Develop Reference

Java Client Server Program Takes input infinitely

I'm new to java socket programming, I'm making a client server program. the server is multithreaded.
when the client connection with server is open. the server sends a text block to the client like this:
connection is open with the server....
Welcome Please Chose one of the following Operations
Insert, Read, Update, Delete
Type Exit to terminate connection.
when I type read or exit or any of the operations it works fine and the server responds.
but the problem happens when I chose an operation i.e insert --> when the server responds and asks me for input and I wanna insert a value, the program keeps taking input infinitely for endless lines I don't know where the problem and how it happens.
it's the same code, the client sends the input as one line when choosing operation but when I chose insert operation and the server is expecting a value it takes it as infinite endless lines.
Client class
public class Client1 {
public static void main(String[] args) throws IOException {
Socket socket=null;
try {
System.out.println("sending connection request to host 127.0.0.1 at port 2000");
socket = new Socket("127.0.0.1", 2000);
System.out.println("connection is open with the server....");
Scanner scn = new Scanner(System.in);
DataOutputStream dos = new DataOutputStream(socket.getOutputStream());
DataInputStream dis = new DataInputStream(socket.getInputStream());
while (true) {
System.out.println(dis.readUTF());
String tosend = scn.nextLine();
dos.writeUTF(tosend);
// If client sends exit,close this connection
// and then break from the while loop
if (tosend.equals("Exit")) {
System.out.println("Closing this connection : " + socket);
socket.close();
System.out.println("Connection closed");
break;
}
String received = dis.readUTF();
System.out.println(received);
}
// closing resources
scn.close();
dis.close();
dos.close();
}
catch (Exception e ){
System.out.println(e);
} finally {
try {
if (socket != null) socket.close();
} catch (Exception e){
System.out.println(e);
}
}
}
Server Class
public class ServerThread extends Thread{
Socket socket ;
DataInputStream dis;
DataOutputStream dos;
ServerThread(Socket socket,DataInputStream dis,DataOutputStream dos ){
this.socket = socket;
this.dis=dis;
this.dos=dos;
}
#Override
public void run(){
String received;
String toreturn;
String welcomeText = """
Welcome Please Chose one of the following Operations
Insert, Read, Update, Delete
Type Exit to terminate connection.""";
while (true){
try {
// Ask user what he wants
dos.writeUTF(welcomeText);
// receive the answer from client
received = dis.readUTF();
if(received.equals("Exit"))
{
System.out.println("Client " + this.socket + " sends exit...");
System.out.println("Closing this connection.");
this.socket.close();
System.out.println("Connection closed");
break;
}
// write on output stream based on the
// answer from the client
switch (received) {
// the problem starts here if I chose insert and wanna print what the user typed, it takes
//input infinitely from the user
case "Insert":
toreturn = "Inserting new info...";
dos.writeUTF(toreturn);
String out = dis.readUTF();
dos.writeUTF("Accepted");
dos.writeUTF(out);
break;
case "Read":
toreturn = "Reading User Info...";
dos.writeUTF(toreturn);
break;
case "Update":
toreturn = "Updating User Info...";
dos.writeUTF(toreturn);
break;
case "Delete":
toreturn = "Deleting User Info";
dos.writeUTF(toreturn);
break;
default:
dos.writeUTF("Unknown User");
break;
}
} catch ( IOException e) {
e.printStackTrace();
}
}
try
{
// closing resources
this.dis.close();
this.dos.close();
}catch(IOException e){
e.printStackTrace();
}
}
}
I don't know how this loop happens especially since the server accepts input correctly when choosing operations but when insert operation is chosen it just takes input infinitely, can anyone help please, i cant implement any operation if this problem persists.

I think its your Client, in the while loop try to remove the second read from server, it's because you once read all what server sents, and when the loop starts again wants to read from server but there is nothing to read and becomes idle.

