creating jar files

8/6/2019 Creating Jar Files

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Creating Jar Files


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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 other files may also be included.

Jar files can be run on Windows by double

clicking jar files if the JVM is installed.


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Why use a JAR File?

Compression:Jar files reduce the size of theoriginal files.

Speed: The applet can be downloaded in one httptransaction.

Security: Jar files can be signed digitally. Userswho recognize the signature can optionally grantpermission to run the file or refuse.

Package sealing: Sealing a package within the jarfile means that all classes defined in that packageare found in the same jar file.


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Flow Chart

Create amanifest.mf file

Create a jar file

Basic jar commands

JAR subdirectories

Execute the jar file


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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


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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


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How to create a jar file

The execution of this command creates

the file:

Foo.jar


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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


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Executing Jar AppletsApplets 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.


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Executing Jar Applets For example, using the applet tag for your

Foo.jar archive, you would use the

following:


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Creating Jar Files:

Advanced meth

od The more formal and complete way to

create a jar file is described in the

following slides.


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Createmanifest.mf

Create a new notepad file named manifest.mf.This file contains a signal line that points out themain class. Here is the signal line.

Main-Class:name of main class

Blank line must be included because someWindows OS versions need it.

For example: myClass is my main Class. Somanifest.mf is following.

Main-Class: myClass


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Jar file and Jar Commands

Next, create a jar file using the jar command injava. Here is the command.

jar cvmf .jarmanifest.mf A.class B.class..

There are spaces between each file and classname.

Here are the basic jar commandsCreate Jar file => jar cf jar-file input-file

View Jar file => jar tf jar-file

Extract Jar file => jar xf jar-file


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JAR Subdirectories

The JAR format also support storing files

in a directory structure. Consider the

following structure.


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JAR Subdirectories


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JAR Subdirectories

If we want keeping the same structure, we

can compress files by typing:

jar cmf Sample.mf Sample.jarSample.class Turtle.class Sample.java

Turtle.java images


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JAR Subdirectories

The contents listing appears as:

META-INF/

META-INF/MANIFEST.MFSample.class

Turtle.class

Sample.java

Turtle.java

images/ images/image1.gif

images/image2.gif

images/image3.gif


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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 jarjar-file


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Common Mistakes

Forgetting to leave a blank line after the

main class declaration in the manifest.mf.

OS may return Fail to load Main-Classmanifest attribute error.

Failing to compress all your files. You

have to type in all the files you want tocompress. Separate these files with a

space.

creating jar files

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