuffixLength());
 
       // Write the data length and the message type.
       int dataLength = frameLength - FRAME_LENGTH_HEADER_SIZE;
@@ -178,17 +179,17 @@ public final class AltsFraming {
       buffer.putInt(MESSAGE_TYPE);
 
       // Move the position back to 0, the frame is ready.
-      buffer.position(0);
+      ((Buffer) buffer).position(0);
       isComplete = true;
     }
 
     /** Resets the state, preparing to construct a new frame. Must be called between frames. */
     private void reset() {
-      buffer.clear();
+      ((Buffer) buffer).clear();
 
       // Save some space for framing, we'll fill that in later.
-      buffer.position(getFramePrefixLength());
+      ((Buffer) buffer).position(getFramePrefixLength());
-      buffer.limit(buffer.limit() - getFrameSuffixLength());
+      ((Buffer) buffer).limit(buffer.limit() - getFrameSuffixLength());
 
       isComplete = false;
     }
@@ -279,7 +280,7 @@ public final class AltsFraming {
       // internal buffer is large enough.
       if (buffer.position() == FRAME_LENGTH_HEADER_SIZE && input.hasRemaining()) {
         ByteBuffer bufferAlias = buffer.duplicate();
-        bufferAlias.flip();
+        ((Buffer) bufferAlias).flip();
         bufferAlias.order(ByteOrder.LITTLE_ENDIAN);
         int dataLength = bufferAlias.getInt();
         if (dataLength < FRAME_MESSAGE_TYPE_HEADER_SIZE || dataLength > MAX_DATA_LENGTH) {
@@ -292,7 +293,7 @@ public final class AltsFraming {
           buffer.order(ByteOrder.LITTLE_ENDIAN);
           buffer.putInt(dataLength);
         }
-        buffer.limit(frameLength);
+        ((Buffer) buffer).limit(frameLength);
       }
 
       // TODO: Similarly extract and check message type.
@@ -300,7 +301,7 @@ public final class AltsFraming {
       // Read the remaining data into the internal buffer.
       copy(buffer, input);
       if (!buffer.hasRemaining()) {
-        buffer.flip();
+        ((Buffer) buffer).flip();
         isComplete = true;
       }
       return isComplete;
@@ -323,7 +324,7 @@ public final class AltsFraming {
 
     /** Resets the state, preparing to parse a new frame. Must be called between frames. */
     private void reset() {
-      buffer.clear();
+      ((Buffer) buffer).clear();
       isComplete = false;
     }
 
@@ -356,9 +357,9 @@ public final class AltsFraming {
       } else {
         int count = Math.min(dst.remaining(), src.remaining());
         ByteBuffer slice = src.slice();
-        slice.limit(count);
+        ((Buffer) slice).limit(count);
         dst.put(slice);
-        src.position(src.position() + count);
+        ((Buffer) src).position(src.position() + count);
       }
     }
   }

alts/src/main/java/io/grpc/alts/internal/AltsHandshakerClient.java

@@ -23,6 +23,7 @@ import com.google.protobuf.ByteString;
 import io.grpc.Status;
 import io.grpc.alts.internal.HandshakerServiceGrpc.HandshakerServiceStub;
 import java.io.IOException;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.security.GeneralSecurityException;
 import java.util.logging.Level;
@@ -199,7 +200,7 @@ class AltsHandshakerClient {
       throw new GeneralSecurityException(e);
     }
     handleResponse(resp);
-    inBytes.position(inBytes.position() + resp.getBytesConsumed());
+    ((Buffer) inBytes).position(inBytes.position() + resp.getBytesConsumed());
     return resp.getOutFrames().asReadOnlyByteBuffer();
   }
 
