Develop Reference

DataInputStream stuck when initialized

I have been trying to send a byte[] array across my client / server application using Sockets, DataInputStream and DataOutputStream, but on the server side, the program just gets stuck when I try to initialise the DataInputStream.
Here is the code on the client side (it works fine):
DataOutputStream datas = new DataOutputStream(connection.getOutputStream());
datas.flush();
byte[] send = identityKeyPair.serialize();
datas.write(send);
datas.flush();
Here is the code on the server side:
reader = new BufferedReader(new InputStreamReader(connection.getInputStream()) );
sender = new PrintWriter(connection.getOutputStream());
newUser = new BasicUserData();
System.out.println("New registration from: " + connection.getInetAddress());
System.out.println("Data:");
String un = reader.readLine();
newUser.USERNAME = un;
System.out.println(newUser.USERNAME);
String pw = reader.readLine();
newUser.PASSWORD = pw;
System.out.println(newUser.PASSWORD);
DataOutputStream dataout = new DataOutputStream(connection.getOutputStream());
System.out.println("Opened data output stream");
DataInputStream receiver = new DataInputStream(connection.getInputStream());
//It gets stuck here, and the program doesn't read anything further
receiver.read();
byte[] id = receiver.readAllBytes();
System.out.println("Opened data input stream");

You are using both connection.getInputStream() and connection.getOutputStream() in two different ways. You should use one and only one way of reading from and writing to streams, don't mix multiple ways.
When you execute reader.readLine(), the BufferedReader called reader will read up-to 8192 characters from the input stream into its buffer, likely consuming all bytes your client has written. This means that when you construct your DataInputStream around that same input stream and try to read it, there is no data available to read, causing the receiver.read() to block waiting for data. That data is never received as your client has sent all its data, which is now buffered in reader.
In addition, unless your client has closed its output stream, InputStream.readAllBytes() will block indefinitely anyway, because it is only finished when the end-of-stream has been reached. And for socket communication, that only happens when the other side closed its output stream.
Change your code so there is only one way of writing data (though not relevant here), and one way of reading data. In addition, you should establish clearly how you need to read and write data in a protocol, so to avoid consuming too much data at the wrong point, and to know how much data you need to read when.

Of course it does nothing but wait as specified in the javadoc a call to DataInputStream#read() blocks the current thread until data can be read from the input stream.
Your reader = BufferedReader(...) uses the same underlying InputStream which means the all the data the client sent is most likely already consumed by the 'login' logic.
Since neither the client nor the server close their respective streams no EOF is emitted either which leads to the stream 'just dangling' around waiting for more data.
Unless your client sends more data the server will wait eternally.
There are two solutions for your issue.
Either thread you application so that the 'await input' logic is in it's own thread or take a look at javas NIO package (more precisely Channels and Selectors)

Java socket writeByte and newline

I got a question for something I can't seem to understand.
If I use the snippet:
DataOutputStream dataoutput = new DataOutputStream(Socket.getOutputStream());
dataoutput.writeBytes(diagnostics);
dataoutput.flush();
The output:diagnostics that is sent from server to client hangs, where even using .flush() will not help. Only closing the socket will release the data packet.
However, if I add a newline character:
dataoutput.writeBytes(diagnostics+"\n");
it works perfectly. And if you duplicate the code,
dataoutput.writeBytes(diagnostics);
dataoutput.writeBytes(diagnostics);
You get the same output as the one with the /newline without duplicate data. Why?
EDIT: Client-side.
incoming = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
response = incoming.readLine();

Like the good lads puts in the comment you should use a flush to force send the payload to destination of the stream. But if that isn't working i suggest to try the following :
DataOutputStream dataoutput = new DataOutputStream(System.out);
dataoutput.writeBytes(diagnostics);
dataoutput.flush();
So now the destination will be the "local" console. and no remote connection is between the source and the destination of the stream, perhaps you can try for yourself if anything is working properly "locally" (and focus in the DataOutputStream behaviour) and then start to tackle the remote streaming process.
Is it not to much, but hope it helps a little!
Greetings!!

How can a server know when the message it receives is actally over?

I'm trying to implement a server and it does something like this when it receives a message:
try{
ObjectInputStream is = new ObjectInputStream(clientSocket.getInputStream());
String message = (String)is.readObject();
ActivateCommand(message,clientSocket);
is.close();
}
and the ActivateCommand:
private void ActivateEwolfCommand(String msg, Socket clientSocket) throws IOException
{
ObjectOutputStream os = new ObjectOutputStream(clientSocket.getOutputStream());
Gson gsonObject = new Gson();
.
//code which makes a Json string
.
os.writeObject(json);
os.close();
}
now, when i tested it with sending a message to the local host with a junit test it worked. But when tried connecting to the server from a C# client, the client connected but the server throw an EOF exception when getting to the point of clientSocket.getInputStream().
I'm guessing it happens because the server did not know when the message is suppose to end but i don't know if thats really the case and if it is, then how can i fix it?

