Develop Reference

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.

Java message is sent to a closed socket

I have a client program that sends messages typed in console to the server. Following some advices, I introduced a check for a closed socket with Socket.checkError(). Nevertheless, for some reason it indicates error only after second failed attempt to send a message.
My code:
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
while (true)
try (
Socket clientSocket = new Socket(hostname, port);
PrintWriter socketOut = new PrintWriter(clientSocket.getOutputStream(), true);
) {
String input;
while ((input=stdIn.readLine())!=null) {
socketOut.println(input);
if (socketOut.checkError()) {
System.out.println("Got socket error");
break;
}
}
} catch (IOException e) {
try {
Thread.sleep(5000);
} catch (InterruptedException e1) {}
}
I shut down (manually) my server side after receiving 'message1'. Therefore, I expect to get the error while trying to send the very next message. Nevertheless it occurs only one message after:
message1
message2
message3
Got socket error
Can anyone explain this behavior and advise me a method to get notification right on the first attempt to send a message in void?

Following some advices, I introduced a check for a closed socket with Socket.checkError().
There is no such method. Clearly you are referring to PrintWriter.checkError().
Nevertheless, for some reason it indicates error only after second failed attempt to send a message.
The reason is that there is both a socket send buffer at the sender and a socket receive buffer at the receiver, and that sending is asynchronous: it therefore isn't possible for the first send to detect an error.
Can anyone explain this behavior and advise me a method to get notification right on the first attempt to send a message in void?
There isn't one. That's the nature of TCP. What you are attempting indicates an application protocol error, and the answer lies in the realm of the application protocol as well: don't have the peer close the socket while this end could still be sending data, OR don't allow this end to send data after the peer has indicated, via the application protocol, that it won't be reading any more data.
Don't use PrintWriter over the network. It suppresses the actual exception. Use BufferedWriter and the write() and newLine() methods.

In ths java library there is no method to check if connection is opened or not. Method like isConnected() and isClosed() check only one side of the connection (where you invoked the method).
From javadoc:
Note: Closing a socket doesn't clear its connection state, which means
this method will return true for a closed socket (see isClosed()) if
it was successfuly connected prior to being closed.
To check if the connection has been really closed simply invoke the read() method (or equivalent) and check if it returns -1.
Note: also if isConnected will work as you like (giving false if the other side of the socket closed the connection or if there is a network problem or similar) the sequence:
if (socket.isConnected()) {
int x = socked.read();
}
will not grant that the x has a value different from -1 or throws an IOException, because the connection could be closed after the isConnected test and before the read operation.
The following code to show how any kind of check on the socket cannot guarantee that a subsequent read will give a valid result.
// Return true because socket communication is enabled
if (myFunctionToCheckIfSocketIsOpen(socket)) {
// Here the peer closed the socket or the network shutdown
// This read will give -1 or throws IOException also if the previous check returned true
int x = socket.read();
}

From the answer of #Davide Lorenzo MARINO I got the idea of employing read(). The only problem that it is blocking. However, one can always run it in another thread, which would modify a class global variable, when read() finally returns -1:
static boolean socketIsAlive;
...
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
while (true)
try (
Socket clientSocket = new Socket(hostname, port);
PrintWriter socketOut = new PrintWriter(clientSocket.getOutputStream(), true);
) {
socketIsAlive=true;
new ConnectionChecker(clientSocket).start();
String input;
while (true) {
if ((input=stdIn.readLine())!=null)
socketOut.println(input);
if (!socketIsAlive) {
System.out.println("Got socket error");
break;
}
}
} catch (IOException e) {
try {
Thread.sleep(5000);
} catch (InterruptedException e1) {}
}
}
...
static public class ConnectionChecker extends Thread{
Socket socket;
public ConnectionChecker(Socket socket) {
this.socket=socket;
}
#Override
public void run() {
try {
if (socket.getInputStream().read()==-1)
socketIsAlive=false;
} catch (IOException e) {}
}
}

My socket keeps closing after socket.connect times out

This is driving me insane!
I'm trying to implement TCP holepunching and as part of this I have a socket trying to connect continuously.
For some reason, after the first mobileSocket2.connect call times out, the second time a socket closed exception comes up. I have no idea why. As far as I can tell, a connection timeout shouldn't close the socket. so why does it say socket closed the second loop?
I bind a socket to the same local IP address and local port earlier on but close that socket.
Any help would be greatly appreciated.
Socket mobileSocket2 = new Socket();
try {
System.out.println("105");
mobileSocket2.setReuseAddress(true);
System.out.println("109");
mobileSocket2.setSoTimeout(50);
mobileSocket2.bind(new InetSocketAddress(myIPAddress.getHostAddress(), myPort));
System.out.println("bound");
}
catch (Exception e) {
System.out.println("caught 104: " + e.toString());
}
while(true){
Thread.sleep(5000);
try{
System.out.println("124");
mobileSocket2.connect(new InetSocketAddress(mobileAddress.getHostAddress(), mobilePort));
System.out.println("connection made: " + mobileSocket);
}
catch(Exception e){
System.out.println("exception 2 caught " + e.toString());
}
}

Your code is invalid.
You can't reconnect a connected Socket. When the connect succeeds you need to add a break statement.
You can't reuse a Socket when a connect() has failed. You have to close it and create a new one.

