JGUI Client Applet

The JGUI is a demonstration of the use of the Java Client Library to build database clients. It is a Java applet which can be run in any Java-enabled browser. We recommend Netscape 3.0 or higher, or Internet Explorer 3.0 or higher. Because Java Applets are subject to security constraints, the JC Applet can only communicate directly with the host from which it was downloaded. Since it must be able to communicate with the PREDATOR server (remember, this applet is going to be a database client), the JGUI must be downloaded from the host on which the database server is running. This is accomplished by starting a HTTP server (a.k.a. web server) on the same machine. You can use _any_ HTTO server of your choice. A number of free web servers are available. On Unix, we use httpd and on WindowsNT, we use the excellent Sambar server. Feel free to use your favorite one. It typically takes some trial and error before you can get a web server to reliably serve your applets (the typical issue is that it should be started in the correct working directory so that the necessary Java classes and HTML files are found in the appropriate subdirectories). The port number you supply to the HTTP server must be different from the database server's port number.

The next step is to create a HTML page which embeds the JGUI applet. We have included a template along with the system (applettry.html). Edit the template file and change the Applet parameter PORT to reflect the port number of your PREDATOR server

Finally, make sure your PREDATOR server is running. Now you can start the JGUI by opening the web page in a Java-enabled browser.

When the applet is loaded, it will connect to the PREDATOR server. After the connection is made, it provides a simple graphical client interface. There is a "Break Connection" button to quit from the server. When the result contains large ADTs like images or audio, the actual large objects are not sent to the client as part of the answer. Instead. a "handle" or identifier is sent. In order to ask to see/play the actual object, you need to click (left mouse button) on the particular cell of the tabular display. The corresponding object is then retrieved from the server and displayed appropriately.

Things to Keep in Mind

Be warned that in most browsers, applets are not reloaded when the "Reload" button is hit. If you run into some initial problem with the JC applet and need to reload it, either restart your browser or use Shift-Reload (with Netscape).

Also please be warned that this applet uses a large number of Java classes that can take a while to load. Most Java VMs will fetch Java classes on an as-needed basis (i.e. pretty much when they actually get invoked). This is meant to minimize network traffic, I suppose. However, this strategy is terrible in terms of latency. The very first time you run the GUI, you will have to twiddle your thumbs while all the classes are downloaded. I tried this from home on a 28.8 line and as my advisor would say, I had time to go down to Colombia, pick myself some choice beans, roast them, and brew myself some really good coffee. There are some improvements possible: one is to use a JAR file. If your browser supports JAR files, try to take advantage of this. Either way, things will be slow the first time, but once the classes are cached, things are smooth.

TroubleShooting

Here are some possible causes of problems, and their symptoms:

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