19.1 Java4620: general information  (Page 8/8)

The bottom line is that with socket programming, it is easy to write code that will cause a stream of bytes to flow in both directions between a clientand a server. This is no more difficult than causing a stream of bytes to flow in both directions between memory and a file on a disk.

However, getting the bytes to flow is the easy part. Beyond that, you must do all of the programming to implement an application protocol that is understoodby both the client and the server.

URL Programming

URL programming occurs at a higher level than socket programming, and in theory represents a very powerful idea.

In theory, by using the URL class, you can open a connection to a resource on the web, specified by a URL object, and simply call the getContent method on that URL object. The content of the resource will then be magically downloaded and will appear as an object on the clientmachine, even if it requires an application protocol that didn't exist when you wrote the program, and contains content that you didn't understand when youwrote the program.

This description may be a bit of an overstatement, but it is pretty close to the claims being made. This is a powerful idea, which may or may not bear fruitin the future.

If fully implemented by browsers, the idea means that you can place new and unusual material on a web site along with special content handlers and protocolhandlers. Then a cooperating browser will use those special handlers to move that material from the web site to the client and interpret its content once itgets there without a requirement to install software (such as plug-ins) on the client computer on a permanent basis.

Here is what Peter van der Linden has to say about this topic in his excellent book titled Just Java 1.1 and Beyond:

"If a browser doesn't recognize a media type, it should be able to download the code to process it from the same place it got the file. If theyever get this working, it will be ... a good thing."

Is it working, or will they ever get it working? I don't know. If it depends on cooperation among all the major players, including the major browser vendors- probably not. Therefore, I don't plan to spend much time on the topic of protocol and content handlers until I see some evidence that it is working tosuch an extent that it is practically useful.

That is not to say that you couldn't use the capability right now if you were developing an intranet and wanted the clients to have access to new and unusualcontent. It would be necessary for you to provide the appropriate protocol and content handlers, and it would probably be necessary for the clients to run Javaapplications written by you instead of standard browsers to access the data.

Also, the URL class provides an alternative way to connect one computer to another and transfer data on a stream basis, so we will see someexamples of retrieving data from a server by obtaining a URL connection, and then opening and servicing I/O streams between the client and the server. Wewill see some sample programs that make use of this technique, but we will also see that it is somewhat redundant with the socket programming approach.

A local area network

This is part of a sub-collection of modules designed for teaching network programming. Therefore, you may find some of the modules more meaningful if youare able to connect two or more computers in a local area network and run the sample programs across the network.

However, it is possible to simulate a network inside a single computer. If you are unable to create an actual network, you should be able to run all of thesample programs by simulating a network in your single computer.

The operating system

While the capabilities of Java are generally independent of the operating system in use, the manner in which an individual computer must be configured fornetwork operation is generally not independent of the operating system.

In those cases where these modules provide instructions for configuring the computer, those instructions will assume a Windows operating system. If you areusing a different operating system, you will need to translate those instructions into your operating system.

What's Next?

We will learn how to use the Java InetAddress class to find the domain name corresponding to an IP address, and to find the IP addresscorresponding to a domain name in our sample program in the next module.

Miscellaneous

This section contains a variety of miscellaneous information.

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