I am trying to make a client send a request to a server and receive a response whilst keeping to connection up.
If i close the socket:
//server side
outToClient.writeBytes("Message to send");
connectionSocket.close();
//client side
serverResponse = inFromServer.readLine();
System.out.println("FROM SERVER: " + serverResponse);
Output on client side
FROM SERVER: Message to send
And after that the connection is lost, obviously.
If i do not close the socket:
//server side
outToClient.writeBytes("Message to send");
//client side
serverResponse = inFromServer.readLine();
System.out.println("FROM SERVER: " + serverResponse);
No output on client side.
The server never sends the message or the client never receives it.
Anyone knows a possible reason for this to be happening?
The client uses a thread to receive messages and a thread to send messages. The client socket is created in the main client thread so receiver and sender threads use the current socket to communicate.
Thanks in advance.
If the client expects to read a line, the server should write a line. A line is terminated by a \n character.
It may also be necessary to flush the stream you are using to send data to the client:
outToClient.writeBytes("Message to send\n");
outToClient.flush();
Without seeing the code it's hard to say if the flush is required or not.
Never, ever write any code on top of TCP without first specifying the protocol. Otherwise, when you have a problem like this, it's impossible to determine which end is at fault.
One side believes it's sending a "message". The other side does not believe the data it receives is a "message". Which is right? Well, if we had a protocol specification, we'd look at its definition of a "message" and see.
But we don't have one, so we can't say. This makes it impossible to construct a correct fix. If you change the sender to send a line, the receiver will still be broken in requiring a line. Or won't it? And you change the receiver to process messages that aren't lines, is that fixing it or breaking it?
It seems in this case that both sides are wrong. The most likely intent is that a message consists of a line of data terminated with a newline. The sender is not sending a newline and the recipient is not insisting on one either (because it accepts the data as a "message" when the connection closes). So unless the design intent really was that a "message" is a chunk of data not including a line ending terminated by either a line ending or the close of a connection, both sides are wrong.
Document the protocol. It's an essential step.
Related
I am trying to set up a program where the server can communicate with multiple clients. The program is written in Java. I got it all working on the same machine so I decided to try LAN. I converted the program to JAR files and I tried connecting my laptop to my PC (both are on the same network). The connection is successful but unfortunately only 1 message arrives to the server. As you can see in the code below, I send multiple messages (Meaning that i write multiple times) via DataOutputStream. One defines the datatype (in the following example 0 means that it's a String) and the other sends the actual message data. I also print the size of the packets in bytes and it always matches the size of the DataOutputStream instance.
DataOutputStream dOut = new DataOutputStream(clientSocket.getOutputStream());
String str = "Hello";
//Type
System.out.println("Type output size: 1");
dOut.writeByte(0);
//Message
System.out.println("Message output size: " + (str.getBytes(StandardCharsets.UTF_8).length + 2));
dOut.writeUTF(str);
System.out.println("Length of all: " + (dOut.size()));
dOut.flush();
So now when the data from the client is sent we need to handle it on the server, which the code below does. It retrieves the InputStream from the Socket called client and inserts it into the DataInputStream. This is were it gets weird on LAN as the stream only contains the first message.
InputStream stream = client.getInputStream();
DataInputStream dIn = new DataInputStream(stream);
while(dIn.available() > 0) {
byte type = dIn.readByte();
switch(type) {
case 0:
System.out.println(dIn.readUTF());
break;
case 1:
System.out.println(dIn.readInt());
break;
case 2:
System.out.println(dIn.readByte());
break;
default:
throw new IllegalStateException("Unexpected value: " + type);
}
}
If you run the Client in the IDE on lets say a laptop connected to the same network and then you run the Server on a PC connected to the same network it will work. However, not if the programs are in JARS.
The actual stacktrace is the following:
java.net.SocketException: Connection reset
at java.base/sun.nio.ch.NioSocketImpl.implRead(NioSocketImpl.java:323)
at java.base/sun.nio.ch.NioSocketImpl.read(NioSocketImpl.java:350)
at java.base/sun.nio.ch.NioSocketImpl$1.read(NioSocketImpl.java:803)
at java.base/java.net.Socket$SocketInputStream.read(Socket.java:966)
at java.base/java.net.Socket$SocketInputStream.read(Socket.java:961)
at
java.base/java.io.DataInputStream.readInt(DataInputStream.java:393)
The stacktrace does not tell me anything, but it points at case 0: in the switch case. It can't read the String as the DataInputStream does not contain any data (I guess?).
I would also like to state that the Server is multithreaded! I have one thread that adds the sockets when they are accepted through ServerSocket.accept() and I use the second (main thread) to read the data sent from clients.
I have selected the code above as I believe that the issue lies within it, however I am new to Socket Programming and I know that some of you would like to see other parts of the code. I will add more relevant code when I am asked.
I do not know why it acts like this, does anyone know why?
What have i tried?
I have tried waiting for packets - but that has only resulted in the
Server looping forever. With waiting for packets I mean not going forward until the DataInputStream contains enough bytes.
