I am working on socket programming and implementing custom request response protocol. For same I have used ObjectInputstream and ObjectOutputstream in java socket API.
The area where I have stucked is to check if data(in my case object) is available to read or not, for this I have tried to use the ObjectInputstream.available() but it is returning 0 even if data is available on stream.
Why is it so?
So I have come up with solution: using exception and handling them in infinitely running loop, so even if exception(Read time out) occurs it will try to read again.
I have doubt is it good practice to do so? Or if any other solution you might have do suggest.
while (true){
try {
request = rpcClient.getRequest();
System.out.println(request);
// use threads to handle request for faster response(make use of request IDs)
rpcClient.sendResponse("test response");
} catch (SocketException e)
{// thrown when connection reset
System.out.println("Connection reset : Server is down.....");
break;
} catch (IOException e){
// thrown when read time out
System.out.println("read time out: listening again");
} catch (ClassNotFoundException e){
e.printStackTrace();
}
}
You shouldn't be using available() in the first place. Disable the read timeout, so you can just let the thread wait until there's something to read (or the connection is broken).
I wouldn't recommend using ObjectStreams for network communications though. It's not very suitable in most cases, considering the header information and other things that gets transferred. You're better off designing your own protocol to use and just send bytes over the network.
That is not a good practice since an infinite loop eats away your CPU time.
I dont quite understand your statement
but it is returning 0 even if data is available on stream
since that isnt the case. If it returns 0, there is no data that can be read from the stream. What makes you so sure there actually is data?
Also: I cant see the code that is calling available(). Could you edit your question?
Related
I'm developing a client (Java)/server(C++) application using TCP sockets.
The protocol I used is composed of Messages beginning by 2 bytes defining the type of what will be the content of the Message.
So basically, the receiving thread waits for data to be received in a loop. But I want to use a timeout with the socket to be notified that the other host takes too long to send data.
receivingSocket.setSoTimeout(durationInMilliseconds);
DataInputStream in = new DataInputStream(receivingSocket.getInputStream());
boolean success = false;
short value = 0;
do {
try {
value = in.readShort();// will throw a SocketTimeoutException in case of timeout, without 2 bytes available from the socket
success = true;
} catch (SocketTimeoutException e) {
/// do something if it happens to often. Otherwise go on with the loop
}
} catch (IOException e) {
/// abort connection in case of other problem
}
} while (!success)
Now, what happens if the receiving thread calls in.readShort() at a point where the socket has got only one byte available in its buffer ? Does this byte remain on the socket's stack ? Or is it lost ? In the first case, I could read it next time I call in.readShort(), otherwise it seems lost for good...
readShort() here is an example, my question stands also for readInt(), ...
Thanks for your help,
It isn't specified. I believe the way the implementation works is that the half data is lost, but in any case there's nothing written that says anything else, so you just have to assume the worst.
However in practice this is very unlikely to happen, provided you observe common sense at the sender.
Explanation
I'm revisiting the project I used to teach myself Java.
In this project I want to be able to stop the server from accepting new clients and then perform a few 'cleanup' operations before exiting the JVM.
In that project I used the following style for a client accept/handle loop:
//Exit loop by changing running to false and waiting up to 2 seconds
ServerSocket serverSocket = new ServerSocket(123);
serverSocket.setSoTimeout(2000);
Socket client;
while (running){ // 'running' is a private static boolean
try{
client = serverSocket.accept();
createComms(client); //Handles Connection in New Thread
} catch (IOException ex){
//Do Nothing
}
}
In this approach a SocketTimeoutException will be thrown every 2 seconds, if there are no clients connecting, and I don't like relying on exceptions for normal operation unless it's necessary.
I've been experimenting with the following style to try and minimise relying on Exceptions for normal operation:
//Exit loop by calling serverSocket.close()
ServerSocket serverSocket = new ServerSocket(123);
Socket client;
try{
while ((client = serverSocket.accept()) != null){
createComms(client); //Handles Connection in New Thread
}
} catch (IOException ex){
//Do Nothing
}
In this case my intention is that an Exception will only be thrown when I call serverSocket.close() or if something goes wrong.
Question
Is there any significant difference in the two approaches, or are they both viable solutions?
I'm totally self-taught so I have no idea if I've re-invented the wheel for no reason or if I've come up something good.
I've been lurking on SO for a while, this is the first time I've not been able to find what I need already.
