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Permanent and persistent Socket connection in java

I've created a client-server connection, something like a chat system. Previously I was using a while loop on the client side, and it was waiting to read a message from the console every time (of course server has a while loop as well to serve forever). But now, I'm trying to first create a connection at the beginning of the session, and then occasionally send a message during the session, so to maintain a permanent and persistent connection.
Currently, without the while loop, the client closes the connection and I don't know how to find a workaround.
Here is the client code:
import java.net.*;
import java.io.*;
public class ControlClientTest {
private Socket socket = null;
// private BufferedReader console = null;
private DataOutputStream streamOut = null;
public static void main(String args[]) throws InterruptedException {
ControlClientTest client = null;
String IP="127.0.0.1";
client = new ControlClientTest(IP, 5555);
}
public ControlClientTest(String serverName, int serverPort) throws InterruptedException {
System.out.println("Establishing connection. Please wait ...");
try {
socket = new Socket(serverName, serverPort);
System.out.println("Connected: " + socket);
start();
} catch (UnknownHostException uhe) {
System.out.println("Host unknown: " + uhe.getMessage());
} catch (IOException ioe) {
System.out.println("Unexpected exception: " + ioe.getMessage());
}
String line = "";
// while (!line.equals(".bye")) {
try {
Thread.sleep(1000);
//TODO get data from input
// line = console.readLine();
line="1";
if(line.equals("1"))
line="1,123";
streamOut.writeUTF(line);
streamOut.flush();
} catch (IOException ioe) {
System.out.println("Sending error: " + ioe.getMessage());
}
// }
}
public void start() throws IOException {
// console = new BufferedReader(new InputStreamReader(System.in));
streamOut = new DataOutputStream(socket.getOutputStream());
}
}
And here is the Server code:
import java.awt.*;
import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
public class ControlServer {
private Socket socket = null;
private ServerSocket server = null;
private DataInputStream streamIn = null;
public static void main(String args[]) {
ControlServer server = null;
server = new ControlServer(5555);
}
public ControlServer(int port) {
try {
System.out
.println("Binding to port " + port + ", please wait ...");
server = new ServerSocket(port);
System.out.println("Server started: " + server);
System.out.println("Waiting for a client ...");
socket = server.accept();
System.out.println("Client accepted: " + socket);
open();
boolean done = false;
while (!done) {
try {
String line = streamIn.readUTF();
// TODO get the data and do something
System.out.println(line);
done = line.equals(".bye");
} catch (IOException ioe) {
done = true;
}
}
close();
} catch (IOException ioe) {
System.out.println(ioe);
}
}
public void open() throws IOException {
streamIn = new DataInputStream(new BufferedInputStream(
socket.getInputStream()));
}
public void close() throws IOException {
if (socket != null)
socket.close();
if (streamIn != null)
streamIn.close();
}
}

I would like to summarize some good practices regarding the stability of TCP/IP connections which I apply on a daily basis.
Good practice 1 : Built-in Keep-Alive
socket.setKeepAlive(true);
It automatically sends a signal after a period of inactivity and checks for a reply. The keep-alive interval is operating system dependent though, and has some shortcomings. But all by all, it could improve the stability of your connection.
Good practice 2 : SoTimeout
Whenver you perform a read (or readUTF in your case), your thread will actually block forever. In my experience this is bad practice for the following reasons: It's difficult to close your application. Just calling socket.close() is dirty.
A clean solution, is a simple read time-out (e.g. 200ms). You can do this with the setSoTimeoutmethod. When the read() method timeouts it will throw a SocketTimeoutException. (which is a subclass of IOException).
socket.setSoTimeout(timeoutInterval);
Here is an example to implement the loop. Please note the shutdown condition. Just set it to true, and your thread will die peacefully.
while (!shutdown)
{
try
{
// some method that calls your read and parses the message.
code = readData();
if (code == null) continue;
}
catch (SocketTimeoutException ste)
{
// A SocketTimeoutExc. is a simple read timeout, just ignore it.
// other IOExceptions will not be stopped here.
}
}
Good practice 3 : Tcp No-Delay
Use the following setting when you are often interfacing small commands that need to be handled quickly.
try
{
socket.setTcpNoDelay(true);
}
catch (SocketException e)
{
}
Good practice 4 : A heartbeat
Actually there are a lot of side scenario's that are not covered yet.
One of them for example are server applications that are designed to only communicate with 1 client at a time. Sometimes they accept connections and even accept messages, but never reply to them.
Another one: sometimes when you lose your connection it actually can take a long time before your OS notices this. Possibly due to the shortcomings described in good practice 3, but also in more complex network situations (e.g. using RS232-To-Ethernet converters, VMware servers, etc) this happens often.
The solution here is to create a thread that sends a message every x seconds and then waits for a reply. (e.g. every 15 seconds). For this you need to create a second thread that just sends a message every 15 seconds. Secondly, you need to expand the code of good practice 2 a little bit.
try
{
code = readData();
if (code == null) continue;
lastRead = System.currentTimeMillis();
// whenever you receive the heart beat reply, just ignore it.
if (MSG_HEARTBEAT.equals(code)) continue;
// todo: handle other messages
}
catch (SocketTimeoutException ste)
{
// in a typical situation the soTimeout is about 200ms
// the heartbeat interval is usually a couple of seconds.
// and the heartbeat timeout interval a couple of seconds more.
if ((heartbeatTimeoutInterval > 0) &&
((System.currentTimeMillis() - lastRead) > heartbeatTimeoutInterval))
{
// no reply to heartbeat received.
// end the loop and perform a reconnect.
break;
}
}
You need to decide if your client or server should send the message. That decision is not so important. But e.g. if your client sends the message, then your client will need an additional thread to send the message. Your server should send a reply when it receives the message. When your client receives the answer, it should just continue (i.e. see code above). And both parties should check: "how long has it been?" in a very similar way.

