JSON Formatting in Data Logging File

I am trying to convert a file that contains data logging parameters to be json format.

Currently it retains the format:

{ X }
{ X }

I know this is not a valid JSON format, so I am trying to make it like this:

[
     { X },
     { X }
]

This is the code I have right know that logs data as this function is called:

public void write(String _rec) { 
    writeLock.writeLock().lock();
    timestamp = System.currentTimeMillis();
    String jsonstr = "{ \"timestamp\":" + timestamp + ", \"record\" : " + _rec + " },\n";
    // Log.d(LOG_TAG, jsonstr);
    try {
        fileWriter.write(jsonstr);
        fileWriter.flush();
    } catch(IOException e) {
        e.printStackTrace();
    }
    writeLock.writeLock().unlock();
} // END write

I added the comma in after the closing }, but I am not sure what to do about the []. Is there a way when I make the file I can insert the [] and then start writing information between them? Is there another solution to automatically insert these brackets?