ResponseFormatter might cause serious performance issue!

[jerome89]

Currently only JSON response is supported, but it can cause serious performance issue.
The following is code block from ResponseFormatter.java.

        for(TimeSeries timeSeries : timeSeriesList) {
            List<String> datapoints = new ArrayList<>();
            for(int i = 0; i < timeSeries.getValues().length; i++) {
                String stringValue;
                if (timeSeries.getValues()[i] == null) {
                    stringValue = "null";
                } else {
                    stringValue = GraphiteUtils.formatDoubleSpecialPlain(timeSeries.getValues()[i]);
                }

                datapoints.add("[" + stringValue + ", " + (timeSeries.getFrom() + timeSeries.getStep() * i) + "]");
            }
            results.add("{\"target\": " + gson.toJson(timeSeries.getName()) + ", \"datapoints\": [" + Joiner.on(", ").join(datapoints) + "]}");
        }
        String responseString = "[" + Joiner.on(", ").join(results) + "]";

Joiner.on(", ").join(datapoints) can be bottleneck if ArrayList 'datapoints' is large because Joiner uses StringBuilder to append each data in String, but the capacity of StringBuilder is not pre-determined, so it continuously copies whole array whenever it exceeds its capacity.

Here is the reference:
https://docs.oracle.com/javase/tutorial/java/data/buffers.html

It is recommend not to use Joiner in this case because it handles arbitrary length of ArrayLists. How about change the code to use StringBuilder directly with proper initialCapacity?