@@ -227,7 +228,7 @@ class AltsHandshakerClient {
       throw new GeneralSecurityException(e);
     }
     handleResponse(resp);
-    inBytes.position(inBytes.position() + resp.getBytesConsumed());
+    ((Buffer) inBytes).position(inBytes.position() + resp.getBytesConsumed());
     return resp.getOutFrames().asReadOnlyByteBuffer();
   }
 

alts/src/main/java/io/grpc/alts/internal/AltsTsiHandshaker.java

@@ -22,6 +22,7 @@ import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Preconditions;
 import io.grpc.alts.internal.HandshakerServiceGrpc.HandshakerServiceStub;
 import io.netty.buffer.ByteBufAllocator;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.security.GeneralSecurityException;
 import java.util.ArrayList;
@@ -151,10 +152,10 @@ public final class AltsTsiHandshaker implements TsiHandshaker {
     ByteBuffer outputFrameAlias = outputFrame;
     if (outputFrame.remaining() > bytes.remaining()) {
       outputFrameAlias = outputFrame.duplicate();
-      outputFrameAlias.limit(outputFrameAlias.position() + bytes.remaining());
+      ((Buffer) outputFrameAlias).limit(outputFrameAlias.position() + bytes.remaining());
     }
     bytes.put(outputFrameAlias);
-    outputFrame.position(outputFrameAlias.position());
+    ((Buffer) outputFrame).position(outputFrameAlias.position());
   }
 
   /**

alts/src/test/java/io/grpc/alts/internal/AltsFramingTest.java

@@ -19,6 +19,7 @@ package io.grpc.alts.internal;
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.fail;
 
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 import java.security.GeneralSecurityException;
@@ -38,7 +39,7 @@ public class AltsFramingTest {
     buffer.order(ByteOrder.LITTLE_ENDIAN);
     buffer.putInt(-1); // write invalid length
     buffer.put((byte) 0); // write some byte
-    buffer.flip();
+    ((Buffer) buffer).flip();
 
     try {
       parser.readBytes(buffer);
@@ -56,7 +57,7 @@ public class AltsFramingTest {
     buffer.order(ByteOrder.LITTLE_ENDIAN);
     buffer.putInt(AltsFraming.getFrameMessageTypeHeaderSize() - 1); // write invalid length
     buffer.put((byte) 0); // write some byte
-    buffer.flip();
+    ((Buffer) buffer).flip();
 
     try {
       parser.readBytes(buffer);
@@ -74,7 +75,7 @@ public class AltsFramingTest {
     buffer.order(ByteOrder.LITTLE_ENDIAN);
     buffer.putInt(AltsFraming.getMaxDataLength() + 1); // write invalid length
     buffer.put((byte) 0); // write some byte
-    buffer.flip();
+    ((Buffer) buffer).flip();
 
     try {
       parser.readBytes(buffer);
@@ -97,7 +98,7 @@ public class AltsFramingTest {
     buffer.putInt(6); // default message type
     buffer.put(new byte[dataLength - AltsFraming.getFrameMessageTypeHeaderSize()]); // write data
     buffer.put((byte) 0);
-    buffer.flip();
+    ((Buffer) buffer).flip();
 
     parser.readBytes(buffer);
 
@@ -116,7 +117,7 @@ public class AltsFramingTest {
     buffer.putInt(dataLength); // write invalid length
     buffer.putInt(6); // default message type
     buffer.put((byte) 0);
-    buffer.flip();
+    ((Buffer) buffer).flip();
 
     parser.readBytes(buffer);
 

alts/src/test/java/io/grpc/alts/internal/AltsHandshakerClientTest.java

@@ -29,6 +29,7 @@ import static org.mockito.Mockito.when;
 
 import com.google.common.collect.ImmutableList;
 import com.google.protobuf.ByteString;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.security.GeneralSecurityException;
 import org.junit.Before;
@@ -178,7 +179,7 @@ public class AltsHandshakerClientTest {
         .thenReturn(MockAltsHandshakerResp.getOkResponse(BYTES_CONSUMED));
 
     ByteBuffer inBytes = ByteBuffer.allocate(IN_BYTES_SIZE);
-    inBytes.position(PREFIX_POSITION);
+    ((Buffer) inBytes).position(PREFIX_POSITION);
     ByteBuffer outFrame = handshaker.startServerHandshake(inBytes);
 
     assertEquals(ByteString.copyFrom(outFrame), MockAltsHandshakerResp.getOutFrame());

alts/src/test/java/io/grpc/alts/internal/AltsTsiHandshakerTest.java

@@ -26,6 +26,7 @@ import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 import com.google.protobuf.ByteString;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import org.junit.Before;
 import org.junit.Test;
@@ -112,7 +113,7 @@ public class AltsTsiHandshakerTest {
     verify(mockServer, never()).startClientHandshake();
     verify(mockServer, never()).next(ArgumentMatchers.<ByteBuffer>any());
     // Mock transport buffer all consumed by processBytesFromPeer and there is an output frame.
-    transportBuffer.position(transportBuffer.limit());
+    ((Buffer) transportBuffer).position(transportBuffer.limit());
     when(mockServer.startServerHandshake(transportBuffer)).thenReturn(outputFrame);
     when(mockServer.isFinished()).thenReturn(false);
 
