Write in Real Time and Remove last character in file JAVA

i'm converting a txt in json, and i'm using json-simple.
I want the file to be written in real time, that is, each line, for this i chose to dont use JsonArray,
because if I use the JSONArray, I have to wait for it to be complete first and then write it to the file.
So I'm only using JsonObjects.
The problem that I have to create a "hidden" JsonArray, for this at the beginning and end of file I add square brackets , and then to each JsonObject I add a comma.
The problem that obviously the comma is printed even at the end of the file before "]", how do I remove the last comma?

    br = new BufferedReader(new FileReader(pathFile + ".txt"));
    JSONObject stringDetails = new JSONObject();
    JSONArray stringList = new JSONArray();
    try (FileWriter file = new FileWriter(pathfile+".json",true)) {
                    file.write("[");
                    while ((line = br.readLine()) != null) {
                       //Miss the code to convert from txt string to json string ...
                        stringDetails.put(stringJson.getKey(), stringJson.getMessage());
                        file.write(String.valueOf(stringDetails)+",");
                        stringDetails = new JSONObject();
                    }
                    file.write("]");
                }

A other question is, using the append (true), in case if the program stopped in anomaly way, all the strings before are saved?

Thanks so much.

I see two possible ways and both are essential the solution to not print it in the first place.

First: use a boolean and an if-statement in the while loop to print the commas before the entries (except for the first one)

boolean isFirst = true;

file.write("[");
while ((line = br.readLine()) != null) {
    //Miss the code to convert from txt string to json string ...
    stringDetails.put(stringJson.getKey(), stringJson.getMessage());

    if (isFirst) {
        isFirst = false;
    } else {
        // print the comma before the new entry
        file.write(",");
    }

    file.write(String.valueOf(stringDetails));
    stringDetails = new JSONObject();
}
file.write("]");

Second: A second approch would be to have a private helper method to print the entry to the file, like:

private static void printEntry(FileWriter file, String line, ... /* what ever else you need*/) {
    //Miss the code to convert from txt string to json string ...
    stringDetails.put(stringJson.getKey(), stringJson.getMessage());
    file.write(String.valueOf(stringDetails));
    stringDetails = new JSONObject();
}

and use this in to extract the writing for the first entry from the while loop, like:

file.write("[");
if ((line = br.readLine()) != null) {
    // print first entry
    printEntry(file, line, ...);

    // print the rest
    while ((line = br.readLine()) != null) {
        file.write(",");
        
        printEntry(file, line, ...);
    }
}
file.write("]");