Hi and thanks in advance,
So I have a program which already creates a socket and can send strings both ways between the server and client using UDP.
However, I need to ask something before I continue with trying to send a file (specifically a text file) over my connection.
Is there a way to physically send an actual file over the connection as apposed to simply sending the files contents, or does sending it contents count as sending the files itself?
I just want to make sure on this before I continue with my program.
Thank you.
A file is not a physical object. It is more an idea of interpreting your disks magnetical (or electrical) state.
A "file" is mainly its content. There is some additional information like permissions, owner, last edit date and so on. But I assume that you don't want to send these information.
I have no idea what the specific goal is you are trying to achieve but it is safe to say, that for most applications it is perfectly fine to think of the contents of a file when saying "file".
Related
I have a Java client/server desktop application, where the communication between client and server is based on Sockets, and the messages exchanged between client and server are serialized objects (message objects, that incapsulate requests and responses).
Now I need to make the client able to upload a file from the local computer to the server, but I can't send the File through the buffer, since the Buffer is already used for exchanging the message objects.
Should i open another stream to send the file, or is there any better way to upload a file for my situation?
I need to make the client able to upload a file from the local computer to the server
- Open a Solely Dedicated Connection to the Server for File uploading.
- Use File Transfer Protocol to ease your work, and moreover its quite easy and reliable to use the Apache's common lib for File uploading and downloading....
See this link:
http://commons.apache.org/net/
You really only have two options:
Open another connection dedicated to the file upload and send it through that.
Make a message object representing bits of a file being uploaded, and send the file in chunks via these message objects.
The former seems simpler & cleaner to me, requiring less overhead and less complicated code.
You can keep your solution and pass the file content as an object, for example as a String - use Base64 encoding (or similar) of the content if it contains troublesome characters
I don't if the question title fits, but here is my problem:
I have a regular webhosting service in hostmonster, with a website built in php.
So I have php script running in a cron job that monitors a xml file for changes, and everytime a new entry comes into that xml file the script stores it in a database.
In the other hand there is java built desktop client, which needs to be noticed ASAP that a new entry is created, for this the client connects to a second php file every second, and this second files tells if there has been changes or not.
The thing is, every 260 connections my I.P gets banned from the server :( and the client crashes, the client will be used by several users.
I contacted support on how to handle this, but they tell me to use a single connection, I tried reusing the UrlConnection but after the first request it just gives null. then I tried with Sockets but no luck. I know there are libraries that manage this but I dont know how are they called. Can someone give me advice?
thank you guys.
Use a long polling method. Hold the connection opened until response arrives. This way you only need to ask for the update once.
PHP may not be the best tool for this job though.
I converted a local application to a Java applet for data security1 but I cannot figure out how to output information to a file. I am inputting files by a URL object but do not know how to output data to a file on the server.
secure in that the person running the program in supervised conditions cannot keep it and run it for someone else (it is a survey I am piloting).
Is there an easy way to do this?
Applets run on the client, not on the server, so an applet can't do this, period. What it can do is send data to a service of some kind on your server, which can then turn around and write the data to a file.
I have written a nice program in Java that connects to a gmail account and download atachments sent to it. Once an attachment has been downloaded, it is marked as read and is not downloaded ever again. This program will have to run in multiple instances with each program downloading unique attachments so that a single attachment is never downloaded twice. The problem is that at the moment if the attachment is of a decent size, one program is still downloading it, when another instance connects and also starts to download the attachment before it has been marked as read.
I have tried checking and setting various flags and checking whether the folder is open, nothing seems to work. Any solutions?
Update: Thank you for the quick answers, sadly IMAP is not an option due to other reasons.
Consider using IMAP instead - it is designed for client-server interaction.
From RFC1939 (Post Office Protocol - Version 3):
POP3 is not intended to provide
extensive manipulation operations of
mail on the server; normally, mail is
downloaded and then deleted. A more advanced (and complex) protocol, IMAP4, is discussed in RFC1730.
I don't think POP3 is made for multiple simultaneous access.
Ask yourself this: do i really need multiple processes accessing the same mailbox?
If you do, you'll have to find a way to have these processes communicate to each other.
Use a common database or server process to coordinate actions.
IMAP does have more options, but i'm not sure if you can "lock" a single mail to mark it as being processed.
