Develop Reference

Java obtain nio SocketChannel connection when the connection is already in use

If another application on the PC is connected to the same remote IP address, a java application will fail to connect properly.
This can also happen when a exits abruptly without closing the socket channel. The connection can be blocked and it is impossible to connect during a subsequent session.
What can I do to ensure that no matter the state of the connection in the underlying OS, my program will connect 100% of the time ?
I am looking for a cross platform solution (Windows & Ubuntu)
public void connect() throws CommunicationIOException {
try {
if (isConnected()) {
return;
}
socket = SocketChannel.open();
socket.socket().connect(new InetSocketAddress(getHostname(), getPort()), getConnectionTimeout());
if (!isConnected()) {
throw new CommunicationIOException("Failed to establish the connection");
}
socket.configureBlocking(false);
} catch (final IOException ex) {
throw new CommunicationIOException(
"An error occurred while connecting to " + getHostname() + " on port " + getPort(), ex);
}
}
.
public boolean isConnected() {
if (socket == null) {
return false;
} else {
return socket.isConnected();
}
}
.
public void close() throws CommunicationIOException {
if (socket != null) {
try {
socket.close();
} catch (final IOException ex) {
throw new CommunicationIOException(
MessageFormat.format(
"An error occurred while attempting to close the connection to {}:{}",
getHostname(), getPort()), ex);
}
}
}

If another application on the PC is connected to the same remote IP address, a java application will fail to connect properly.
No it won't, unless the server is improperly programmed.
This can also happen when a exits abruptly without closing the socket channel.
No it can't, again unless something is improperly programmed.
The connection can be blocked
No it can't.
and it is impossible to connect during a subsequent session.
No it isn't.
What can I do to ensure that no matter the state of the connection in the underlying OS, my program will connect 100% of the time ?
Nothing in this life will give you a 100% guarantee. However your fears as expressed above are baseless.

ObjectInputStream hangs both server and client even after flushing

I've written a basic TCP chat in Java and when testing locally (i.e. localhost) it's been working fine. I can connect, type messages to myself and receive without issues. I can connect and disconnect several times a second.
However, when trying to connect through my router's external IP for whatever reason it hangs when trying to initialize the ObjectInputStream on not only the Client side, but also Server side. So I'm guessing it's something to do with my Router firewall, on which I added a firewall rule. I did only create a firewall rule for the specific port 1777 where I make my connection, is it possible that the data sent when flushing ObjectOutputStream is sent on another port? Which wouldn't make sense I guess considering the socket is bound to a specific port.
I am flushing after initializing the ObjectOutputStream and I am creating that before creating the ObjectInputStream, on both sides. What doesn't make sense to me is that it allows for a connection to be made.
I guess code may be irrelevant in this case, but here it is anyway:
This Thread is run at all times, and waits for connections then starts a new Thread for handling the connecting client:
public void run() {
while (connected) {
try (ServerSocket serverSocket = new ServerSocket(port)) {
Socket socket = serverSocket.accept();
new ClientHandler(socket);
} catch (IOException e) {
...
}
}
}
Constructor of ClientHandler class:
public ClientHandler(Socket socket) throws IOException {
outputStream = new ObjectOutputStream(socket.getOutputStream());
outputStream.flush();
inputStream = new ObjectInputStream(socket.getInputStream());
welcomeClient();
start(); // Start listening for messages.
}
This is where the ClientHandler Thread hangs.
Here is Client side code:
// Get IP, port and stuff
socket = new Socket(ip, port);
outputStream = new ObjectOutputStream(socket.getOutputStream());
outputStream.flush();
inputStream = new ObjectInputStream(socket.getInputStream());
// Write a couple objects and read some objects.
They both hang on "inputStream = new ObjectInputStream(socket.getInputStream());"
What could be the problem? I can also connect through my local IP, which makes me think it's still an outbound->inbound firewall issue for whatever reason.

Error 'null' in sockets connection , Android [duplicate]

This question already has answers here:
How can I fix 'android.os.NetworkOnMainThreadException'?
(66 answers)
Closed 7 years ago.
What I was trying to do:
I was trying to build a test app, for now, simply establishing connection between the app on Android phone (4.2.2)(as client) and a java application running on pc (windows 8)(as server) via sockets connection.
What I've done already:
I've made the programs for both client and server in java on pc and tested them positively (Connection got established).
The network:
Both my phone and pc are connected to wifi at my home.ipconfig on pc shows address 192.168.56.1 while on logging into router it shows address of my pc to be 192.168.0.108 (Probably I don't understand networking :P).
The code:
client(Android)
public void connectPC(View view)
{
try
{
clientSocket = new Socket("192.168.0.108",1025);
outstream = clientSocket.getOutputStream();
instream = clientSocket.getInputStream();
data_out = new DataOutputStream(outstream);
data_in = new DataInputStream(instream);
statusView.setText("Connected!");
}
catch (Exception e)
{
statusView.setText("Error: "+e.getMessage());
}
}
The Server:
import java.net.*;
import java.io.*;
public class ServerSide extends Thread
{
ServerSocket serverSocket;
public ServerSide(int port) throws IOException
{
serverSocket = new ServerSocket(1025);
serverSocket.setSoTimeout(10000);
}
public void run()
{
while(true)
{
System.out.println("Waiting for client on port : "+ serverSocket.getLocalPort());
Socket server;
try {
server = serverSocket.accept();
System.out.println("Connected to : " +server.getRemoteSocketAddress());
} catch (IOException e) {
// TODO Auto-generated catch block
System.out.println(e.getMessage());
}
}
}
public static void main(String... args)
{
int port=6066;
try
{
Thread t = new ServerSide(port);
t.start();
}
catch(IOException e)
{
System.out.println(e.getMessage());
}
}
}
The Problem:- The connection simply doesn't establish, the catch block shows e.getMessage() as null.
PS I've tried 192.168.56.1 ip address too. And added uses permission in manifest file
Any help in this regard please..!

