So I did this client server program in java for my college mini project. Note that this is just a small module of a big project I'm working on. I need a string to be sent from the client to the server. The server will return back the string as it is back to the client. (The code will be modified later such that the string is processed before sending back). The client will send a string whenever needed to the server. Thus it means it is compulsory for the server to be running for indefinite time.
The problem I face here is that my server works perfectly only for the first time when the client sends a string. If I run the client the second time with a different string, I get back the same string I sent to the server previously!
Here is my server program:
public class Server {
public static boolean x = true;
public static String reply;
public static void main(String a[]) throws Exception {
System.out.println("Entered server console..");
Socket echoSocket = null;
ServerSocket serverSocket = null;
PrintWriter out = null;
BufferedReader in = null;
System.out.println("Initializing Connection..");
boolean runFlag = true;
try {
serverSocket = new ServerSocket(77);
while (runFlag) {
echoSocket = serverSocket.accept();
out = new PrintWriter(echoSocket.getOutputStream(), true);
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
while (x) {
in = new BufferedReader(new InputStreamReader(echoSocket.getInputStream()));
reply = in.readLine();
if (reply != null) {
x = false;
}
}
System.out.println("received: " + reply);
out.println(reply);
System.out.println("sent back: " + reply);
stdIn.close();
}
} catch (Exception e) {
System.out.println("Exception in starting server: " + e.getMessage());
} finally {
out.close();
in.close();
echoSocket.close();
}
}
}
Here is my Client program:
public class Client {
public static String reply,temp;
public static boolean x=true;
public Client()
{
temp="lala";
}
public Client(String t)
{
temp=t;
}
public static void main(String[] args) throws IOException {
Socket echoSocket = null;
PrintWriter out = null;
BufferedReader in = null;
try {
echoSocket = new Socket("localhost", 77);
out = new PrintWriter(echoSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(echoSocket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Don't know about host: localhost.");
System.exit(1);
} catch (IOException e) {
System.err.println("Couldn't get I/O for the connection to: localhost.");
System.exit(1);
}
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
temp="lala"; //this is the string to be sent
out.println(temp);
while (x) {
reply= in.readLine();
if(reply!=null)
{
x=false;
}
}
System.out.println("reply: "+reply);
out.close();
in.close();
stdIn.close();
echoSocket.close();
}
}
Can anyone help me find what the problem here is?
while (x) {
in = new BufferedReader(new InputStreamReader(echoSocket.getInputStream()));
reply = in.readLine();
if (reply != null) {
x = false;
}
}
Your server enters this loop the first time a client connects, and it sets the reply String to some input from the client. However, it never enters this loop again, as x's value never changes back to true.
When you accept a request, the x will be set false and never become true.
Please initial the x when you enter the loop.
What's more,if you use a socket between client and server, please move the
echoSocket = serverSocket.accept();
out of the first loop.And you can use echoSocket to communicate.Then you will
keep the long connection.
Related
I have a multithreaded client-server system which works back and forth with the client communicating first and the server replying.
However, for two specific clients, I need them to constantly check if there is data held in the input stream before proceeding when the user makes an input.
The program is a car park management system. When the car park is full(0 spaces available) and a car arrives at an entrance client, the system forms a queue of clients waiting to grant entry. When a car leaves the car park, the first client in the queue is removed and added to a BlockingQueue for that specific entrance client. I have created a direct output output stream for each of the entrance clients. So when a BlockingQueue is not empty, data is taken from this queue and output is sent to the stream of that specific client.
However, the problem is - the entrance client which was queued should automatically read its InputStream and print the data to grant access, but instead it causes an error and crashes. I think what is happening is that when the system first starts, the is the client is stuck waiting to read data which initially doesn't exist because it would require some sort of input at the first stage, causing an error.
How do I fix this so that the client reads and prints the input stream(whether it be specific data such as contains the word "queue") IF there is data available else to continue IF the user makes an input.