Either reopen the socket when the first exception is caught, or set the timeout to 0, which the javadoc says it is interpreted as 'infinite'. I didn't look at the Socket and SocketImpl code, but I'm fairly confident that, since Socket is a Closeable (and also an AutoCloseable), the implementation is along the lines of
public void connect(InetAddress address, int port) throws IOException {
try {
// connect to the other endpoint
} catch(/*any relevant exception*/) {
throw new IOException(/* ... */);
} finally {
this.close();
}
}

socket programming code not running forever

I have a 2 nodes that should always communicate with each other, but they don't seem to talk for more than 1 interaction. After successfully sending and receiving 1 message, they stop.
My code looks like this:
The initiator:
try {
Socket client = new Socket(ip, port);
OutputStream toNode = client.getOutputStream();
DataOutputStream out = new DataOutputStream(toNode);
out.writeUTF("Start:Message");
System.out.println("Sent data");
InputStream fromNode = client.getInputStream();
DataInputStream in = new DataInputStream(fromNode);
if(in.readUTF().equals("Return Message")) {
System.out.println("Received data");
out.writeUTF("Main:Message");
System.out.println("Sent data again");
}
else
System.out.println("Error");
client.close();
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
The responder:
while(true) {
Socket server;
try {
server = s.accept();
DataInputStream in = new DataInputStream(server.getInputStream());
String msg = in.readUTF();
String[] broken_msg = msg.split(":");
if(broken_msg.length > 0)
System.out.println("Looping");
String ret;
if (broken[0].equalsIgnoreCase("Start")) {
ret = "Return Message";
DataOutputStream out = new DataOutputStream(server.getOutputStream());
out.writeUTF(ret);
}
else if (broken[0].equalsIgnoreCase("Main")) {
//Do Useful work
}
} catch (IOException e) {
e.printStackTrace();
}
}
My output looks like this:
Looping
and:
Sent data
Received data
Sent data again

You are looping around the accept() call, accepting new connections, but your actual I/O code only reads one message per connection. You need an inner loop around the readUTF() calls, handling the entire connection until EOFException is thrown. Normally all the I/O for a connection is handled in a separate thread.

In order for programs to do repetitive actions, you would generally use looping of some sort, including for loops, while loops and do-while loops. For something like this where you don't know how many times you'd need to communicate in advance, then you would need to use a while loop, not a for loop.
Having said that, you have no while loops whatsoever inside of your connection code... so without code that would allow continued communication, your program will stop, exactly as you've programmed it to do.
Solution: fix this. Put in while loops where continued communication is needed.

Array of Sockets Java

I'm creating server and client java applications. I would like to create an array to store my sockets in. I'm using eclipse, and when I type in this line:
Socket[] sockets = new Socket[3];
Eclipse gives me an error saying "The resource type Socket[] does not implement java.lang.AutoCloseable".
How can I fix this?
Thank you
Try/Catch Statement:
try (
Socket[] sockets = new Socket[3]; //Line giving me error
ServerSocket serverSocket =
new ServerSocket(Integer.parseInt(ip));
Socket clientSocket = serverSocket.accept();
ServerClient client = new ServerClient(clientSocket);
PrintWriter out =
new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(
new InputStreamReader(clientSocket.getInputStream()));
//User input
BufferedReader stdIn =
new BufferedReader(
new InputStreamReader(System.in))
) {
String inputLine;
while ((inputLine = in.readLine()) != null) {
out.println(inputLine);
}
} catch (IOException e) {
System.out.println("Exception caught when trying to listen on port "
+ port + " or listening for a connection");
System.out.println(e.getMessage());
continue;
} catch (Exception e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}

While Socket class itself implements AutoCloseable interface, array of Sockets - does not.
To put it in simple terms: you cannot open or close an array.

The resources defined in a try-with-resources block must all be auto-closeable. That's what it's for. Socket[] is not AutoCloseable, so it cannot be defined there. Move the declaration before the try. Ditto for any other resources you get the error on. Don't treat it as a general declaration block. It isn't.

When I run your code I recevie this error message
Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - incompatible types: try-with-resources not applicable to variable type
(java.net.Socket[] cannot be converted to java.lang.AutoCloseable)
I advice you not to use try catch block with resources when you want to define your socket array.
try (
your rest of code
) {
define your array here ---> Socket[] sockets = new Socket[3];
String inputLine;
while ((inputLine = in.readLine()) != null) {
out.println(inputLine);
}
} catch (IOException e) {
your rest of code
Note: Socket class implements Closeable and AutoCloseable , yet array cannot be defined in try block like you tried to do

You can outsmart this problem easily, just create a class containing a socket connection, then build an array of this class object.
Build the class:
Class example
{
Socket con;
The constructor and extra code here
...
}
Then just build the array:
example[] arr=new example[3];