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How to run a simple socket server that allows incoming connections?

I am trying to build a very simple socket server in JAVA that my Flash application can listen to. I am using this tutorial. Everything seems to be working - the JAVA code is compiled and the server is running.
My question is: how can external applications send messages to this server using just an IP address and a port number? My goal is that flash can listen to socket messages sent by an external application.
The Java code:
import java.io.*;
import java.net.*;
class SimpleServer {
private static SimpleServer server;
ServerSocket socket;
Socket incoming;
BufferedReader readerIn;
PrintStream printOut;
public static void main(String[] args) {
int port = 8080;
try {
port = Integer.parseInt(args[0]);
} catch (ArrayIndexOutOfBoundsException e) {
// Catch exception and keep going.
}
server = new SimpleServer(port);
}
private SimpleServer(int port) {
System.out.println(">> Starting SimpleServer");
try {
socket = new ServerSocket(port);
incoming = socket.accept();
readerIn = new BufferedReader(
new InputStreamReader(
incoming.getInputStream()));
printOut = new PrintStream(incoming.getOutputStream());
printOut.println("Enter EXIT to exit.\r");
out("Enter EXIT to exit.\r");
boolean done = false;
while (!done) {
String str = readerIn.readLine();
if (str == null) {
done = true;
} else {
out("Echo: " + str + "\r");
if(str.trim().equals("EXIT"))
done = true;
}
incoming.close();
}
} catch (Exception e) {
System.out.println(e);
}
}
private void out(String str) {
printOut.println(str);
System.out.println(str);
}
}

Maybe I don't understand correctly your problem description, but if you create the server in Java, it listens to its port and not your Flash application. If you want your Flash application to wait for messages from other applications, it must have a server role and listen to a TCP port the same way as this Java server does.
You can connect to and test the given Java server easily by telnet program (available in all operating systems) by providing a host name or an IP address and a port as parameters:
telnet 127.0.0.1 8080
Any other application can connect in a similar way, using just a hostname/IP address and a port. For example in Java, you can create a client socket:
Socket clientSocket = new Socket("localhost", 8080);
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

By not specifying an IP address for your socket, it will listen on 0.0.0.0 (all interfaces).
In fact, that will usually be your computer's IP / the server's IP.
Assuming that your application runs on your computer at home, there are three cases that cover most of the connection situations:
Connecting from the same machine:
Use 127.0.0.1:8080
Connecting from the same LAN (e.g. your brother's PC):
Use your LAN IP (e.g. 192.168.1.4:8080)
Connecting from WAN (outside your routers LAN) (internet e.g.):
Use your WAN IP.(e.g. 84.156.74.194). There are plenty websites, that tell you your WAN IP like this
You may have to setup your router, to forward the port 8080 to your PC
For simple connection tests, one could use a telnet client.

I think you are missing the point of client/server socket applications.
If you are building the socket server (with whatever programming language you chose), you will then need to connect with (a) socket client(s) to this server. After a connection is successfully established (persistent) between the client and the server, you can start what ever kind of communication you have implemented between them.
The server always acts as the passive, the client as active part in a socket server/client constellation.

I was checking the link that you are referring to. In that, the procedure to create a stand-alone server is mentioned which is the code that you have pasted as well.
According to the link, the application acts as the client and uses the XMLSocket methods to connect to this server. This application is the flash application that you are talking about. As mentioned in the link, by using the following code any flash application can connect and talk to the server:
var xmlsock:XMLSocket = new XMLSocket();
xmlsock.connect("127.0.0.1", 8080);
xmlsock.send(xmlFormattedData);
When you mention
My goal is that flash can listen to socket messages sent by an external application.
its actually the flash application that is the client and it cannot listen unless programmed to act as a server. I hope this provides some clarity!

Sending an object through a socket in grails using console Web GUI

I have two application servers in two Hosts. Each host has a Local address IP (172.x.x.x)
I want to send an object from 172.x.x.x:8080 to 172.y.y.y:8080 using Java sockets
Server Side(172.x.x.x:8080)
def myObject="Mar7ben bil World"
ServerSocket ss = new ServerSocket(8080);
Socket socket = ss.accept();
ObjectOutputStream out = new ObjectOutputStream(socket.getOutputStream());
out.writeObject(myObject);
out.close();
Client Side(172.y.y.y:8080 )
Socket socket = new Socket('172.x.x.x',8080);
ObjectInputStream inp = new ObjectInputStream(socket.getInputStream());
Object o = inp.readObject();
obj= o;
inp.close();
socket.close();
i get the following error message when i run code of server side on console :
http://172.x.x.x:8080/myApp/console
java.net.BindException: Address already in use
at java.net.PlainSocketImpl.bind(PlainSocketImpl.java:383)
at java.net.ServerSocket.bind(ServerSocket.java:328)
at java.net.ServerSocket.<init>(ServerSocket.java:194)
at java.net.ServerSocket.<init>(ServerSocket.java:106)

Your Grails app listen 8080 port (http://172.x.x.x:8080/myApp/console). You have to change Server Side port to something else, 8081 for example.
P.S. 808x ports are usually used for HTTP services. If you aren't making a HTTP server, it's better to chose another port. See list of well-known ports: http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers#Well-known_ports

Sockets in Java...?