@@ -127,7 +128,7 @@ public class AltsTsiHandshakerTest {
     verify(mockServer, never()).next(ArgumentMatchers.<ByteBuffer>any());
     // Mock transport buffer all consumed by processBytesFromPeer and output frame is empty.
     // Expect processBytesFromPeer return False, because more data are needed from the peer.
-    transportBuffer.position(transportBuffer.limit());
+    ((Buffer) transportBuffer).position(transportBuffer.limit());
     when(mockServer.startServerHandshake(transportBuffer)).thenReturn(emptyOutputFrame);
     when(mockServer.isFinished()).thenReturn(false);
 
@@ -174,7 +175,7 @@ public class AltsTsiHandshakerTest {
     when(mockClient.isFinished()).thenReturn(false);
 
     handshakerClient.getBytesToSendToPeer(transportBuffer);
-    transportBuffer.position(transportBuffer.limit());
+    ((Buffer) transportBuffer).position(transportBuffer.limit());
     assertFalse(handshakerClient.processBytesFromPeer(transportBuffer));
   }
 

alts/src/test/java/io/grpc/alts/internal/FakeTsiTest.java

@@ -27,6 +27,7 @@ import io.netty.buffer.ByteBuf;
 import io.netty.util.ReferenceCounted;
 import io.netty.util.ResourceLeakDetector;
 import io.netty.util.ResourceLeakDetector.Level;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.security.GeneralSecurityException;
 import java.util.ArrayList;
@@ -86,11 +87,11 @@ public class FakeTsiTest {
 
       byte[] transportBufferBytes = new byte[TsiTest.getDefaultTransportBufferSize()];
       ByteBuffer transportBuffer = ByteBuffer.wrap(transportBufferBytes);
-      transportBuffer.limit(0); // Start off with an empty buffer
+      ((Buffer) transportBuffer).limit(0); // Start off with an empty buffer
 
-      transportBuffer.clear();
+      ((Buffer) transportBuffer).clear();
       clientHandshaker.getBytesToSendToPeer(transportBuffer);
-      transportBuffer.flip();
+      ((Buffer) transportBuffer).flip();
       assertEquals(
           FakeTsiHandshaker.State.CLIENT_INIT.toString().trim(),
           new String(transportBufferBytes, 4, transportBuffer.remaining(), UTF_8).trim());
@@ -99,14 +100,14 @@ public class FakeTsiTest {
       assertFalse(transportBuffer.hasRemaining());
 
       // client shouldn't offer any more bytes
-      transportBuffer.clear();
+      ((Buffer) transportBuffer).clear();
       clientHandshaker.getBytesToSendToPeer(transportBuffer);
-      transportBuffer.flip();
+      ((Buffer) transportBuffer).flip();
       assertFalse(transportBuffer.hasRemaining());
 
-      transportBuffer.clear();
+      ((Buffer) transportBuffer).clear();
       serverHandshaker.getBytesToSendToPeer(transportBuffer);
-      transportBuffer.flip();
+      ((Buffer) transportBuffer).flip();
       assertEquals(
           FakeTsiHandshaker.State.SERVER_INIT.toString().trim(),
           new String(transportBufferBytes, 4, transportBuffer.remaining(), UTF_8).trim());
@@ -115,14 +116,14 @@ public class FakeTsiTest {
       assertFalse(transportBuffer.hasRemaining());
 
       // server shouldn't offer any more bytes
-      transportBuffer.clear();
+      ((Buffer) transportBuffer).clear();
       serverHandshaker.getBytesToSendToPeer(transportBuffer);
-      transportBuffer.flip();
+      ((Buffer) transportBuffer).flip();
       assertFalse(transportBuffer.hasRemaining());
 