I hope this makes sense, I tried to make it as clear as possible.
Server class:
public class Server {
public static void main(String[] args) throws IOException {
//Create the shared objects in the global scope...
int groundFloor = 0; //SET TO 0 FOR TESTING
int firstFloor = 0;
SharedState SharedStateObject = new SharedState(groundFloor,firstFloor);
//Sets up the server socket on port 4444
ServerSocket serverSocket = null;
try
{
serverSocket = new ServerSocket(4444);
System.out.println("Car Park Server started." + "\n");
}
catch (IOException e) {
System.err.println("Could not start server on specified port.");
System.exit(-1);
}
//Got to do this in the correct order with only four clients!
ServerThread GroundFloorEntrance = new ServerThread(serverSocket.accept(), "GroundFloorEntrance", SharedStateObject);
ServerThread FirstFloorEntrance = new ServerThread(serverSocket.accept(), "FirstFloorEntrance", SharedStateObject);
ServerThread GroundFloorExit1 = new ServerThread(serverSocket.accept(), "GroundFloorExit1", SharedStateObject);
ServerThread GroundFloorExit2 = new ServerThread(serverSocket.accept(), "GroundFloorExit2", SharedStateObject);
GroundFloorEntrance.start();
FirstFloorEntrance.start();
GroundFloorExit1.start();
GroundFloorExit2.start();
serverSocket.close();
//Loop for granting queued clients access
while(true)
{
BlockingQueue<String> queuedGroundAccess = SharedStateObject.getQueuedGround();
BlockingQueue<String> queuedFirstAccess = SharedStateObject.getQueuedFirst();
if(!queuedGroundAccess.isEmpty())
{
Socket clientSocket = GroundFloorEntrance.clientSocket();
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
try
{
out.println(queuedGroundAccess.take());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
if(!queuedFirstAccess.isEmpty())
{
Socket clientSocket = FirstFloorEntrance.clientSocket();
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
try
{
out.println(queuedFirstAccess.take());
} catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
}
}
Client
public class GroundFloorEntrance {
#SuppressWarnings("resource")
public static void main(String[] args) throws IOException {
// Set up the socket, in and out variables
Socket clientSocket = null;
PrintWriter out = null;
BufferedReader in = null;
int port = 4444;
String serverName = "localhost";
String clientID = "Ground Floor Entrance";
try {
clientSocket = new Socket(serverName, port);
out = new PrintWriter(clientSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Don't know about host: " + serverName);
System.exit(1);
} catch (IOException e) {
System.err.println("Couldn't get I/O for the connection to: "+ port);
System.exit(1);
}
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
String fromServer = null;
String fromUser = null;
System.out.println("Initialised " + clientID + " client and IO connections");
//I THINK THE ISSUE IN THE FOLLOWING STRUCTURE:
while (true) {
fromServer = in.readLine();
if(fromServer != null && fromServer.contains("Queue: "))
{
System.out.println(fromServer);
}
fromUser = stdIn.readLine();
if (fromUser != null) {
out.println(fromUser);
}
fromServer = in.readLine();
System.out.println(fromServer);
}
}
}
A problem is in this loop. When you write fromServer = in.readLine(); it stop execution of your program and waits for data to be entered from server.
while (true) {
fromServer = in.readLine();
if(fromServer != null && fromServer.contains("Queue: "))
{
System.out.println(fromServer);
}
fromUser = stdIn.readLine();
if (fromUser != null) {
out.println(fromUser);
}
fromServer = in.readLine();
System.out.println(fromServer);
}
What you can do with that? You should read data from server in another thread to prevent blocking main thread while waiting for data. Like that:
new Thread(new MyRunnable(fromServer)).start();
And MyRunnable will look like this:
public class MyRunnable implements Runnable {
private Scanner scanner;
public MyRunnable(Scanner scanner) {
this.scanner = scanner;
}
#Override
public void run() {
while (true) {
if (scanner.hasNextLine()) {
System.out.println(scanner.nextLine());
}
}
}
}
If you will have some questions, please ask.