I have disabled Nagels Algorithm through setTCPNoDelay(false).
Tried to send different datatypes, but that also failed
I tried changing the first packet to a String which resulted in the String showing up in the DataInputStream.
I have tried portforwarding the port used and I have tried disabling the firewall on both computers.
Update 1
I have been taking advice from the comments which has led a to a few discoveries:
Closing the DataOutputStream successfully sends all packets to the client.
It is also possible to build your own buffer and decode it in the server. However, it is still not possible to send any more messages after this.
It worked as a JAR because IntelliJ was being nice (Eclipse threw the same error when running in IDE)
Update 2:
I think this post is relevant. It states that SocketException is sent when a client closes it's socket "ungracefully". And because my Client closes (as it is not in a loop) and I don't close the socket properly - it will close "ungracefully" and the data will be lost. Hence the error.
The issue is now solved, and the solution is quite logical. My client does not operate in a loop, rather it sends the data and closes the program. That sounds fine, but I forgot to properly close the socket of the client.
The reason why the second 'packet' never arrived was due to me doing this tiny mistake. The packet was on it's way through the local network but the client socket improperly closed it's socket before the packet arrived to the server, which is why I got a SocketException error. See this.
I solved the issue by putting socket.close() ,where socket is the client's socket, after I had sent all the messages I wanted to send.
I'm a novice in Java socket communication, I'm trying to connect 3 clients to a server without the use of threads as the number of clients connected to the server will as be 3. I have written a condition in the server to tokenize the received input from the client and send write to the respective stream. But the message isn't reaching the server so I'm not sure whether the client isn't writing to the stream or the server is unable to receive the data. I'm trying to establish a cryptographic protocol between the connected clients for demonstration purposes.
I was using BufferedReader and PrintStream earlier since I was testing with keyboard input, then I changed to data streams still doesn't seem to work
Here my code at Server.java to forward messages, the control doesn't appear to come to this loop at all(not sure).
while(true){
String recvd=cin2.readLine();
System.out.println("At server: "+recvd);
StringTokenizer st = new StringTokenizer(recvd);
String MsgToSend=st.nextToken();
String recipient=st.nextToken();
if(recipient.equalsIgnoreCase("client1")){
cout2.writeUTF(MsgToSend);
}
else if(recipient.equalsIgnoreCase("client2")){
cout.writeUTF(MsgToSend);
}
else if(recipient.equalsIgnoreCase("client3")){
cout3.writeUTF(MsgToSend);
}
}
Here is my client-side code,
while(i==0){
String s1="A";
String s2="B";
String s3="client1";
String toSend=String.join(" ",s1+s2,s3);
System.out.println("toSend :"+toSend);
sout.writeUTF(toSend);
sout.flush();
i++;
}
Receiving Client,
while (true){
s=sin.readLine();
System.out.print("Server : "+s+"\n");
}
Not sure whether the client is unable to write or the server is unable to read. Any suggestions or solutions to correct the code?
The socket navigation logic in your server file seems to be strange as you are manually creating a string with keyword client1 and then tokenizing it for navigation. By this approach your flow would get static and establishment of a protocol gets difficult.
Please see:
You are sending the data to client 1 by forming the string AB client1, then your server file's first line is using client 2 socket reading. (Sending data from client1 socket to client1 socket, not right).
Please change that to client1 socket reading, use prinln() and readline() to send and read socket data and then check if your flow is fine!
Also, for the protocol flow, you have to send the data to the other two sockets and not to the one that is sending.
For eg: if you are sending the data from client 1 to client 2, as per your approach, you have to for a string AB client2 and send to the server, tokenize it and navigate the flow to client 2.
Hope this helps!
Thanks,
Areed
Kindly find in the details of assignment:
Start a TCP Server.
Start multiple clients to connect to the server.
On connect :
Server gives each connection a random unique name
Server starts sending PING messages to client as : “PING client abc123 at time hhmmss” every 5 seconds.
Client receives the PING messages and prints them to console.
On connect :
Client starts sending “Time request” messages every 10 seconds
Server gets the message and responds : “Client abc123, the time is hhmmss”
Client gets the time messages and prints them to console.
Things to note :
Use plain sockets (not NIO)
Make sure that the socket handling code is reusable
Make sure that the time request reads and writes are asynchronous.
Ensure that ping and all other reads and writes are also asynchronous
Ensure that ping and time response messages do not intersperse or corrupt each other
We are not here to write a program for you, but here is some stuff you can use to solve most of the problems in your project.
First off you are using TCP in Java so you will be using the following objects:
Socket - Client connection
ServerSocket - Server host
There is documentation on how to use these here.
Server side, you want multiple connections and you will want the rest of the program to run. This means multitasking. The ServerSocket should run on a separate thread. Then it should be put in a do-while loop to accept() connections. The server will then wait there until a client socket connects. When a connection is made you will need to create a new thread to let the ServerSocket accept more clients. Make sure when multi threading to use synchronized methods.
