What is a Jar File? Java archive (jar) files are compressed files - - PowerPoint PPT Presentation

Creating Jar Files

Based on slides by: Jin Hung, Gregory Olds, George Blank, Sun Java Web Site

What is a Jar File?

• Java archive (jar) files are compressed files that

can store one or many files.

• Jar files normally contain java or class files, but
• ther files may also be included.
• Jar files can be run on Windows by double

clicking jar files if the JVM is installed.

Why use a JAR File?

• Compression:Jar files reduce the size of the original files.
• Speed: The applet can be downloaded in one http

transaction.

• Security: Jar files can be signed digitally. Users who

recognize the signature can optionally grant permission to run the file or refuse.

• Package sealing: Sealing a package within the jar file

means that all classes defined in that package are found in the same jar file.

Flow Chart

• Create a manifest.mf file
• Create a jar file
• Basic jar commands
• JAR subdirectories
• Execute the jar file
• Common mistakes
• Bibliography

How to create a jar file: Simple Method

• The jar command a utility that comes with the

JDK.

• The format of the basic format of the jar

command is:

jar cf jar-file input-files

How to create a jar file

• Take as an example the Java application

made from the files

– foo.class – foobar.class

• To make a jar file use the command

jar cvf foo.jar foo.class foobar.class

• Note that each file is separated by a space

How to create a jar file

• The execution of this command creates the

file:

– foo.jar

Executing Jar Application

• Jar applications can be run with the following

basic command:

– java -jar jar-file

• So to run our foo.jar if it was an application

use:

– java –jar foo.jar

Executing Jar Application

• Many Operating systems such as the Mac OS

will allow the execution of jar applications by simply double clicking on them.

Executing Jar Applets

• Applets are invoked from applet tags in HTML

code.

• To execute an Applet simply add the name of

the jar file into the applet tag is under the archive parameter.

Executing Jar Applets

• For example, using the applet tag for your

Foo.jar archive, you would use the following:

<applet code=TicTacToe.class archive=“foo.jar" width=120 height=120> </applet>

Creating Jar Files: Advanced method

• The more formal and complete way to create

a jar file is described in the following slides.

• Note that the more complex your process, the

more difficulty a beginning Java user is going to have with it.

Create manifest.mf

Create a new notepad file named manifest.mf. This file contains two lines that define the author and the main class and a third line that is blank: Created By:<space>Name ( student number ) Main-Class:<Space>name of main class <blank line> For example: for the demo program manifest.mf is: Created By: Hugh Murrell (7525684562) Main-Class: BigIntegerDemo <blank line>

Jar file and Jar Commands

• Next, create a jar file using the “jar” command in
• java. Here is the command.

jar cvfm <the jar file name>.jar manifest.mf

a.class b.class……..

There are spaces between each file and class name.

§ Here are the basic jar commands

Create Jar file => jar cf jar-file input-file View Jar file => jar tf jar-file Extract Jar file => jar xf jar-file

JAR Subdirectories

• The JAR format also support storing files in a

directory structure. Consider the following structure:

JAR Subdirectories

JAR Subdirectories

• If we want to keep the same structure, we can

compress files by typing: jar cvfm Sample.jar Sample.mf Sample.class

Turtle.class Sample.java Turtle.java images

JAR Subdirectories

• The contents listing appears as:

META-INF/ META-INF/MANIFEST.MF Sample.class Turtle.class Sample.java Turtle.java images/ images/image1.gif images/image2.gif images/image3.gif

Executing the Jar File

• There are two ways to execute jar files.
• 1. If an OS can read *.jar as javaw.exe or

java.exe, users can double click jar files on the window. Otherwise:

• 2. In a command window, type in

java –jar jar-file

Viewing manifest information

To view then contents of your manifest file I will issue the shell command:

• unzip –p demo.jar META-INF/MANIFEST.MF

the –p option for unzip causes output to be sent to the console.

Common Mistakes

• Forgetting to leave a blank line after the main class

declaration in the manifest.mf. OS may return “Fail to load Main-Class manifest attribute” error in Windows machines.

• Failing to compress all your files. You have to type in

all the files you want to compress. Separate these files with a space.

Bibliography

• Packaging Programs in JAR Files, and

Running JAR-Packaged Software http://java.sun.com/docs/books/tutorial/ deployment/jar/index.html