I want to write a client-sever program in which server and client send messages to each other. First, my server send a message to client, then the client reply. Next, my server send another message, the client reply. The problem is, on my first message induced by the server, the client does not respond.
My server:
public class Server {
public void go() {
try {
ServerSocket serverSocket = new ServerSocket(9999);
System.out.println("Server listening ...");
Socket socket = serverSocket.accept();
try (
PrintWriter printWriter = new PrintWriter(socket.getOutputStream());
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
) {
String input;
printWriter.println(new Scanner(System.in).nextLine());
while ((input = bufferedReader.readLine()) != null) {
System.out.println(input);
printWriter.println(new Scanner(System.in).nextLine());
if(input == "Bye") break;
}
}
} catch (IOException ioException) {
ioException.printStackTrace();
}
}
public static void main(String[] args) {
Server server = new Server();
server.go();
}
}
My client:
public class Client {
public void go() {
try {
try (
Socket socket = new Socket("localhost", 9999);
PrintWriter printWriter = new PrintWriter(socket.getOutputStream());
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
) {
String input;
while ((input = bufferedReader.readLine()) != null) {
System.out.println("1");
System.out.println(input);
printWriter.println(new Scanner(System.in).nextLine());
}
}
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Client client = new Client();
client.go();
}
}
Are there some problems with my code?
Your client connects and then blocks reading a line from the server.
Your server accepts the connection and then blocks reading a line from the client.
I don't know what you were expecting to happen next, but it won't. Somebody needs to send something.
Your code seems to be fine. You just need to push the infrastructure by calling flush() after writing:
printWriter.flush();
I have a client class and a server class.
If client sends message to server, server will send response back to the client, then client will print all the messages it received.
For example,
If Client sends "A" to Server, then Server will send response to client
"1111". So I use readLine() in client class to read the message from server, then client print "1111" in the console.
If Client sends "B" to Server, then Server will send response to client
"2222\n 3333". So the expected printing output from client is:
"2222"
"3333"
So the response message from server to client may have 1 line or 2 lines depending on the message it send from client to server.
My question is that how I can use readLine() to read the message that send from server to client. More specifically, if I use the following codes,
String messageFromServer;
while(( messageFromServer = inputStreamFromServer.readLine()) != null) {
println(messageFromServer);
}
It will only print the first line, and will not print anything else even if I keep sending message from client to server, because readLine() will stops once it has read the first line.
update:
More specifically, I am looking for some methods in the client class to read message that contains 1 or multiple lines from server at a time. I am wondering if there are any ways to do it in client side if I don't want to change the format of the message that sent from server to client.
update 2
To make my question more clear, I will put some sample codes in the following:
This is server:
import java.net.*;
import java.io.*;
public class Server {
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = null;
try {
serverSocket = new ServerSocket(1234);
} catch (IOException e) {
System.err.println("Could not listen on port: 1234.");
System.exit(1);
}
Socket clientSocket = null;
try {
clientSocket = serverSocket.accept();
} catch (IOException e) {
System.err.println("Accept failed.");
}
System.out.println("Connected");
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String textFromClient =null;
String textToClient =null;
textFromClient = in.readLine(); // read the text from client
if( textFromClient.equals("A")){
textToClient = "1111";
}else if ( textFromClient.equals("B")){
textToClient = "2222\r\n3333";
}
out.print(textToClient + "\r\n"); // send the response to client
out.flush();
out.close();
in.close();
clientSocket.close();
serverSocket.close();
}
}
The client:
public class Client {
public static void main(String[] args) throws IOException {
Socket socket = null;
PrintWriter out = null;
BufferedReader in = null;
BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
try {
socket = new Socket("localhost", 1234);
out = new PrintWriter(socket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Don't know about host");
} catch (IOException e) {
System.err.println("Couldn't get I/O for the connection");
}
System.out.println("Connected");
String textToServer;
while((textToServer = read.readLine())!=null){
out.print(textToServer + "\r\n" ); // send to server
out.flush();
String messageFromServer =null;
while(( messageFromServer = textToServer=in.readLine()) != null){
System.out.println(messageFromServer);
}
}
out.close();
in.close();
read.close();
socket.close();
}
private static void debug(String msg)
{
System.out.println("Client: " + msg);
}
}
You shouldn't need to change the format of the data sent by the server, and readLine() should work, but I suspect that the server is not flushing or closing the OutputStream after writing the response which could possibly explain things.
