JAR Files and Manifest Files for Java Beans

JAR Files

Before developing your own Bean, it is necessary for you to understand JAR (Java Archive) files, because tools such as the BDK expect Beans to be packaged within JAR files. A JAR file allows you to efficiently deploy a set of classes and their associated resources. 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 of these pieces can be placed into one JAR file.

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 these elements were produced by a specific organization or individual.

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

The JAR Utility

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

                     jar options files

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

If a manifest file such as Yxz.mf is available, it can be used with the following command:

                                jar cfm Xyz.jar Yxz.mf *.class *.gif

Tabulating the Contents of a JAR File

The following command lists the contents of Xyz.jar:

                               

                               jar tf Xyz.jar

Extracting Files from a JAR File

The following command extracts the contents of Xyz.jar and places those files in the

current directory:

                              jar xf Xyz.jar

Updating an Existing JAR File

The following command adds the file file1.class to Xyz.jar:

                            jar -uf Xyz.jar file1.class

Recursing Directories

The following command adds all files below directoryX to Xyz.jar:

                            jar -uf Xyz.jar -C directoryX *

Manifest Files

A developer must provide a manifest file to indicate which of the components in a JAR file are Java Beans. An example of a manifest file is provided in the following listing. It defines a JAR file that contains four .gif files and one .class file. The last entry is a Bean.

Name: sunw/demo/slides/slide0.gif
Name: sunw/demo/slides/slide1.gif
Name: sunw/demo/slides/slide2.gif
Name: sunw/demo/slides/slide3.gif
Name: sunw/demo/slides/Slides.class
Java-Bean: True

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”.