You could wrap a thread around the connection and have it periodically send a status to keep the line open, say every 30 seconds or whatever. Then, when it actually has data to send it would reset the keep alive to be 30 seconds after the last transmission. The status could be helpful to see if the client is still alive anyway, so at least it can be a useful ping.
Also, you should change your server code, you appear to only handle one connection at the moment. You should loop and when a socket connection comes in spawn a thread to handle the client request and go back to listening. I may be reading to much into what may just be your test code, though.

Make the client socket connection wrapped around a thread. Use a blocking queue to wait for messages. There should only be a single sender queue throughout your application, so use a singleton pattern.
e.g.
QueueSingleton queue = QueueSingleton.getSenderQueue();
Message message = queue.take() // blocks thread
send(message); //send message to server
When you need to send a message to the server, you can use the blocking queue to send the message.
QueueSingleton queue = QueueSingleton.getSenderQueue();
queue.put(message)
The client thread will wake up and process the message.
For maintaining the connection, use a timer task. This is special type of thread that calls a run method repetitively at specified periods. You can use this to post a message, a ping message, every so often.
For processing the received message, you could have another thread, waiting for messages on another blocking queue (receiver queue). The client thread will put the received message on this queue.

Thread pool in a web server in Java

I have my multithread web server and now i wish to implement a thread pool, however even after looking about it i don't get how can i do it in my code :(
Could someone help me get it better?
I really need to understand how what i read can be used here, because i don't see the connection and how that works.
import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
public class WebServer {
static class RequisicaoRunnable implements Runnable {
private Socket socket;
RequisicaoRunnable(Socket socket) {
this.socket = socket;
}
#Override
public void run() {
try {
//System.out.println("connection from " + socket.getInetAddress().getHostName());
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
//System.out.println("READING SOCKET...");
String str = in.readLine();
String[] arr = str.split(" ");
if (arr != null && arr.length > 2) {
while(!str.equals("")) {
//System.out.println(str);
str = in.readLine();
}
if (arr[0].equals("GET")) {
//System.out.println("REQUESTED RESOURCE: " + arr[1]);
String nomeArquivo = arr[1];
if (arr[1].startsWith("/")) {
nomeArquivo = nomeArquivo.substring(1);
}
if (nomeArquivo.equals("")) {
nomeArquivo = "index.html";
}
File f = new File(nomeArquivo);
if (f.exists()) {
FileInputStream fin = new FileInputStream(f);
socket.getOutputStream().write("HTTP/1.0 200 OK\n\n".getBytes());
byte[] buffer = new byte[1024];
int lidos;
do {
lidos = fin.read(buffer);
if (lidos > 0) {
socket.getOutputStream().write(buffer, 0, lidos);
}
} while (lidos > 0);
fin.close();
} else {
socket.getOutputStream().write("HTTP/1.0 404 Not Found\n\n".getBytes());
socket.getOutputStream().write("<html><body>HTTP/1.0 404 File Not Found</body></html>\n\n".getBytes());
}
} else {
socket.getOutputStream().write("HTTP/1.0 501 Not Implemented\n\n".getBytes());
}
}
socket.close();
} catch (IOException e) { }
}
}
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = new ServerSocket(8080);
System.out.println("waiting connections....");
while (true) {
Socket socket = serverSocket.accept();
RequisicaoRunnable req = new RequisicaoRunnable(socket);
new Thread(req).start();
}
}
}