Is the call to readLine() hanging? Are you in control of the server code? If so, can you include it?
Revised classes that work as I believe you expect:
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
public class ClientServerTest2
{
public static void main(String[] args) throws Exception
{
Thread serverThread = new Thread(new Server());
serverThread.start();
Thread clientThread = new Thread(new Client());
clientThread.start();
serverThread.join();
clientThread.join();
}
private static class Server implements Runnable
{
#Override
public void run()
{
ServerSocket serverSocket = null;
try
{
serverSocket = new ServerSocket(1234);
Socket clientSocket = null;
clientSocket = serverSocket.accept();
debug("Connected");
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String textFromClient = null;
String textToClient = null;
textFromClient = in.readLine(); // read the text from client
debug("Read '" + textFromClient + "'");
if ("A".equals(textFromClient))
{
textToClient = "1111";
}
else if ("B".equals(textFromClient))
{
textToClient = "2222\r\n3333";
}
debug("Writing '" + textToClient + "'");
out.print(textToClient + "\r\n"); // send the response to client
out.flush();
out.close();
in.close();
clientSocket.close();
serverSocket.close();
}
catch (Exception e)
{
e.printStackTrace();
}
}
private static void debug(String msg)
{
System.out.println("Server: " + msg);
}
}
private static class Client implements Runnable
{
#Override
public void run()
{
Socket socket = null;
PrintWriter out = null;
BufferedReader in = null;
BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
try
{
socket = new Socket("localhost", 1234);
out = new PrintWriter(socket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
debug("Connected");
String textToServer;
textToServer = read.readLine();
debug("Sending '" + textToServer + "'");
out.print(textToServer + "\r\n"); // send to server
out.flush();
String serverResponse = null;
while ((serverResponse = in.readLine()) != null)
debug(serverResponse); // read from server and print it.
out.close();
in.close();
read.close();
socket.close();
}
catch (IOException e)
{
e.printStackTrace();
}
}
}
private static void debug(String msg)
{
System.out.println("Client: " + msg);
}
}
Change while(( messageFromServer = inputStreamFromServer.readLine() != null) to while(( messageFromServer = inputStreamFromServer.readLine()) != null)
Actually this shouldn't even compile....
It's a work around.
If you want to send multiple strings like in your case : "2222\n 3333".
You can send them by adding a seperator character (like :) between two strings : "2222: 3333".
Then you can call write from server side as
clientOut.write("2222: 3333\n");
On client side parse recieved String :
messageFromServer = inputStreamFromServer.readLine();
String strArray[] = messageFromServer.split(":");
strArray[0] : 2222
strArray[0] : 3333
I have two Java applications, where an Android client connects to a server on a computer and sends a message using BufferedWriter over websockets.