You need to print the stacktrace rather than just the exception message. That will give you more information to debug the problem ... including the name of the exception, and the place where it was thrown.
Also, it is a bad idea to catch Exception and attempt to recover from it. Catching Exception could catch all sorts of exceptions that you were never expecting. Recovering from exceptions that you weren't expecting is risky ... because you cannot be sure it is a safe thing to do. It is typically better to let the application die ...

I would like to give you some suggestions:
use asynctask in android for networking activities otherwise NetworkOnMainThreadException occur because it is good to run all time consuming activities in background.Also keep in mind do all task in doBackgroung function of asynctask and then update and publish result with help of onPostExecute() and onProgress().
If you are not using asynctask then simply use thread and perform all networking activity on separate thread.
In java software ,track IP address by using Enumeration instead of InetAddress because Enumeration will show all IP address on network and probably you will find answer of your question.(Try to connect to all the IP that is shown by Enumeration method and connection is established with suitable one automatically)

manage socket connection in Java over tcp

I wrote a client which basically just open a socket and send content over the connection. ( the content follows the Http protocol)
The problem I'm facing regards to the question - how and when should i close the connection.
The issue is that the connection sometime closes too early ("FIN" is sent to the server before the server answered).
in this case the server's answer is lost.
I tried to use Thread.sleep before closing the connection but nothing seems to affect the time between the content is sent and the "FIN" message is sent. (viewed in Wireshark)
The answer sometimes arrive and sometimes not ( race condition).
How can i delay the "FIN" message so i won't miss the server's response?
i added the relevant class. The relevant function is sendContentOverSocket
public class SocketClient {
private String hostName;
private int portNumber;
private Socket ConnectionSocket;
public void init(String hostName, int portNumber){
this.hostName = hostName;
this.portNumber = portNumber;
this.ConnectionSocket=createSocketConnection();
}
private Socket createSocketConnection() {
Socket socket = null;
try {
socket = new Socket(this.hostName, this.portNumber);
return socket;
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return socket;
}
public void sendContentOverSocket(String content) {
try {
PrintWriter out = new PrintWriter(
ConnectionSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(
ConnectionSocket.getInputStream()));
out.print(content);
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
out.close();
in.close();
ConnectionSocket.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}

TCP works with a concept called a half close.
When you close the socket that is an indication that you are not going to send anymore.
In your explanation I see "FIN is sent to the server before the server answered", if you are the client, that would mean that you have performed a close on the socket.
If you expect a result from a server within a certain time frame you need some kind of timing mechanism, possibly making use of select in combination with a timeout.
If the server closes his end of the connection, you detect this by receiving bytes in receive. Usually this means that you have to close the socket too.
So in conclusion there is 3 reasons for you to close the socket :
the server closes his end of the socket basically saying i am not going to send anymore
you have waited for a while and you are tired of waiting and decide to close the socket yourself.
any other error conditions but usually they all appear like receiving 0 bytes or a negative number.

You should close the connection after you've read the response, of course. Difficult to see the mystery here. No sleeps. If you don't read the response (a) you can't know whether the request succeeded or failed, and (b) the server is liable into encounter an exception as well.
Your code is poor quality. All those methods should propagate exceptions instead of catching them internally and returning null.

In case of Java 7, since all three classes, i.e. Socket, PrintWriter, BufferedReader, implement AutoCloseable and based on the fact, that you want to close socket right after you invoke sendContentOverSocket(String content) try to use the following code:
public class SocketClient {
private String hostName;
private int portNumber;
public void init(String hostName, int portNumber) {
this.hostName = hostName;
this.portNumber = portNumber;
}
public void sendContentOverSocket(String content) {
try (Socket socket = new Socket(this.hostName, this.portNumber);
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()))) {
out.print(content);
} catch(IOException e) {
//Appropriate exception handler
}
}
}
In this case Java will close all resources properly by itself.
If you use Java 6 or earlier try to use try-finally block instead:

solved.
i now understand how it works.
what i was missing in the client is the attempt to read from the input Stream.
When you try to read
while ((inputFromServer = in.readLine()) != null)
the client waits for input. The only thing that will break this loop is the server closing the connection.
after that happens you can safely close the connection on the client side. No need to delay the FIN and such...

Java SocketException. Why is it closed?

I'm trying to get a threaded chatserver working. But my socket is closing and I have no clue why it is.
In the server class I create a new ClientHandler
addHandler(new ClientHandler(this, incoming));
addHandler starts the thread and adds the new ClientHandler to an ArrayList in server.
incoming is the client socket.
public ClientHandler(Server serverArg, Socket sockArg) {
server = serverArg;
client = sockArg;
System.out.println(client.isClosed());
}
There is nothing called between
public void run() {
try {
System.out.println(client.isClosed());
in = new Scanner(client.getInputStream());
out = new PrintWriter(client.getOutputStream(), true);
announce();
System.out.println("Waiting for input");
boolean done = false;
while(!done && in.hasNextLine()) {
System.out.println("There is input!");
server.broadcast(clientName + ": " + in.nextLine());
}
} catch (Exception e) {
System.out.println(e.toString());
}
}
When I run this code. This is the output:
false
true
java.net.SocketException: Socket is closed
Why is it closing immediately after creating this class / before starting it?

The connection is being closed by your code between when you add the handler and when the run() method is called. I suggest you add a breakpoint to the close method in the JDK and see where it is called. Or have a look at the code after you add the handler.

You can use a networking sniffer like wireshark or tcpdump.
To find out that if the connection broke due to the server side.
I guess that maybe the client side close the connection.