Idea behind the Thread pool is that create a specified number of threads at start and then assign task to them. Alternatively removing headache of creating threads each time.
I was implemented it a little some days ago, here is what I done.
Create some threads at start they share a request queue
Threads are constantly looking for queue and when a request come one
of the thread dispatch the request and perform action
The Queue will be synchronized 3.
Here are some queue methods
Queue#add(); //add the socket at the end
Queue#removeFront();//remove socket
Queue#isEmpty();//boolean if queue is empty
Queue#size(); //return size of queue
Queue#getMaxSize();//get maximum allowed size for queue
Your Request processing runnable
public class Processor implements Runnable {
private Queue<Socket> requests;
private boolean shut;
Processor(Queue<Socket> requests) {
this.requests = requests;
shut = false;
}
#Override
public void run() {
while(!shut) {
if(requests.isEmpty()) {
try{
Thread.sleep(#rendomeTimemill);
} catch(InterruptedException e){}
}else {
Socket skt = Queue.removeFront();
try {
//System.out.println("processing request from " + socket.getInetAddress().getHostName());
//do you want
} catch (Exception e) {
} finally {
if(skt != null) {
try{ skt.close(); skt = null; } catch(IOException ex){}
}
}
}
}
}
public void stopNow() {
shut = true;
Thread.interrupt();
}
}
in your main thread
create a queue to put requests
//start your server socket
Queue<Socket> requests = new Queue<Socket>();
Start worker thread pool
Precessor []workers = new Processor[NUM_WORKER];
for(int i=0;i<NUM_WORKER; i++) {
worker[i] = new Processor(requests);
Thread th = new Thread(worker[i]);
th.strat();
}
in request listening
//while loope that run forever
// accept socket
if(requests.size() == requests.getMaxSize()) {
socket.getOutputStream().write("HTTP/1.0 505 Error\n\n".getBytes());
socket.getOutputStream().write("<html><body>Try again</body></html>\n\n".getBytes());
socket.close();
} else {
requests.add(socket);
}
when you want to shout down server
for(int i=0;i<NUM_WORKER; i++) {
worker[i].stopNow();
}
Note: My concern was not the HTTP headers, so i m not specific, but you must implement the complete HTTP header e.g. Content-type, Content-length etc.

JDK might be a good place to start
An Executor or ExecutorService should is what you're looking for. Reading material:
http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ExecutorService.html
The examples in there are pretty complete I think, but here's an example using the code you posted:
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = new ServerSocket(8080);
System.out.println("waiting connections....");
ExecutorService pool = Executors.newCachedThreadPool();
while (true) {
Socket socket = serverSocket.accept();
RequisicaoRunnable req = new RequisicaoRunnable(socket);
pool.execute(req);
}
}
We create an executor service that is backed by a cached thread pool. You can swap this out for any type of pool you like by changing the type of executor service you get from Executors:
http://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Executors.html
In the example I've given we use a cached thread pool which should create new threads as needed but re use old ones as they become available (finish whatever they were executing). If you look through the methods provided in that class you can create Executor services that are backed by various types of thread pool e.g. single thread, fixed number of threads, etc.
The example above should work as is, but if you want to change how the thread pool works try another thread pool type.
The cached thread pool will mean each connection will immediately be serviced, however it can create an unbounded number of threads.
on the other hand if you wanted the executor to use a blocking queue as suggested by fge you could try a fixed thread pool instead:
Executors.newFixedThreadPool(x)
you get the blocking queue for free with that.

You can use, for instance, a BlockingQueue. This is the basis for a producer/consumer scenario.
In your case:
the producer holds the server socket; it accepts new client sockets and pushes the client sockets onto the queue;
the consumers grab client sockets from the queue and process requests.
On top of all that, you can also use a bounded queue; you can try and push a new client socket to the queue; if the queue is full you can then default to a "no can't do" consumer.
Scenarios are many. There is not one answer.

