Passing an array of JSON objects trough sockets in Java

Question

I have a simple client/server application that need to send some JSON objects (that are generated at runtime) through socket to the server.

On the client I have (for test purposes): drawList is a list of Draw objects (with 4 elements).

Socket client = new Socket(serverName, serverPort);
Gson gson = new Gson();
DataOutputStream out = new DataOutputStream(client.getOutputStream());
for (int i=0; i<drawList.size(); i++){
    out.writeUTF(gson.toJson(drawList.get(i)));
    out.flush();
}
out.close();
client.close();

On the server side I have:

Socket server = serverSocket.accept();
BufferedReader in = new BufferedReader(new InputStreamReader(server.getInputStream()));
String inputLine;

while((inputLine = in.readLine()) != null){             
    List<Draw> drawList = DrawUtils.getDrawList(inputLine);             
    for (Draw draw : drawList){
        processDraw(draw);
    }
}

Follows the getDrawList method

public static List<Draw> getDrawList(String input){
    List<Draw> drawList = new ArrayList<Draw>();
    Gson gson = new Gson();
    JsonParser jParser =  new JsonParser();
    JsonArray jArray = jParser.parse(input).getAsJsonArray();

    for (JsonElement obj : jArray){
        drawList.add(gson.fromJson(obj, Draw.class));
    }
    return drawList;
}

The points here are two:

Even if I'm flushing the output channel on client for each "Draw object", at server side I'm getting only one big String with all my objects. Maybe at this point I'm missing some socket concept...
Since I'm getting all my objects in one string, I'm trying then to parse it as a Json array, but I'm getting an exception "com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON". Using debugger I checked the input String and it seems ok except for a strange "A" separating my objects.

Any clues? Thanks!

Kayaman · Accepted Answer · 2013-11-14 13:56:31Z

2

Don't use writeUTF(), or in fact DataOutputStream at all. The writeUTF() writes the length of the String in front of the text values (the 'A' you're noticing), so it's not suitable for sending general textual data (i.e. writeUTF() can only be used with readUTF() and vice-versa).

Use a new BufferedWriter(new OutputStreamWriter(mySocketOutputStream, "UTF-8"))); instead.

answered Nov 14, 2013 at 13:56

Kayaman

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2 Comments

LucasM Over a year ago

Thank a lot @Kayaman! Now it's working ;) Another point is that the BufferedWriter does not have a writeln method therefore I need to manually add a "\n" character at the end position of my JSON object in order to be able to read it on server side using the InputStreamReader.readline(), right?

Kayaman Over a year ago

Exactly. Luckily that's not a huge problem :)

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