I'm building a chat program where hosts are connected via sockets and talk to each other using ObjectInput and ObjectOutput streams. A host builds a string from keyboard input and sends it to the other hosts along with an array of ints.
After a host has successfully read a message via readObject(), the while(true) loop continues and that host hangs on the very next call of readObject(). I can only surmise that this is because indata.available() is returning true even after reading whatever was in it, and when it tries to read again before something else has been sent, it blocks (waits).
A snippet of the relevant code is below. I've done some research and found that I can't flush or empty an input stream. I also can't close it - because of the nature of the constantly-running chat program, it needs to stay open to continue reading.
Also, I understand that I'm checking indata.available() and then reading in using inputs.readObject(). I thought this was the proper way to do it, but correct me if I'm wrong.
I'm not sure what to do about it! I need indata.available() to return 0 if I haven't written an object to the stream.
private InputStream[] indata;
private ObjectInputStream[] inputs;
private ObjectOutputStream[] outputs;
private int[] stamps;
// Establish connections via sockets between 3 hosts, serverless
while (true) {
// Build a message
for (all hosts that aren't myself) {
if ( i != rank ) {
outputs[i].writeObject( message );
outputs[i].writeObject( stamps );
outputs[i].flush( );
outputs[i].reset( );
}
}
// Read a message in from a host that sent one
for (all hosts that aren't myself) {
if (indata[j].available() > 0) {
String message = (String)inputs[j].readObject();
int[] senderStamps = (int[])inputs[j].readObject();
}
}
}
Some additional information, for clarification:
I'm using available() because the instructor used it in his code and I'm not allowed to change it. Further, the call to available() worked as intended when only one object (the string) was being sent - the only code that was there on the sending side was "writeObject" and "flush". It was my job to add the code to send the array, and when I do that, I also have to add the code to reset() the ObjectOutputStream (or I get other problems - when the array is sent, modified, and then sent again without calling reset() between sendings, the original, unmodified version is sent instead of the newly modified one).
I can't just block on read, as a process that's blocked on read cannot write to the other hosts, and I need to be able to write even when a host has nothing to read.
Also, we aren't allowed to use multiple threads.
I figured out what was happening, but I'm not sure I understand why.
When I called reset() after sending, and then read the objects in at the receiving end, there was still 1 byte left in the stream. This is what was causing me to fall into the if-clause and then block when reading again (because there was really no object to read).
I had to call reset because I was sending a persistent object (the array). I noticed that I didn't have to call reset when I was only sending the string, and the difference between the string and the array was that the array was a data member of the class while the string was being created anew each time the loop ran.
So, I created a non-persistent copy of the array that I wanted to send, and sent that copy. When I did this, I didn't have to call reset (still don't understand why). Further, after reading from the input stream, there were 0 bytes left inside, so it never put the program in a position where readObject would block.
I think I kinda understand why I don't have to reset when sending non-persistent objects, but I don't understand why reset() was causing data to be left in the corresponding input stream.
Either way, it works as intended now.
Related
I have a C++ client using QT and a JAVA server and I have successfully written from the client to the server but I cannot write from the server to the client. My code:
QString
Client::readTCP ( )
{
socketTCP->waitForReadyRead();
QTextStream in (socketTCP);
return in.readAll() ;
}
// Later on
qDebug() << Client::readTCP();
But no matter what method I choose I can't get a response from the server. The server code is as follows:
DataOutputStream output = new DataOutputStream (SOCKET.getOutputStream());
output.writeBytes ( "myString" );
ANSWER:
It works either because I changed in.readAll() to in.readLine() or it is because I waited a couple seconds after the server started before sending a message.
The QTextStream::readAll function attempts to read the entire contents of the stream. Either this message is or isn't the entire contents of the stream.
If this message isn't the entire contents of the stream, then it should not return. It would be a serious error if readAll returned only a part of the contents of the stream despite the fact that it's specified to return all the contents.
If this is the entire contents of the stream, then the server is broken. If it doesn't close the socket, how can the client know it's received the entire contents? Unless there's some other way to indicate end of message, it has to be indicated by closing the stream, and you don't show the server closing the stream.
