We Keep Coding

Reading message fails if client not inside a while loop

I have a small TCP server and client where the server is performing inside a while(true) loop meanwhile my client isn't. However when i send a message from the client to the server, the server isn't able to read it. Below is the code and error:
SERVER:
ServerSocket serverSocket = new ServerSocket(port);
System.out.println("[SERVER]: Server launched on port " + port);
while (true) { // the while loop makes the server continuously accept client request
Socket clientSocket = serverSocket.accept();
System.out.println("New client connected: " + clientSocket);
PrintWriter writer = new PrintWriter(clientSocket.getOutputStream());
BufferedReader reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
System.out.println(reader.readLine());
}
CLIENT:
Socket socket = new Socket("localhost", 9101);
PrintWriter out = new PrintWriter(socket.getOutputStream());
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
out.println("lol");
ERROR:
Exception in thread "main" java.net.SocketException: Connection reset
The error occurs when we try to read data in the server. With all that being said, if i surround the out.println() method in the client in a while(true) loop the error goes away and the code works fine. So i'm wondering why the i get this exception when the server is inside an infinite loop but the client isn't.

Bug when create new Thread for a Socket

I'm using this code to constantly accept when a new Socket connect to Server:
while (true) {
Socket socket = null;
socket = ss.accept();
System.out.println("A client is connect...\n");
DataInputStream in = new DataInputStream(socket.getInputStream());
DataOutputStream out = new DataOutputStream(socket.getOutputStream());
Thread acceptClient = new Thread(new ClientHandler(socket, in, out));
acceptClient.start();
}
But when a connection is created, I see this in the console:
A client is connect...
A client is connect...
A client is
connect...
I don't understand why this line is displayed 3 times. Can anyone explane?

Java socket hanging when inside of java servlet

So I've been trying to figure out how to send messages between my arduino and my java servlet (tomcat) and have been bumping into some problems. I'm using this code in my java servlet:
ServerSocket server;
//socket server port on which it will listen
int port = 9876;
String message = "";
//create the socket server object
server = new ServerSocket(port);
System.out.println("Server socket created");
//keep listens indefinitely until receives 'exit' call or program terminates
while(true){
System.out.println("Waiting for client request...");
//creating socket and waiting for client connection
Socket socket = server.accept();
BufferedReader buff = new BufferedReader(new InputStreamReader (socket.getInputStream()));
System.out.println("Input stream established");
message = buff.readLine();
System.out.println("Message Received: " + message);
buff.close();
socket.close();
//terminate the server if client sends exit request
if(message.equalsIgnoreCase("exit")) break;
}
System.out.println("Shutting down Socket server!!");
//close the ServerSocket object
server.close();
But it's hanging on the
Socket socket = server.accept();
line. I assume this is because the messages i'm sending from the arduino aren't arriving. On the arduino side of things, this is my client:
if (client.connect(ip, 9876)) {
Serial.println("connected");
client.println("12345678");
Serial.println("Message sent");
} else {
Serial.println("connection failed");
}
Not much to it. Now, I've tried sending this as a HTTP POST request, but without success (surely i'm doing it incorrectly, i just don't know how to do it, i've tried for a while). I know this is probably horrifically written, but i'm very open to learning from you guys today.
The weird thing is that this works exactly how i want it to when i put the java server code in a new regular java project. Can anyone help me figure this out? Why is it hanging in the servlet but not in a regular java project?

client requesting from web server

I have coded a simple java client to send requests and receive data from my my java web server. The server is capable of handling persistent connections and everything works fine when I use browser to send requests however when I send requests using my client it only works with non persistent connections. when I use my java client to send requests it would receive the data requested from the server and then just freezes. My code for java client:
public static void main(String[] args) throws UnknownHostException, IOException {
// TODO code application logic here
String sentence;
String modifiedSentence;
BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("127.0.0.1", 6789);
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
System.out.println("Enter the file name that you would like to request from Server:");
sentence = inFromUser.readLine();
System.out.println("would you like to have a persistent connection (yes/no):");
String sentence1 = inFromUser.readLine();
if(sentence1.equals("yes")){sentence1="Connection: keep-alive";}
else{sentence1="Connection: close";}
outToServer.writeBytes("GET /"+sentence + "\r\n");
outToServer.writeBytes(sentence1+"\r\n");
outToServer.writeBytes("\r\n");
while ((modifiedSentence=inFromServer.readLine()) != null)
{
System.out.println("FROM SERVER: " + modifiedSentence);
}
System.out.println("done");
clientSocket.close();
}

My guess is that you try to modifiedSentence=inFromServer.readLine() after the server closed the connection (which closes the socket and the other streams as well) and you get a java.net.SocketException.
Try surrounding it with try/catch and print the error message / stacktrace

client acting both a as a client and server - JAVA

I have an interesting question. I am trying to establish a peer to peer connection which means a client process acts both as a server and client. Ideally, it should have a client socket (Socket class) and a server socket(Server Socket class). Now I tried to use this concept but it does not work. Please take a look at it:
public static void main(String argv[]) throws Exception
{
Socket clientSocket = null;
BufferedReader inFromUser = new BufferedReader( new InputStreamReader(System.in));
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
System.out.println("Enter the server port u want to be assigned to this peer:");
sentence = inFromUser.readLine();
System.out.println("writing current port to client = "+ sentence);
outToServer.writeBytes("p~"+sentence + "\n" );
int serverport = Integer.parseInt(sentence);
ServerSocket server = new ServerSocket(serverport);
Socket client;
//client
System.out.println("enter port no of the server port of an other peer:");
int msg=Integer.parseInt(inFromUser.readLine());
clientSocket = new Socket("localhost", msg);
outToServer = new DataOutputStream(clientSocket.getOutputStream());
outToServer.writeBytes("hi");
while(true)
{
//server port listens infinitely and spawns new thread
System.out.println("inside true");
client = server.accept();
Thread serverThread = new Thread(new acceptconnection1(client));
serverThread.start();
}}}
class acceptconnection1 implements Runnable {
BufferedReader inFromClient, inn;
DataOutputStream ds;
Socket socket;
int peersocket;
String clientSentence;
int serverport ;
Socket clientSocket = null;
acceptconnection1 (Socket socket,) throws IOException{
this.socket = socket;
inn = new BufferedReader (new InputStreamReader(System.in));
inFromClient =new BufferedReader(new InputStreamReader(socket.getInputStream()));
ds = new DataOutputStream(socket.getOutputStream());
}
#Override
public void run () {
String cs,a;
try {
System.out.println("waiting for connection ");
if(( clientSentence = inFromClient.readLine())!=null)
{
System.out.println("Message from other peer" + clientSentence);
}
} catch (IOException ex) {
Logger.getLogger(acceptconnection1.class.getName()).log(Level.SEVERE, null, ex);
}}}
Output:
when I create two client processes,
o/p of client1:
Enter the server port u want to be assigned to this peer:
1111
writing current port to client = 1111
hi enter port no u want to connect to:
2222
inside true
inside true
waiting for connection
Enter the server port u want to be assigned to this peer:
2222
writing current port to client = 2222
hi enter port no u want to connect to:
1111
inside true
inside true
waiting for connection
what happens is both of them wait for connections. how do i solve this?

You have a deadlock condition. To result this, create the ServerSocket first so the Socket has something to talk to. Create the Socket which will connect but do nothing until accepted. Then accept connections.
BTW: You don't need to create two connections for traffic to pass both ways. Once a connection has been established, you can use that one connection as client-server, or server-server or what ever.