So, I guess that this has been answered before, but I couldn't find the question, so forgive me.
I have a rather basic chat client-server pair, of which the server is multithreaded to allow for several clients to connect at the same time. The server code looks like this...
private void loop(int port) {
// Opens a port for connections.
ServerSocket serverSocket = null;
Socket clientSocket = new Socket();
try {
serverSocket = new ServerSocket(port);
System.out.println("Server running in port " + port);
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
// Listens for a connection
while (onlineState == true && serverSocket != null) {
if (cur_players < max_players) {
try {
clientSocket = serverSocket.accept();
System.out.println(clientSocket.getInetAddress() + " has connected to the port " + clientSocket.getPort());
cur_players++;
new Thread(new SocketThread( clientSocket, Chat.getOpenSeat() )).start();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
Here, what Chat.getOpenSeat() does is browse an array(boolean seats[]) through for an open spot in the chat buffer array (String buffer[][]) and returns an integer for the spot, then marking it taken. However, when I access the buffer array from the threads, the thread only finds the messages it has added itself. Below is the corresponding code.
toClient = c.poll(bufferSocket); // Retrieves the top-most message from the seat's sub-array,
// then bumps the remaining messages up in the sub-array.
if (toClient != null) {
out.println(toClient); // Sends the message through the Socket.
System.out.println("Message was sent.");
toClient = null;
}
Curiously enough, the threads can access the seats[] array without any trouble, finding the currently active seats and correctly giving all the corresponding sub-arrays their messages. Here's also the bit of code I use to add a new message to the array:
public void offer(String msg) {
for (int seat = 0; seat < Server.max_players; seat++) {
if (seats[seat] == true) {
for (int i = 0; i < 100; i++) {
if (msgBuffer[seat][i] == null) {
msgBuffer[seat][i] = msg;
System.out.println("Message: '" + msg + "' was buffered for the Seat " + seat + ".");
break;
}
}
}
}
}
So, how do I add Strings to an array that is commonly read-write accessible to all of my threads?
Just ask if you need to see more of the code.
Make the array volatile, and it will behave concurrent.
More on volatile: http://www.javamex.com/tutorials/synchronization_volatile.shtml.
Related
I am coding client-server multithread calculator using java, socket programming.
There's any syntax error, but msgs cannot be received from server.
I think
receiveString = inFromServer.readLine()
does not works. This code is in Client program, in the while(true) loop.
What is the problem?
Here is my full code.
SERVER
import java.io.*;
import java.net.*;
public class Server implements Runnable
{
static int max = 5; //maximum thread's number
static int i = 0, count = 0; //i for for-loop, count for count number of threads
public static void main(String args[]) throws IOException
{
ServerSocket serverSocket = new ServerSocket(6789); //open new socket
File file = new File("src/serverinfo.dat"); //make data file to save server info.
System.out.println("Maximum 5 users can be supported.\nWaiting...");
for(i=0; i <= max; i++) { new Connection(serverSocket); } //make sockets - loop for max(=5) times
try //server information file writing
{
String dataString = "Max thread = 5\nServer IP = 127.0.0.1\nServer socket = 6789\n";
#SuppressWarnings("resource")
FileWriter dataFile = new FileWriter(file);
dataFile.write(dataString);
}
catch(FileNotFoundException e) { e.printStackTrace(); }
catch(IOException e) { e.printStackTrace(); }
}
static class Connection extends Thread
{
private ServerSocket serverSocket;
public Connection(ServerSocket serverSock)
{
this.serverSocket = serverSock;
start();
}
public void run()
{
Socket acceptSocket = null;
BufferedReader inFromClient = null;
DataOutputStream msgToClient = null;
String receiveString = null;
String result = "", sys_msg = "";
