Suppose I have following Java code on a server:
Socket aSocket = new Socket(aInetaddr, aPort); //aSocket is a client i m connecting to
InputStream input = aSocket.getInputStream();
Socket bSocket = new Socket(bInetaddr, bPort); //bSocket is a client i m connectin to
Now what I would like to achieve is, to pass the InputStream "input" from the aSocket to the bSocket. I have no information about whats coming from the InputStream, or how it is used in the end.
The whole transfer of data has to pass my server, I can't make a direct connection between the 2 clients.
Get the output stream for the second socket and then you can do this trivially using Apache Commons IOUtils.copy(). Amongst many variants there's one that takes an input and output stream as arguments.
Related
I'm doing a comparison between some C functions used in network programming, and their Java counterpart. Most of them I can find in documents about Socket, ServerSocket, InetAddress classes.
However I can't seem to find listen(), recv(), send() and getaddrinfo() in Java. As far as I go, I see that most Java client-server programs do not require them, as you can just write the byte/message and flush them directly to the other end, using flush() or PrintWriter().
Do I understand this right, and are there any equivalent functions to those three?
When you create a ServerSocket, the underlying listen function gets called when you bind to a port, either through the constructor or though the bind method.
The recv and send functions are called by reading from and writing to the InputStream and OutputStream respectively that are attached to a Socket instance and returned by the getInputStream and getOutputStream methods.
Listening and receiving can be implemented in multiple different ways. Java is object-oriented so the logic is usually represented by a specific object type.
Take a look at Reading from and Writing to a Socket tutorial which gives a few examples on working with sockets, e.g. using InputStream and OutputStream to interact with Socket:
try (
Socket echoSocket = new Socket(hostName, portNumber);
PrintWriter out =
new PrintWriter(echoSocket.getOutputStream(), true);
BufferedReader in =
new BufferedReader(
new InputStreamReader(echoSocket.getInputStream()));
BufferedReader stdIn =
new BufferedReader(
new InputStreamReader(System.in))
)
Hello stack overflow world, I've been struggling with the most straight forward and common problem within Java IO, for some time, and now need your help to tackle it.
Check out this piece of code I have in a try block, within a thread.run():
// connect to client socket, and setup own server socket
clientSocket = new Socket(serverHostname, CLIENT_PORT);
//send a test command to download a file
String downloadFileName = "sample.txt";
DataOutputStream dataOutputStream = new DataOutputStream(clientSocket.getOutputStream());
System.out.println("Sending a request to download file : " + downloadFileName + " from user: Arsa node"); //todo: replace with node user later
dataOutputStream.writeUTF("D/sample.txt");
//close socket if host isn't detected anymore, and if socket doesn't become null suddenly
dataOutputStream.flush();
dataOutputStream.close();
System.out.println("****File has been sent****");
in = new DataInputStream(clientSocket.getInputStream());
byte[] retrievedFileData = new byte[8036];
if (in.readInt() > 0) {
System.out.println("Starting file download!");
in.read(retrievedFileData);
System.out.println("File data has been read, converting to file now");
//closing input stream will close socket also
in.close();
}
clientSocket.close();
2 Main questions that have been confusing me to death:
Why does dataOutputStream.close() need to be run for writeUTF to actually send my string to the server socket, I find that when I don't have dos.close(), data isn't retrieved on the other side, further because I close it, I no longer can read from the socket - as it seems the socket connection becomes closed when the Output Stream is previously closed...
What's a better way, following some sort of pattern to do this? For context, all I'm trying to do is write the filename I'm looking to download to my client, then read the response right away, which I expect to be bytes with the file, any error handling I will consider as a part of my development.
Overall, it shouldn't be complicated to write something to a socket, then read and ingest it's response...which doesn't seem to be the case here,
any help would be greatly appreciated! If the ServerSocket code snippet is needed I'm happy to share.
The observed behavior is just a side-effect of close(), as it calls flush() before closing to make sure any buffered data is sent. To solve your problem, you need to call the flush() method instead of closing.
This behavior is not unique to DataOutputStream: a lot of other OutputStream (or Writer) implementations apply buffering, and you will need to flush when you want to ensure the data is sent to the client, written to disk or otherwise processed.
BTW: The DataOutputStream and DataInputStream is for a very specific type of data serialization protocol that is particular to Java. You may want to consider carefully if this is the right protocol to use.
I want to transmit an integer number throught a socket.
I have a python script that open a socket and wait for requests, in the other side there is a Java servlet that send a request at this socket to obtain a value.
All the communication works well the problem is that the value obtained from java cause a EOFexception. I've tried some convertion and formats in the python script (I'm not expert of this language) but nothing works.
Instead, if I use in python a echo socket, picking all that socket receive and send back to java, all working fine.
But I need to send a simple value.
The following is the portion of code that I use, I've tried with float, this is with a String value:
[Python socket related istructions]
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.bind(server_address)
sock.listen(1)
connection = sock.accept()
[Python echo socket working]
data = connection.recv(16)
if data:
connection.sendall(data)
instead I want to do:
[Python return a example value]
connection.sendall('55')
That working if I send request from a client like Hercules but cause a EOFexception in Java
[Java Servlet - send request]
socket = new Socket(address, port);
dataInputStream = new DataInputStream(socket.getInputStream());
String content = dataInputStream.readUTF(); //EOFexception
Destination.value = content;
Solved the problem by using InputStream instead of DataInputStream, write the python socket to send string as byte and changed the java receiver to read byte stream and convert in to string (good format for my purpose).