Create ServerSocket like this: new ServerSocket(port); This will open server on int port.
On the client side we will also use multithreading. The Socket will need to be on a seprate thread and to create a Socket to connect to an ip and port do the following: new Socket(ip, port) (ip is a String)
Once connected both the server and client will need to communicate. An easy way to do this is to send a message back and forth. You can't just not send because the server or client will pause and wait on receive and not be able to send. I will get to this issue in a minute.
One way to send would be to use a PrintWriter. Create a new PrintWriter with the Socket output stream. From here you can send by printing line with PrintWriter.
Make sure you flush or autoFlush your PrintWriter or else it will keep sending the same message over and over.
You can then receive a message by using BufferedReader by creating a new InputStreamReader from the socket input stream. Doing this you can use readLine(). Remember this will pause until something is received.
Doing this on both the server and client you can send back and forth. Once at this point you could easily do the rest of your requested assignment.
As I mentioned earlier, this method will result in the server and clients sending and receiving over and over even if the message is empty. Doing the following can prevent this and use less cpu and bandwidth as it will only send when it has a message to send:
(I created this diagram for a forum post for a project update I have created once)
The green lines represent received data going somewhere and the red represent data from the program going to the sender thread to be sent.
Notice on this diagram that there is a thread for receiving and sending. This allows one to send messages while the other receives, so the sender won't pause if the receiver does pause to wait to receive a message.
Other good resources:
Simple Chat Program
enter link description here
I'm using this program to test a PULL socket with ROUTER. I create/bind a ROUTER, then connect a PULL socket with an identity to it; the ROUTER then sends a message addressed specifically for the client using its identity (basic zeromq enveloping)
Test Program
public static void main(String[] o){
ZContext routerCtx = new ZContext();
Socket rtr = routerCtx.createSocket( ZMQ.ROUTER);
rtr.setRouterMandatory(true);
rtr.bind("tcp://*:5500");
ZContext clientCtx = new ZContext();
Socket client1 = clientCtx.createSocket( ZMQ.PULL);
client1.setIdentity("client1".getBytes());
client1.connect("tcp://localhost:5500");
try{
//Thread.currentThread().sleep(2000);
rtr.sendMore("client1");
rtr.sendMore("");
rtr.send("Hello!");
System.out.println( client1.recvStr());
System.out.println("Client Received: " + client1.recvStr());
}catch(Exception e1){
System.out.println( "Could not send to client1: " + e1.getMessage());
}
routerCtx.destroy();
clientCtx.destroy();
}
Results
The expected result is to print Client Received: Hello!", but instead the ROUTER throws an exception consistent with unaddressable message; I'm using setRouterMandatory(true) to throw that exception under such circumstances, however, the client explicitly sets an identity and the server sends to that identity, so I don't understand why the exception is raised.
Temporary Fix
If I add a slight delay by uncommenting Thread.currentThread().sleep(2000);, the message is delivered successfully, but I despise using sleeps and waits, it creates messy and brittle code, but more importantly, doesn't answer the "why?"
Questions
Why is this happening? It was my understanding that "late joining" applied only to PUB/SUB sockets.
Is PULL with ROUTER an invalid socket combination? I'm using it for a chat program, and aside from this issue, it works great.
Why is this happening?
You have a race condition. The client1.connect call starts the connection process, but there is no guarantee the actual connection is established when you call rtr.sendMore("client1");. Your sleep() workaround pretty much proves this.
Changing PULL to DEALER is a step in the right direction, because DEALER can send and receive. In order to avoid the need for sleeps and waits you would have to change your protocol. A simple change to the code above would be to have the DEALER connect and then immediately send a "HELLO" message to the ROUTER (could be just an empty message). The router code must be redesigned such that it does nothing until it receives a HELLO message from the DEALER. Once you have received the HELLO message you know the connection is successfully established and you can safely send your chat messages.
Also, this protocol eliminates the need for your router to know the client id in advance. Instead you can extract it from the HELLO message. A message from a DEALER to a ROUTER is guaranteed to be a multi-part message and the first part is the client ID.
My objective is to implement a simple request-response pattern based on Java sockets
that is used to request an object from a server.
It should work like this:
The client sends a message to the server, which the server evaluates. Depending on what
he received, a certain function is called. This part works.
The server writes the requested data into the ObjectOutputStream. This also works, at least
I didn't receive an error.
The client reads the data from the input stream until he receives a CLOSE message which makes the
program quit the while-loop. This does not work as it should.
Here are some critical code fragments:
// Client (Sending request) *** WORKS
objectOutputStream.writeInt(GET_OBJECT);
objectOutputStream.flush();
// Server (After receipt of the message) *** WORKS
objectOutputStream.writeInt(object);
objectOutputStream.writeInt(CLOSE);
// Client (Reading the answer from the server) *** WRONG
while(true){
int i = objectInputStream.readInt();
if(i == CLOSE)
break;
}
You have flush at the client side, and the message is received by the server;
you lack flush at the server side, and the message is not received by the client.
I notice a pattern in these two facts...