Oracle

• The Java™ Archive (JAR) file format enables you to bundle multiple files into a single archive file.
• Typically a JAR file contains the class files and auxiliary resources associated with applets and applications.
• JAR file benefits:
  • Security: You can digitally sign the contents of a JAR file
  • Compression
  • Packaging for extensions: The extensions framework provides a means by which you can add functionality to the Java core platform
• JAR files are packaged with the ZIP file format (lossless data compression)

jar cf jar-file input-file(s)

• Options and arguments:
  • c indicates that you want to create a JAR file
  • f indicates that you want the output to go to a file rather than to stdout
  • jar-file name of resulting JAR file. By convention .jar
  • input-file(s) is a space-separated list of files that you want to include in your JAR file (can contain the wildcard * symbol).
  • If any of the "input-files" are directories, the contents of those directories are added to the JAR archive recursively.
• This generate a compressed JAR and a default manifest file

• m Used to include manifest information from an existing manifest file

jar cmf existing-manifest jar-file input-file(s)

java -jar jar-file

• -jar flag tells the launcher that the application is packaged in the JAR file format.
• 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

• classname, is the name of the class that is the application's entry point.

• The manifest is a special file that can contain information about the files packaged in a JAR file
• When you create a JAR file, it automatically receives a default manifest file
• There can be only one manifest file in an archive, and it always has the pathname META-INF/MANIFEST.MF
• The default manifest file simply contains the following:

Manifest-Version: 1.0
Created-By: 1.7.0_06 (Oracle Corporation)

• Manifest's entries take the form of "header: value" pairs.

• Use the m command-line option to add custom information to the manifest during creation of a JAR file.
• Typically, modifying the default manifest involves adding special-purpose headers to the manifest that allow the JAR file to perform a particular desired function.
• You must first prepare a text file containing the information you wish to add to the manifest.
• Warning: The text file from which you are creating the manifest must end with a new line or carriage return. The last line will not be parsed properly if it does not end with a new line or carriage return.

jar cfm jar-file manifest-addition input-file(s)

Main-Class: classname

• classname is the name of the class that is your application's entry point
  • class having a method with signature public static void main(String[] args)

• We first create a text file named Manifest.txt with the following contents:

Main-Class: MyPackage.MyClass

• Warning: The text file must end with a new line or carriage return.
• We then create a JAR file named MyJar.jar by entering the following command:

jar cfm MyJar.jar Manifest.txt MyPackage/*.class

• Classes

public class A {
    public static void main(String[] args) {
        System.out.println("Class A: main");
        new B();
    }
}

public class B {
    public B() {
        System.out.println("new B object");
    }
}

• Manifest.txt

Main-Class: A

javac A.java
jar cfm A.jar Manifest.txt A.class B.class
java -jar A.jar

• 'e' flag (for 'entrypoint') creates or overrides the manifest's Main-Class attribute
• Use it to specify the application entry point without editing or creating the manifest file.

jar cfe app.jar MyApp MyApp.class

• If the entrypoint class name is in a package it may use a '.' (dot) character as the delimiter
• For example, if Main.class is in a package called foo the entry point can be specified in the following ways:

jar cfe Main.jar foo.Main foo/Main.class

• You may need to reference classes in other JAR files from within a JAR file.
• You specify classes to include in the Class-Path header in manifest file

Class-Path: jar1-name jar2-name directory-name/jar3-name