Hello @peng.xiaoliang

RFA Java package provides the GenericOMMParser class which is used to read and initialize dictionaries and parse any OMM messages. Hence, you may modify GenericOMMParser class to get your preference fields from the response and keep them in the map. Then, you can create them as your preference format i.e. JSON. The example source code is here:

 //Add the method to GenericOMMParser.java to get CURRENCY, BID and ASK field
    public static HashMap<String,String> parseOMMData(OMMData data)
    {
    	HashMap<String,String> preferredFields = new HashMap<String,String>();
    	String valueStr;
    	if(CURRENT_DICTIONARY==null) 
	//make sure that RDMdictionary is loaded already
    		CURRENT_DICTIONARY = getDictionary(1);
        for (Iterator iter = ((OMMIterable)data).iterator(); iter.hasNext();)
        {
            OMMEntry entry = (OMMEntry)iter.next();
            if(entry.getType()==OMMTypes.FIELD_ENTRY) {
            	OMMFieldEntry fe = (OMMFieldEntry)entry;
            	FidDef fiddef = CURRENT_DICTIONARY.getFidDef(fe.getFieldId());
            	OMMData aValue = null;
            	if(fe.getFieldId() == 22 || fe.getFieldId()==25) { 
		//BID or ASK field
            		aValue=  fe.getData(fiddef.getOMMType());
            		if(!aValue.isBlank()) 
			//not blank value, get value as a String
            			valueStr = aValue.toString();
            		else
            			valueStr = ""; //otherwise, set blank value
            		preferredFields.put(fiddef.getName(), valueStr);
            	}
            	else if(fe.getFieldId() == 15)//CURRENCY which is enum type
                {
            		aValue=fe.getData(fiddef.getOMMType());
            		if(!aValue.isBlank()) 
			//not blank value, get String of the enum value 
            			valueStr=CURRENT_DICTIONARY.expandedValueFor((short)15, ((OMMEnum)aValue).getValue());  
            		else
            			valueStr = ""; //otherwise, set blank value
            		preferredFields.put(fiddef.getName(), valueStr);
                }	
            }
        }
        return preferredFields;
    }
...
//in your application at the method to process data(item events)
public void processEvent(Event event)
{
	...
	OMMItemEvent ie = (OMMItemEvent)event;
        OMMMsg respMsg = ie.getMsg();
        GenericOMMParser.parse(respMsg);
      //If it is Refresh or Update data message, get data
        if((respMsg.getMsgModelType() == RDMMsgTypes.MARKET_PRICE) && 
        		( respMsg.getMsgType() == OMMMsg.MsgType.REFRESH_RESP || respMsg.getMsgType() == OMMMsg.MsgType.UPDATE_RESP))
        {
        	//Get CURRENCY, BID and ASK field
        	HashMap<String,String> preferredFields= GenericOMMParser.parseOMMData(respMsg.getPayload());
        	//Create JSON format
        	//{"CURRENCY": "JPY","BID":"108.87","ASK":"108.88"}  
        	StringBuffer jsonDataFormatStrBuf = new StringBuffer("{");
        	if(preferredFields.containsKey("CURRENCY"))
        		jsonDataFormatStrBuf.append("\"CURRENCY\":\"" + preferredFields.get("CURRENCY")+"\",");
        	if(preferredFields.containsKey("BID")) 
        		jsonDataFormatStrBuf.append("\"BID\":\""+ preferredFields.get("BID"));
        	if(preferredFields.containsKey("ASK")) 	
        		jsonDataFormatStrBuf.append("\",\"ASK\":\"" + preferredFields.get("ASK") + "\"}");
        	if (jsonDataFormatStrBuf.toString().length() > 1) //contain at least 1 field
        		System.out.println("Json Data format is " + jsonDataFormatStrBuf.toString());	 
        } 

The example Refresh message output:

Msg Type: MsgType.REFRESH_RESP
	…
	Payload: 2060 bytes
		FIELD_LIST
			FIELD_ENTRY 1/PROD_PERM: 213
			FIELD_ENTRY 2/RDNDISPLAY: 153
			FIELD_ENTRY 3/DSPLY_NAME: BARCLAYS     LON
			FIELD_ENTRY 5/TIMACT: 09:34
			FIELD_ENTRY 11/NETCHNG_1: 0.20
			FIELD_ENTRY 12/HIGH_1: 109.26
			FIELD_ENTRY 13/LOW_1: 108.80
			FIELD_ENTRY 15/CURRENCY: JPY (392)
			FIELD_ENTRY 17/ACTIV_DATE: 25 APR 2018
			FIELD_ENTRY 19/OPEN_PRC: 108.80
			FIELD_ENTRY 21/HST_CLOSE: 108.81
			FIELD_ENTRY 22/BID: 109.01
			FIELD_ENTRY 23/BID_1: 109.02
			FIELD_ENTRY 24/BID_2: 109.02
			FIELD_ENTRY 25/ASK: 109.04
			FIELD_ENTRY 26/ASK_1: 109.04
			FIELD_ENTRY 27/ASK_2: 109.04
…
Json Data format is {"CURRENCY":"JPY","BID":"109.01","ASK":"109.04"}

The example Update message output:

	Msg Type: MsgType.UPDATE_RESP
	…
	Payload: 212 bytes
		FIELD_LIST
			FIELD_ENTRY 22/BID: 109.02
			FIELD_ENTRY 393/PRIMACT_1: 109.02
			FIELD_ENTRY 25/ASK: 109.05
			FIELD_ENTRY 275/SEC_ACT_1: 109.05
			…
Json Data format is {"BID":"109.02","ASK":"109.05"}

Note: an Update message contains only change fields so CURRENCY may not in the Update message.

If you would like to get only your preference fields(CURRENCY, BID and ASK) from the server which is ADS(one component of TREP), you can use Dynamic View feature which allows ADS sends only fields specified in the request message to your application. For more details and example application, please refer to How to request particular fields of an item (RIC) via RFA API

I think the latest TREP software supports JSON format, so you get the market data updates already in JSON. But I have not still tested it.

In that case you do not use RFA but another mechanism to connect to TREP, send requests and get JSON data.