When there is no more data available on the InputStream to read() - that basically is what causes an EOF. How much data is available is determined by the client - the data that it writes to the Socket's OutputStream on its side appears as such on the InputStream of Socket of the server side. You can call InputStream.available() to get an estimate of number of bytes that can be still read().
However your code is trying to read an Object using an ObjectInputStream - this class has its own protocol to read a serialized byte stream and convert that to an object - if it does not find the bytes to complete the task this can throw the EOF exception. If your client is in C# - the format of bytes this writes for a serialized object will definitely not be the same as expected by the ObjectInputStream on the server side.

This is why it's a bad idea to create your own client-server protocol with a socket and object streams. Many people have spent many years bringing you, oh, well:
SOAP
REST
RMI
Hessian
CORBA
Thrift
and the multitude of other protocols out there. Surely one of them, if not 5 or 6, is good enough to solve your problem, including all issues of framing.

If you want to send strings over a socket, then an ObjectInputStream or ObjectOutputStream isn't the right stream implementation. These stream implementations use Java object serialization. Even if you serialize String instances, the resulting bytes are not the same as plain string to byte conversion with the appropriate character encoding.
And a C# application doesn't understand Java serialization at all.
Consider using a PrintWriter to write strings to your stream and a BufferedReader for reading.
PrintWriter writer = new PrintWriter(new OutputStreamWriter(clientSocket.getOutputStream(), "UTF-8"));
writer.println(...);
BufferedReader reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream(), "UTF-8"));
String line = reader.readLine();
Then you can read and write strings line by line.
This is only a starting point. If you want to implement your own protocol you have to pay attention to some more points. As an example you can read the specifications for some TCP protocols like POP3, FTP or HTTP 1.0.

Writing to Socket outputStream w/o closing it

I'd like to write some messages to the server.
Each time, for the tramsmitting only, I'm closing the outputStream and reopen it when I have to send the next message.
os.write(msgBytes);
os.write("\r\n".getBytes());
os.flush();
os.close();
How Can I keep this Socket's OutputStream, os, open and still be able to send the message?
Thanks.

I am missing something here. If you don't call close, it will not close. For example,
os.write(msgBytes);
os.write("\r\n".getBytes());
os.flush();
// Do something
os.write("more message");
os.flush();
// When you are finally done
os.close();

In most protocols, the server accepts som kind of EOF symbol. Send such a symbol instead of closing the stream.
For example, IRC servers interpret "\r\n" as the end of a message. This would be 2 messages on one open OutputStream:
PrintStream printStream = new PrintStream(socket.getOutputStream());
printStream.print("JOIN #channel1\r\n");
printStream.flush( );
printStream.print("JOIN #channel2\r\n");
printStream.flush( );
Also, you should wrap your outputStream with DataOutputStream. This wrapper creates more portable output. Plain OutputStream can cause trouble with certain primitive datatypes if server and client have different computer architectures.

Wrap the Socket's OutputStream in a PrintWriter and call the PrintWriter's println method.
PrintWriter pw = new PrintWriter(socket.getOutputStream());
....
pw.println(message); // call repeatedly to send messages.
....
pw.close(); // when finished writing to server and want to close conn.

I have found the problem and it lays on the client's side.
On the client, I have used -
count = inputStream.read(buffer)) > -1
That means, the client waits till server's outputStream closed, and then processing the incoming data.

Java socket programming - stream get stuck

I am currently working on a simple proxy server, which receives http request from browser, process it, then forward it to the desire web server.
I try to get the request from the input stream of the socket connected by the browser, everything is fine except that the stream get stuck after receiving the last block of data.
My code is in fact very simple, as shown below:
ServerSocket servSocket = new ServerSocket(8282);
Socket workSocket = servSocket.accept();
InputStream inStream = workSocket.getInputStream();
byte[] buffer = new byte[1024];
int numberRead = 0;
while ((numberRead = inStream.read(buffer, 0, 1024)) != -1){
System.out.println(new String(buffer));
}
The loop simply cannot exit, even the request reception is finished.
Is there any method to workaround this problem?
Thanks in advance for any advice.

As in InputStream javadoc the method will block until the data is available or the EOF is encountered. So, the other side of Socket needs to close it - then the inStream.read() call will return.
Another method is to send the size of message you want to read first, so you know ahead how many bytes you have to read. Or you can use BufferedReader to read from socket in line-wise way. BufferedReader has a method readLine() which returns every time a line is read, which should work for you as HTTP protocol packages are nice divided into lines.

It will cycle until the connection is closed, and the client is probably waiting for HTTP response from you and doesn't close it.

The browser is waiting for a response before it closes the connection.
Your read-method on the other hand will block until the stream/connection is closed or new data is received.

Not a direct solution according to your current code.
As HTTP is a line based protocol, you might want to use a Buffered Reader and call readLine() on it.

The when a http request comes in it will always be concluded with a blank line, for example:
GET /someFile.html HTTP/1.1
Host: www.asdf.com
After sending that request the client connection will then wait for a response from the server before closing the connection. So if you want to parse the request from the user you are probably better off using a BufferedReader and reading full lines until you reach a lines of text that is blank line.