protobuf Java example with 4-bytes UTF-8 characters.

A.java

import xiaofeng.A.Foo;

public class A {
  public static void main(String[] argv) throws Exception {
    Foo foo = Foo.newBuilder()
        .setValue("\uD852\uDF62\uD83D\uDE00")
        .build();
    byte[] data = foo.toByteArray();
    for (byte b : data) {
      System.out.println(String.format("\\x%02X", b));
    }
  }
}

a.proto

syntax = "proto2";

package xiaofeng;

message Foo {
  optional string value = 1;
}