I'll repeat the advice I always give when I see problems like this -- do not ever implement a network protocol until you specify that protocol in a protocol specification. Otherwise, it's not possible to fix problems where the server and client disagree because there's no way to know which end is right. Here, the server and client disagree over how the end of a message is to be marked, and without a protocol specification to refer to, there's no way to know which end to fix.
If you had a protocol specification, you could just look at the section that explains how the ends of messages are marked and detected. Then you could fix whichever end doesn't follow the specification. (Or, if the specification doesn't say how, then fix the specification! Clearly, this has to happen somehow and it's the specification's job to explain how.)
In Java, after sending data to buffer, flush it. Output streams have flush() method which forces any data left in stream to be written/sent. Try that if using readAll() on client side.
Also, readLine() is advised if you know how much data will be sent. You can loop trough in.readLine() until it gets null. Also readLine() will remove any \n or \r\n.
I have no experience with Qt so I cannot say if you are reading correctly from the socket, but with Java I use PrintStream's method println when sending text, and a VS C++ Client receives it just fine using recv.
Also, you may want to check if the packet is actually sent over the socket using Wireshark.
I've implemented a C server and a Java client which communicate to each other through TCP. The problem is when the client sends a string, the recv() call in C server just reads a character and returns immediately. If I put a breakpoint at recv() and then do a step-over, the server receives the whole string that was sent.
Here's the recv call I'm using in C
char tempBuffer[256] = {'\0'};
int retVal = recv(hClientSocket, tempBuffer, sizeof(tempBuffer), 0);
And here's the Java client.
clientSocket = new Socket("127.0.0.1", 24886);
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
outToServer.writeBytes("alert(\"hi\")");
My question is, is there a way in which I can make this work without having to put a while loop in my C server code to receive each character separately? What's the best way to do this?
I implemented the C++ client given here and it works fine. So I'm guessing it's some problem in Java+C++ interoperability. Thanks!
Although what Scott M says is true on a Java side ( about using flush ), you will have to rework your whole communication protocol.
You will have to either tell to C-code, how many bytes are expected to be sent (this will be the easiest to track), or use some sort of termination character, to recognize that the string input is over ( e.g. null terminator byte ).
There is absolutely no guarantee that the network layer will not chop the transition arbitrarily, so code defensively and expect recv always to recieve a partial data, which you will have to assemble for your internal consumption.
Try flushing the java stream. They are normally buffered, which will cause some delay in the writing of bytes to a stream. Long enough to mess up code at full speed, short enough to send the whole string while debugging.
With TCP there's no way around "putting a while loop around recv()". TCP is a stream protocol built on top of limited-size packet service (IP). TCP does its own "packetization", and is actually very smart about it. This implies that you never know how many bytes the next read from the socket will return.
The well-known strategy that works is to design your application-level protocol so the message itself tells how big it is, then read in a loop until you have full message to process.
Having said that, there's a kludge, namely MSG_WAITALL flag for the recv(2) call, that you can use to wait for given amount of data, assuming you know how much to expect.
I once tried by having a header indicating length of string .. In that way din have to check for the end of message..
I am currently using a non-blocking SocketChannel (Java 1.6) to act as a client to a Redis server. Redis accepts plain-text commands directly over a socket, terminated by CRLF and responds in-like, a quick example:
SEND: 'PING\r\n'
RECV: '+PONG\r\n'
Redis can also return huge replies (depending on what you are asking for) with many sections of \r\n-terminated data all as part of a single response.
I am using a standard while(socket.read() > 0) {//append bytes} loop to read bytes from the socket and re-assemble them client side into a reply.
NOTE: I am not using a Selector, just multiple, client-side SocketChannels connected to the server, waiting to service send/receive commands.
What I'm confused about is the contract of the SocketChannel.read() method in non-blocking mode, specifically, how to know when the server is done sending and I have the entire message.
I have a few methods to protect against returning too fast and giving the server a chance to reply, but the one thing I'm stuck on is:
Is it ever possible for read() to return bytes, then on a subsequent call return no bytes, but on another subsequent call again return some bytes?
Basically, can I trust that the server is done responding to me if I have received at least 1 byte and eventually read() returns 0 then I know I'm done, or is it possible the server was just busy and might sputter back some more bytes if I wait and keep trying?
If it can keep sending bytes even after a read() has returned 0 bytes (after previous successful reads) then I have no idea how to tell when the server is done talking to me and in-fact am confused how java.io.* style communications would even know when the server is "done" either.
