Adv Java [JAR File Creation]

♠ Posted by Unknown in Adv Java at 01:01

What is JAR files?

A JAR file allows you to efficiently deploy a set of classes and their associated resource.  For example, a developer may build a multimedia application that uses various sound and image files. A set of Beans can control how and when this information is presented. All these pieces can be placed into one JAR files.

JAR technology makes it much easier to deliver and install software. Also, the elements in a JAR file are compressed, which makes downloading a JAR file much faster than separately downloading several uncompressed files. Digital signatures may also be associated with the individual elements in a JAR file. This allows a consumer to be sure that a specific organization or individual produced these elements.

            Note : The package java.util.zip contains classes that read write JAR files.

Manifeast Files :

A developer must provide a manifest file to indicate which of the components in a Jar file are Java Beans. A manifest file may reference several .class files. If a .class file is a Java Bean, its entry must be immediately followed by the line “Java-Bean: True”.

Introspection :

Introspection is the process of analyzing a Bean to determine its capabilities. This is an essential feature of the Java Beans API, because it allows an application builder tool to present information about a component to a software designer. Without introspection, the Java Beans technology could not operate.

There are two ways in which the developer of a Bean can indicate which of its properties, events, and methods should be exposed by an application builder tool. With the first method, simple naming conventions are used. these allow the introspection mechanisms to infer information about a Bean. In the second way, an additional class is provided that explicitly supplies this information.

The JAR Utility :

A utility is used to generate a JAR file. Its syntax is shown here:

         

          jar option files

Option	Description
c	A new archive is to be created.
C	Change directories during command execution.
f	The first element in the file list is the name of the archive that is to be created or accessed.
i	Index information should be provided.
M	The second element in the file list is the name of the external manifest file.
M	Manifest file not created.
t	The archive contents should be tabulated.
u	Update existing JAR file.
v	Verbose output should be provided by the utility as it executes.
x	Files are to be extracted from the archive. (If there is only one file, that is the name of the archive, and all files in ti are extracted. Otherwise, the first element in the file list is the name of the archive, and the remaining elements in the list are the files that should be extracted from the archive.)
0	Do not use compression.

Creating a JAR file :

The following command creates a JAR file named xyz.jar that contains all of the .class and .gif files in the current directory:

         

          jar cf xyz.jar *.class *.gif