Develop Reference

Java socket port number changes on its own

Client
try {
Socket sock = new Socket("localhost", Integer.parseInt(args[0]));
System.out.println(sock.getLocalAddress());
System.out.println(sock.getLocalPort());
ObjectInputStream ois = new ObjectInputStream(sock.getInputStream());
ObjectOutputStream oos = new ObjectOutputStream(sock.getOutputStream());
oos.writeInt(55);
ois.readInt();
} catch (Exception e) {e.printStackTrace();}
Server
try {
ServerSocket ss = new ServerSocket(1234);
Socket sock = ss.accept();
System.out.println(sock.getLocalAddress());
System.out.println(sock.getLocalPort());
ObjectInputStream ois = new ObjectInputStream(sock.getInputStream());
ObjectOutputStream oos = new ObjectOutputStream(sock.getOutputStream());
System.out.println(ois.readInt());
oos.close();
} catch (Exception e) {e.printStackTrace();}
I run the server and then the client, in that order.
I pass 1234 as a command-line argument to the client. In other words, I execute threads this way
java server
java client 1234
Doing that prints these to the console
// server
/127.0.0.1
1224
// client
/127.0.0.1
50261
Neither threads move on from this point.
What's going on here?

Java socket port number changes on its own
No it doesn't. You're looking at two different sockets and two different ports. A connection consists of two endpoints. The localPort of the client socket is 50261, which is the outbound port, and the localPort of the accepted socket at the server is 1234, which is the same as the port being listened at.
Have a look at the respective getRemotePort() values. You will see that the client's is 1234 and the server's is 50261, or rather that it agrees with the client's localPort, whatever it is next time you run it.
Neither threads move on from this point. What's going on here?
You need to construct the ObjectOutputStream before the ObjectInputStream, for reasons explained in the Javadoc and in many answers here, such as this one.

Java Socket Client Unable to Connect to C# Socket Server

I have a use case, where I have to send data from a .NET application, to a Java application and I'm trying to do this using sockets. I have a server created using C# -
TcpListener tcpListener = null;
IPAddress ipAddress = Dns.GetHostEntry("localhost").AddressList[0];
try
{
// Set the listener on the local IP address
// and specify the port.
tcpListener = new TcpListener(ipAddress, 13);
tcpListener.Start();
Console.WriteLine("The server is running at port 13...");
Console.WriteLine("The local End point is -" +
tcpListener.LocalEndpoint);
output = "Waiting for a connection...";
Console.WriteLine(output);
Socket s = tcpListener.AcceptSocket();
Console.WriteLine("Connection accepted from " + s.RemoteEndPoint);
}
catch (Exception e)
{
output = "Error: " + e.ToString();
Console.WriteLine(output);
}
}
On the Java side, I have created a socket which listens to the same host and port -
Socket socket;
try {
socket = new Socket( "localhost", 13);
System.out.println("Connection established");
BufferedReader input =
new BufferedReader(new InputStreamReader(socket.getInputStream()));
String answer = input.readLine();
System.out.println(answer);
socket.close();
} catch (Exception e) {
e.printStackTrace();
}
I'm getting the below error - java.net.ConnectException: Connection refused: connect. I have used telnet localhost 13, to check if my server is really running, and it is. So i don't think it could be an issue with server not running or firewalls, since both are running locally. Any ideas on how to resolve this?

I tried your code and I had exactly the same problem. Your C# TCP Server only binds to the IPv6 interface (check e.g. the resource monitor listening addresses). If you change your server to new TcpListener(IPAddress.Any, 13) it should work.

How to keep Socket Connection until Server Process it?

I am writing Socket program , Here Client Sends a String through Stream , Server Process it and writes back to Client. My problem is, after Server process the String , it Writes back to Stream but in client It can't able to read the Stream its showing exception as Exception in while: java.net.SocketException: socket closed Here is my code,
Client ,
public void run() {
while (true) {
try {
// Open your connection to a server, at port 1231
s1 = new Socket("localhost", 1231);
OutputStream s1out = s1.getOutputStream();
DataOutputStream dos = new DataOutputStream(s1out);
InputStream in=s1.getInputStream();
DataInputStream dis=new DataInputStream(in);
String s = br.readLine();
dos.writeUTF(s);
dos.flush();
dos.close();
System.out.println(dis.readUTF());//it is the String from Server after processing
dis.close();
} catch (IOException ex) {
// Logger.getLogger(SimpleClient.class.getName()).log(Level.SEVERE, null, ex);
System.out.println("Exception in while: " + ex);
}
}
In Server
public void run()
{
while(true){
try {
System.out.println("Waiting for connect to client");
s1=serverSocket.accept();
s1In = s1.getInputStream();
dis = new DataInputStream(s1In);
out=s1.getOutputStream();
dos=new DataOutputStream(out);
String clientData=dis.readUTF();
//processing task String
dos.writeUTF("Bus Registered Successfully");
dos.flush();
}
}
Here I am not able to read Bus Registered Successfully at client side . How to Solve this.?