Please feel free to suggest completely different approaches =3
The problem with second approach is that the server will die if an exception occurs in the while loop.
The first approach is better, though you might want to add logging exceptions using Log4j.
while (running){
try{
client = serverSocket.accept();
createComms(client);
} catch (IOException ex){
// Log errors
LOG.warn(ex,ex);
}
}
Non-blocking IO is what you're looking for. Instead of blocking until a SocketChannel (non-blocking alternative to Socket) is returned, it'll return null if there is currently no connection to accept.
This will allow you to remove the timeout, since nothing will be blocking.
You could also register a Selector, which informs you when there is a connection to accept or when there is data to read. I have a small example of that here, as well as a non-blocking ServerSocket that doesnt use a selector
EDIT: In case something goes wrong with my link, here is the example of non-blocking IO, without a selector, accepting a connection:
class Server {
public static void main(String[] args) throws Exception {
ServerSocketChannel ssc = ServerSocketChannel.open();
ssc.configureBlocking(false);
while(true) {
SocketChannel sc = ssc.accept();
if(sc != null) {
//handle channel
}
}
}
}
The second approach is better (for the reasons you mentioned: relying on exceptions in normal program flow is not a good practise) allthough your code suggests that serverSocket.accept() can return null, which it can not. The method can throw all kinds of exceptions though (see the api-docs). You might want to catch those exceptions: a server should not go down without a very good reason.
I have been using the second approach with good success, but added some more code to make it more stable/reliable: see my take on it here (unit tests here). One of the 'cleanup' tasks to consider is to give some time to the threads that are handling the client communications so that these threads can finish or properly inform the client the connection will be closed. This prevents situations where the client is not sure if the server completed an important task before the connection was suddenly lost/closed.
I've got some problems with Android Bluetooth stuff.
When I call
bytes = mmInStream.read(buffer);
It usually works as it should.
On the Cat B15 smartphone however, the read method sometimes blocks forever, even though the connection is still running and data should be arriving.
I have temporarily solved the problem by this code:
while (true) {
int available = 0;
try {
available = mInStream.available();
} catch (IOException e) {}
if (available > 0) {
try {
bytes = mInStream.read(buffer);
ioExceptionsCounter = 0;
// [send the obtained bytes to the UI activity]
// ...............
} catch (IOException e) {
++ioExceptionsCounter;
if (ioExceptionsCounter >= 4) {
break;
}
}
}
try {
Thread.sleep(10);
} catch (InterruptedException e) {}
}
I dont think that the ioExceptionsCounter is really necessary but there was some complaints that it sometimes disconnects without reason so I thought one IOException might not be enough to close the connection.
What I really don't like about this is that it uses polling. It does work now on the Cat phone but it doesn't make me happy that all the other devices now execute this ugly code.
Do you have any ideas why this could be? Is this just a bug of the phone? By the way it runs ICS but it is definitely device specific.
I'm inclined to think that you are encountering a hardware-specific bug.
The various InputStream.read() methods are documented to block until at least one byte is read, or the end of the stream is detected, or an error occurs. If the read() sometimes blocks forever for you in the event that no bytes are available when it is first invoked, then that's definitely a bug outside your own code.
Also, it's highly questionable to ignore any number of IOExceptions, either from available() or from read(). After the stream throws an exception you cannot be confident of the integrity of anything you afterward might manage to read from it. I would normally expect such attempts at further reading also to throw IOExceptions. If you are getting spurious IOExceptions on the Cat B15, such that simply retrying your read() successfully obtains the correct data, then that is also a bug (maybe another facet of the same one).
I am currently implementing a web proxy but i have run into a problem.I can parse my request from the browser and make a new request quite alright but i seem to have a problem with response.It keeps hanging inside my response loop
serveroutput.write(request.getFullRequest());
// serveroutput.newLine();
serveroutput.flush();
//serveroutput.
//serveroutput.close();
} catch (IOException e) {
System.out.println("Writting tothe server was unsuccesful");
e.printStackTrace();
}
System.out.println("Write was succesful...");
System.out.println("flushed.");
try {
System.out.println("Getting a response...");
response= new HttpResponse(serversocket.getInputStream());
} catch (IOException e) {
System.out.println("tried to read response from server but failed");
e.printStackTrace();
}
System.out.println("Response was succesfull");
//response code
public HttpResponse(InputStream input) {
busy=true;
reader = new BufferedReader(new InputStreamReader(input));
try {
while (!reader.ready());//wait for initialization.
