Is there a way to check for incoming connections to ServerSocket, so my server would not stop when waiting for someone to connect.
I need something like
try {
if (server.thereisarequest()) {
Socket socket = server.accept();
}
}
Are you sure you want this? Having the code presented, only one thread is waiting. Run another threads which handle accepted connections so that your server doesn't stop.
You can also use java non-blocking asynchronous I/O (nio and nio2), but in the background there is a thread on duty anyway.
Keep it simple... use an infinite while loop to make server look for the incoming request... thats how most of the servers works..
while (true) {
incoming = s.accept(); // incoming is of type Socket.
}
You can create a new thread to accept connections.
Then you have 2 choices:
Process the socket in a new thread
Put the sockets in a queue (ConcurrentLinkedQueue) and check it queue from you main thread.
Related
I'm trying to write an emulator for a single-threaded physical product. It accepts one long-lived connection and any other connections get a single error message (in the same thread).
I know I can use java.net with two threads:
Thread 1 - start ServerSocket on port XXXX and wait for accept(). For the first connection create a Socket and Thread #2, and for other connections produce an error message.
Thread 2 - process the Socket IO.
But how can I do it with one thread, so it behaves more like the physical product (ie. repeatedly attempting connections would starve the thread from dealing with the first connection)?
Trying not to use third-party libraries, but can do if that's the only option.
Thanks!
Unfortunately the common java.net.ServerSocket as well as the java.nio.channels.ServerSocketChannel only feature a blocking method to accept incoming connections. However the java.nio package features many other classes and methods to handle I/O-Operations in a single thread by multiplexing the opened channels.
This approach would still enforce a dedicated Thread for the accepting ServerSocketChannel but you could handle every accepted connection in a single thread.
In comparison the ServerSocket approach needs one new thread for each new connection.
Imgaine you connect 100 clients using a ServerSocket then you will end up with 101 thread. With a ServerSocketChannel you could end up using only 2 thread.
Still programming is often about a tradeoff between complexity/flexibility and performance. So keep that in mind.
A possible solution I could think of could look like this:
public static void main( String[] args ) throws IOException
{
int portNr = 8080;
ExecutorService es = Executors.newSingleThreadExecutor();
ChannelHandler ch = new ChannelHandler();
es.execute( ch );
// Starting server:
ServerSocketChannel serv = ServerSocketChannel.open();
// Bind socket to Port
serv.socket().bind(new InetSocketAddress(portNr));
while( serverAlive )
{
ch.addChannel(serv.accept());
}
serv.close();
}
How you actually process the new added SocketChannel depends on your application. And so does the ChannelHandler#addChannel method.
It seems like you should accept the single connection and then close the ServerSocket. Any future connect attempts will then get a connection refusal. When the long-lived connection ends, create a new ServerSocket, accept one more connection, ... Rinse and repeat.
EDIT If you have to deliver an error message as per your comment below, you have to accept the connection to send it over, and if you have to do I/O and accepting all in one thread you have to use java.nio.channels.ServerSocketChannel/SocketChannel, non-blocking mode, and a Selector.
Using a ServerSocket with one Thread is not a good idea at all. WHY?
you know socket.accept() waits till the next client connects, so the Thread is blocked and if you only have one Thread your whole Programm is blocked till a Client connects.
May you explain why you try to do it single threaded?
I'm in the process of writing a messaging program, and I'm running into a spot where I'm having trouble understanding how to pass a socket over to a new thread for handling outbound messages via TCP. I'm currently using UDP packets for messages coming from a client, to the server, which, being UDP, doesn't require very much processing, as it's simply listening for incoming packets, before it de-serializes the objects, and processes them as needed in a separate thread. My problem now is, I'm setting up a client initiated TCP socket for reverse traffic, from the server to the assorted clients that connect. I've done a bit of research, and I already understood that each client should have their own thread for handling outgoing messages, along with another thread simply for accepting the incoming connections. I'm unsure of how to actually achieve this, and I've done some research into the topic.
I've found this: http://docs.oracle.com/javase/tutorial/networking/sockets/clientServer.html
The resource above basically verified my original suspicion that this would have to be handled by threads dedicated to the client. They included psuedo code here, which is representing my listener thread.
while (true) {
accept a connection;
create a thread to deal with the client;
}
I'm a bit of a visual learner, and I have been searching for some type of an example where this is done. I'm unsure of what variable I'd be passing over to the thread that keeps the original connection open, and pushes data back to clients. I'm also having a little bit of trouble grasping whether it even keeps the same socket open, or if a new one needs to be established, which then, makes me believe a firewall could interfere, but I know that won't be the case.
Can somebody explain this for me in detail? If possible, an example would be greatly appreciated!
I'll be likely replying and commenting on responses in about 15-30 minutes from the time this is posted.
What you are doing sounds correct. I typically implement a server like this (simplified version with no tracking of the clients and so on):
#Override
public void run() {
//start listening on the port
try {
serverSocket = new ServerSocket(port);
logger.info("Listening for connections on port " + port);
} catch (IOException e) {
logger.error("Cannot start SocketListener on port " + port + ". Stopping.", e);
return;
}
while (!stopped) {
try {
//wait for connection
Socket newSocket = serverSocket.accept();
ClientThread client = new ClientThread(newSocket);
Thread clientThread = new Thread(client, MEANINGFUL_THREAD_ID);
clientThread.start();
} catch ...