-      transportBuffer.clear();
+      ((Buffer) transportBuffer).clear();
       clientHandshaker.getBytesToSendToPeer(transportBuffer);
-      transportBuffer.flip();
+      ((Buffer) transportBuffer).flip();
       assertEquals(
           FakeTsiHandshaker.State.CLIENT_FINISHED.toString().trim(),
           new String(transportBufferBytes, 4, transportBuffer.remaining(), UTF_8).trim());
@@ -131,14 +132,14 @@ public class FakeTsiTest {
       assertFalse(transportBuffer.hasRemaining());
 
       // client shouldn't offer any more bytes
-      transportBuffer.clear();
+      ((Buffer) transportBuffer).clear();
       clientHandshaker.getBytesToSendToPeer(transportBuffer);
-      transportBuffer.flip();
+      ((Buffer) transportBuffer).flip();
       assertFalse(transportBuffer.hasRemaining());
 
-      transportBuffer.clear();
+      ((Buffer) transportBuffer).clear();
       serverHandshaker.getBytesToSendToPeer(transportBuffer);
-      transportBuffer.flip();
+      ((Buffer) transportBuffer).flip();
       assertEquals(
           FakeTsiHandshaker.State.SERVER_FINISHED.toString().trim(),
           new String(transportBufferBytes, 4, transportBuffer.remaining(), UTF_8).trim());
@@ -147,9 +148,9 @@ public class FakeTsiTest {
       assertFalse(transportBuffer.hasRemaining());
 
       // server shouldn't offer any more bytes
-      transportBuffer.clear();
+      ((Buffer) transportBuffer).clear();
       serverHandshaker.getBytesToSendToPeer(transportBuffer);
-      transportBuffer.flip();
+      ((Buffer) transportBuffer).flip();
       assertFalse(transportBuffer.hasRemaining());
     } catch (GeneralSecurityException e) {
       throw new AssertionError(e);

alts/src/test/java/io/grpc/alts/internal/MockAltsHandshakerResp.java

@@ -20,6 +20,7 @@ import static java.nio.charset.StandardCharsets.UTF_8;
 
 import com.google.protobuf.ByteString;
 import io.grpc.Status;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 import java.security.SecureRandom;
@@ -62,7 +63,7 @@ class MockAltsHandshakerResp {
     buffer.order(ByteOrder.LITTLE_ENDIAN);
     buffer.putInt(frameSize);
     buffer.put(TEST_OUT_FRAME.getBytes(UTF_8));
-    buffer.flip();
+    ((Buffer) buffer).flip();
     return ByteString.copyFrom(buffer);
   }
 

alts/src/test/java/io/grpc/alts/internal/TsiTest.java

@@ -26,6 +26,7 @@ import io.grpc.alts.internal.TsiFrameProtector.Consumer;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.Unpooled;
 import io.netty.buffer.UnpooledByteBufAllocator;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.security.GeneralSecurityException;
 import java.util.ArrayList;
@@ -83,7 +84,7 @@ public final class TsiTest {
 
     byte[] transportBufferBytes = new byte[transportBufferSize];
     ByteBuffer transportBuffer = ByteBuffer.wrap(transportBufferBytes);
-    transportBuffer.limit(0); // Start off with an empty buffer
+    ((Buffer) transportBuffer).limit(0); // Start off with an empty buffer
 
     while (clientHandshaker.isInProgress() || serverHandshaker.isInProgress()) {
       for (TsiHandshaker handshaker : new TsiHandshaker[] {clientHandshaker, serverHandshaker}) {
@@ -94,9 +95,9 @@ public final class TsiTest {
           }
           // Put new bytes on the wire, if needed.
           if (handshaker.isInProgress()) {
-            transportBuffer.clear();
+            ((Buffer) transportBuffer).clear();
             handshaker.getBytesToSendToPeer(transportBuffer);
-            transportBuffer.flip();
+            ((Buffer) transportBuffer).flip();
           }
         }
       }

core/src/main/java/io/grpc/internal/CompositeReadableBuffer.java

@@ -102,18 +102,16 @@ public class CompositeReadableBuffer extends AbstractReadableBuffer {
 
   @Override
   public void readBytes(final ByteBuffer dest) {
-    // Use Buffer instead of ByteBuffer for JDK 9+ compatibility.
-    final Buffer destAsBuffer = dest;
     execute(new ReadOperation() {
       @Override
       public int readInternal(ReadableBuffer buffer, int length) {
         // Change the limit so that only lengthToCopy bytes are available.
-        int prevLimit = destAsBuffer.limit();
+        int prevLimit = dest.limit();
-        destAsBuffer.limit(destAsBuffer.position() + length);
+        ((Buffer) dest).limit(dest.position() + length);
 