OK, the idea is simple enough. You main loop currently creates a new RequisicaoRunnable object and a new Thread to run it each time it gets a connection from a client. The idea behind a thread pool is to avoid creating new Threads each time.
In the simplest version of a thread pool, you create a blocking queue, and you create and start a fixed number of worker threads before you enter your main loop. The main loop will look almost exactly the same as what you have now, but instead of starting a Thread to run each new RequisicaoRunnable, it will simply add the new object to the queue.
Your worker threads are all the same:
while (! shutdownHasBeenRequested()) {
RequisicaoRunnable requisicaoRunnable = getATaskFromTheQueue();
requisicaoRunnable.run();
}
That way, each new task (client) will be executed (handled) by the next available thread from your pool.
If this is a homework assignment then you'll pretty much want to implement what I described, filling in some details as needed.
If it's not homework, then consider using a java.util.concurrent.ThreadPoolExcecutor() instead. No point in re-inventing the wheel when there's a perfectly good wheel right there waiting to be used.
Edit: as fge said, one improvement would be to send back a quick "sorry, try again later" response when new connections are coming in faster than you can handle them. When the queue has too many pending connections in it (i.e., when you hit the limit of a BoundedQueue), that's when you know to bail out and send the "try again later" response.

Socket programming. Program creating 2 connections instead of just 1

I am building a client/server application, for some socket programming exercise.
Below is construction + run method of my server class. The server awaits a respond from the client, which in this case is just a string.
The problem is that it seems to make two connections when the client respond. From my print statements i can see that all the code in the run method is run twice, and then the first line once again.
Why would dateServer.accept(); accept a connection for only one client request?
public Server() throws Exception {
dateServer = new ServerSocket(3001);
System.out.println("Server lytter på port 3000.");
this.start();
}
public void run() {
while (true) {
try {
System.out.println("waiting for client to request");
Socket client = dateServer.accept();
System.out.println("connection established");
Connect c = new Connect(client);
clients.add(c);
this.sleep(5000);
} catch (Exception e) {
}
}
}
--EDIT--
Client code that talks to server (Message is a simple "wrapper" class"):
System.out.println("Write to server:");
String name = scanner.nextLine();
Message message = new Message(name, null);
oos.writeObject(message);
oos.flush();

If all the prints happen twice there must have been two connections. The first line prints again after that because you're in a loop.
NB:
Never ignore exceptions: especially IOExceptions.
The sleep is completely pointless. accept() will block while there are no incoming connections. You are literally wasting time here.

How to create a simultaneous connection between a server and a client on the same network in Java?

I am trying to create a MapleStory type game for my computer science final. It's basically a 2D RPG played over LAN. My question is how would I get the connection between two computers to be simultaneous?
class MagicServer extends Thread
{
private ServerSocket serverSocket;
public MagicServer(int port) throws IOException
{
serverSocket = new ServerSocket(port);
//serverSocket.setSoTimeout(10000);
}
public void run()
{
Scanner kb = new Scanner(System.in);
while(true)
{
try
{
System.out.println("Waiting for client on port " + serverSocket.getLocalPort() + "...");
Socket server = serverSocket.accept();
System.out.println("Just connected to " + server.getRemoteSocketAddress());
DataInputStream in = new DataInputStream(server.getInputStream());
DataOutputStream out = new DataOutputStream(server.getOutputStream());
System.out.println(in.readUTF());
for(int i=0; i<5; i++)
{
System.out.println(in.readUTF());
out.writeUTF(kb.nextLine());
}
server.close();
}
catch(SocketTimeoutException s)
{
System.out.println("Socket timed out!");
break;
}
catch(IOException e)
{
e.printStackTrace();
break;
}
}
}
public static void main(String [] args)
{
int port = 2001;
try
{
Thread t = new MagicServer(port);
t.start();
}catch(IOException e)
{
e.printStackTrace();
}
}
}
I was experimenting with this and I can only send/recieve messages between two computers in order (i.e. server sends to client then client sends to server) and I cannot go out of order. How would I do this?

You have to de-couple your reads/writes by using multiple threads or through interlocked queues to manage your work.
In you loop, you force the serialization by doing a read then a write in the code.
There's no reason you couldn't spawn a reader thread that feeds into a queue to perform work by worker threads and writes happen on another thread. You just need to have synchronization on the work queues.
You can also poll the socket to see if there's any data available to read and if not, send any data waiting to be written.
There's more exotic ways to do it, and plenty of examples, search around and see what your comfortable with.
There's a similar thread here

You could use multiple threads to have a connection one way in one thread on one port and a connection the other way on a different port in the other thread. This could introduce some synchronization issues though, so I would recommend rethinking your design so that communication only needs to occur in one direction at a time (you can switch back and forth as often as you like if need be).