Trying to Get a new ServerSocket to open fails

I was given the below code by my teacher for a class. I ran it one or twice and it worked fine. However I suddenly cannot get it to run from the command prompt on Windows 8 anymore. No matter what port I specify it just prints "Opening port..." and never continues. No exception is ever thrown. I have disabled my firewall and antivirus and it does not seem to work. I have added a print statement as the first line of the try catch block and it will print but it just will not create the new Socket. I am sure it is something in my Windows settings but I am unsure as to what or how to resolve it.
// Server program
// File name: "TCPServer.java"
import java.io.*;
import java.net.*;
public class TCPServer
{
private static ServerSocket servSock;
public static void main(String[] args)
{
System.out.println("Opening port...\n");
try{
// Create a server object
servSock = new ServerSocket(Integer.parseInt(args[0]));
}
catch(IOException e){
System.out.println("Unable to attach to port!");
System.exit(1);
}
do
{
run();
}while (true);
}
private static void run()
{
Socket link = null;
try{
// Put the server into a waiting state
link = servSock.accept();
// Set up input and output streams for socket
BufferedReader in = new BufferedReader(new InputStreamReader(link.getInputStream()));
PrintWriter out = new PrintWriter(link.getOutputStream(),true);
// print local host name
String host = InetAddress.getLocalHost().getHostName();
System.out.println("Client has estabished a connection to " + host);
// Receive and process the incoming data
int numMessages = 0;
String message = in.readLine();
while (!message.equals("DONE"))
{
System.out.println(message);
numMessages ++;
message = in.readLine();
}
// Send a report back and close the connection
out.println("Server received " + numMessages + " messages");
}
catch(IOException e){
e.printStackTrace();
}
finally{
try{
System.out.println("!!!!! Closing connection... !!!!!\n" + "!!! Waiting for the next connection... !!!");
link.close();
}
catch(IOException e){
System.out.println("Unable to disconnect!");
System.exit(1);
}
}
}
}

This code works fine. The problem is the code for the client. The answer to your problem is already written in a comment in your code.
// Put the server into a waiting state
link = servSock.accept();
The server goes into a waiting state until it gets a connection. The client is the one that would be getting the error since it did not connect. If the client was working correctly the code would continue and you would get the additional output.