         // Write the bytes and restore the original limit.
         buffer.readBytes(dest);
-        destAsBuffer.limit(prevLimit);
+        ((Buffer) dest).limit(prevLimit);
         return 0;
       }
     }, dest.remaining());

core/src/main/java/io/grpc/internal/ReadableBuffers.java

@@ -210,8 +210,7 @@ public final class ReadableBuffers {
    * A {@link ReadableBuffer} that is backed by a {@link ByteBuffer}.
    */
   private static class ByteReadableBufferWrapper extends AbstractReadableBuffer {
-    // Use Buffer instead of ByteBuffer for JDK 9+ compatibility.
+    final ByteBuffer bytes;
-    final Buffer bytes;
 
     ByteReadableBufferWrapper(ByteBuffer bytes) {
       this.bytes = Preconditions.checkNotNull(bytes, "bytes");
@@ -225,19 +224,19 @@ public final class ReadableBuffers {
     @Override
     public int readUnsignedByte() {
       checkReadable(1);
-      return ((ByteBuffer) bytes).get() & 0xFF;
+      return bytes.get() & 0xFF;
     }
 
     @Override
     public void skipBytes(int length) {
       checkReadable(length);
-      bytes.position(bytes.position() + length);
+      ((Buffer) bytes).position(bytes.position() + length);
     }
 
     @Override
     public void readBytes(byte[] dest, int destOffset, int length) {
       checkReadable(length);
-      ((ByteBuffer) bytes).get(dest, destOffset, length);
+      bytes.get(dest, destOffset, length);
     }
 
     @Override
@@ -248,10 +247,10 @@ public final class ReadableBuffers {
 
       // Change the limit so that only length bytes are available.
       int prevLimit = bytes.limit();
-      bytes.limit(bytes.position() + length);
+      ((Buffer) bytes).limit(bytes.position() + length);
 
       // Write the bytes and restore the original limit.
-      dest.put((ByteBuffer) bytes);
+      dest.put(bytes);
       bytes.limit(prevLimit);
     }
 
@@ -260,11 +259,11 @@ public final class ReadableBuffers {
       checkReadable(length);
       if (hasArray()) {
         dest.write(array(), arrayOffset(), length);
-        bytes.position(bytes.position() + length);
+        ((Buffer) bytes).position(bytes.position() + length);
       } else {
         // The buffer doesn't support array(). Copy the data to an intermediate buffer.
         byte[] array = new byte[length];
-        ((ByteBuffer) bytes).get(array);
+        bytes.get(array);
         dest.write(array);
       }
     }
@@ -272,9 +271,9 @@ public final class ReadableBuffers {
     @Override
     public ByteReadableBufferWrapper readBytes(int length) {
       checkReadable(length);
-      ByteBuffer buffer = ((ByteBuffer) bytes).duplicate();
+      ByteBuffer buffer = bytes.duplicate();
       ((Buffer) buffer).limit(bytes.position() + length);
-      bytes.position(bytes.position() + length);
+      ((Buffer) bytes).position(bytes.position() + length);
       return new ByteReadableBufferWrapper(buffer);
     }
 
@@ -285,7 +284,7 @@ public final class ReadableBuffers {
 
     @Override
     public byte[] array() {
-      return ((ByteBuffer) bytes).array();
+      return bytes.array();
     }
 
     @Override

core/src/test/java/io/grpc/internal/CompositeReadableBufferTest.java

@@ -23,6 +23,7 @@ import static org.mockito.Mockito.verify;
 
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import org.junit.After;
 import org.junit.Before;
@@ -119,17 +120,17 @@ public class CompositeReadableBufferTest {
     ByteBuffer byteBuffer = ByteBuffer.allocate(EXPECTED_VALUE.length());
     int remaining = EXPECTED_VALUE.length();
 
-    byteBuffer.limit(1);
+    ((Buffer) byteBuffer).limit(1);
     composite.readBytes(byteBuffer);
     remaining--;
     assertEquals(remaining, composite.readableBytes());
 
