Receive an object over TCP/IP

Question

I am going to write a program over TCP/IP and I should send objects by client or by server, It is going right when I want to send or receive strings but when I am trying to read an object:

private Socket client;

public ThreadedClient(Socket client) {
    this.client = client;
}

@Override
public void run() {
        try {
            ObjectInputStream objIn = new ObjectInputStream(client.getInputStream());
            while(true){
                try {
                    Object fromClient = objIn.readObject();

                } catch (ClassNotFoundException e) {e.printStackTrace();}
            }
        } catch (IOException e) {e.printStackTrace();}
    }

I receive an exception:

java.io.StreamCorruptedException: invalid stream header: 306E6165
    at java.io.ObjectInputStream.readStreamHeader(Unknown Source)
    at java.io.ObjectInputStream.<init>(Unknown Source)
    at org.bihe.serverSocket.ThreadedClient.run(Server.java:137)
    at java.lang.Thread.run(Unknown Source)

and it refers to this line:

    ObjectInputStream objIn = new ObjectInputStream(client.getInputStream());

It is my server code:

            ServerSocket ss = new ServerSocket(8800);
            while(true){
                Socket newClient = ss.accept();

                System.out.println(">>>> Client number " + (++counter) + " connected.");
                OutputStream outputStream = newClient.getOutputStream();
                PrintWriter sender = new PrintWriter(outputStream);
                sender.println(true);
                sender.flush();
                ThreadedClient client = new ThreadedClient(newClient);
                clients.add(client);
                new Thread(client).start();

Client side code:

sc = new Socket("127.0.0.1", 8800);
            InputStream inputStream = sc.getInputStream();
            Scanner scanner = new Scanner(inputStream);
            boolean s = scanner.nextBoolean();
            if(s){
                System.out.println("Client connected successfully.");
                return true;
            }else{
                System.out.println("Ohhh, Some problem happened, try again later!");
            }

Can anyone explain me what is happening, what is this exception and why I received this exception?

Answer 1

Just get rid of sending and receiving the Boolean. It's redundant. If there was some problem creating the connection, the socket wouldn't get created: an exception would be thrown instead. You're confusing everything with multiple streams on the same socket. Don't do that.

In your read-object loop, you need to catch EOFException separately, and when you get it, close the socket and exit the loop. If you get any other IOException, log it, close the socket, and exit the loop.

Answer 2

If you want to send object over network you must serialize your objects.

Check this question: How To send an object over TCP in Java

Java Serialization: Serialization

You could do it like this:

import java.net.*;  
import java.io.*;  

class testobject implements Serializable {  
  int value;  
  String id;  

  public  testobject(int v, String s ){  
       this.value=v;  
       this.id=s;  
    }  

}  

public class SimpleServer  {  
  public static void main(String args[]) {  
  int port = 2002;  
  try {  
    ServerSocket ss = new ServerSocket(port);  
    Socket s = ss.accept();  
    InputStream is = s.getInputStream();  
    ObjectInputStream ois = new ObjectInputStream(is);  
    testobject to = (testobject)ois.readObject();  
    if (to!=null){System.out.println(to.id);}  
    System.out.println((String)ois.readObject());  
    is.close();  
    s.close();  
    ss.close();  
}catch(Exception e){System.out.println(e);}  
}  
}  

import java.net.*;  
import java.io.*;  
   public class SimpleClient {  
   public static void main(String args[]){  
     try{  
     Socket s = new Socket("localhost",2002);  
     OutputStream os = s.getOutputStream();  
     ObjectOutputStream oos = new ObjectOutputStream(os);  
     testobject to = new testobject(1,"object from client");  
     oos.writeObject(to);  
     oos.writeObject(new String("another object from the client"));  
     oos.close();  
     os.close();  
     s.close();  
   }catch(Exception e){System.out.println(e);}  
  }  
}  

Answer 3

If you'd like to achieve good performance and send object then you definitely should use Google Protobuf

It allows you to define messages in simple .proto files. Then you use bundled compiler to generate Java classes which will be serialized and sent.

Also better idea is to use Netty over plain Java sockets. This prevent you from writing a lot of boilerplate code and define simple serialization/deserialization pipelines. Take a look at user-guide.