try
{
while(true)
{
acceptSocket = serverSocket.accept(); // 접속수락 소켓
count++;
inFromClient = new BufferedReader(new InputStreamReader(acceptSocket.getInputStream()));
msgToClient = new DataOutputStream(acceptSocket.getOutputStream());
System.out.println(count + "th client connected: " + acceptSocket.getInetAddress().getHostName() + " " + count + "/" + max);
System.out.println("Waiting response...");
while(true)
{
if (count >= max+1) // if 6th client tries to access
{
System.out.println("Server is too busy. " + max + " clients are already connected. Client access denied.");
sys_msg = "DENIED";
msgToClient.writeBytes(sys_msg);
acceptSocket.close();
count--;
break;
}
try{ msgToClient.writeBytes(result); }
catch(Exception e) {}
try{ receiveString = inFromClient.readLine(); }
catch(Exception e) // if receiveString = null
{
System.out.println("Connection Close");
count--;
break;
}
System.out.println("Input from client : " + receiveString);
try
{
if(receiveString.indexOf("+") != -1) { result = cal("+", receiveString); }
else if(receiveString.indexOf("-") != -1) { result = cal("-", receiveString); }
else if(receiveString.indexOf("/") != -1) { result = cal("/", receiveString); }
else if(receiveString.indexOf("*") != -1) { result = cal("*", receiveString); }
else if(receiveString.indexOf("+") == -1 || receiveString.indexOf("-") == -1 || receiveString.indexOf("*") == -1 || receiveString.indexOf("/") == -1) { result = "No INPUT or Invalid operation"; }
}
catch(Exception e){ result = "Wrong INPUT"; }
try{ msgToClient.writeBytes(result); }
catch(Exception e) {}
}
}
}
catch(IOException e) { e.printStackTrace(); }
}
}
private static String cal(String op, String recv) //function for calculating
{
double digit1, digit2; //first number, second number
String result = null;
digit1 = Integer.parseInt(recv.substring(0, recv.indexOf(op)).trim());
digit2 = Integer.parseInt(recv.substring(recv.indexOf(op)+1, recv.length()).trim());
if(op.equals("+")) { result = digit1 + " + " + digit2 + " = " + (digit1 + digit2); }
else if(op.equals("-")) { result = digit1 + " - " + digit2 + " = " + (digit1 - digit2); }
else if(op.equals("*")) { result = digit1 + " * " + digit2 + " = " + (digit1 * digit2); }
else if(op.equals("/"))
{
if(digit2 == 0){ result = "ERROR OCCURRED: Cannot be divided by ZERO"; }
else{ result = digit1 + " / " + digit2 + " = " + (digit1 / digit2); }
}
return result;
}
#Override
public void run() {
// TODO Auto-generated method stub
}
}
-----------------------------------------------------------------
CLIENT
import java.io.*;
import java.net.*;
public class Client {
public static void main(String args[]) throws IOException
{
Socket clientSocket = null;
BufferedReader userInput = new BufferedReader(new InputStreamReader(System.in));
BufferedReader inFromServer = null;
DataOutputStream msgToServer = null;
String sendString = "", receiveString = "";
try
{
clientSocket = new Socket("127.0.0.1", 6789); //make new clientSocket
inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
msgToServer = new DataOutputStream(clientSocket.getOutputStream());
System.out.println("Input exit to terminate");
System.out.println("Connection Success... Waiting for permission");
while(true)
{
receiveString = inFromServer.readLine();
if(receiveString.equals("DENIED"))
{
System.out.println("Server is full. Try again later.");
break;
}
else { System.out.println("Connection permitted."); }
System.out.print("Input an expression to calculate(ex. 3+1): ");
sendString = userInput.readLine();
if(sendString.equalsIgnoreCase("exit")) //when user input is "exit" -> terminate
{
clientSocket.close();
System.out.println("Program terminated.");
break;
}
try { msgToServer.writeBytes(sendString); }
catch(Exception e) {}
try { receiveString = userInput.readLine(); }
catch(Exception e) {}
System.out.println("Result: " + receiveString); //print result
}
}
catch(IOException e)
{
e.printStackTrace();
}
}
}
You've set up your server socket stack wrong.
Your code will make 5 threads, each calling accept on a serversocket.
