Java Glossary : remote file access

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remote file access

The java.io.File class won't let you read or write files on some other remote machine, e.g. Applets can't read or write files on the server, even if they are signed. The File class in an application may let you read or write files on some other machine on a LAN, but that depends on the underlying operating system to make the files on the other workstations on the LAN look those files like local files. It won't do you any good to concoct filenames that look like URLs. The File class is only for reading local files!

But why? Why can't you just use an URL as your filename, and read and write files on some remote machine? The problem is you don't want people out on the Internet reading and writing your files, or files on your server, without monitoring very carefully just what they are doing. You don't want to give any old Tom, Dick or Harriet out on the Internet the keys to read or write any file on your system whenever they want.

Unfortunately, the Java file system has no security features. There is no way to identify yourself, to prove your identity, or to limit your read/write access to a given set of files or directories. It presumes unlimited read/write access. This is just not good enough for remotely reading and writing files.

Servers typically don't provide a means for remote machines to directly read or write their files. When they do, such as in Novell NFS servers, there is a whole layer of security tacked on top.

Instead, servers provide a pot pourri of indirect ways to read and write the remote files. In each case you run some program on the server that talks via the Internet to the remote computer, and does any file reading and writing on its behalf. e.g.

Canadian Mind Products FileTransfer classes let you copy files from a variety of sources, including jars, to a variety of targets, both remote and local. They use HTTP protocol where necessary.
CGI (Common Gateway Interface) lets you communicate with a servlets running on the server that will read or write files for you and send back results. See Base64 to convery your binary data to only printable characters.
HTTP (Hypertext Transfer Protocol) HTTP GET allows reading of a selected set of files, HTTP POST lets you send a query and get back information. HTTP PUT lets you upload a file, though the PUT feature is rarely supported.
JDBC (Java DataBase Connectivity) lets you communicate with an SQL server running on the server to read and write database files.
Sockets let you send raw streams of bytes back and forth between remote machine and server. The program on the server can then use that information to read and write files. If you compress your data, this about as efficient as it gets.
RMI (Remote Method Invocation) lets you send Java objects back and forth between the remote computer and a server. Each can run methods on the other published objects. These methods may read and write files.
EJB (Enterprise Java Beans) are like RMI plus servlets, plus transaction logging and backout.
FTP (File Transfer Protocol) allows file upload/download. For efficiency, send only zipped files, bundling many small files into one big one. FTP is a slower protocol than HTTP but supports recovery there is a disconnect in mid transfer.
CORBA (Common Object Request Broker Architecture) is like a heavy duty RMI that supports languages other than Java. It is considerably more complicated to use.
Jini. I'm told you can use this for intermachine communication, but I have never tried it.
JavaSpaces. I'm told you can use this for intermachine communication, but I have never tried it.

Each of these schemes has is own built-in security to be sure the general public can only see the files you want them too, and that authorised users can only indirectly read and write the files you want them too.

Java P2P Unleashed: With JXTA, Web Services, XML, Jini, JavaSpaces, and J2EE

0-672-32399-0

Robert Flenner, Michael Abbott, Toufic Boubez, Frank Cohen, Navaneeth Krishnan, Alan Moffet, Rajam Ramamurti, Bilal Siddiqui, Frank Sommers

Covers a variety of the more exotic techniques for inter-computer communication with Java.

CGI ¤ CORBA ¤ EJB ¤ file io ¤ FileTransfer classes ¤ FTP ¤ HTTP ¤ JavaSpaces ¤ JDBC ¤ JINI ¤ JSP ¤ RMI ¤ Servlet ¤ socket

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