}
}
where serverSocket is a ServerSocket instance variable and stopped is a flag I use to stop the listener thread.
So to answer your questions in the comment, you normally pass the Socket object to each client thread so that that thread can work with the input and output stream and handle closing of the socket and so on. Once you "accept" a socket connection, you do not need to recreate the ServerSocket, you simply call .accept() again to start waiting for a new connection.
In most cases, you will need to keep track of all client threads in your server so that you can stop the server gracefully or do broadcasts for example.
What does the server.accept() method return when no new socket is formed, i.e., when no new connection is made? Is it possible to go to next line of code while sever.accept() is waiting for a new connection?
If you want to do something while the server is waiting for a connection you can use multiple threads. In a single-threaded application you cannot call a function and continue with your work without waiting it to return: either you are waiting for the server to accept a connection, or you are doing other computations.
A possible alternative to threads is setting the SO_TIMEOUT socket option on the server socket. This makes the call to accept throw an exception if a connection is not received within the timeout, allowing you to go to the next line.
For example:
ServerSocket ss = new ServerSocket(8989);
ss.setSoTimeout(10000); // 10 seconds
Socket clientSocket;
try {
clientSocket = ss.accept();
// process connection from client
} catch (SocketTimeoutException ste) {
// connection was not received,
// do something else
}
Another alternative is using non-blocking IO and the Selector class. Here's an example of a non-blocking socket server written this way.
No. server.accept() is a blocking method and it will wait.
From the javadoc
Listens for a connection to be made to this socket and accepts it. The
method blocks until a connection is made.
I coded a little TCP thread Server, which creates a new thread for every server.accept(). Nearly everything works great, but I have problems to kill all threads per interrupt. (I use a ServiceExecutor to manage the threads. Therefore I use the shutdownNow method to reach the interrupt-methods) The Worker-instances use a BufferedReader and it's readline-method to receive and compute the input. AFAIK the readline blocks and would not react on an interrupt, but how to stop it?
while(!isInterrupted()){
try {
clientSocket = this.serverSocket.accept();
} catch(IOException e){
break;
}
this.threadPool.execute(new ThreadWorker(clientSocket));
}
threadPool.shutdownNow();
try{
serverSocket.close();
}catch(IOException e){
//todo
}
I tried to close the ServerSocket to kill the Input/Output Streams, but it didn't work as expected.
A couple alternatives:
1) If you are closing the whole app, and there is nothing of importance to explicitly close, call System.Exit(0). HEALTH WARNING - doing this causes some developers to have apoplectic fits and post endlessly about 'cleaning up gracefully'.
2) Keep a thread-safe list of all client sockets in the accept() thread. Pass a reference to this list as part of your client context that is passed to the client<>server threads. Add new connections to the list in the accept() thread. When a client thread detects a disconnect, remove its entry from the list. When you want to close all clients, iterate the list and close the client sockets - this will cause the readline method to return early, with an error, in the client threads.
I'm accepting a connection from a client and then passing that connected socket off to another object, however, that socket needs to be non-blocking. I'm trying to use getChannel().configureBlocking(false) but that does not seem to be working. It needs to be non-blocking because this the method below is called every 100ms. Is there some other way that I should be making this non-blocking? Thanks for any help!
public void checkForClients() {
DataOutputStream out;
DataInputStream in;
Socket connection;
InetAddress tempIP;
String IP;
try {
connection = serverSocket.accept();
connection.getChannel().configureBlocking(false);
System.err.println("after connection made");
in = new DataInputStream(connection.getInputStream());
out = new DataOutputStream(connection.getOutputStream());
tempIP = connection.getInetAddress();
IP = tempIP.toString();
System.err.println("after ip string");
// create a new user ex nihilo
connectedUsers.add(new ConnectedUser(IP, null, connection, in, out));
System.err.println("after add user");
} catch (SocketTimeoutException e) {
System.err.println("accept timeout - continuing execution");
} catch (IOException e) {
System.err.println("socket accept failed");
}
}
Two things:
Why aren't you using a ServerSocket if you're listening for connections?
If you want to accept multiple clients you want to use a loop.
The basic structure of a multi-client server is:
while (true) {
// accept connections
// spawn thread to deal with that connection
}
If the issue is blocking on the accept() call, well that's what accept() does: it blocks waiting for a connection. If that's an issue I suggest you have a separate thread to accept connections.
See Writing the Server Side of a Socket.
I would expect your code to block on the accept call, never getting to the configureBlocking call.
I typically spin off a separate thread for each socket connection, and let it block until a connection is actually made/accepted This allows the main thread to continue unblocked while it is waiting for client connections.
If you're looking for non-blocking sokets, my suggestion is to use Selectors and ServerSocketChannels with the NIO package.
http://java.sun.com/j2se/1.4.2/docs/guide/nio/
If the typical blocking socket doesn't give you the availability you need (a connection every 100ms does seem tight). You should look at a non-blocking socket. Here is a tutorial. You can also look at Apache MINA to make this easier.
One approach is to use an I/O loop (event loop) in a single threaded environment. Take a look at Deft web server for inspiration. (Especially the start() method in IOLoop)