The idea is to have a single ServerSocket (and not 5, as in your example). Then, this single serversocket (running in a single thread that handles incoming sockets flowing out of this serversocket) will call .accept which will block (freeze the thread) until a connection is made, and will then return a Socket object. You'd then spin off a thread to handle the socket object, and go right back to the accept call. If you want to 'pool' (which is not a bad idea), then disassociate the notion of 'handles connections' from 'extends Thread'. For example, implement Runnable instead. Then pre-create the entire pool (for example, 10 threads), have some code that lets you 'grab a thread' from the pool and 'return a thread' to the pool, and now the serversocket thread will, upon accept returning a socket object, grab a thread from the pool (which will block, thus also blocking any incoming clients, if every thread in the pool is already taken out and busy handling a connection), until a thread returns to the pool. Alternatively, the serversocket code checks if the pool is completely drained and if so, will put on a final thread the job of responding to that client 'no can do, we are full right now'.
I'm not sure if you actually want that; just.. make 1 thread per incoming socket is a lot simpler. I wouldn't dive into pool concepts until you really need them, and if you do, I'd look for libraries that help manage them. I think further advice on that goes beyond the scope of this question, so I'll leave the first paragraph as an outlay of how ServerSocket code ought to work, for context.
I have 2 buttons in my client with a button listener each.
In my firt button listener I am sending a String over the socket and I am getting back an array of integers after it is spanwed. No problem there. Here is my code.
public void rollDice() {
try {
DataOutputStream sout1 = new DataOutputStream(socket.getOutputStream());
String line = "dice";
PrintStream out1 = new PrintStream(sout1);
out1.println(line);
} catch (UnknownHostException e1) {
e1.printStackTrace();
} catch (IOException e1) {
e1.printStackTrace();
}
}
With the second listener I want t send first a string to put the server into the right state, and after I want to send an integer to continue the process. Here is my code but it doesn't seem to work. Server is printing a random number, even if I send a "2".
public void sendDice() {
try {
DataOutputStream sout2 = new DataOutputStream(socket.getOutputStream());
String line = "pick";
PrintStream out2 = new PrintStream(sout2);
out2.println(line);
out2.write(diceListLength);
} catch (UnknownHostException e1) {
e1.printStackTrace();
} catch (IOException e1) {
e1.printStackTrace();
}
}
Here is the server's side.
public void run() {
boolean running = true;
try {
// Create streams for reading / writing lines of text to the socket
DataInputStream input = new DataInputStream(s.getInputStream());
DataInputStream inputInt = new DataInputStream(s.getInputStream());
ObjectOutputStream output = new ObjectOutputStream(s.getOutputStream());
// Print a message:
System.out.println("\nClient from: " + s.getInetAddress() + " port " + s.getPort());
while(running) {
String st = input.readLine();
if (st.equals("dice")) {
for (int i = 0; i < diceRolled.length - number; i++) {
diceRolled[i] = (int) ( 1 + Math.random() * 6);
System.out.print(diceRolled[i] + " ");
}
output.writeObject(diceRolled);
output.reset();
} else if (st.equals("pick")) {
number = inputInt.readInt();
System.out.print(number);
}
}
} catch (IOException e) {
System.out.println("Error: " + e.getMessage());
// Always be sure to close the socket
} finally {
try {
if (s != null) {
System.out.println(s.getLocalSocketAddress() + " closed.");
s.close();
}
} catch (Exception e) {
System.out.println("Error: " + e.getMessage());
}
}
}
Try setting autoFlush on the PrintStream when you create it... a single integer will not be sent until newline or buffer full.
From java's documentation
autoFlush - A boolean; if true, the output buffer will be flushed whenever a byte array is written, one of the println methods is invoked, or a newline character or byte ('\n') is written
Also useful:
Use line-based messaging, i.e. the second message type can be "pick:4" (check with st.startsWith("pick")) and then parse the integer. With your code, you can easily end up loosing state. (Single-line messages are "pseudo-atomic").
Don't create DataInputStreams in every listener method, make them object variables (same for PrintStreams...). There's no need to (re)create objects in every click.
This question already has answers here:
Java detect lost connection [duplicate]
(9 answers)
Java socket API: How to tell if a connection has been closed?
(9 answers)
Closed 9 years ago.
