Why does the JsonWriter throw an Incomplete document exception in GSON?

Question

I want to send json over a socket stream but I constantly encounter this error, when closing the connection and I have no idea why. I am trying to close a JsonWriter and I get an IOException even though I never used the JsonWritter. Here is the exception in question:

Exception in thread "main" java.lang.RuntimeException: java.io.IOException: Incomplete document
    at Main.close(Main.java:34)
    at Main.socketTest(Main.java:23)
    at Main.main(Main.java:12)
Caused by: java.io.IOException: Incomplete document
    at com.google.gson.stream.JsonWriter.close(JsonWriter.java:610)
    at Main.close(Main.java:31)
    ... 2 more

And this is the code for the server socket:

    public class Main {
    public static void main(String[] args) {
        Main main = new Main();
        main.socketTest();
    }

    Socket socket;
    JsonWriter jsonWriter;

    public void socketTest() {
        try (ServerSocket serverSocket = new ServerSocket(3030)){
            Socket socket = serverSocket.accept();
            this.socket = socket;
            jsonWriter = new JsonWriter(new OutputStreamWriter(socket.getOutputStream(), "UTF-8"));
            jsonWriter.close();
            socket.close();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

The client code is very basic and it justs establishes connection with server:

public class ClientTest {
    public static void main(String[] args) {
        ClientTest clientTest = new ClientTest();
        clientTest.connect();
    }

    public void connect() {
        try {
            Socket socket = new Socket("localhost", 3030);

        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

I hope anyone can help me because I have been struggling with this issue for days.

Answer 1

An "empty" stream or string is not a valid JSON document for the com.google.gson.stream.JsonWriter class:

To encode your data as JSON, create a new JsonWriter. Each JSON document must contain one top-level array or object.

So you must write an array or object, otherwise you get the "Incomplete document" error message as indicated in documentation of close():

Throws:

IOException - if the JSON document is incomplete.