Jar Files

General Information

Introduction and Purpose

The Java Archive (JAR) file format bundles multiple files into a single archive file. The file format is based on ZIP and the ZLIB compression format, so the Java jar tool can be used as a (command line) replacement for pkzip that works on Unix platforms. Typically a JAR file will contain the class files and auxiliary resources associated with applets and applications -- not just class files.

Why?

• Aggregation of programs and libraries
• Compresion
• Security and authorization through digitially signed jars
• Java features: extensions (eg. Java3D and JavaMail), package sealing and package versioning

Jar files can be elements on a class path:

java -cp dir/dir1:lib1.jar:dir/dir2:lib2.jar:. Application

Using the Jar Tool

Operation	Command
Creating a JAR file	jar cf jar-file input-file(s)
Viewing the contents of a JAR file	jar tf jar-file
Extracting the contents of a JAR file	jar xf jar-file
Extracting specific files from a JAR file	jar xf jar-file archived-file(s)
Merging manifest information	jar cmf manifest-addition jar-file input-file(s)
Updating existing jar	jar uf jar-file input-file(s)
Creating a JAR file (Java 1.6)	jar cef entry jar-file input-file(s)
To run an application packaged as a JAR file (version 1.2 -- requires Main-Class manifest header)	java -jar app.jar
To invoke an applet packaged as a JAR file	<applet code=AppletClassName.class archive="JarFileName.jar" width=width height=height> </applet>

Manifest entries:

main	Main-Class: Main Main-Class: pack.ClassName
class path	Class-Path: http://www.univ.edu/file.jar Class-Path: file:/directory/subdirectory/ Class-Path: file:/directory/subdirectory/file.jar Class-Path: file1.jar file2.jar

Executing Jar Files

java -jar jar-file

Before this command will work, however, the runtime environment needs to know which class within the JAR file is the application's entry point.

To indicate which class is the application's entry point, you must add a Main-Class header to the JAR file's manifest. The header takes the form:

Main-Class: classname

javac Main.java
echo "Main-Class: Main" >! mc.mf   # create a manifest file entry
jar -cvmf mc.mf main.jar Main.class
added manifest
adding: Main.class(in = 414) (out= 284)(deflated 31%)
rm mc.mf             # file no longer needed
java -jar main.jar   # jar file now executes without mentioning main

Applet in jar file:

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

JAR URL

jar:<url>!/{entry}

jar:http://www.foo.com/bar/baz.jar!/COM/foo/Quux.class
jar:file:/export/home/faculty/stansif/public_html/java/applets/image/view.jar!/image.gif

Accessing resources

final java.net.URL url = X.class.getResource ("/dir/image.png");
final ImageIcon icon = new ImageIcon (url);

Sealed Packages

A package can be sealed by adding the Sealed header beneath the header naming the package that's to be sealed.:

Name: myCompany/myPackage/
Sealed: true

The default manifest created by the Jar tool does not contain any Sealed headers, of course, because packages are not sealed by default. To seal a package, you therefore have to add the Sealed header yourself. To insert the Sealed header in a JAR file's manifest, you first need to write a text file containing the appropriate headers. The file you write doesn't have to be a complete manifest file; it can contain just enough information for the Jar tool to know where and what information to merge into the default manifest.

Package Versioning

java.lang.Package (Java API documentation) (local copy)

class Main {
    public static void main(String[] args) {
        Package[] pkgs = Package.getPackages();
        for (int i=0; i<pkgs.length; i++) {
            System.out.println(pkgs[i].getName());
            System.out.println("  Specification Title:     " + pkgs[i].getSpecificationTitle());
            System.out.println("  Specification Version:   " + pkgs[i].getSpecificationVersion());
            System.out.println("  Specification Vendor:    " + pkgs[i].getSpecificationVendor());
            System.out.println("  Implementation Title:    " + pkgs[i].getImplementationTitle());
            System.out.println("  Implementation Version:  " + pkgs[i].getImplementationVersion());
            System.out.println("  Implementation Vendor:   " + pkgs[i].getImplementationVendor());
            if (pkgs[i].isSealed()) {
                System.out.println("  Sealed: true");
            }
        }
    }
}

Java Extensions

Reading and Writing Jar Files in Java

• JarFiles.java -- a simple example of writing a jar file
• ReadJar.java
• WriteJar.java