How to convert from Json to Protobuf?

With proto3 you can do this using JsonFormat. It parses directly from the JSON representation, so there is no need for separately calling MyMessage.parseFrom(...). Something like this should work:

JsonFormat.parser().merge(json_string, builder);

//You can use this for converting your input json to a Struct / any other Protobuf Class    

import com.google.protobuf.Struct.Builder;
import com.google.protobuf.Struct;
import com.google.protobuf.util.JsonFormat;
import org.json.JSONObject;

JSONObject parameters = new JSONObject();

Builder structBuilder = Struct.newBuilder();
JsonFormat.parser().merge(parameters.toString(), structBuilder);

// Now use the structBuilder to pass below (I used it for Dialog Flow V2 Context Management)

You can convert json string to Proto using builder and json String

Example :

YourProto.Builder protoBuilder = YourProto.newBuilder();
JsonFormat.parser().merge(JsonString, protoBuilder);

If you want to ignore unknown json field then

 YourProto.Builder protoBuilder = YourProto.newBuilder();
 JsonFormat.parser()..ignoringUnknownFields().merge(JsonString, protoBuilder);

 

Another way is, to use mergeFrom method from ProtocolBuffer

Example :

YourProto.Builder protoBuilder = YourProto.newBuilder();
protoBuilder.mergeFrom(JsonString.getBytes());

Once it execute, you will get all the data in protoBuilder from json String

Since someone asked about getting the exception "com.google.protobuf.InvalidProtocolBufferException: JsonObject" when following Adam's advice--I ran into the same issue. Turns out it was due to the google protobuf timestamps. They are being serialized as an object containing two fields "seconds" and "nanos", since this isn't production code, I just got around this by parsing the JSON using jackson, going through the JSON object recursively and changing every timestamp from an object to a string formatted as per RFC 3339, I then serialized it back out and used the protobuf JSON parser as Adam has shown. This fixed the issue. This is some throwaway code I wrote (in my case all timestamp fields contain the word "timestamp", this could be more robust, but I don't care):

public Map<String, Object> fixJsonTimestamps(Map<String, Object> inMap) {
    Map<String, Object> outMap = new HashMap<>();
    for(String key : inMap.keySet()) {
        Object val = inMap.get(key);
        if(val instanceof Map) {
            Map<String, Object> valMap = (Map<String, Object>)val;
            if(key.toLowerCase().contains("timestamp") && 
                    valMap.containsKey("seconds") && valMap.containsKey("nanos")) {
                if(valMap.get("seconds") != null) {
                    ZonedDateTime d = ZonedDateTime.ofInstant(Instant.ofEpochSecond((int)valMap.get("seconds")).plusNanos((int)valMap.get("nanos")),
                            ZoneId.of("UTC"));
                    val = d.format(DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"));
                }
                
            } else {
                val = fixJsonTimestamps(valMap);
            }
        } else if(val instanceof List && ((List) val).size() > 0 && 
                ((List) val).get(0) instanceof Map) {
            List<Map<String, Object>> outputList = new ArrayList<>();
            for(Map item : (List<Map>)val) {
                outputList.add(fixJsonTimestamps(item));
            }
            val = outputList;
        }
        outMap.put(key, val);
    }
    return outMap;
}

Not the most ideal solution but it works for what I am doing, I think I saw someone recommend using a different timestamp class.

Had to convert the following JSON to Protobuf. Used the following online tool to get the correct response Online Json to Protobuf Converter Tool
Input -

{
    "boolean": true,
    "color": "blue",
    "object": {
      "a": "b",
      "c": "d"
    },
    "string": "Hello World - JavaInUse"
}

Output -

fields {
  key: "boolean"
  value {
    bool_value: true
  }
}
fields {
  key: "color"
  value {
    string_value: "blue"
  }
}
fields {
  key: "object"
  value {
    struct_value {
      fields {
        key: "a"
        value {
          string_value: "b"
        }
      }
      fields {
        key: "c"
        value {
          string_value: "d"
        }
      }
    }
  }
}
fields {
  key: "string"
  value {
    string_value: "Hello World - JavaInUse"
  }
}