android platform channels: moved to direct buffers for c <-> java interop

shell/platform/android/io/flutter/embedding/engine/FlutterJNI.java

@@ -814,7 +814,7 @@ public void setPlatformMessageHandler(@Nullable PlatformMessageHandler platformM
   @SuppressWarnings("unused")
   @VisibleForTesting
   public void handlePlatformMessage(
-      @NonNull final String channel, byte[] message, final int replyId) {
+      @NonNull final String channel, ByteBuffer message, final int replyId) {
     if (platformMessageHandler != null) {
       platformMessageHandler.handleMessageFromDart(channel, message, replyId);
     }
@@ -825,7 +825,7 @@ public void handlePlatformMessage(
   // Called by native to respond to a platform message that we sent.
   // TODO(mattcarroll): determine if reply is nonull or nullable
   @SuppressWarnings("unused")
-  private void handlePlatformMessageResponse(int replyId, byte[] reply) {
+  private void handlePlatformMessageResponse(int replyId, ByteBuffer reply) {
     if (platformMessageHandler != null) {
       platformMessageHandler.handlePlatformMessageResponse(replyId, reply);
     }

shell/platform/android/io/flutter/embedding/engine/dart/DartMessenger.java

@@ -75,14 +75,18 @@ public void send(
 
   @Override
   public void handleMessageFromDart(
-      @NonNull final String channel, @Nullable byte[] message, final int replyId) {
+      @NonNull final String channel, @Nullable ByteBuffer message, final int replyId) {
     Log.v(TAG, "Received message from Dart over channel '" + channel + "'");
     BinaryMessenger.BinaryMessageHandler handler = messageHandlers.get(channel);
     if (handler != null) {
       try {
         Log.v(TAG, "Deferring to registered handler to process message.");
-        final ByteBuffer buffer = (message == null ? null : ByteBuffer.wrap(message));
-        handler.onMessage(buffer, new Reply(flutterJNI, replyId));
+        handler.onMessage(message, new Reply(flutterJNI, replyId));
+        if (message != null && message.isDirect()) {
+          // This ensures that if a user retains an instance to the ByteBuffer and it happens to
+          // be direct they will get a deterministic error.
+          message.limit(0);
+        }
       } catch (Exception ex) {
         Log.e(TAG, "Uncaught exception in binary message listener", ex);
         flutterJNI.invokePlatformMessageEmptyResponseCallback(replyId);
@@ -96,13 +100,18 @@ public void handleMessageFromDart(
   }
 
   @Override
-  public void handlePlatformMessageResponse(int replyId, @Nullable byte[] reply) {
+  public void handlePlatformMessageResponse(int replyId, @Nullable ByteBuffer reply) {
     Log.v(TAG, "Received message reply from Dart.");
     BinaryMessenger.BinaryReply callback = pendingReplies.remove(replyId);
     if (callback != null) {
       try {
         Log.v(TAG, "Invoking registered callback for reply from Dart.");
-        callback.reply(reply == null ? null : ByteBuffer.wrap(reply));
+        callback.reply(reply);
+        if (reply != null && reply.isDirect()) {
+          // This ensures that if a user retains an instance to the ByteBuffer and it happens to
+          // be direct they will get a deterministic error.
+          reply.limit(0);
+        }
       } catch (Exception ex) {
         Log.e(TAG, "Uncaught exception in binary message reply handler", ex);
       } catch (Error err) {

shell/platform/android/io/flutter/embedding/engine/dart/PlatformMessageHandler.java

@@ -6,11 +6,12 @@
 
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
+import java.nio.ByteBuffer;
 
 /** Handler that receives messages from Dart code. */
 public interface PlatformMessageHandler {
   void handleMessageFromDart(
-      @NonNull final String channel, @Nullable byte[] message, final int replyId);
+      @NonNull final String channel, @Nullable ByteBuffer message, final int replyId);
 
-  void handlePlatformMessageResponse(int replyId, @Nullable byte[] reply);
+  void handlePlatformMessageResponse(int replyId, @Nullable ByteBuffer reply);
 }

shell/platform/android/io/flutter/plugin/common/BinaryCodec.java

@@ -18,8 +18,31 @@
 public final class BinaryCodec implements MessageCodec<ByteBuffer> {
   // This codec must match the Dart codec of the same name in package flutter/services.
   public static final BinaryCodec INSTANCE = new BinaryCodec();
+  /**
+   * A BinaryCodec that returns direct ByteBuffers from `decodeMessage` for better performance.
+   *
+   * @see BinaryCodec.BinaryCodec(boolean)
+   */
+  public static final BinaryCodec INSTANCE_DIRECT = new BinaryCodec(true);
 
