Question

Currently I am writing data in JSON format, one message at a time like this:

String[] arr = strLine.split(",");

   out.append("data: {\n");
   out.append("data: \"c1\": " + arr[0].toString()+ ",\n");
   out.append("data: \"c2\": " + arr[1].toString()+ ",\n" );
   out.append("data: }\n\n");
   out.flush();

But, now I want to out.flush only when 5 such messages are there. So, how do I do this? create Json messages? with separate data:{ } body. And if I do so how do I access pairs of c1, c2 at client side with javascript? And what are the alternatives.

PS: When I say 5 values, I mean 5 pair of values of c1 and c2.

And how do I access values using javascript at client side, if I do something like this

out.append("data: [\n");
for(int j=0; j<5; j++) {
     String[] arr = strLine.split(",");
     if (j!=0) out.append(",\n");
     out.append("\{"c1\": " + arr[0].toString()+ ",\n");
     out.append(\"c2\": " + arr[1].toString()+"}" );
     out.flush();
  }
//HEY MR, LOOK BELOW ME
//HEY MR, LOOK BELOW ME
//HEY MR, LOOK BELOW ME
//HEY MR, LOOK BELOW ME
out.append("]\n"); <---- HEY RIGHT HERE 
//HEY MR, LOOK ABOVE ME
//HEY MR, LOOK ABOVE ME
//HEY MR, LOOK ABOVE ME
//HEY MR, LOOK ABOVE ME
out.flush();

Working JS code as requested ( calling registerSSE() on body load):

function registerSSE(){
  var source = new EventSource ('http://localhost:8080/SSE_Test1/ReadCsv');
  source.addEventListener('message', function(e) {
  var data = JSON.parse(e.data);
  console.log(data.timestamp, data.c1, data.c2, data.c3, data.c4);
}

And in servlet you just need to append this line:

response.setContentType("text/event-stream");

Answer 1

Please don’t manually generate/parse JSON strings. In Java, use org.json and in Javascript, use JSON.parse and JSON.stringify.

MDN: parse and stringify

For outputting pairs of 5, you’d do something like:

String[] arr = strLine.split(",");
JSONArray array = new JSONArray();
for(int i=0, j=0;i<arr.length;i+=2,j++){
   JSONObject dataObj = new JSONObject();
   dataObj.put("c1", arr[i]);
   dataObj.put("c2", arr[i+1]);
   array.put(j, dataObj);
   if ((j > 0) && (j % 5 ==0)) {
       out.append("data: " +array.toString() + "\n");
       out.flush();
       array = new JSONArray(); // start with a new one
   }
}

To access it on the client side, given your EventSource source:

source.addEventListener('message', function(e) {
    var data = JSON.parse(e.data); // data is an array of {c1:...,c2:...} objects
    for (var i = 0; i < data.length; i++) { // it should be 5 in length
        console.log('c1 is: '+data[i].c1+' and c2 is '+data[i].c2);
    }
}