Develop Reference

What is the proper way to send a message to a particular client in a server-client structure?

I am building this program where a server sends different order to different clients. One part of the program sends the order to the server, the server processes to which client the message goes to and then it sends it to the particular client it should go.
Here is the code where the message is send:
#Override
public void run() {
boolean active = true;
String oldMsgToDron = "";
String to_print = "";
BufferedWriter bw = new BufferedWriter(osw);
while(active) {
try {
//msgToDron is declared in DronController as: public static volatile String msgToDron;
to_print = DronController.msgToDron;
if(to_print.equals(oldMsgToDron) == false) {
System.out.println("Message we are sending to the dron: " + to_print);
if(to_print.length()>45 && to_print.startsWith("!whisper") && dronId.equals(to_print.substring(9, 45))) {
bw.write("Server whispers:" + to_print.replace("!whisper " + dronId, ""));
bw.newLine();
bw.flush();
System.out.println("Message we sent to the dron: " + to_print);
}
else if (to_print.startsWith("!whisper") == false){
bw.write(to_print);
bw.newLine();
bw.flush();
System.out.println("A message for all drones has been sent.");
}
try{Thread.sleep(5);}catch(InterruptedException ex){ex.printStackTrace();}
oldMsgToDron = "";
to_print = "";
DronController.msgToDronEsclavo = "";
}
}catch(IOException e) {
try {
active = false;
if(bw != null)
bw.close();
}catch(IOException f) {e.printStackTrace();f.printStackTrace();break;}
}
}
}
My problem is that the message is processed inside the thread that controls the connection to the client and at the end of the loop the message is changed to an empty string. This is so I can send the same order twice in a row and the server isn't constantly sending the same messages to the client.
The thing is that by doing it this way if I have 3 or more connections it starts to give me problems because whenever one of the threads reaches the end they turn the variables DronController.msgToDronEsclavo, to_print and oldMsgToDron to empty strings before the other threads had time to check if the message was for the client they have assigned.
How can I avoid that the different threads change the variable before the others have finished? Are there any kind of design pattern for this kind of structures? What are some good practices with this type of programs?
Also, I've thought about making the client check if the message sent is for them or not but I find this solution unsecure.

Usually it is the client who connects to the server via network - to request data or to send data. The server has no reliable way to know when the client would connect the next time, or trigger a connection itself.
If you want to send data from the server to the client there used to be patterns that the client would poll for data, or have always one request waiting for the next packet from the server. You may want to read about AJAX and Websocket
Another pattern would be to install a webhook, which essentially gives the server a URL at which the client can be reached. This in essence swaps the server to acting as client and the client to acting as server for this data transfer.
Well, the above was written related to your question title. Now looking at the content, you are asking how to prevent that threads impact each other.
Learn Java's concepts on thread synchronization. Here is a nice tutorial.

Error while reading data through socket communication

Following scenario that explains my problem.
I've a PLC that acts as a server socket program. I've written a Client Java program to communicate through socket communication with the PLC.
Steps that take place in this process are:
1) For each second my Client program happen to communicate with the PLC, read the data in stream, store the data temporarily in a ByteArrayOutputStream and closing both input stream and socket. Following snippet gives the idea
try {
socket = new Socket(host, port);
is = socket.getInputStream();
outputBuffer = new ByteArrayOutputStream();
byte[] buffer = new byte[1024];
int read;
if((read = is.read(buffer)) != -1) {
outputBuffer.write(buffer, 0, read);
}
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
System.out.println("Before closing the socket");
try {
is.close();
socket.close();
} catch (IOException e) {
e.printStackTrace();
}
System.out.println("After closing the socket");
} catch (Exception e) {
e.printStackTrace();
}
}
2) Processing stored data according to my requirement is what I'm trying to do. So for every 1 second, client program connects to Server, read the data(if data is present), store the data, close socket and process it. And it has to happen for a very long run, probably till the Server program is on. And that may happen till for every few weeks.
3) Problem what I'm facing is, I'm able to run the above show for 1-2 hours, but from then, Client Program unable to fetch the data from the Server Program(PLC in this case), though both are connected through socket. I.e 128 bytes of data present, but Client program isn't able to read that data. And this started happening after program run successfully for almost 2hours
4) Please find the brief code which may help for you to look into.
public class LoggingApplication {
public static void main(String[] args) throws NumberFormatException {
if (args.length > 0 && args.length == 2) {
String ipAddress = mappingService.getIpAddress();
int portNo = (int) mappingService.getPortNo();
ScheduledExecutorService execService = Executors.newScheduledThreadPool(1);
execService.schedule(new MyTask(execService, ipAddress, portNo, mappingService), 1000, TimeUnit.MILLISECONDS);
} else {
throw new IllegalArgumentException("Please pass IPAddress and port no as arguments");
}
}
}
Runnable Code:
public class MyTask implements Runnable {
public ScheduledExecutorService execService;
private String ipAddress;
private int portNo;
private ConfigurationMappingService mappingService;
private MySocketSocketUtil mySocketSocketUtil;
public MyTask(ScheduledExecutorService execService, String ipAddress, int portNo, ConfigurationMappingService mappingService) {
this.execService = execService;
this.ipAddress = ipAddress;
this.portNo = portNo;
this.mappingService = mappingService;
}
public void run() {
MySocketSocketUtil mySocketSocketUtil = new MySocketSocketUtil(ipAddress, portNo);
execService.schedule(new MyTask(execService, ipAddress, portNo, mappingService), 1000, TimeUnit.MILLISECONDS);
mySocketSocketUtil.getData(); //It's able to fetch the data for almost 2 hours but from then, it's just getting empty data and it's keep on giving empty data from then. and so on.
/*
*
*Some code
*/
}
}
Here's where, I'm having the problem
mySocketSocketUtil.getData(); is able to fetch the data for almost 2 hours but from then, it's just getting empty data and it's keep on giving empty data from then. and so on. It's a big question I know, And I want to understand what might have gone wrong.
Edit: I'm ignoring the condition to check end of the stream and closing a socket based on it is because, I knew I'm going to read first 1024 bytes of data only always. And So, I'm closing the socket in finally block