String line;
while ((line = reader.readLine()) != null) {
fullResponse += "\r\n" + line;
}
reader.close();
fullResponse = "\r\n" + fullResponse.trim() + "\r\n\r\n";
} catch (IOException`` e) {
e.printStackTrace();
}
busy = false;
}
You're doing a blocking, synchronous read on a socket. Web servers don't close their connections after sending you a page (if HTTP/1.1 is specified) so it's going to sit there and block until the webserver times out the connection. To do this properly you would need to be looking for the Content-Length header and reading the appropriate amount of data when it gets to the body.
You really shouldn't be trying to re-invent the wheel and instead be using either the core Java provided HttpURLConnection or the Appache HttpClient to make your requests.
while (!reader.ready());
This line goes into an infinite loop, thrashing the CPU until the stream is available for read. Generally not a good idea.
You are making numerous mistakes here.
Using a spin loop calling ready() instead of just blocking in the subsequent read.
Using a Reader when you don't know that the data is text.
Not implementing the HTTP 1.1 protocol even slightly.
Instead of reviewing your code I suggest you review the HTTP 1.1 RFC. All you need to do to implement a naive proxy for HTTP 1.1 is the following:
Read one line from the client. This should be a CONNECT command naming the host you are to connect to. Read this with a DataInputStream, not a BufferedReader, and yes I know it's deprecated.
Connect to the target. If that succeeded, send an HTTP 200 back to the client. If it didn't, send whatever HTTP status is appropriate and close the client.
If you succeeded at (2), start two threads, one to copy all the data from the client to the target, as bytes, and the other to do the opposite.
When you get EOS reading one of those sockes, call shutdownOutput() on the other one.
If shutdownOutput() hasn't already been called on the input socket of this thread, just exit the thread.
If it has been called already, close both sockets and exit the thread.
Note that you don't have to parse anything except the CONNECT command; you don't have to worry about Content-length; you just have to transfer bytes and then EOS correctly.
I'm creating a small server using java.nio, but when trying to stress test it I keep getting messages about the connection being reset on the server side, or more specifically:
apr_socket_recv: An established connection was aborted by the software in your host machine
I've tried to narrow it down to the most simple of loops, but still no luck. I can get the error after a hundred or so connections, or maybe just after 1 or 2.
Here's the server loop:
byte[] response = ("HTTP/1.1 200 OK\r\n"
+ "Server: TestServer\r\n"
+ "Content-Type: text/html\r\n"
+ "\r\n"
+ "<html><b>Hello</b></html>").getBytes();
SocketChannel newChannel = null;
while (active) {
try {
//get a new connection and delegate it.
System.out.print("Waiting for connection..");
newChannel = serverSocketChannel.accept();
System.out.println("ok");
newChannel.configureBlocking(true);
newChannel.write(ByteBuffer.wrap(response));
}
catch (IOException e) {
e.printStackTrace();
}
finally {
try {
newChannel.close();
} catch (IOException ex) {
Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
I've tried checking if the write didn't write all requested byte, but it seemingly does. Interestingly enough, calling System.gc() after each newChannel.close() makes the problem disappear (but in return, it's horribly slow). So either I'm not releasing all resources I should release, or the application just needs a pause..
I'm losing all of my best years on this. Oh, and by the way.. if I ignore writing to the channel and just close after I accept the connection, the problem still doesn't go away.
Well I found it out, so I might as well share it.
My app needed a pause. It was simply going too fast, and closing the connection before the client had written all of its request data. The fix would be to keep on reading until the entire HTTP request had been received. D'oh.. lesson learned.
From the docs for SocketChannel#Write (emphasis mine):
An attempt is made to write up to r bytes to the channel, where r is
the number of bytes remaining in the buffer, that is, src.remaining(),
at the moment this method is invoked.
[...]
Returns: The number of bytes written, possibly zero.
It's up to you to check the return value from the write call (which you're not doing presently), and issue successive write calls until the whole of the buffer has been sent. Something like this, I guess:
ByteBuffer toWrite = ByteBuffer.wrap(response);
while (toWrite.remaining() > 0) {
newChannel.write(toWrite);
}
You'll obviously get aborts if you don't write all of your response data and then just close the socket.