java I/O blocks thread on server side

i m working on client socket connection. client is a GPRS hardware device. i m receiving request from this client on my serversocket and then opening multiple threads. my problem is that when device/client close the socket then my IO detects that throws an exception but when i put off the battery from the device while sending the request to the serversocket it is blocked without throwing any exception. somebody suggested me to use setSOTimeout() to comeout from the blocking thread.
i have tried Socket.setSOTimeout(int) . but this is not working out in my case. i m sending my code properly.
// inside main class---
public class ServerSocketClass{
public ServerSocketClass() {
try{
serverSocket = new ServerSocket(port);//creating a serversokcet on a port
System.out.println("Server waiting for client on port " +
serverSocket.getLocalPort());
} catch (Exception e) {
e.printStackTrace();
handleExceptions("errorServerSocketOpen.txt", e);
System.exit(0);
}
try {
while (true) {
Socket clientSocket = serverSocket.accept();//accepting the client connection and //creating client socket
new AcceptConnection(clientSocket);//calling the constructor of other //class to open a new thread
System.out.println("constructor called.....");
}
} catch (Exception e) {
handleExceptions("errorClientSocketOpen.txt", e);
}
}
}
//inside other class---
public class AcceptConnection implements Runnable {
public AcceptConnection(Socket socket) {
this.clientSocket = socket;
if (clientSocket != null) {
new Thread(this).start();
}
public void run() {
// clientSocket.setSoTimeout(60000);// this is what i added.timeout on each client socket opened in threads
InputStream inputStream = clientSocket.getInputStream();
DataOutputStream dataOutputStream = new DataOutputStream(clientSocket.getOutputStream());
byte[] mainBuffer = new byte[2048];
int len = -1, totalLength = 0;
debugInfo = " GOING TO READ FROM SOCKET.... " + "\n";
while ((len = inputStream.read(mainBuffer)) > -1) {
totalLength = len;
}//end of while
} }//end of other class
now my problem is that when multiple threads are opened and i send the data from client than after 60 seconds it closes the main socket and stops receiving the data.and a readtimeout error occurs.
please help me out and tell me how my objective could be fulfilled.
thanks in advance
**
#stephen
**
ok stephen got it what u r trying to say... u r right on your statement "Well yes ... that's what you told it to do."May be i m not able to make u understand my problem or i m not getting the setSoTimeout() logic as i m newbie in java.
i would like to ask one more time...
this is how i m creating a new client socket for each client connection and opening a new thread for each client connection.
while (true) {
Socket clientSocket = serverSocket.accept();//accepting the client connection and //creating client socket
new AcceptConnection(clientSocket);//calling the constructor of other class to open a new thread
System.out.println("constructor called.....");
}
public void run() {
// clientSocket.setSoTimeout(60000);// this is what i added.timeout on each uclient socket opened in threads
........................................
.......................................
}
now i want to say if i m opening a new thread for a new client connection and i m separately putting setSoTimeout() on each client connection object then if a particular thread A is blocked on I/O while reading then after a timeout set in setSoTimeout(50000) ,say 50 sec ,only thread A should come out of read and give the exception, not other threads running simultaneously say B,C,D.but in my case after a timeout all threads returns after giving exception and in fact any new client connection gives the same error and server application stops receiving any data on read.
i want only thread A should give exception and come out from read without affecting other client socket objects(or threads).
now i hope i have told u everything about my confusion and problem.
please help me out and thanks a lot.

now my problem is that when multiple threads are opened and i send the data from client than after 60 seconds it closes the main socket and stops receiving the data.and a readtimeout error occurs.
Well yes ... that's what you told it to do.
As I think I said in my answer to your previous question, you cannot distinguish between these cases:
The client has no data to send for a period.
The network is partitioned for a period.
The client has disappeared from the network.
Another option that you have is to call Socket.setKeepAlive() to enable TCP keepalives. This causes the a special "keepalive" handshake to be performed periodically to ensure that the TCP/IP connection is still alive. Unfortunately, it doesn't look like you can set the TCP/IP keepalive interval in Java.
EDIT
NIO won't help. When I said "cannot" above ... I meant that it is PHYSICALLY IMPOSSIBLE to do it. Here's an analogy to help you understand.
The only communication between Outer
WoopWoop and the rest of the world is
by letter. Once a week my friend in
Outer WoopWoop (who lives alone) posts
a letter to me to fill me in on the
gossip. Last week I didn't receive a
letter from my friend. How can I tell if:
my friend has died,
my friend had no gossip last week,
the Outer WoopWoop postal workers have been on strike, or
all of the above?
The correct answer is that I cannot tell.