socket = new Socket(host, port);
if(socket != null && socket.isConnected())
It is impossible for socket to be null or socket.isConnected() to be false at this point. Don't write pointless code.
if((read = is.read(buffer)) != -1) {
outputBuffer.write(buffer, 0, read);
};
Here you are ignoring a possible end of stream. If read() returns -1 you must close the socket. It will never not return -1 again. This completely explains your 'empty data':
from then, it's just getting empty data and it's keep on giving empty data from then, and so on
And you should not create a new Socket unless you have received -1 or an exception on the previous socket.
} else {
System.err.println("Socket couldn't be connected");
}
Unreachable: see above. Don't write pointless code.

You should never disconnect from the established connection. Connect once in the LoggingApplication. Once the socket is connected keep it open. Reuse the socket on the next read.

I think there are couple of points you need to fix before getting to the solution to your problem. Please try to follow the following suggestions first:
As #EJP said this code block is not needed.
if(socket != null && socket.isConnected()) {
also you are using a byte array of length 1024 and not using while or for loop to read the data stream. Are you expecting only a block of data which will never exceed 1024 bytes?
byte[] buffer = new byte[1024];
int read;
if((read = is.read(buffer)) != -1) {
This is also not needed as it is unreachable.
} else {
System.err.println("Socket couldn't be connected");
}
Can you explain the data stream behavior you are expecting?
Last but not the least is.read(buffer) is a blocking call so if there is no data to read yet, it will hold the thread execution at that point.
Please try to answer the questions I have asked.
#KishoreKumarKorada from your description in the comment section, it seems like you are monitoring the data change on server side. Socket stream works in a read-once fashion. So,
First thing is, you need to request from server every time and the server needs to RESEND the data on every request.
Second, the way you presented is more like you are operating on byte level, which is not very good way to do that unless you have any legitimate reason to do so. The good way is to wrap the data in JSON or XML format and send it over the stream. But to reduce bandwidth consumption, you may need to operate on byte stream sometimes. You need to decide on that.
Third, for monitoring the data change, the better way is to use some timestamp to compare when the data has changed on the server side and what is the timestamp stored on the client side, if they match, data has not changed. Otherwise fetch the data from the server side and update the client side.
Fourth, when there is data available that you are not able to read, can you debug the ins.read(...) statement to see if its getting executed and the execution goes inside the if block or if statement is evaluated to false? if true then examine the read value and let me know what you have found?
Thanks.

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.

Async reading in server-client: separate client messages

I'm implementing a simple server using AsynchronousServerSocketChannel. For testing purposes, I created a tiny client prototype that sends two messages, "hi" and "stackoverflow", then disconnects. On server side, I read the arrived messages and print them to standard output. When the client executed, I'm expecting to receive:
message [hi], bytecount 2
message [stackoverflow], bytecount 13
The problem is, that sometimes both messages already arrived when server invokes reading callback so I get
message [histackoverflow], bytecount 15
instead.
The question is, if it is possible to ensure on server side that the messages arrive separately and if yes, how to do it?
Here's my CompletionHandler prototype that handles client connections:
class CommunicationHandler implements CompletionHandler<AsynchronousSocketChannel, Void> {
private final AsynchronousServerSocketChannel server;
public CommunicationHandler(final AsynchronousServerSocketChannel server) {
this.server = server;
}
#Override
public void failed(Throwable ex, Void attachment) {}
#Override
public void completed(final AsynchronousSocketChannel client, Void attachment) {
// handle client messages
final ByteBuffer buffer = ByteBuffer.allocateDirect(Server.BUFFER_SIZE);
final Session session = new Session();
try {
client.read(buffer, session, new CompletionHandler<Integer, Session>() {
#Override
public void completed(Integer byteCount, final Session currSession) {
if (byteCount == -1) {
return;
}
buffer.flip();
// TODO forward buffer to message handler (probably protocol?)
System.out.println("message [" + convertToString(buffer) + "], byteCount " + byteCount);
buffer.clear();
// read next message
client.read(buffer, currSession, this);
}
#Override
public void failed(Throwable ex, final Session currSession) {}
});
}
// accept the next connection
server.accept(null, this);
}
ByteBuffer to String conversion:
public static String convertToString(ByteBuffer bb) {
final byte[] bytes = new byte[bb.remaining()];
bb.duplicate().get(bytes);
return new String(bytes);
}
Here is a test client prototype:
public class Client {
public final void start() {
try (AsynchronousSocketChannel client = AsynchronousSocketChannel.open();) {
Future<Void> connCall = client.connect(InetAddress.getByName("127.0.0.1"), 8060));
connCall.get();
// client is now connected
// send greeting message
Future<Integer> writeCall = client.write(Charset.forName("utf-8").encode(CharBuffer.wrap("hi")));
writeCall.get();
// Thread.sleep(5000L);
writeCall = client.write(Charset.forName("utf-8").encode(CharBuffer.wrap("stackoverflow")));
writeCall.get();
client.close();
} catch (IOException e) {
} catch (InterruptedException ex) {
} catch (ExecutionException ex) {
}
}