I'm writing a Java client/server application. It should allow clients to send text data to the server. This kind of communication should be repeatable many times using the same connection.
I write it like this:
// On a server:
ServerSocket serverSocket = new ServerSocket(port);
Socket socket = serverSocket.accept();
socket.setKeepAlive(true);
BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
if (reader.ready()) {
for (String line = reader.readLine(); line != null; line = reader.readLine()) {
// do something with line
}
}
// On a client:
Socket socket = new Socket(host, port);
socket.setKeepAlive(true);
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
writer.write("Some data from client to server");
writer.flush();
The problem is: I can't read on a server before I close OutputStream on a client. Or I can't open OutputStream on a client again, if it was already closed. How can I do continuous sending and reading of data?
You need two threads at both ends, one for reading data and other one for writing data.
The problem is: I can't read on a server before I close OutputStream on a client.
Yes you can. You just can't get to the case where readLine() returns null. It isn't the same thing.
Or I can't open OutputStream on a client again, if it was already closed.
Of course not. You have to create a new Socket.
How can I do continuous sending and receiving of data?
I don't understand the question. The code you posted doesn't attempt that.
If your goal is to send many mesages over the same socket connection, these messages will have to be delimited by an application-level protocol. In other words, you won't be able to rely on any system calls like reader.ready() or reader.readLine() == null to detect the end of the message on te server.
One way to achieve this is to begin each message with its length in characters. The server will then read exactly that number of charecters, and then stop and wait for a new message. Another is to define a special character sequence which concludes each message. The server will react to reading that particular sequence by ending the reading of the current message and returning to the "wait for new message" state. You must ensure that this sequence never appears in the message itself.
I'd like to fetch a webpage, just fetching the data (not parsing or rendering anything), just catch the data returned after a http request.
I'm trying to do this using the high-level Class Socket of the JavaRuntime Library.
I wonder if this is possible since I'm not at ease figuring out the beneath layer used for this two-point communication or I don't know if the trouble is coming from my own system.
.
Here's what my code is doing:
1) setting the socket.
this.socket = new Socket( "www.example.com", 80 );
2) setting the appropriate streams used for this communication.
this.out = new PrintWriter( socket.getOutputStream(), true);
this.in = new BufferedReader( new InputStreamReader( socket.getInputStream() ) );
3) requesting the page (and this is where I'm not sure it's alright to do like this).
String query = "";
query += "GET / HTTP/1.1\r\n";
query += "Host: www.example.com\r\n";
...
query += "\r\n";
this.out.print(query);
4) reading the result (nothing in my case).
System.out.print( this.in.readLine() );
5) closing socket and streams.
If you're on a *nix system, look into CURL, which allows you to retrieve information off the internet using the command line. More lightweight than a Java socket connection.
If you want to use Java, and are just retrieving information from a webpage, check out the Java URL library (java.net.URL). Some sample Java code:
URL ur = new URL("www.google.com");
URLConnection conn = ur.openConnection();
InputStream is = conn.getInputStream();
String foo = new Scanner(is).useDelimiter("\\A").next();
System.out.println(foo);
That'll grab the specified URL, grab the data (html in this case) and spit it out to the console. Might have to tweak the delimiter abit, but this will work with most network endpoints sending data.
Your code looks pretty close. Your GET request is probably malformed in some way. Try this: open up a telnet client and connect to a web server. Paste in the GET request as you believe it should work. See if that returns anything. If it doesn't it means there is a problem with the GET request. The easiest thing to do that point would be write a program that listens on a socket (more or less the inverse of what you're doing) and point a web browser to localhost:[correct port] and see what the web browser sends you. Use that as your template for the GET request.
Alternatively you could try and piece it together from the HTTP specification.
I had to add the full URL to the GET parameter. To make it work. Although I see you can specify HOST also if you want.
Socket socket = new Socket("youtube.com",80);
PrintWriter out = new PrintWriter(new BufferedWriter(new
OutputStreamWriter(socket.getOutputStream())));
out.println("GET http://www.youtube.com/yts/img/favicon_48-vflVjB_Qk.png
HTTP/1.0");
out.println();
out.flush();
Yes, it is possible. You just need to figure out the protocol. You are close.
I would create a simple server socket that prints out what it gets in. You can then use your browser to connect to the socket using a url like: http://localhost:8080. Then use your client socket to mimic the HTTP protocol from the browser.
Not sure why you're going lower down than URLConnection - its designed to do what you want to do: http://download.oracle.com/javase/tutorial/networking/urls/readingWriting.html.
The Java Tutorial on Sockets even says: "URLs and URLConnections provide a relatively high-level mechanism for accessing resources on the Internet. Sometimes your programs require lower-level network communication, for example, when you want to write a client-server application." Since you're not going lower than HTTP, I'm not sure what the point is of using a Socket.