Android app to server Socket program not reading String from app

Question

I have an android app which will send a string to a server using the following code:

package com.example.testapp;


import java.io.BufferedWriter;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.Socket;
import java.net.UnknownHostException;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class MainActivity extends Activity {

    private Socket socket;

    private static final int SERVERPORT = 5000;
    private static final String SERVER_IP = "192.168.1.125";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);     

        new Thread(new ClientThread()).start();
    }

    public void onClick(View view) {
        try {
            EditText et = (EditText) findViewById(R.id.EditText01);
            String str = et.getText().toString();
            PrintWriter out = new PrintWriter(new BufferedWriter(
                    new OutputStreamWriter(socket.getOutputStream())),
                    true);
            out.println(str);
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    class ClientThread implements Runnable {

        @Override
        public void run() {

            try {
                InetAddress serverAddr = InetAddress.getByName(SERVER_IP);

                socket = new Socket(serverAddr, SERVERPORT);

            } catch (UnknownHostException e1) {
                e1.printStackTrace();
            } catch (IOException e1) {
                e1.printStackTrace();
            }

        }

    }
}

In server side I am running a java program like the follows:

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.ClassNotFoundException;
import java.net.ServerSocket;
import java.net.Socket;

public class SocketServerExample {

    //static ServerSocket variable
    private static ServerSocket server;
    //socket server port on which it will listen
    private static int port = 5000;

    public static void main(String args[]) throws IOException, ClassNotFoundException{
        //create the socket server object
        server = new ServerSocket(port);
        //keep listens indefinitely until receives 'exit' call or program terminates
        while(true){
            System.out.println("Waiting for client request");
            //creating socket and waiting for client connection
            Socket socket = server.accept();
            //read from socket to ObjectInputStream object
            ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
            //convert ObjectInputStream object to String
            String message = (String) ois.readObject();
            System.out.println("Message Received: " + message);
            //create ObjectOutputStream object
            ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
            //write object to Socket
            oos.writeObject("Hi Client "+message);
            //close resources
            ois.close();
            oos.close();
            socket.close();
            //terminate the server if client sends exit request
            if(message.equalsIgnoreCase("exit")) break;
        }
        System.out.println("Shutting down Socket server!!");
        //close the ServerSocket object
        server.close();
    }

}

But it is not reading the String which I send from the android app. Instead when I submit from app, the java program shows the following errors:

Exception in thread "main" java.io.StreamCorruptedException: invalid stream header: 54657374
    at java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:803)
    at java.io.ObjectInputStream.<init>(ObjectInputStream.java:298)
    at SocketServerExample.main(SocketServerExample.java:29)

How can I fix this. Where is my error lays?? Is it on Server side or client side?? Please help me guys.

Answer 1

PROBLEM ANALYSIS:

You use a PrintWriter for sending from the client, but you use an ObjectInputStream on the receiving side of the server. These two are not compatible.

You have to use a pair of Writer and Reader together or a pair of ObjectOutputStream and ObjectInputStream, but you can't mix them.

SOLUTION:

We're going to use pairs of Writer and Reader. The client side already uses a Writer and a Reader, so we only need to change the server side.

On the server side, instead of the ObjectInputStream, use a BufferedReader:

BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
reader.readLine();

Moreover, use a PrintWriter instead of the ObjectOutputStream on the server side for sending back to the client:

PrintWriter writer = new PrintWriter(new BufferedWriter(
                new OutputStreamWriter(socket.getOutputStream())),
                true);
writer.println(str);

Alltogether, then the server side looks like this:

import java.io.IOException;
import java.io.BufferedWriter;
import java.io.BufferedReader;
import java.io.OutputStreamWriter;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.lang.ClassNotFoundException;
import java.net.ServerSocket;
import java.net.Socket;

public class SocketServerExample {

    //static ServerSocket variable
    private static ServerSocket server;
    //socket server port on which it will listen
    private static int port = 5000;

    public static void main(String args[]) throws IOException, ClassNotFoundException{
        //create the socket server object
        server = new ServerSocket(port);
        //keep listens indefinitely until receives 'exit' call or program terminates
        while(true){
            System.out.println("Waiting for client request");
            //creating socket and waiting for client connection
            Socket socket = server.accept();

            // //read from socket to ObjectInputStream object
            // ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
            // //convert ObjectInputStream object to String
            // String message = (String) ois.readObject();
            // System.out.println("Message Received: " + message);

            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            String message = reader.readLine();
            System.out.println("Message Received: " + message);

            // //create ObjectOutputStream object
            // ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
            // //write object to Socket
            // oos.writeObject("Hi Client "+message);

            PrintWriter writer = new PrintWriter(new BufferedWriter(
                new OutputStreamWriter(socket.getOutputStream())),
                true);
            writer.println("Hi Client "+message);

            //close resources
            reader.close();
            writer.close();
            socket.close();
            //terminate the server if client sends exit request
            if(message.equalsIgnoreCase("exit")) break;
        }
        System.out.println("Shutting down Socket server!!");
        //close the ServerSocket object
        server.close();
    }

}