Jars

"Jars"
Java Archive Files

Overview

A "jar", or Java archive, is simply a file that conains a number of .class files. (This is its typical use. It can be coerced to hold other files.)
Unlike other compressed file formats (such as .zip or .tar), the Java compiler and virtual machine know the jar format.

We'll learn how to see what's in a jar, create a jar, extract files from a jar. The jar command has additional options (see it manual page).

	  jar -cvf game.tar X.class Y.class Z.class     // create a jar
	  jar -tvf game.tar                             // table of contents (see what's in a jar)
	  jar -xvf game.tar filename                    // extract a file from a jar

To tell the Java compiler and/or JVM where a jar is, add the name of the jar file to your CLASSPATH. (In DrJava, Edit-Preferences, Resource Locations, Extra Classpath, Add)

How to See What's in a Jar:

Run the jar command and give it the "table of contents", "verbose", and "file" options. It will print the names of the files in the jar:
```
	   % jar  -tvf  game.jar  
	   
```

How to Make a Jar Available to the JVM by adding it to the Classpath:

On the command line, use java's classpath option:

           % jar  -cp game.jar:.  ClassWithMainMethod

Add the jar name via your IDE's (e.g. Drjava's Reources, Extra Classes)
Add the jar to your environment (e.g. using the CLASSPATH environment variable).

How to Create a Jar:

Use the "jar" command, shown below with the options 'c'reate, 'v'erbose, and 'f'ile. Note that the syntax is similar to the Unix "tar" command. The command below creates (or overwrites) a file called someName.jar whose contents include a copy of all the .class files found in the current directory. The original .class files remain unchanged.
```
	   % jar  -cvf  game.jar  *.class 
	   
```
The Java compiler and JVM can find the jar (when resolving/finding class names) if the jar is in the same directory where javac/java is run, or if the jar is in a directory in the invoker's CLASSPATH variable.
If you want the Jar to be run as an application (e.g. to put all the class files for a game you've written in to a jar and run the game), the JVM needs to know which class' main method to use as a starting point. To "tell it":
1. Create a "manifest" file that contains the name of the class which has the special main method. E.g. a file called "manifest.txt" with contents like this:
```
	   Main-Class: NameOfClassWhoseMainMethodShouldBeRun
	   
```
2. Create the Jar:
```
	   jar  -cvmf  manifest.txt  myApplication.jar  *.class
	   
```
3. Run the Jar as an application from the command line:
```
	   % java -jar myApplication.jar
	   
```
4. Depending on the operating system, you may be able to simply click or double-click on a jar's icon to run the application contained in it.
Can a directory be put in to a jar?
Only if the directory is a package. One or more packages can be put in to a jar. (See next bullet)

To add a package to a jar:

At the top of each class file:

                   package package_name;
        Example 1: package chessgame;   
        Example 2: package net.coolgames.chessgame

To designate a main() method for the jar, create a manifest file in which a line should appear like so:

                   Main-Class: package_name.NameOfClassWhoseMainMethodShouldBeRun
        Example 1: Main-Class: chesswiz.Main
        Example 2: Main-Class: net.coolgames.chesswiz.Main

Create the jar:

                   jar  -cvmf  manifest.txt  jar_name.jar  package_directory_pathname
        Example 1: jar  -cvmf  manifest.txt  Chess.jar  chessgame
        Example 2: jar  -cvmf  manifest.txt  Chess.jar  net/coolgames/chessgame

How to Extract a File from a Jar:

	   % jar  -xvf  game.jar  filename

"Jars"Java Archive Files

Overview

"Jars"
Java Archive Files