How do I read a Json from a socket? Java

Question

I am doing a Server/Client network. The messages are send as Json Objects. I have a method for writing the Json-Object but my method for reading the Json-Object doesnt work. I dont have any exceptions but there is no output like excpected. Something must be wrong with the bufferedReader. I dont know how to get that Json-Object from the socket who sent it. Method for writing:

public void writeMessage(JSONObject json) {
        try {
            PrintWriter printWriter = new PrintWriter(
                    new OutputStreamWriter(
                            socket.getOutputStream())); 
            printWriter.print(json);
            printWriter.flush();

        } catch (IOException writeMessageException) {
            System.out.println("!");

        }
    }

method for reading the message/ receiving the message:

private static void readMessageFromServer(Socket socket) {
        try (BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()))) 
    {
         StringBuilder sb = new StringBuilder();
            String readLine;
            while ((readLine = br.readLine()) != null) {
                sb.append(readLine);
            }
            JSONObject js = new JSONObject(sb.toString());
        String action1 = (String) js.get("action1");
    System.out.println(action1);
    } catch(IOException e) {
        System.out.println("!");
    }
    }

Thank you :)

What is `JSONObject`? From which package and/or library it comes? Try to take a look at result of `json.toString()` since that is what will be sent by `printWriter.print(json);` — Pshemo, Jul 07 '22 at 20:07
BTW `catch(IOException e) { System.out.println("!"); }` is bad idea, unless you really don't want to know *what* is wrong with your application. Usually we use `e.printStackTrace();` instead to help us debug the issue (more info: [What is a stack trace, and how can I use it to debug my application errors?](https://stackoverflow.com/q/3988788)) — Pshemo, Jul 07 '22 at 20:14
Your receiving code is expecting the JSON to be line-oriented, but JSON is not required to have line breaks in it. The receiving code is also reading until the connection is disconnected, but the sender code is not closing the socket after sending the JSON. The best way to handle this situation is to have the sender send the JSON's length before sending the actual JSON, and then have the receiver stop reading when it has read the specified amount. — Remy Lebeau, Jul 08 '22 at 00:08

ARCHER · Answer 1 · 2022-07-08T07:14:47.357

When reading data in bytes, we need to define our own protocol for communication between server and client. The simplest protocol which we can define is called TLV (Type Length Value). It means that every *message written to the socket is in the form *of the Type Length Value.

So we define every message sent as:

A 1 byte character that represents the data type, like s for String A 4 byte integer that indicates the length to the data And then the actual data, whose length was just indicated

DataInputStream in = new DataInputStream(new BufferedInputStream(socket.getInputStream()));
char dataType = in.readChar();
int length = in.readInt();
    if(dataType == 's') {
        byte[] messageByte = new byte[length];
        boolean end = false;
        StringBuilder dataString = new StringBuilder(length);
        int totalBytesRead = 0;
        while(!end) {
            int currentBytesRead = in.read(messageByte);
            totalBytesRead = currentBytesRead + totalBytesRead;
            if(totalBytesRead <= length) {
                dataString
                  .append(new String(messageByte, 0, currentBytesRead, StandardCharsets.UTF_8));
            } else {
                dataString
                  .append(new String(messageByte, 0, length - totalBytesRead + currentBytesRead, 
                  StandardCharsets.UTF_8));
            }
            if(dataString.length()>=length) {
                end = true;
            }
        }
    }

More detail info can be found here https://www.baeldung.com/java-inputstream-server-socket

Are you quoting someone else's answer? If so, you should give credit where it is due — Remy Lebeau, Jul 08 '22 at 00:09

elkapita · Answer 2 · 2022-07-07T22:01:02.937

Consider using javax websocket.

javax.websocket client simple example

You can simply implement MessageHandler and specify the type parameter you expecting to receive using POJOs.

    @Override
public void onOpen(Session session, EndpointConfig config) {
    session.addMessageHandler(new MessageHandler.Whole<MyJsonObject>() {

        @Override
        public void onMessage(MyJsonObject message) {
        //do something
        }
    });
}

That way you are creating a listener (@OnMessage) and handling each message as soon as it received. You can also implement MessageHandler with String type parameter and handle parsing the json yourself inside OnMessage method;

@Override
public void onOpen(Session session, EndpointConfig config) {
    session.addMessageHandler(new MessageHandler.Whole<String>() {

        @Override
        public void onMessage(String message) {
            MyJsonObject jObj = new ObjectMapper().readValue(message, MyJsonObject.class);
        }
    });
}

If you want to stick with java net socket consider switching to Jackson Object Mapper from JSONObject, that way you can convert input stream into any object:

     private static void readMessageFromServer(Socket socket){
        InputStream in = socket.getInputStream();
        MyObject obj = new ObjectMapper().readValue(in, MyObject.class); 
     }

more javax webSocket examples

How do I read a Json from a socket? Java

2 Answers2