I need to build an application which can receive data from over a network and use this data to do some unrelevant things with.
Here's a piece of code to make clear what I'm doing.
On the server side:
static Socket client = null;
static ServerSocket ss = null;
if (ss != null) {
ss.close();
}
ss = new ServerSocket(5513);
isrunning = true;
System.out.println("Waiting for client...");
client = ss.accept();
System.out.println("Client accepted.");
BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
And the client side:
Socket client = null;
PrintWriter out = null;
try {
client = new Socket("hostname", 5513);
out = new PrintWriter(client.getOutputStream(), true);
}
Please note that this is just a piece of the code. There are no errors in the code.
After running the server-sided piece of code, it correctly waits for the client to connect.
Now here comes the problem. As soon as I try to connect from the client side, I'm getting a "connection refused"-error.
HOWEVER, I found something on the internet whoch told me to try telnetting from the client side. For example, let the server-sided IP be 192.168.1.1. So, after using this command:
telnet 192.168.1.1 5513
I actually get a connection with the server. The command will launch an empty screen, and everything I manually type in the command line will be sent to the server-side after pressing enter (checked with debugging).
So, I can manually connect to the server-side and send some data, but my code refuses to connect.
Anyone who knows what I am doing wrong?

Is this the code you're actually using?
client = new Socket("hostname", 5513);
Try changing it to:
client = new Socket("192.168.1.1", 5513);

client = new Socket("hostname", 5513);
Hostname needs to represent the IP Address you're connecting to. If you're trying to connect to yourself, it would be "localhost"
Also, the server is not listening for the client AT ALL TIMES, there must be a while loop so the server listens and accepts connections.
while (true) {
client = ss.accept();
out = new PrintWriter(client.getOutputStream(), true);
//You should probably assign it to a seperate thread to handle stuff for this client
}
And I should explain on why you're getting that particular error. When something says that the connection is refused, it usually means that the IP Address you want to connect to knows your sending a connection and is blocking it because it was not listening for that connection. Basically, when the server closed, you stopped listening for the client, so anything that came in on that port would be blocked. Of course, the other case could be that Java was blocked on your firewall and an exception should be made for it. Although this is rarely the case if what you're trying to accomplish is over a LAN.

You're not actually using "hostname" in your Socket object in the client are you?
It should the 192.168.1.1.

Are you on Windows? and If so have you added java.exe and javaw.exe to Firewall with inbound and outbound enabled? and have you added a rule for 5513 to your Firewall?
If yes Windows but no Firewall settings, that's your answer, open up your Firewall.

Java Socket Programming

i have a txt file with students name and marks for subjects. i send this file from client to server using
Socket clientSocket = new Socket("127.0.0.1",5432);
OutputStream os = clientSocket.getOutputStream();
os.write(clientWriteArr,0,clientWriteArr.length);
and read this file at server using
ServerSocket sock = new ServerSocket(5432);
Socket serverSocket = sock.accept();
InputStream is = serverSocket.getInputStream();
is.read(serverReadArr,0,serverReadArr.length);
i am modifying the file contents upto this all is working fine.
after this i want to send back this file back to client but i am not getting file at the client and also not getting any exception

You can leave the original socket open from which you read the file, and then write the result to the same socket before closing it. This would be a standard request/response model like what is used for HTTP, and is convenient because the server does not need to know how to connect back to the client. Give us some code for more detailed advice.

You need the the "server" to open a socket connection back to the "client" to send data back. The "client" has to be listening on the port that the "server" wants to connect to.
"Client" and "server" have dual roles in this case.
What exception do you get?

Your server side code should be like:
ServerSocket serverSocket = new ServerSocket(8999);
Socket socket = serverSocket.accept();
DataInputStream in = new DataInputStream(socket.getInputStream());
DataOutputStream out = new DataOutputStream(socket.getOutputStream());
Here, in : you can read the data sent by client.
out: you can write data to client
Your client code should be like:
Socket socket = new Socket("localhost", 8999);
DataInputStream in = new DataInputStream(socket.getInputStream());
DataOutputStream out = new DataOutputStream(socket.getOutputStream());
Here, in you can send data to server.
out, you can read the data sent by server.
Reading data from input stream:
while (true) {
int c = in.read();
}
when you call in.read(), it will block current thread until it reads something.
Writing data to output stream:
out.write(data);

Why it cannot find getInputStream?

I have this code:
ServerSocket serverSideSocket = new ServerSocket(1234);
serverSideSocket.accept();
BufferedReader in = new BufferedReader(new InputStreamReader(serverSideSocket.getInputStream()));
And compiler writes me that it cannot find "getInputStream". I do not understand why. In the beginning of my code I do import java.net.*.

Calling of accept returns instance of Socket which has required method getInputStream.
The code might look like this:
ServerSocket serverSideSocket = new ServerSocket(1234);
Socket socket = serverSideSocket.accept();
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
Great tutorial how to work with sockets in java: http://java.sun.com/docs/books/tutorial/networking/sockets/index.html

This because conceptually a ServerSocket doesn't provide a direct connection object that can be used to send and receive data. A ServerSocket is a tool that you can use with the .accept() method to let it listen on the choosen port and generate a new real connection when a client tries to connect.
That's why you can't get an InputStream from a ServerSocket. Since many clients can connect to the same server, every client will make the server socket generate a new Socket (that is an opened TCP connection) that is returned from .accept() through which you can send and receive using its InputStream and OutputStream.