-  private BinaryCodec() {}
+  private final boolean returnsDirectByteBufferFromDecoding;
+
+  private BinaryCodec() {
+    this.returnsDirectByteBufferFromDecoding = false;
+  }
+
+  /**
+   * A constructor for BinaryCodec.
+   *
+   * @param returnsDirectByteBufferFromDecoding `true` means that the Codec will return direct
+   *     ByteBuffers from `decodeMessage`. Direct ByteBuffers will have better performance but will
+   *     be invalid beyond the scope of the `decodeMessage` call. `false` means Flutter will copy
+   *     the encoded message to Java's memory, so the ByteBuffer will be valid beyond the
+   *     decodeMessage call, at the cost of a copy.
+   */
+  private BinaryCodec(boolean returnsDirectByteBufferFromDecoding) {
+    this.returnsDirectByteBufferFromDecoding = returnsDirectByteBufferFromDecoding;
+  }
 
   @Override
   public ByteBuffer encodeMessage(ByteBuffer message) {
@@ -28,6 +51,13 @@ public ByteBuffer encodeMessage(ByteBuffer message) {
 
   @Override
   public ByteBuffer decodeMessage(ByteBuffer message) {
-    return message;
+    if (returnsDirectByteBufferFromDecoding) {
+      return message;
+    } else {
+      ByteBuffer result = ByteBuffer.allocate(message.capacity());
+      result.put(message);
+      result.rewind();
+      return result;
+    }
   }
 }

shell/platform/android/io/flutter/plugin/common/MessageCodec.java

@@ -26,6 +26,10 @@ public interface MessageCodec<T> {
   /**
    * Decodes the specified message from binary.
    *
+   * <p><b>Warning:</b> The ByteBuffer is "direct" and it won't be valid beyond this call. Storing
+   * the ByteBuffer and using it later and will lead to a {@code java.nio.BufferUnderflowException}.
+   * If you want to retain the data you'll need to copy it.
+   *
    * @param message the {@link ByteBuffer} message, possibly null.
    * @return a T value representation of the bytes between the given buffer's current position and
    *     its limit, or null, if message is null.

shell/platform/android/platform_view_android_jni_impl.cc

@@ -828,15 +828,16 @@ bool RegisterApi(JNIEnv* env) {
 
   g_handle_platform_message_method =
       env->GetMethodID(g_flutter_jni_class->obj(), "handlePlatformMessage",
-                       "(Ljava/lang/String;[BI)V");
+                       "(Ljava/lang/String;Ljava/nio/ByteBuffer;I)V");
 
   if (g_handle_platform_message_method == nullptr) {
     FML_LOG(ERROR) << "Could not locate handlePlatformMessage method";
     return false;
   }
 
   g_handle_platform_message_response_method = env->GetMethodID(
-      g_flutter_jni_class->obj(), "handlePlatformMessageResponse", "(I[B)V");
+      g_flutter_jni_class->obj(), "handlePlatformMessageResponse",
+      "(ILjava/nio/ByteBuffer;)V");
 
   if (g_handle_platform_message_response_method == nullptr) {
     FML_LOG(ERROR) << "Could not locate handlePlatformMessageResponse method";
@@ -1107,11 +1108,10 @@ void PlatformViewAndroidJNIImpl::FlutterViewHandlePlatformMessage(
       fml::jni::StringToJavaString(env, message->channel());
 
   if (message->hasData()) {
-    fml::jni::ScopedJavaLocalRef<jbyteArray> message_array(
-        env, env->NewByteArray(message->data().GetSize()));
-    env->SetByteArrayRegion(
-        message_array.obj(), 0, message->data().GetSize(),
-        reinterpret_cast<const jbyte*>(message->data().GetMapping()));
+    fml::jni::ScopedJavaLocalRef<jobject> message_array(
+        env, env->NewDirectByteBuffer(
+                 const_cast<uint8_t*>(message->data().GetMapping()),
+                 message->data().GetSize()));
     env->CallVoidMethod(java_object.obj(), g_handle_platform_message_method,
                         java_channel.obj(), message_array.obj(), responseId);
   } else {
@@ -1141,10 +1141,9 @@ void PlatformViewAndroidJNIImpl::FlutterViewHandlePlatformMessageResponse(
                         nullptr);
   } else {
     // Convert the vector to a Java byte array.
-    fml::jni::ScopedJavaLocalRef<jbyteArray> data_array(
-        env, env->NewByteArray(data->GetSize()));
-    env->SetByteArrayRegion(data_array.obj(), 0, data->GetSize(),
-                            reinterpret_cast<const jbyte*>(data->GetMapping()));
+    fml::jni::ScopedJavaLocalRef<jobject> data_array(
+        env, env->NewDirectByteBuffer(const_cast<uint8_t*>(data->GetMapping()),
+                                      data->GetSize()));
 
     env->CallVoidMethod(java_object.obj(),
                         g_handle_platform_message_response_method, responseId,

shell/platform/android/test/io/flutter/embedding/engine/dart/DartMessengerTest.java