As the others have mentioned, POP3 isn't really intended for this kind of scenario.
If you absolutely have to use POP3, I'd suggest downloading all the e-mail to an intermediate server which sorts the messages and makes them available for each of the other clients.
It sounds like you're just trying to distribute the processing of the e-mails. If that's the case, you can just have each client connect to your intermediate server to retrieve the next available message.
I'm not sure what your constraints are, but you may even want to consider receiving the attachments some other way besides e-mail. If people are uploading files, you could set up a web form that automatically sends each file to the next available instance of your application for processing.
If you need to stay with a POP3 connection, you could keep a local database of previously downloaded message ids. Then new instances could check against that before downloading again. The best solution is just to use IMAP, though, as IMAP is able to set the read/unread flags before downloading.
You could mark the mail as read before starting the download, and then start downloading it.
I need a simple application, preferably a cross-platform one, that enables sending of files between two computers.
It just need to accept and send the files, and show a progress bar. What applications could I use or how could I write one?
Sending and Receiving Files
The sending and receiving of a file basically breaks down to two simple pieces of code.
Recieving code:
ServerSocket serverSoc = new ServerSocket(LISTENING_PORT);
Socket connection = serverSoc.accept();
// code to read from connection.getInputStream();
Sending code:
File fileToSend;
InputStream fileStream = new BufferedInputStream(fileToSend);
Socket connection = new Socket(CONNECTION_ADDRESS, LISTENING_PORT);
OutputStream out = connection.getOutputStream();
// my method to move data from the file inputstream to the output stream of the socket
copyStream(fileStream, out);
The sending piece of code will be ran on the computer that is sending the code when they want to send a file.
The receiving code needs to be put inside a loop, so that everytime someone wants to connect to the server, the server can handle the request and then go back to waiting on serverSoc.accept().
To allow sending files between both computers, each computer will need to run the server (receiving code) to listen for incoming files, and they will both need to run the sending code when they want to send a file.
Progress Bar
The JProgressBar in Swing is easy enough to use. However, getting it to work properly and show current progress of the file transfer is slightly more difficult.
To get a progress bar to show up on a form only involves dropping it onto a JFrame and perhaps setting setIndeterminate(false) so hat it shows that your program is working.
To implement a progress bar correctly you will need to create your own implementation of a SwingWorker. The Java tutorials have a good example of this in theirlesson in concurrency.
This is a fairly difficult issue on its's own though. I would recommend asking this in it's own question if you need more help with it.
Woof is a cool Python script that might work for you:
http://www.home.unix-ag.org/simon/woof.html
I would strongly consider using FTP. Apache has a FTP client and a server
Edit: spdenne's suggestion of HTTP is also good, especially if everyone has Java 6. If not, you can use something like Tiny Java Web Server.
You can write one by using Socket programming in Java. You would need to write a Server and a Client program. The server would use a ServerSocket to listen for connections, and the Client would use a Socket to connect to that server on the specified port.
Here's a tutorial: http://www.javaworld.com/jw-12-1996/jw-12-sockets.html
Sun's Java 6 includes a light-weight HTTP server API and implementation. You could fairly easily use this to serve your file, using URLConnection to obtain it.
Check out this tutorial, it's a really basic example. You would probably also want to send control headers prior to the actual file being sent, containing the size of the file, filename, etc.
Alternatively, base it on an existing protocol, like this project.
Can you install FTP servers on (one of) your machines ?
If you can, you will just have to use a FTP client (FileZilla for example, which have a progress bar).
Two popular apps are "scp" and "rsync". These are standard on Linux, are generally available on Unix and can be run on Windows under cygwin, although you may be able to find windows-native apps that can do it as well. (PuTTY can serve as an SCP client).
For any sort of pc-to-pc file transfer, you need to have a listener on the destination PC. This can be a daemon app (or Windows system process), or it can be a Unix-style "superserver" that's configured to load and run the actual file-copy app when someone contacts the listening port.
SCP and one of the rsync modes do require that there be some sort of remote login capability. Rsync can also publish resources that it will handle directory. Since the concept of a Windows "remote login" isn't as well-established as it is under Linux, this may be preferable. Plus it limits remote access to defined sources/targets on the destination machine instead of allowing access to any (authorized) part of the filesystem.
To transfer over a network more efficiently. Take a look at this article that explains efficient data transfer through zero copy