Develop Reference

How to return files from a Server through OutputStream?

I have the following server called Server.class and what exactly does, it receives text from the client and then sends that exact text back to the client. I'm just learning and I used the server from this website http://www.javaworld.com/article/2077322/core-java/core-java-sockets-programming-in-java-a-tutorial.html.
The Client in my case is the browser so i don't need to implement it.
What I want to get is when a client asks for a file like: localhost:8888/myfile.txt
I should give him back that file, so he can download it or see it from the browser.
I tried to use a BufferedReader
BufferedReader br = new BufferedReader(new InputStreamReader(is));
is is a variable from InputStream
So now i can know what the client introduced doing String clientData = br.readLine()
But how can I get the file (myfile.txt) from that string and return that file so it can be observed or downloaded in the browser??
The Server:
import java.io.*;
import java.net.*;
public class Server {
public static void main(String args[]) {
// declaration section:
// declare a server socket and a client socket for the server
// declare an input and an output stream
ServerSocket echoServer = null;
String line;
DataInputStream is;
PrintStream os;
Socket clientSocket = null;
// Try to open a server socket on port 8888
// Note that we can't choose a port less than 1023 if we are not
// privileged users (root)
try {
echoServer = new ServerSocket(8888);
}
catch (IOException e) {
System.out.println(e);
}
// Create a socket object from the ServerSocket to listen and accept
// connections.
// Open input and output streams
try {
clientSocket = echoServer.accept();
is = new DataInputStream(clientSocket.getInputStream());
os = new PrintStream(clientSocket.getOutputStream());
// As long as we receive data, echo that data back to the client.
while (true) {
line = is.readLine();
os.println(line);
}
}
catch (IOException e) {
System.out.println(e);
}
}
}

TCP IP Client receives no input but packet sent from server

I have a java client program that sends a command to server and server sends back an acknowledgement and a response string.
My client program gets the input stream length from client socket as
0. But, I used wireshark to investigate. Wireshark logs show that the server has sent back a response to my ip. Somehow, my client is unable
to read it.
My Client
public class Client {
private static final String SERVER_ADDRESS = "192.168.64.79";
private static final int TCP_SERVER_PORT = 6669;
public void connect(String command) {
BufferedReader in;
try {
// Socket skt = new Socket("50.128.128.254", 6669);//ip,port
Socket clientSocket = new Socket(SERVER_ADDRESS, TCP_SERVER_PORT);// ip,port
System.out.println(" client Socket created ..Enter command : ");
PrintWriter outToServer = new PrintWriter(clientSocket.getOutputStream(), true);
String ToServer = command;
outToServer.println(ToServer);
in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
System.out.print("Received string : length "+clientSocket.getInputStream().available()+"\n response:");
// while (!in.ready()) {
// }
System.out.println(in.readLine()); // Read one line and output it
// System.out.print("'\n");
in.close();
clientSocket.close();
System.out.print("connection closed");
} catch (Exception e) {
System.out.print("Whoops! It didn't work!\n");
e.printStackTrace();
}
}
}
WireShark results:
9bytes of data sent from server to my client
Client output
Java client receives inputstream length 0

Writing a telnet client in java - how to listen for new data?

