private int read4Bytes() throws IOException {
    if (this.dataLengthBuffer.remaining() > 0) {
      return this.rpcServer.channelRead(channel, this.dataLengthBuffer);
    } else {
      return 0;
    }
  }
Reads calls from a connection and queues them for handling.
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/SimpleServerRpcConnection.java