Well there are many things not right in your program. But first let me answer your question ... you are closing the socket just after writing the stream ... so server throws exception, just remove dos.close(); just after the dos.flush();. It will run fine.
Now back to the programming practices ...
1) Server should accept the connection in a while(true) loop and then make a new thread. So following statement should not be the part of run method.
System.out.println("Waiting for connect to client");
s1=serverSocket.accept();
s1In = s1.getInputStream();
dis = new DataInputStream(s1In);
out=s1.getOutputStream();
dos=new DataOutputStream(out);
2) There is no need of run method in client. Because Every new client will be a new program that has its own variables and socket.

A quick look shows me that the reason the socket is closed is because you used dos.close().
Closing a DataInputStream (or PrintStream, or any similiar stream) will close the underlying socket.
Just take out dos.close().
You can also move the dos.close() to the very end of the try block. As a general rule, don't close anything related to the socket until you're done with the socket.

EOF Exception in a thread using sockets

I have the code below:
while (true)
{
lengthInput = instr.readByte();
// Other code
}
The thing is that I'm using a client to send information to the socket, but after it finishes I got EOF Exception and it brokes the thread, what I need is to manages this and dont stop the thread, because I need to send more information and be able to read it.
Thanks in advance for your help.

I guess the problem is related to your socket initialization. You probably need to check if your client socket indeed successfully create a socket and bind to a specified port. You may also check your client really send data to the outstream and flush to the server side. I have a small project on Android emulators with socket communication. Both my client and serve extends from Java Thread class. Maybe you can gain some idea seeing my code below.
The client side
try {
socket = new Socket(InetAddress.getByAddress(new byte[]{10, 0, 2, 2}),
Integer.parseInt(remote_port));//note we must keep the addr#10.0.2.2
// write out
out = new ObjectOutputStream(socket.getOutputStream());
out.writeObject(out_msg);
out.flush();
// read in
in = new ObjectInputStream(socket.getInputStream());
socket.setSoTimeout(1000);
in_msg.set_message((Message)in.readObject());
// close all
out.close();
in.close();
socket.close();
return true;
}catch(InterruptedIOException E){}
The server side
while (true) {
try {
// read in message
Socket ClientSocket = serverSocket.accept();
Message out_msg = new Message();
// read in message
ObjectInputStream in = new ObjectInputStream(ClientSocket.getInputStream());
Message in_msg = (Message) in.readObject();
//Log.d(TAG, "recv" + " content:" + in_msg.msg2Str());
message_process(in_msg, out_msg);
// write out message
ObjectOutputStream out = new ObjectOutputStream(ClientSocket.getOutputStream());
out.writeObject(out_msg);
out.flush();
} catch(Exception E){}
}

server connections

I'm doing my own server in Java. I need to do one socket connection and receive a lot of object from client. I did it, it works, but I don`t know if this is the best(fastest) solution. Here is example of my code:
try {
serverSocket=new ServerSocket(18234, 1000);
} catch (IOException e) {
System.out.print("Server failed..");
e.printStackTrace();
}
Object x;
ObjectInputStream ois;
System.out.println("Waiting for connection...");
Socket connection= serverSocket.accept();
System.out.println("Connection received from " + connection.getInetAddress().getHostName());
while(true){
ois = new ObjectInputStream(connection.getInputStream());
x=ois.readObject();
System.out.println(x.getString());
if(x.getString().equals("END")) break;
}
Problem is, that when I try receive new object, all time I must do new ObjectInputStream.. Is this solution correct? I must do really fast server and all time doing new ObjectInputStream is too expensive in my opinion.

Use the same ObjectOutputStream and ObjectInputStream for the life of the socket, at both ends. Your statement about being obliged to use a new one each object is incorrect.