Java Programing

Compression makes downloading classes over a network much faster. A quick survey of the SDK distribution shows that a typical class file shrinks by about 40 percent when it is compressed. Text files such as arbitrary HTML or ASCII containing English words often compress by as much as 75 percent to one-quarter of their original size. (On the other hand, image files don’t get smaller when compressed; both of the common image formats have compression built in.) Compression is not the only advantage that a JAR file has for transporting files over a network. For an application with many components, the amount of time it takes to transport all of the parts may be less significant than the time involved in setting up the connections and making the requests for them. This is especially important for applets loaded via the Web. The typical web browser has to make a separate HTTP request for each class or data file. An applet comprising 100 classes, for example, would require at least 100 separate trips to the web server to gather all its parts. Placing all the classes in a single JAR file enables them to be downloaded in a single transaction. Eliminating the overhead of making HTTP requests is likely to be a big savings, since individual class files tend to be small, and a complex applet could easily require many of them. 3.5.2 The jar Utility The jar utility provided with the SDK is a simple tool for creating and reading JAR files. Its user interface isn’t friendly; it mimics the Unix tar (tape archive) command. If you’re familiar with tar, you’ll recognize the following incantations: jar -cvf jarFile path [ path ] [ .. . ] Create jarFile containing path(s) jar -tvf jarFile [ path ] [ … ] List the contents of jarFile, optionally showing just path(s) jar -xvf jarFile [ path ] [ … ] Extract the contents of jarFile, optionally extracting just path(s) In these commands, the letters c, t, and x tell jar whether it is creating an archive, listing an archive’s contents, or extracting files from an archive. The f means that the next argument will be the name of the JAR file on which to operate. The v tells jar to be more verbose when displaying information about files. In verbose mode you can get information about file sizes, modification times, and compression ratios. Subsequent items on the command line (i.e., anything aside from the letters telling jar what to do and the file on which jar should operate) are taken as names of archive items. If you’re creating an archive, the files and directories you list are placed in it. If you’re extracting, only the filenames you list are extracted from the archive. (If you don’t list any files, jar extracts everything in the archive.) For example, let’s say we have just completed our new game: “spaceblaster.” All the files associated with the game are in three directories. The Java classes themselves are in the spaceblaster/game directory; spaceblaster/images contains the game’s images; and spaceblaster/docs contains associated game data. We can pack all of this in an archive with this command: % jar cvf spaceblaster.jar spaceblaster - 62

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