As you guys know read never returns -1 unless the connection is dead and these are standard long-lived DB connections, so I won't be closing and opening them on each request.
I know a popular response (atleast for these NIO questions) have been to look at Grizzly, MINA or Netty -- if possible I'd really like to learn how this all works in it's raw state before adopting some 3rd party dependencies.
Thank you.
Bonus Question:
I originally thought a blocking SocketChannel would be the way to go with this as I don't really want a caller to do anything until I process their command and give them back a reply anyway.
If that ends up being a better way to go, I was a bit confused seeing that SocketChannel.read() blocks as long as there aren't bytes sufficient to fill the given buffer... short of reading everything byte-by-byte I can't figure out how this default behavior is actually meant to be used... I never know the exact size of the reply coming back from the server, so my calls to SocketChannel.read() always block until a time out (at which point I finally see that the content was sitting in the buffer).
I'm not real clear on the right way to use the blocking method since it always hangs up on a read.
Look to your Redis specifications for this answer.
It's not against the rules for a call to .read() to return 0 bytes on one call and 1 or more bytes on a subsequent call. This is perfectly legal. If anything were to cause a delay in delivery, either because of network lag or slowness in the Redis server, this could happen.
The answer you seek is the same answer to the question: "If I connected manually to the Redis server and sent a command, how could I know when it was done sending the response to me so that I can send another command?"
The answer must be found in the Redis specification. If there's not a global token that the server sends when it is done executing your command, then this may be implemented on a command-by-command basis. If the Redis specifications do not allow for this, then this is a fault in the Redis specifications. They should tell you how to tell when they have sent all their data. This is why shells have command prompts. Redis should have an equivalent.
In the case that Redis does not have this in their specifications, then I would suggest putting in some sort of timer functionality. Code your thread handling the socket to signal that a command is completed after no data has been received for a designated period of time, like five seconds. Choose a period of time that is significantly longer than the longest command takes to execute on the server.
If it can keep sending bytes even after a read() has returned 0 bytes (after previous successful reads) then I have no idea how to tell when the server is done talking to me and in-fact am confused how java.io.* style communications would even know when the server is "done" either.
Read and follow the protocol:
http://redis.io/topics/protocol
The spec describes the possible types of replies and how to recognize them. Some are line terminated, while multi-line responses include a prefix count.
Replies
Redis will reply to commands with different kinds of replies. It is possible to check the kind of reply from the first byte sent by the server:
With a single line reply the first byte of the reply will be "+"
With an error message the first byte of the reply will be "-"
With an integer number the first byte of the reply will be ":"
With bulk reply the first byte of the reply will be "$"
With multi-bulk reply the first byte of the reply will be "*"
Single line reply
A single line reply is in the form of a single line string starting with "+" terminated by "\r\n". ...
...
Multi-bulk replies
Commands like LRANGE need to return multiple values (every element of the list is a value, and LRANGE needs to return more than a single element). This is accomplished using multiple bulk writes, prefixed by an initial line indicating how many bulk writes will follow.
Is it ever possible for read() to return bytes, then on a subsequent call return no bytes, but on another subsequent call again return some bytes? Basically, can I trust that the server is done responding to me if I have received at least 1 byte and eventually read() returns 0 then I know I'm done, or is it possible the server was just busy and might sputter back some more bytes if I wait and keep trying?
Yes, that's possible. Its not just due to the server being busy, but network congestion and downed routes can cause data to "pause". The data is a stream that can "pause" anywhere in the stream without relation to the application protocol.
Keep reading the stream into a buffer. Peek at the first character to determine what type of response to expect. Examine the buffer after each successful read until the buffer contains the full message according to the specification.
I originally thought a blocking SocketChannel would be the way to go with this as I don't really want a caller to do anything until I process their command and give them back a reply anyway.
I think you're right. Based on my quick-look at the spec, blocking reads wouldn't work for this protocol. Since it looks line-based, BufferedReader may help, but you still need to know how to recognize when the response is complete.
I am using a standard
while(socket.read() > 0) {//append
bytes} loop
That is not a standard technique in NIO. You must store the result of the read in a variable, and test it for:
-1, indicating EOS, meaning you should close the channel
zero, meaning there was no data to read, meaning you should return to the select() loop, and
a positive value, meaning you have read that many bytes, which you should then extract and remove from the ByteBuffer (get()/compact()) before continuing.