Simple Client-Server Application - Socket problems

I'm writing a simple client-server application using TCP Sockets . It works with the multi-threading principle to allow for several client connections to the same server.
I'm having some trouble figuring out some of the errors I get with the sockets, i'm fairly new in this environment as you will probably tell.
I'll show you the code I have, and the output i get from it, but basically the problem lies in the very connecting of the clients to the server, and I ran through all the code but still can't find what's wrong with it.
Server:
public static ArrayList<String> userList = new ArrayList<String>();
public static int index;
public static String date;
public static void main(String args[]) throws Exception {//inicio main
ServerSocket server = new ServerSocket(6500); //Create socket on port 6500
System.out.println ("Server started on port 6500");
while (true){ //Waiting for clients
System.out.println("Server waiting for client connections..");
Socket socket = null;
BufferedReader reader = new BufferedReader(new FileReader("C:\\UNIV\\Redes\\workspace\\Copy of Ex_4.3_Teste\\lists\\blacklist.txt"));
String line = null;
socket = server.accept();
// Blacklist verification
while ((line = reader.readLine()) != null) {
if (line.equals(socket.getInetAddress().toString())) {
System.out.println("IP Blacklisted: " + socket.getInetAddress().toString());
System.out.println("Closing connection to " + socket.getInetAddress().toString());
PrintStream checkBlack = new PrintStream(socket.getOutputStream(),true);
checkBlack.println("***BLACKLISTED***");
reader.close();
checkBlack.close();
socket.close();
break;
}
}//End of Blacklist Verification
//Sending feedback in case of approved client
try {
PrintStream checkBlack = new PrintStream(socket.getOutputStream(),true);
checkBlack.println("***NBLACKLISTED***");
checkBlack.close();
} catch (SocketException e) {
}
userList.add(socket.getInetAddress().toString()); //Add connected user's IP to USERLIST
System.out.println("New connection..");
System.out.println("Size of UserList: " + userList.size());
Thread t = new Thread(new EchoClientThread(socket));
t.start(); //Starting Client Thread
}//End of Waiting for Clients
}//End of Main
public static class EchoClientThread implements Runnable{
private Socket s;
public EchoClientThread(Socket socket) {
this.s = socket;
}
public void run() {
String threadName = Thread.currentThread().getName(); //Thread Name
String stringClient = s.getInetAddress().toString(); //Client IP
System.out.println("Connected to " + stringClient);
try{
BufferedReader input = new BufferedReader(
new InputStreamReader(s.getInputStream()));
PrintStream output = new PrintStream(
s.getOutputStream(),true);
String line;
while ((line = input.readLine()) !=null) { //Input Cycle
System.out.println (stringClient+": "+threadName+": "+line); //Print command from client
if (line.equalsIgnoreCase("9")){ //Exit
break;
}
else if (line.equalsIgnoreCase("1")){ //Send List of Online Users
System.out.println("Option 1: Sending list of online users to " + stringClient);
output.println(" ");
output.println("List of Online Users:");
output.println(" ");
for(int i=0;i<userList.size();i++){
output.println(userList.get(i));
}
}
else if (line.equalsIgnoreCase("2")) { //Send message to a single user
System.out.println("Nothing here yet..");
}
else if (line.equalsIgnoreCase("3")) { //Send message to all the online users
System.out.println("Nothing here yet..");
}
else if (line.equalsIgnoreCase("4")){ //Send User Blacklist
System.out.println("Option 4: Sending user blacklist to " + stringClient);
BufferedReader reader = new BufferedReader(new FileReader("C:\\UNIV\\Redes\\workspace\\Copy of Ex_4.3_Teste\\lists\\blacklist.txt"));
String lineRead = null;
output.println(" ");
output.println("User Blacklist:");
output.println(" ");
while ((lineRead = reader.readLine()) != null) {
output.println(lineRead);
}
reader.close();
}
else{
output.println("Unknown command.");
}
output.println("***CLOSE***"); //Closes client's input cycle
output.println("***NBLACKLISTED***"); //Sending feedback in case of approved client
}//Input Cycle End
output.println("See you later!");
input.close(); //Closes inputStream
output.close(); //Closes outputStream
s.close(); //Closes Socket
}
catch (Exception e){
System.err.println("Server Side Error!");
System.out.println(e);
}
userList.remove(s.getInetAddress().toString());
System.out.println("Client "+ stringClient+" was disconnected!");
}//End of run()
}//End of EchoClientThread
}//End of EchoServerThread
Client:
public static void main(String args[]) throws Exception {
if (args.length !=1){
System.err.println ("usage: java EchoClient2 <host>");
System.exit(1);
}
String host = args[0];
int port = 6500;
String cmd, line;
Socket socket = new Socket(host,port);
BufferedReader input = new BufferedReader(
new InputStreamReader(socket.getInputStream()));
PrintStream output = new PrintStream(socket.getOutputStream(),true);
while( true ) {//Input cycle
Scanner scan = new Scanner (System.in);
if (input.readLine().equals("***BLACKLISTED***")) {
System.out.println("IP is Blacklisted");
break;
}
System.out.println(" ");
System.out.println("CLIENT MENU");
System.out.println(" ");
System.out.println("1 - List on-line users");
System.out.println("2 - Send message to a single user");
System.out.println("3 - Send message to all on-line users");
System.out.println("4 - List Blacklisted Users");
System.out.println("9 - Exit");
System.out.println(" ");
System.out.print(host+":"+port+"#>"); //Command prompt
cmd = scan.nextLine(); //Scanning command to send to the server
output.println(cmd); //Sending command to the server
if ( cmd.equalsIgnoreCase("9")){
System.out.println("Exiting..");
break;
}
try {
while (!(line = input.readLine()).equals("***CLOSE***")) { //Input Cycle
System.out.println (line); //Prints server answer
}
} catch (Exception e) {
System.err.println("Client Side Error!");
System.out.println(e);
break;
}
}//End of Cycle
System.out.println("Connection Terminated");
input.close(); //Closes inputStream
output.close(); //Closes outputStream
socket.close(); //Closes Socket
}
}
So the server starts fine with the following output:
Server started on port 6500
Server waiting for client connections..
But as soon as I try to connect with the client, this happens:
Server Side:
Server started on port 6500
Server waiting for client connections..
New connection..
Size of UserList: 1
Server waiting for client connections..
Connected to /127.0.0.1
java.net.SocketException: Socket is closed
Server Side Error!
Client /127.0.0.1 was disconnected!
On the client side, though, it still shows the input menu, and the command prompt, like so:
CLIENT MENU
1 - List on-line users
2 - Send message to a single user
3 - Send message to all on-line users
4 - List Blacklisted Users
9 - Exit
127.0.0.1:6500#>
And when I input something on the Client Side prompt, i get:
127.0.0.1:6500#>1
Client Side Error!
java.net.SocketException: Software caused connection abort: recv failed
Connection Terminated
I know what the errors mean, Socket is closed is pretty much self-explanatory, but i just can't find wheres the code problem that makes the socket close.
Any help is much appreciated.

You have your blacklist mechanism not quite right.
When you close a stream associated with the socket it will close the socket as well.
So the server is closing any socket that it gets and then hands it on to a thread,
which tries to use the socket and fails.
// Blacklist verification
while ((line = reader.readLine()) != null) {
// blah blah blah
}//End of Blacklist Verification
//Sending feedback in case of approved client
try {
PrintStream checkBlack = new PrintStream(socket.getOutputStream(),true);
checkBlack.println("***NBLACKLISTED***");
checkBlack.close(); // <== why are you closing the stream?
} catch (SocketException e) {
}
try this instead
// Blacklist verification
while ((line = reader.readLine()) != null) {
// blah blah blah
}//End of Blacklist Verification
//Sending feedback in case of approved client
try {
socket.getOutputStream().write("***NBLACKLISTED***\n".getBytes());
} catch (SocketException e) {
e.printStackTrace();
}
A debugger is your friend.