I'm attempting to implement a Telnet client in Java, which will just get data from a telnet server.
I've been looking for ages now and have found a lot of things, but not particularly what I need - for example the apache commons client example, which seems to send a lot of commands, which is just confusing me to be honest.
So I therefore thought it would be easier to just to write my own client which connects to the server using a socket.
public class TelnetClient {
private String host;
public TelnetClient(String host) {
this.host = host;
}
public void getData(){
Socket s = new Socket();
PrintWriter s_out = null;
BufferedReader s_in = null;
try {
s.connect(new InetSocketAddress("www.google.com" , 80));
System.out.println("Connected");
//writer for socket
s_out = new PrintWriter( s.getOutputStream(), true);
//reader for socket
s_in = new BufferedReader(new InputStreamReader(s.getInputStream()));
}
//Host not found
catch (UnknownHostException e) {
System.err.println("Don't know about host : " + host);
System.exit(1);
} catch (IOException e) {
e.printStackTrace();
}
//Send message to server
String message = "GET / HTTP/1.1\r\n\r\n";
s_out.println( message );
System.out.println("Message send");
//Get response from server
String response;
try {
while ((response = s_in.readLine()) != null) {
System.out.println( response );
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
But I cant actually test this with the server currently so Im just using google.com, however I want to change it to listen continuously for new lines of data on the server.
Basically my question is, am I going about this the wrong way - am I being naive by just using sockets to access the telnet server and am I underestimating what a telnet client/server should be, thanks for any help.
Also if anyone has any good/simple examples of a telnet client, that would be very useful!!

I've been holding off on answering because there are a couple different things going on here -- Google doesn't have a telnet server running on port 80, it's a web (HTTP) server. You're connecting to the webserver with your telnet client and trying to talk over HTTP with plain text. HTTP and telnet are two different protocols.
So there is a mismatch between what you want to do and what these Java telnet clients are designed to do -- namely connect to a remote shell.
Based on what you've been saying, I think you can get it done much more easily by just making an HTTP request. There's a dead simple solution in this answer:
import java.net.*;
import java.io.*;
public class URLConnectionReader {
public static void main(String[] args) throws Exception {
URL yahoo = new URL("http://www.yahoo.com/");
URLConnection yc = yahoo.openConnection();
BufferedReader in = new BufferedReader(
new InputStreamReader(
yc.getInputStream()));
String inputLine;
while ((inputLine = in.readLine()) != null)
System.out.println(inputLine);
in.close();
}
}
Sockets are complicated stuff. Take a look at this tutorial if you want to dig deeper in to them. But they're going to be a huge pain to work with and probably not help you get what you need done.

Using the same client socket to send multiple messages to server socket

There's a million examples on using Java sockets out there - and every one is the same!
Every one shows a client socket being created, some text being sent, and the socket closed.
I am writing some test code. I want my client to loop round and send quite a few messages. It seems silly to close the client socket each time and re-create, so I thought I would just create one client socket, loop round and send data on the same socket. The thing is though - my server socket does not print out what it has received until the last message has been sent by the client and the client socket closed.
Server:
Socket sock;
ClientConnection client;
ServerSocket ss = new ServerSocket(portNumber);
ss.setSoTimeout(0); // 0=infinite
while (true) {
sock = ss.accept();
client = new ClientConnection(sock);
new Thread(client).start();
// ClientConnection reads from sock, prints, and closes sock
}
ClientConnection (a separate class on the Server side):
public class ClientConnection implements Runnable
{
private Socket m_socket;
private BufferedReader m_in = null;
public ClientConnection(Socket socket)
{
m_socket = socket;
try {
InputStream inStream = socket.getInputStream();
m_in = new BufferedReader(new InputStreamReader(inStream));
}
catch (IOException ioe) {
ioe.printStackTrace();
}
}
public String getMessage()
{
String line = null;
StringBuffer completeMessage = new StringBuffer();
try {
while ((line = m_in.readLine()) != null)
{
completeMessage.append(line);
}
}
catch (IOException ioe) {
ioe.printStackTrace();
return "";
}
return completeMessage.toString();
}
public void run()
{
try {
String message = getMessage();
System.out.println("Received: " +message);
}
finally
{
try {
m_socket.close();
}
catch (IOException ioe) {
ioe.printStackTrace();
}
}
}
}
Client:
socket = new java.net.Socket(m_destination, m_portNumber);
outputStream = socket.getOutputStream();
printStream = new java.io.PrintStream(outputStream);
while (more-stuff-to-send)
{
printStream.print(text);
printStream.print("\n");
printStream.flush();
}
prinStream.close();
socket.close();
ClientConnection is created by the server when I start the client, but it does not print what has been sent until the client is done sending.
I feel like I'm missing the point somewhere along the line. Chat examples are quite common, so if I had a chat client then every message it wanted to send to a chat server it would create a client socket, send the message, and close the socket? Just doesn't seem right somehow.
Thank you.