The client:
try {
toast("Sending...");
Socket sock = new Socket(ip, PORT);
OutputStream os = sock.getOutputStream();
BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(os));
bw.flush();
bw.write("Hello Server!");
toast("Connected!");
} catch (UnknownHostException e) {
toast(e.getMessage());
} catch (IOException e) {
toast(e.getMessage());
}
The server:
public static void main(String[] args) {
ServerSocket server;
ConnectionThread ct;
Socket s;
ExecutorService es = Executors.newSingleThreadExecutor();
try {
System.out.println("Starting server...");
server = new ServerSocket(1337);
s = server.accept();
ct = new ConnectionThread(s);
es.execute(ct);
} catch (IOException ex) {
ex.printStackTrace();
}
}
The ConnectionThread class:
public class ConnectionThread implements Runnable {
private Socket sock;
private InputStream is;
private BufferedReader br;
private boolean online;
public ConnectionThread(Socket s) {
System.out.println("Creating connection thread.");
this.sock = s;
online = true;
}
#Override
public void run() {
String input = "";
try {
System.out.println("Starting to read...");
is = sock.getInputStream();
br = new BufferedReader(new InputStreamReader(is));
while (online) {
input = br.readLine();
if(input != null){
System.out.print("Received message: ");
System.out.println(input);
}
}
br.close();
is.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
When I run the server, and then the client, the client will show the "Connected!" toast, and the server's output will be:
Starting server...
Creating connection thread.
Starting to read...
So, it seems like the connection is actually being made, but the message does not arrive. Does anybody know why this could be happening?
Your server is expecting a complete line terminated by a newline. Try:
bw.write("Hello Server!");
bw.newLine();
Do it like this...
String s = new String();
while ((br.readLine())!=null) {
s = s+br.readLine();
System.out.print("Received message: ");
System.out.println(input);
}
}
And
bw.println("Hello Server");
I notice that you don't send an endline on your client, so the BufferedReader.readline() will never return, because it cannot match the \n-character. Try it again with
bw.write("Hello Server!\n");
on the client side.
Hi I am trying to understand how Sockets work can implement a multiplayer side to a monopoly game.I understood how to create the connection, but now it seems I have trouble sending and receiving the data between the client and the server .Here is my code:
Client code:
public class EchoClient
{
public static void main(String[] args)
{
Socket echoSocket = null;
PrintWriter out = null;
BufferedReader in = null;
try
{
echoSocket = new Socket("127.0.0.1", 5000);;
out = new PrintWriter(echoSocket.getOutputStream());
in = new BufferedReader(
new InputStreamReader(echoSocket.getInputStream()));
BufferedReader stdIn = new BufferedReader(
new InputStreamReader(System.in));
String userInput;
while ((userInput = stdIn.readLine()) != null)
{
out.println(userInput);
}
out.close();
in.close();
stdIn.close();
echoSocket.close();
}
catch (UnknownHostException e)
{
System.err.println("Don't know about host: taranis");
}
catch (IOException e)
{
System.err.println("Couldent get I/O for "
+ " the connection to : taranis.");
}
}
}
Server code:
public class ServerSide
{
ServerSocket connect;
Socket connection;
PrintWriter out;
BufferedReader in;
public void go()
{
try
{
connect = new ServerSocket(5000);
connection = connect.accept();
in = new BufferedReader(
new InputStreamReader(connection.getInputStream()));
String userInput;
while ((userInput = in.readLine()) != null)
{
System.out.println("echo: " + in.readLine());
}
}
catch (IOException e)
{
System.out.println(e);
}
}
public static void main(String[] args)
{
new ServerSide().go();
}
}
I was trying to create here a simple connection between a client and a server.On the client side when the user inputs data I want it to be sent to the server and then print it it on the server console.It seems that the way I wright the code it isent working what did I do wrong?
First of all you need to declare a console object and then print on it.
Then don't forget to:
After every print on the console at the server side you need to flush the stream so the all the data will be printed.
Your code looks fine to me. Usually with sockets and keyboard input, you run into the case where the reader.readLine() hangs because it is still trying to read input from the other side. Typically, i will put an empty out.println() at the end of my client so the server will terminate the reading while loop. I've tried flush() before as Mike suggested but that seems to not work for me.
On the client, you're missing out.flush; after the out.println.
On the server,
while ((userInput = in.readLine()) != null) {
System.out.println("echo: " + userInput); // not in.readLine
}