It's been a long time, but . . .
I am currently using a non-blocking SocketChannel
Just to be clear, SocketChannels are blocking by default; to make them non-blocking, one must explicitly invoke SocketChannel#configureBlocking(false)
I'll assume you did that
I am not using a Selector
Whoa; that's the problem; if you are going to use non-blocking Channels, then you should always use a Selector (at least for reads); otherwise, you run into the confusion you described, viz. read(ByteBuffer) == 0 doesn't mean anything (well, it means that there are no bytes in the tcp receive buffer at this moment).
It's analogous to checking your mailbox and it's empty; does it mean that the letter will never arrive? was never sent?
What I'm confused about is the contract of the SocketChannel.read() method in non-blocking mode, specifically, how to know when the server is done sending and I have the entire message.
There is a contract -> if a Selector has selected a Channel for a read operation, then the next invocation of SocketChannel#read(ByteBuffer) is guaranteed to return > 0 (assuming there's room in the ByteBuffer arg)
Which is why you use a Selector, and because it can in one select call "select" 1Ks of SocketChannels that have bytes ready to read
Now there's nothing wrong with using SocketChannels in their default blocking mode; and given your description (a client or two), there's probably no reason to as its simpler; but if you want to use non-blocking Channels, use a Selector
I am using Java NIO's SocketChannel to write : int n = socketChannel.write(byteBuffer); Most of the times the data is sent in one or two parts; i.e. if the data could not be sent in one attemmpt, remaining data is retried.
The issue here is, sometimes, the data is not being sent completely in one attempt, rest of the data when tried to send multiple times, it occurs that even after trying several times, not a single character is being written to channel, finally after some time the remaning data is sent. This data may not be large, could be approx 2000 characters.
What could be the cause of such behaviour? Could external factors such as RAM, OS, etc cause the hindarance?
Please help me solve this issue. If any other information is required please let me know.
Thanks
EDIT:
Is there a way in NIO SocketChannel, to check, if the channel could be provided with data to write before actual writing. The intention here is, after attempting to write complete data, if some data hasn't been written on channel, before writing the remaining data can we check if the SocketChannel can take any more data; so instead of attempting multiple times fruitlessly, the thread responsible for writing this data could wait or do something else.
TCP/IP is a streaming protocol. There is no guarantee anywhere at any level that the data you send won't be broken up into single-byte segments, or anything in between that and a single segment as you wrote it.
Your expectations are misplaced.
Re your EDIT, write() will return zero when the socket send buffer fills. When you get that, register the channel for OP_WRITE and stop the write loop. When you get OP_WRITE, deregister it (very important) and continue writing. If write() returns zero again, repeat.
While using TCP, we can write over sender side socket channel only until the socket buffers are filled up and not after that. So, in case the receiver is slow in consuming the data, sender side socket buffers fill up and as you mentioned, write() might return zero.
In any case, when there is some data to be sent on the sender side, we should register the SocketChannel with the selector with OP_WRITE as the interested operation and when selector returns the SelectionKey, check key.isWritable() and try writing on that channel. As mentioned by Nilesh above, don't forget to unregister the OP_WRITE bit with the selector after writing the complete data.
Related posts didn't answer my question.
I have a server which does something like:
EVERY TWO SECONDS DO:
if the inputstream is not null {
if inputStream.available() is 0
{
return
}
print "handling input stream"
handleTheInputStream();
}
Even after my client disconnects, the server doesn't recognize it through an IOException. The other post said that I would see an End-of-Stream character. However, that is not the case, since after my client disconnects I never see "handling input stream" which indicates that no data is available.
Perhaps something is wrong with the way I currently understand how this works.
Please help.
Don't use available() - that says whether or not there's currently data available, not whether there will be data available in the future. In other words, it's the wrong tool to use to detect disconnection.
Basically you should call read() (and process the data) until it returns -1, at which point it means the client has disconnected.
If this is done using sockets, you may want to check the Socket class's various instance methods, such as isClosed() or isInputShutdown().
Of course, this assumes that the method operating on this stream has access to the Socket object and not just the InputStream.