Socket Should Only Read One Message

if i send messages as in the code below, is there a way to read only one message, if the client doesn't know how long it is (or if he only knows the maximum length)?in the documentation it says, that the method read(byte[]) reads until no data ist available or end of file is decteted. What does the latter mean? Can i extend the array that i sent by a byte that signals the end? Here is my Code:

Server

public class Server implements Runnable{
 ServerSocket server;
HashMap<Short,Socket> clients;
 public Server(){
clients = new HashMap<>();

[code]....

Both are startet in seperate threads. At the moment the first output of the client is "12 bytes read", and then continously "-1 bytes read"(because the server closes).