@@ -1,11 +1,13 @@
 package io.flutter.embedding.engine.dart;
 
+import static junit.framework.TestCase.assertEquals;
 import static junit.framework.TestCase.assertNotNull;
 import static junit.framework.TestCase.assertTrue;
 import static org.mockito.Matchers.any;
 import static org.mockito.Mockito.mock;
 
 import io.flutter.embedding.engine.FlutterJNI;
+import io.flutter.plugin.common.BinaryMessenger;
 import io.flutter.plugin.common.BinaryMessenger.BinaryMessageHandler;
 import java.nio.ByteBuffer;
 import org.junit.Test;
@@ -46,9 +48,80 @@ public void itHandlesErrors() {
         .onMessage(any(ByteBuffer.class), any(DartMessenger.Reply.class));
 
     messenger.setMessageHandler("test", throwingHandler);
-    messenger.handleMessageFromDart("test", new byte[] {}, 0);
+    messenger.handleMessageFromDart("test", ByteBuffer.allocate(0), 0);
     assertNotNull(reportingHandler.latestException);
     assertTrue(reportingHandler.latestException instanceof AssertionError);
     currentThread.setUncaughtExceptionHandler(savedHandler);
   }
+
+  @Test
+  public void givesDirectByteBuffer() {
+    // Setup test.
+    final FlutterJNI fakeFlutterJni = mock(FlutterJNI.class);
+    final DartMessenger messenger = new DartMessenger(fakeFlutterJni);
+    final String channel = "foobar";
+    final boolean[] wasDirect = {false};
+    final BinaryMessenger.BinaryMessageHandler handler =
+        (message, reply) -> {
+          wasDirect[0] = message.isDirect();
+        };
+    messenger.setMessageHandler(channel, handler);
+    final ByteBuffer message = ByteBuffer.allocateDirect(4 * 2);
+    message.rewind();
+    message.putChar('a');
+    message.putChar('b');
+    message.putChar('c');
+    message.putChar('d');
+    messenger.handleMessageFromDart(channel, message, /*replyId=*/ 123);
+    assertTrue(wasDirect[0]);
+  }
+
+  @Test
+  public void directByteBufferLimitZeroAfterUsage() {
+    // Setup test.
+    final FlutterJNI fakeFlutterJni = mock(FlutterJNI.class);
+    final DartMessenger messenger = new DartMessenger(fakeFlutterJni);
+    final String channel = "foobar";
+    final ByteBuffer[] byteBuffers = {null};
+    final int bufferSize = 4 * 2;
+    final BinaryMessenger.BinaryMessageHandler handler =
+        (message, reply) -> {
+          byteBuffers[0] = message;
+          assertEquals(bufferSize, byteBuffers[0].limit());
+        };
+    messenger.setMessageHandler(channel, handler);
+    final ByteBuffer message = ByteBuffer.allocateDirect(bufferSize);
+    message.rewind();
+    message.putChar('a');
+    message.putChar('b');
+    message.putChar('c');
+    message.putChar('d');
+    messenger.handleMessageFromDart(channel, message, /*replyId=*/ 123);
+    assertNotNull(byteBuffers[0]);
+    assertTrue(byteBuffers[0].isDirect());
+    assertEquals(0, byteBuffers[0].limit());
+  }
+
+  @Test
+  public void directByteBufferLimitZeroAfterReply() {
+    // Setup test.
+    final FlutterJNI fakeFlutterJni = mock(FlutterJNI.class);
+    final DartMessenger messenger = new DartMessenger(fakeFlutterJni);
+    final ByteBuffer message = ByteBuffer.allocateDirect(4 * 2);
+    final String channel = "foobar";
+    message.rewind();
+    message.putChar('a');
+    message.putChar('b');
+    message.putChar('c');
+    message.putChar('d');
+    final ByteBuffer[] byteBuffers = {null};
+    BinaryMessenger.BinaryReply callback =
+        (reply) -> {
+          assertTrue(reply.isDirect());
+          byteBuffers[0] = reply;
+        };
+    messenger.send(channel, null, callback);
+    messenger.handlePlatformMessageResponse(1, message);
+    assertEquals(0, byteBuffers[0].limit());
+  }
 }

shell/platform/android/test/io/flutter/plugin/platform/PlatformViewsControllerTest.java

@@ -630,12 +630,10 @@ public void checkInputConnectionProxy__falseIfViewIsNull() {
     assertFalse(shouldProxying);
   }
 
-  private static byte[] encodeMethodCall(MethodCall call) {
+  private static ByteBuffer encodeMethodCall(MethodCall call) {
     final ByteBuffer buffer = StandardMethodCodec.INSTANCE.encodeMethodCall(call);
     buffer.rewind();
-    final byte[] dest = new byte[buffer.remaining()];
-    buffer.get(dest);
-    return dest;
+    return buffer;
   }
 
   private static void createPlatformView(