What determines in this code what is sent back to the client? TCP Sockets

In the code below, what determines what will be sent back to the client (the PHP page). I am trying to alter this so that it sends a variable back to the PHP page with an error message that is defined based on actions made in my java code.
Edit: To answer some questions, what I am trying to do is this.
Send a string to the java script with a socket and convert it to a variable to be used in the java script. It will run through some if statements and I need to set the error statements to a variable lets say "reply". I need to send "reply" then back to the PHP file.
public class MyJavaServer {
public static void main(String[] args) {
int port = 20222;
ServerSocket listenSock = null; //the listening server socket
Socket sock = null; //the socket that will actually be used for communication
try {
listenSock = new ServerSocket(port);
while (true) { //we want the server to run till the end of times
sock = listenSock.accept(); //will block until connection recieved
BufferedReader br =
new BufferedReader(new InputStreamReader(sock.getInputStream()));
BufferedWriter bw =
new BufferedWriter(new OutputStreamWriter(sock.getOutputStream()));
String line = "";
while ((line = br.readLine()) != null) {
bw.write("PHP said: " + line + "\n");
bw.flush();
}
//Closing streams and the current socket (not the listening socket!)
bw.close();
br.close();
sock.close();
}
} catch (IOException ex) {
ex.printStackTrace();
}
}
}

If I get your question right, the line where the answer gets sent to the peer is
bw.write("PHP said: " + line + "\n");
which writes the given string to bw.

Why SocketException?

I have Client class and Server class but when i run both main methods and then nothing will happen and when i stop running ,this exception will be occurred. why?? please help me,how can I fix it???
my Client class:
public class Client {
static BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
try {
Socket c = new Socket("localhost", 5001);
BufferedReader read = new BufferedReader(new InputStreamReader(c.getInputStream()));
BufferedWriter write = new BufferedWriter(new OutputStreamWriter(c.getOutputStream()));
String string = reader.readLine();
write.write(string, 0, string.length());
write.newLine();
write.flush();
System.out.println(read.readLine());
} catch (Exception e) {
System.err.println(e);
}
}}
my Server class:
public class Server{
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
ServerSocket s = null;
try {
s = new ServerSocket(5001);
System.out.println("listening...");
Socket so = s.accept();
BufferedReader read = new BufferedReader(new InputStreamReader(so.getInputStream()));
BufferedWriter write = new BufferedWriter(new OutputStreamWriter(so.getOutputStream()));
while (true) {
String string = read.readLine();
System.out.println(string);
String answer = "I got" + string + "from you!";
write.write(answer, 0, answer.length());
write.newLine();
write.flush();
}
} catch (IOException ex) {
Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
}
}}
stacktrace in server cpnsole:
run:
listening...
system connected
Hello
Dec 19, 2009 12:58:15 PM server.Main main
SEVERE: null
java.net.SocketException: Connection reset
at java.net.SocketInputStream.read(SocketInputStream.java:168)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:264)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:306)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158)
at java.io.InputStreamReader.read(InputStreamReader.java:167)
at java.io.BufferedReader.fill(BufferedReader.java:136)
at java.io.BufferedReader.readLine(BufferedReader.java:299)
at java.io.BufferedReader.readLine(BufferedReader.java:362)
at server.Main.main(Main.java:37)
BUILD SUCCESSFUL (total time: 9 seconds)
in Client console:
run:
Hello
I gotHellofrom you!
BUILD SUCCESSFUL (total time: 4 seconds)

Your client connects to the server, sends some data, reads the response and terminates. That's ok.
But your server waits for a client, reads its data, writes a response and then tries to read some data from the client again. But the client has closed the connection. So the server gets the exception you described.
To fix this (on server side), you have to do the Socket so = s.accept(); within your while loop. And don't forget to close the socket at the end of the loop.

First. A BufferedWriter isn't useful with Sockets. Use a PrintWriter witch flushes automatically.
PrintWriter writer = new PrintWriter(socket.getOutputStream(), true);// true means "flush automatically"
writer.println(reader.readLine());
// Now you don't have to add a newline and flush.
Edit
Secondly. The reason why the exception is thrown is because the client closes the Socket after writing your input to the Server. The server is still waiting for another String to read. But he can't because the Socket is closed. You don't close it literally. But the program ends there. So Java think: "Nothing left to do, exit". By exiting, the connection closes.
To solve it you have to put the communication in a while(true) loop and, to stop the connection in a correct way, send an "end-of-connection" message.
Client side:
while (true)
{
String userinput = reader.readLine(); // From System.in
writer.writeln(userinput);
if (userinput.equals("end"))
{
socket.close();
break; // break out of the while(true) loop
}
}
Server side:
while (true) {
String socketinput = reader.readLine();
if (socketinput.equals("end"))
{
socket.close();
break; // Break out of the while(true) loop.
}
... // Handle the socketInput
}
That is also what "Connection reset" means.
Martijn.