-    byteBuffer.limit(byteBuffer.limit() + 5);
+    ((Buffer) byteBuffer).limit(byteBuffer.limit() + 5);
     composite.readBytes(byteBuffer);
     remaining -= 5;
     assertEquals(remaining, composite.readableBytes());
 
-    byteBuffer.limit(byteBuffer.limit() + remaining);
+    ((Buffer) byteBuffer).limit(byteBuffer.limit() + remaining);
     composite.readBytes(byteBuffer);
     assertEquals(0, composite.readableBytes());
     assertEquals(EXPECTED_VALUE, new String(byteBuffer.array(), UTF_8));

core/src/test/java/io/grpc/internal/ReadableBufferTestBase.java

@@ -21,6 +21,7 @@ import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
 
 import java.io.ByteArrayOutputStream;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.util.Arrays;
 import org.junit.Test;
@@ -86,7 +87,7 @@ public abstract class ReadableBufferTestBase {
     ReadableBuffer buffer = buffer();
     ByteBuffer byteBuffer = ByteBuffer.allocate(msg.length());
     buffer.readBytes(byteBuffer);
-    byteBuffer.flip();
+    ((Buffer) byteBuffer).flip();
     byte[] array = new byte[msg.length()];
     byteBuffer.get(array);
     assertArrayEquals(msg.getBytes(UTF_8), array);
@@ -98,7 +99,7 @@ public abstract class ReadableBufferTestBase {
     ReadableBuffer buffer = buffer();
     ByteBuffer byteBuffer = ByteBuffer.allocate(2);
     buffer.readBytes(byteBuffer);
-    byteBuffer.flip();
+    ((Buffer) byteBuffer).flip();
     byte[] array = new byte[2];
     byteBuffer.get(array);
     assertArrayEquals(new byte[]{'h', 'e'}, array);

cronet/src/main/java/io/grpc/cronet/CronetClientStream.java

@@ -42,6 +42,7 @@ import io.grpc.internal.TransportTracer;
 import io.grpc.internal.WritableBuffer;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.nio.charset.Charset;
 import java.util.ArrayList;
@@ -218,7 +219,7 @@ class CronetClientStream extends AbstractClientStream {
         ByteBuffer byteBuffer;
         if (buffer != null) {
           byteBuffer = ((CronetWritableBuffer) buffer).buffer();
-          byteBuffer.flip();
+          ((Buffer) byteBuffer).flip();
         } else {
           byteBuffer = EMPTY_BUFFER;
         }
@@ -471,7 +472,7 @@ class CronetClientStream extends AbstractClientStream {
     @Override
     public void onReadCompleted(BidirectionalStream stream, UrlResponseInfo info,
         ByteBuffer buffer, boolean endOfStream) {
-      buffer.flip();
+      ((Buffer) buffer).flip();
       if (Log.isLoggable(LOG_TAG, Log.VERBOSE)) {
         Log.v(LOG_TAG, "onReadCompleted. Size=" + buffer.remaining());
       }

cronet/src/test/java/io/grpc/cronet/CronetClientStreamTest.java

@@ -43,6 +43,7 @@ import io.grpc.internal.TransportTracer;
 import io.grpc.internal.WritableBuffer;
 import io.grpc.testing.TestMethodDescriptors;
 import java.io.ByteArrayInputStream;
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.nio.charset.Charset;
 import java.util.ArrayList;
@@ -176,7 +177,7 @@ public final class CronetClientStreamTest {
     // 5 writes are called.
     verify(cronetStream, times(5)).write(isA(ByteBuffer.class), eq(false));
     ByteBuffer fakeBuffer = ByteBuffer.allocateDirect(8);
-    fakeBuffer.position(8);
+    ((Buffer) fakeBuffer).position(8);
     verify(cronetStream, times(2)).flush();
 
     // 5 onWriteCompleted callbacks for previous writes.
@@ -294,7 +295,7 @@ public final class CronetClientStreamTest {
     ArgumentCaptor<ByteBuffer> bufferCaptor = ArgumentCaptor.forClass(ByteBuffer.class);
     verify(cronetStream, times(1)).write(bufferCaptor.capture(), isA(Boolean.class));
     ByteBuffer buffer = bufferCaptor.getValue();
-    buffer.position(request.length());
+    ((Buffer) buffer).position(request.length());
     verify(cronetStream, times(1)).flush();
 
     // Receive response header