I have a java program with Socket. I need to check if client has disconnected. I need a example how to do that. I have researched but I don't understand. So can someone make example code and explane everything.
sorry for bad English
my code:
package proov_server;
//SERVER 2
import java.io.*;
import java.net.*;
class server2 {
InetAddress[] kasutaja_aadress = new InetAddress[1000];
String newLine = System.getProperty("line.separator");
int kliendiNr = 0;
int kilene_kokku;
server2(int port) {
try {
ServerSocket severi_pistik = new ServerSocket(port);
System.out.println("Server töötab ja kuulab porti " + port + ".");
while (true) {
Socket pistik = severi_pistik.accept();
kliendiNr++;
kasutaja_aadress[kliendiNr] = pistik.getInetAddress();
System.out.println(newLine+"Klient " + kliendiNr + " masinast "
+ kasutaja_aadress[kliendiNr].getHostName() + " (IP:"
+ kasutaja_aadress[kliendiNr].getHostAddress() + ")");
// uue kliendi lõime loomine
KliendiLoim klient = new KliendiLoim(pistik,kliendiNr);
// kliendi lõime käivitamine
klient.start();
}
}
catch (Exception e) {
System.out.println("Serveri erind: " + e);
}
}
DataOutputStream[] väljund = new DataOutputStream[1000];
DataInputStream[] sisend = new DataInputStream[1000];
int klient = 0;
int nr;
// sisemine klass ühendusega tegelemiseks
class KliendiLoim extends Thread {
// kliendi pistik
Socket pistik;
// kliendi number
KliendiLoim(Socket pistik2,int kliendiNr) {
nr = kliendiNr;
this.pistik = pistik2;
}
public boolean kontroll(){
try{
System.out.println("con "+pistik.isConnected());
System.out.println("close "+pistik.isClosed());
if(pistik.isConnected() && !pistik.isClosed()){
//System.out.print(con_klient);
return true;
}
}catch(NullPointerException a){
System.out.println("Sihukest klienti pole!!!");
}
kliendiNr --;
return false;
}
public void run() {
try {
sisend[nr] = new DataInputStream(pistik.getInputStream()); //sisend
väljund[nr] = new DataOutputStream(pistik.getOutputStream()); //väljund
}catch (Exception ea) {
System.out.println(" Tekkis erind: " + ea);
}
while(true){
try{
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.print("Sisesta k2sk: ");
String k2sk = null;
k2sk = br.readLine();
/*
String command;
if(k2sk.indexOf(" ") < 0){
command = k2sk;
}else{
command = k2sk.substring(0, k2sk.indexOf(" "));
}
*/
String[] words = k2sk.split("\\s+");
for (int i = 0; i < words.length; i++) {
words[i] = words[i].replaceAll(" ", "");
}
switch(words[0]){
case "suhtle":
if(väljund.length > klient && väljund[klient] != null)
{
väljund[klient].writeUTF("1");
}else{
väljund[klient] = null;
sisend[klient] = null;
System.out.println("Sihukest klienti pole");
}
break;
case "vaheta":
try{
int klinetnr = Integer.parseInt(words[1]);
//if(kontroll(klinetnr) ){
klient = Integer.parseInt(words[1]);
//}
}
catch(NumberFormatException e){
System.out.println("See pole number!!! ");
}
break;
case "kliendid":
if(kliendiNr != 0){
for(int i=1;i <= kliendiNr;i++){
if(kontroll()){
System.out.println("Klient:"+i+" ip: " + kasutaja_aadress[i] );
}else{
System.out.println("Pisi");
väljund[klient] = null;
sisend[klient] = null;
}
}
System.out.println(newLine);
}else{
System.out.println("Kiente pole");
}
break;
}
System.out.println(kliendiNr);
}catch(SocketException a){
System.out.println("Klient kadus");
}
catch(Exception e){
System.out.println(" Viga: " + e);
}
}
}
}
public static void main(String[] args) {
new server2(4321);
}
}
If the client has disconnected properly:
read() will return -1
readLine() returns null
readXXX() for any other X throws EOFException.
The only really reliable way to detect a lost TCP connection is to write to it. Eventually this will throw an IOException: connection reset, but it takes at least two writes due to buffering.
A related thread on Stackoverflow here along with the solution. Basically, the solution says that the best way to detect a client-server disconnect is to attempt to read from the socket. If the read is successfully, then the connection is active.If an exception is thrown during the read there is no connection between the client and the server. Alternatively it may happen that the socket is configured with a timeout value for the read operation to complete. In case, this timeout is exceeded a socket timeout exception will be thrown which can be considered as either the client is disconnected or the network is down.