In addition to the possibility of getting two (or even more) writes in one read, for larger messages (usually about 3k or more) you can get one write split over several reads. TCP is a stream protocol and does not preserve record boundaries, unless by chance: What is a message boundary? There are two solutions that work in general, although with async channel I think you'll need to do your own buffer management which may be confusing and hard to test:
add an explicit length field before each record
add a delimiter after each record when there is a byte not otherwise used, or an escape can be used to distinguish data from the delimiter
and several others that have been tried:
as your comment suggests, wait long enough that the first request has always been read before the second is sent. On the local networks and test systems used by developers this usually is a few milliseconds or even less; on the real Internet it is fairly often several seconds, sometimes minutes, and in theory can be hours or even days.
if records are never longer than a few fragments (maybe 10k or so) use UDP (available in Java as DatagramSocket but not as a NIO channel AFAICS) and implement your own protocols to handle message loss, duplication and reordering (which is hard to do and often ends up failing in some obscure cases that were discovered and avoided or fixed in TCP 30 years ago)
use SCTP (not available in Java at all AFAICS, and not too many other systems either)
Aside: your test client sends data in UTF-8, but new String (byte[]) uses the default encoding which is platform-dependent and not necessarily UTF-8. I'm not sure it's guaranteed but in practice all usable encodings include ASCII as a subset, and your example data is ASCII. But if you want to support actual UTF-8 data code for it.

Receiving UDP in Java without dropping packets

I have a library which I need to improve since it is dropping to many packets. I want to receive a RTP-Stream, but the streamer is sending bursts of 30-40 packets within a millisecond (MJPEG-Stream). I can see the packets being complete when monitoring traffic in Wireshark. But when trying to receive them in Java, I lose a lot of those packets.
I have already been able to improve the libraries behavior by implementing a ring buffer that would constantly get filled whenever a packet is available and a separate reader thread that reads from this buffer. But I'm still not able to get all the packets from my socket that I can see in wireshark. Through RTP sequence numbers I can monitor in the reader thread if the packet processed is the one expected.
The following code is handling packet receiving:
private volatile byte[][] packetBuffer = new byte[1500][BUFFER_SIZE];
private volatile DatagramPacket[] packets = new DatagramPacket[BUFFER_SIZE];
private volatile int writePointer = 0;
public void run() {
Thread reader = new RTPReaderThread();
reader.start();
while (!rtpSession.endSession) {
// Prepare a packet
packetBuffer[writePointer] = new byte[1500];
DatagramPacket packet = new DatagramPacket(packetBuffer[writePointer], packetBuffer[writePointer].length);
// Wait for it to arrive
if (!rtpSession.mcSession) {
// Unicast
try {
rtpSession.rtpSock.receive(packet);
} catch (IOException e) {
if (!rtpSession.endSession) {
e.printStackTrace();
} else {
continue;
}
}
} else {
// Multicast
try {
rtpSession.rtpMCSock.receive(packet);
} catch (IOException e) {
if (!rtpSession.endSession) {
e.printStackTrace();
} else {
continue;
}
}
}
packets[writePointer] = packet;
this.incrementWritePointer();
synchronized (reader) {
reader.notify();
}
}
}
What I already know:
I know that UDP is allowed to lose packets, but I still want to
achieve the best possible result. If wireshark can see the packet, I
want to be able to retrieve it as well, if possible.
I know that the ring buffer is never full while losing packets, so
this doesn't make me lose packets either. I tried with BUFFER_SIZES
of 100 and even 1000, but I already lose the first packets before a
total of 1000 packets has been send.
So the question is: what is best practice to receive as many packets as possible from a DatagramSocket? Can I improve handling of packet bursts?

Try setting the SO_RCVBUF size on the datagram socket with rtpSock.setReceiveBufferSize(size). This is only a suggestion to the OS, and the OS may not honor it, especially if it is too large. But I would try setting it to (SIZE_OF_PACKET * 30 * 100), where 30 is for the number of packets in a burst, and 100 is a guess of the number of milliseconds where you will not be able to keep up with the arrival speed.
Note that if your code cannot keep up with processing at the arrival speed in general, the OS has no choice but to drop packets.