Introduction to Java : Distributing Applets part-2

.zip, .cab, and .jar

Distribution History

As we noted earlier, for most of the history of Java development, there was really no packaging. For the most part, the developer was required to put each class file as a separate file on the web server in the same hierarchy as the packages defined in your Java files. For example, if you had a com.test.utility package you would need a directory hierarchy of com/test/utility under the directory with the main applet class.

Unfortunately, this meant that in order for a web browser to get all of the supporting files it needed, it would have to open up a separate HTTP connection for each file. Given the state of web traffic, site overload, and any number of Internet gremlins, this could add all sorts of delays to the download. What’s more, the files transferred were not compressed in any way.

Archiving Applets with Netscape’s ZIP Format

Initially, Netscape came to the rescue by allowing developers to put class files and supporting files like images and data in a single uncompressed ZIP file. Although, files were not compressed, this was certainly a valuable distribution option. Any web surfer who used Netscape Navigator to view a Java applet need only download a single ZIP file.

Thus, most developers created an uncompressed ZIP file and placed it along with the raw class files on the server. That way, Navigator users could get the speedier ZIP file and surfers using other Java-enabled browsers could still have the separate class files available to them. The HTML now evolved to something like this:

Compressing Archives with Microsoft’s CAB Format

Nipping at the heels of Netscape, Microsoft soon introduced the CAB format to even the score, and as usual, make things even more complex for the developer. CAB files followed the same logic as Netscape’s ZIP archive format. The developer could archive their entire applet into one file and use the CABBASE parameter to direct Internet Explorer browsers where they could pick up the code (this is discussed in Chapter Two). The HTML looked something like this:

However, CAB files had one extra goody. Internet Explorer supported compressed CAB files. This made transferring large Java applets much more efficient because large files could be compressed into much smaller ones.

But, as we mentioned, this only added another level of complexity to the chores necessary for developers wishing to distribute their applications. Specifically, a developer now needed to have (1) all the raw files available for less popular browsers such as Hot Java, (2) a ZIP file for Netscape Navigator users, and (3) a CAB file for Microsoft Internet Explorer users.

Signing Compressed Archives with Netscape’s JAR Format

Not to be outdone, and already deeply ensconced in the battle over web supremacy, Netscape and Sun joined together to develop the JAR (Java ARchive) format. Distributed as part of the JDK 7, JAR is a platform-independent file format (based on the ZIP file format) that allows a developer to bundle and compress a Java applet and its requisite components (class files, images and sounds) into a single file.

Further, the JAR Format allowed applet authors to digitally sign individual entries in a JAR file to authenticate their origin. You create and manipulate JAR files using the jar utility program that has command line arguments similar to those of the popular UNIX tar program and then reference the archive using HTML such as the following:

Preparing your Applet for the Web

Currently, distribution is fairly inconvenient for the developer. To support Internet Explorer, it is best to create a CAB file. Although the most recent versions of Internet Explorer support JAR files, the CAB format works across the widest range of versions. Further, if you want to sign them, you may want to use CAB.

To support the newest browsers from Netscape and Sun you should create a JAR file. To support older versions of Netscape Navigator, you’ll need to include a ZIP file.

Finally, you should leave the class files in their raw form for any miscellaneous browsers that have been slower to adopt the newer technologies.

So how do you go about preparing all of these compressed archives? In the next sections we will walk you through the process so it can be as painless as possible.