The post also talks about using the isReachable method - refer InetAddress documentation. However, this method only tells us whether a remote host is reachable or not. This may just one of the reasons for the client to disconnect from the server. You wont be able to detect disconnection due to client crash or termination using this technique.
:) I'm trying to make an Android app which will scan an entire network for a specific open port. I am using Android Studio (Windows) and the emulator. I am able to scan the network for 90 seconds before my program stops running. I don't receive any logcat errors and I lose the stack and program counter in the debugger. I cannot restart the program without first restarting the emulator. It looks like I lose any connection I had to the emulator.
I should also mention that removing the socket connect line results in the task running indefinitely. Has anyone experienced this kind of time out before?
Any pointers in the right direction would be much appreciated!!
Jenny
private class getNetworkState extends AsyncTask<Integer, Integer, Integer> {
#Override
protected Integer doInBackground(Integer... params) {
for (int subnet2 = 216; subnet2 < 220; subnet2++)
{
for (int subnet = 0; subnet < 255; subnet++)
{
// open a tcp socket
String server = String.format("192.168.%d.%d", subnet2, subnet);
Socket socket = new Socket();
try
{
socket.connect(new InetSocketAddress(server, port), timeOut);
System.out.println("Network state of " + server + " == " + socket.isConnected());
socket.close();
}
catch (Exception e)
{
System.out.println("Network state of " + server + " == " + e);
}
}
}
return 1;
}
How often are you seeing exceptions in socket.connect(...)? The way the code exists, failed connects will throw an Exception and the socket will not be closed.
I think a better practice would be to close the socket in a finally block. If the cause of your problems is a ton of open sockets, this might help clear up the issue.
private class getNetworkState extends AsyncTask<Integer, Integer, Integer> {
#Override
protected Integer doInBackground(Integer... params) {
for (int subnet2 = 216; subnet2 < 220; subnet2++)
{
for (int subnet = 0; subnet < 255; subnet++)
{
// open a tcp socket
String server = String.format("192.168.%d.%d", subnet2, subnet);
Socket socket = new Socket();
try
{
socket.connect(new InetSocketAddress(server, port), timeOut);
System.out.println("Network state of " + server + " == " + socket.isConnected());
}
catch (Exception e)
{
System.out.println("Network state of " + server + " == " + e);
} finally {
socket.close();
}
}
}
return 1;
}
I'm trying to code a UDP client to receive packets from a server that is broadcasting on the local network. The problem is the receive method isn't blocking and waiting for a packet to arrive.
Instead, it's receiving null or empty packets.
I've tried to use .setSoTimeout(0), which supposedly will tell the receive to block until it receives a packet, but it doesn't.
Does anyone know how to fix this?
Here's the code:
while (search == true) {
InetAddress addr = InetAddress.getByName("0.0.0.0");
DatagramSocket sock = new DatagramSocket(1355);
sock.setSoTimeout(0);
byte[] recebe = new byte[1024];
sock.setBroadcast(true);
System.out.println("entrou1");
DatagramPacket packet = new DatagramPacket(recebe, recebe.length);
System.out.println("entrou2");
sock.receive(packet);
String info = new String(packet.getData());
System.out.println("tamanho: " + info.length());
if (info.trim().equals("") == false && info != null) {
System.out.println("entrou aqui");
System.out.println("info recebida:" + info + ":fsadfsfs");
String servs[] = info.split("\n");
list1.clear();
servidores.clear();
for (int i = 0; i < servs.length; i++) {
System.out.println("vec: " + servs[i]);
if (servs[i].trim().equals("")) {
System.out.println("break;");
break;
} else {
String aux = servs[i].substring(0, servs[i].lastIndexOf("->"));
System.out.println("aux: " + aux);
list1.add(aux);
servidores.add(servs[i]);
}
}
}
System.out.println("info:\n" + info);
sock.close();
synchronized (obj) {
try {
obj.wait();
} catch (InterruptedException ex) {
Logger.getLogger(AcederPartilhaGUI.class.getName()).log(Level.SEVERE, null, ex);
}
}
}