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下面一起看下 RPC 请求消息的发送和应答接收相关代码。

RPC 请求调用源码分析

请求调用主要有两步：请求 Frame 构造和 Frame 发送，请求 Frame 构造代码如下所示（ClientCallImpl 类）：

public void sendMessage(ReqT message) {
    Preconditions.checkState(stream != null, "Not started");
    Preconditions.checkState(!cancelCalled, "call was cancelled");
    Preconditions.checkState(!halfCloseCalled, "call was half-closed");
    try {
      InputStream messageIs = method.streamRequest(message);
      stream.writeMessage(messageIs);
...

使用 PB 对请求消息做序列化，生成 InputStream，构造请求 Frame：

private int writeUncompressed(InputStream message, int messageLength) throws IOException {
    if (messageLength != -1) {
      statsTraceCtx.outboundWireSize(messageLength);
      return writeKnownLengthUncompressed(message, messageLength);
    }
    BufferChainOutputStream bufferChain = new BufferChainOutputStream();
    int written = writeToOutputStream(message, bufferChain);
    if (maxOutboundMessageSize >= 0 && written > maxOutboundMessageSize) {
      throw Status.INTERNAL
          .withDescription(
              String.format("message too large %d > %d", written , maxOutboundMessageSize))
          .asRuntimeException();
    }
    writeBufferChain(bufferChain, false);
    return written;
}

Frame 发送代码如下所示：

public void writeFrame(WritableBuffer frame, boolean endOfStream, boolean flush) {
      ByteBuf bytebuf = frame == null ? EMPTY_BUFFER : ((NettyWritableBuffer) frame).bytebuf();
      final int numBytes = bytebuf.readableBytes();
      if (numBytes > 0) {
        onSendingBytes(numBytes);
        writeQueue.enqueue(
            new SendGrpcFrameCommand(transportState(), bytebuf, endOfStream),
            channel.newPromise().addListener(new ChannelFutureListener() {
              @Override
              public void operationComplete(ChannelFuture future) throws Exception {
                if (future.isSuccess()) {
                  transportState().onSentBytes(numBytes);
                }
              }
            }), flush);

NettyClientHandler 接收到发送事件之后，调用 Http2ConnectionEncoder 将 Frame 写入 Netty HTTP/2 协议栈（NettyClientHandler 类）：

private void sendGrpcFrame(ChannelHandlerContext ctx, SendGrpcFrameCommand cmd,
      ChannelPromise promise) {
    encoder().writeData(ctx, cmd.streamId(), cmd.content(), 0, cmd.endStream(), promise);
  }

RPC 响应接收和处理源码分析

响应消息的接收入口是 NettyClientHandler，包括 HTTP/2 Header 和 HTTP/2 DATA Frame 两部分，代码如下（NettyClientHandler 类）：

private void onHeadersRead(int streamId, Http2Headers headers, boolean endStream) {
    NettyClientStream.TransportState stream = clientStream(requireHttp2Stream(streamId));
    stream.transportHeadersReceived(headers, endStream);
    if (keepAliveManager != null) {
      keepAliveManager.onDataReceived();
    }
  }

如果参数 endStream 为 True，说明 Stream 已经结束，调用 transportTrailersReceived，通知 Listener close，代码如下所示（AbstractClientStream2 类）：

if (stopDelivery || isDeframerStalled()) {
        deliveryStalledTask = null;
        closeListener(status, trailers);
      } else {
        deliveryStalledTask = new Runnable() {
          @Override
          public void run() {
            closeListener(status, trailers);
          }
        };
      }

读取到 HTTP/2 DATA Frame 之后，调用 MessageDeframer 的 deliver 对 Frame 进行解析，代码如下（MessageDeframer 类）：

private void deliver() {
    if (inDelivery) {
      return;
    }
    inDelivery = true;
    try {
          while (pendingDeliveries > 0 && readRequiredBytes()) {
        switch (state) {
          case HEADER:
            processHeader();
            break;
          case BODY:
            processBody();
...

将 Frame 转换成 InputStream 之后，通知 ClientStreamListenerImpl，调用 messageRead(final InputStream message)，将 InputStream 反序列化为响应对象，相关代码如下所示（ClientStreamListenerImpl 类）：

public void messageRead(final InputStream message) {
      class MessageRead extends ContextRunnable {
        MessageRead() {
          super(context);
        }
        @Override
        public final void runInContext() {
          try {
            if (closed) {
              return;
            }
            try {
              observer.onMessage(method.parseResponse(message));
            } finally {
              message.close();
            }

当接收到 endOfStream 之后，通知 ClientStreamListenerImpl，调用它的 close 方法，如下所示（ClientStreamListenerImpl 类）：

private void close(Status status, Metadata trailers) {
      closed = true;
      cancelListenersShouldBeRemoved = true;
      try {
        closeObserver(observer, status, trailers);
      } finally {
        removeContextListenerAndCancelDeadlineFuture();
      }
    }

最终调用 UnaryStreamToFuture 的 onClose 方法，set 响应对象，唤醒阻塞的调用方线程，完成 RPC 调用，代码如下（UnaryStreamToFuture 类）：

public void onClose(Status status, Metadata trailers) {
      if (status.isOk()) {
        if (value == null) {
          responseFuture.setException(
              Status.INTERNAL.withDescription("No value received for unary call")
                  .asRuntimeException(trailers));
        }
        responseFuture.set(value);
      } else {
        responseFuture.setException(status.asRuntimeException(trailers));
      }