使用文件传输和混合加密技术聊天客户端 - 服务器

Question

I'm trying to realize a chat client-server (in local) that permits to exchange text and files. I utilize java.security and java.crypto to implement hybrid cryptography (and the tool Swing). I exchange text in a serialized way (using ObjectInputStream and ObjectOutputStream ), inserting it (after I crypted it with an apposite function) in byte[] format in Message (that is an object that i created and that is effectively exchanged between sockets):

 import java.io.Serializable;

 public class Message implements Serializable{ 
 private static final long serialVersionUID = 1L;
 byte[] data;

 Message(byte[] data){ 
    this.data = data;
  }

 byte[] getData(){
    return data;
  }
 }

Chat works fine until I exchange only Message . Now I'm trying to implement file transfer (first I tried to implement file transfer from server to client, without cryptography), therefor I took a file "selectedFile" with FileChoser and I sent it to the client thanks ObjectOutputStream 's method writeObject(selectedFile) . On the client side I recognized if the object that is arrived is File or Message with:

class ListenFromServer extends Thread{

   public void run(){
   while(true){

        try{

      if((Message.class.isInstance(sInput.readObject()))==true){ //I verify if the recived object belongs to Message class
        m=(Message) sInput.readObject();//sInput is an instance of ObjectInputStream class, connected to client socket's InputeStream
        decryptMessage(m.getData()); //function that decrypts the content of m and inserts the result in a String that after I append in a text area
         }
          else
          {
              File recivedFile= (File) sInput.readObject();
              File saveFile=new File(path+"/"+ recivedFile.getName());
              save(recivedFile,saveFile);//function that insert the recived file in a specific folder 
             System.out.println("file ricevuto");
          }

        } catch (ClassNotFoundException ex) {
           System.out.println("INFO: Classe non trovata durante la lettura dell'oggetto Messaggio: " + ex.getMessage() + "\n");

        } catch(IOException e){
           System.out.println("Connection closed");
           break;
        }
        catch(NullPointerException ne){
        ne.printStackTrace();
        ne.getCause();
           System.out.println("Errore"+ ne.getMessage());
        }

The problem is that file is recived by the client only clicking twice server's "sendFile" button , moreover now this problem regards also the action of sending text to client, because client receives Message object only when I send it twice (I use two different methods to send a Message object and a File object).

This problem doesn't occur when I eliminate instruction:

if((Message.class.isInstance(sInput.readObject()))==true){
  ...
}

I ask you how to overcome this problem, or if there is a better way to distinguish File and Message objects in reception.

Answer 1

You're actually reading two objects in sequence, not one.

sInput.readObject()

This is an order to read an object. You give this twice in sequence, so that's two requests to reads different objects.

To fix this, just read the object once, test the type of the object, and cast it when appropriate:

Object inputObject = sInput.readObject();     // Read the object once
if (inputObject instanceof Message) {         // If the object is a message
  Message m = (Message) inputObject;          //   cast as a Message
  ...                                         //   use the Message m
} else if (inputObject instanceof File) {     // else if it's a file
  File f = (File) inputObject;                //   cast as a File
  ...                                         //   use the File f
}