client = new ClientConnection(sock);
You are passing the socket in constructor.
so you shouldn't do:
socket = new java.net.Socket(m_destination, m_portNumber);
just cache that vatiable from contructor as : this.sock = sock;
getting the reader and the writer is ok, also the server is ok.
I would use a Vector to be synchromized queue for sending messages, and the while (more-stuff-to-send) loop would check the queue and id empty than sleep, if has something to send, than pop the first and sent it while he must do stuff, or socket is closed my the client.

Can a process connect to a socket that itself created?

EDIT: The code below throws no exception but has no output and hangs. It should output "Test message". In main(), we start a thread that's given a server socket listening on a random port. The main thread the tries to connect and communicate with the ServerSocket on that same random port, but is apparently failing. Why?
public class IntraProcSockTest {
private static int port;
private class Listener extends Thread {
public Listener() {
}
public void run() {
try {
ServerSocket serverSocket = new ServerSocket(0);
port = serverSocket.getLocalPort();
Socket socket = serverSocket.accept();
BufferedReader in;
String fromClient;
in = new BufferedReader(new InputStreamReader(
socket.getInputStream()));
while ((fromClient = in.readLine()) != null) {
System.out.println("From client: " + fromClient);
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
public IntraProcSockTest() {
new Listener().start();
}
public static void main(String[] args) {
new IntraProcSockTest();
try {
Thread.sleep(5000);
Socket socket = new Socket("localhost", port);
PrintWriter socketOut = new PrintWriter(socket.getOutputStream());
socketOut.println("Test message");
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}

A process can connect to a socket created by itself, there is no problem. Show us the code that throws an exception and/or more details about the exception.
First of all, be careful not to specify a local port for the client socket (the one connecting to the other which is listening). Let the OS choose a random port. Remember that any socket is identified by four elements (remote host, local host, remote port, local port), if you bind both the server socket and the client socket on the same local port, let it be 4498, both sockets are defined as follows: (localhost, localhost, 4498, 4498) and this doesn't work. I suspect this might be your problem.
To avoid such problems, client sockets are often bound to a random port, chosen by the OS. Show us your code, expecially the part in which the client sockets gets created and connects to the server socket.
And about IPC, it is not always bad to use sockets as an inter-process or even intra-process communication technique. The performance is worse, obviously, and you might loose some code readability, but your software will be easily portable to a network (distributed) application. It's up to your plans, it's not like IPC sockets == bad.

To create a Socket connection in one thread you can.
ServerSocket ss = new ServerSocket(0); // open a random free port.
Socket c = new Socket(ss.getInetAddress(), ss.getLocalPort());
Socket s = ss.accept();
ss.close();
final byte[] bytes = "Hello World!".getBytes();
final OutputStream out = c.getOutputStream();
out.write(bytes.length);
out.write(bytes);
final DataInputStream in = new DataInputStream(s.getInputStream());
int len = in.read();
final byte[] b = new byte[len];
in.readFully(b);
System.out.println(new String(b));
c.close();
s.close();
If all you want is IPC within a Process, a socket is not the fastest or simplest way to go. Try using a Pipe (NIO) or PipeInput/OutputStream (IO). Its faster and simpler.
Pipe pipe = Pipe.open();
SinkChannel sink = pipe.sink();
SourceChannel source = pipe.source();
or
PipedOutputStream output = new PipedOutputStream();
PipedInputStream input = new PipedOutputStream(output);
BTW: You can connect a client and server Socket in the same thread, however
Using an Exchanger is 10x faster, and using a ring buffer is faster again.
If you want convenience, using an ExecutorService is the best way to deleagte work to a background pool of threads. This can still perform millions of tasks per second.