Java Archive

The Java Archive (JAR) file format enables you to bundle multiple files into a single archive file.

To process/manipulate/create/extract jar file use

jar[option] jar-file command

Name

The name of the specification.

Specification-Title

The title of the specification.

Specification-Version

The version of the specification.

Specification-Vendor

The vendor of the specification.

Implementation-Title

The title of the implementation.

Implementation-Version

The build number of the implementation.

Implementation-Vendor

The vendor of the implementation

Main-class

helps us to refer the class name of the executable jar

Sealed

If you want to guarantee that all classes in a package come from the same code source, use JAR sealing.

Class-Path

The value of this attribute specifies the relative URLs of the extensions or libraries that this application or extension needs

 #MANIFEST.MF

Manifest-Version: 1.0
Ant-Version: Apache Ant 1.6.5
Created-By: 1.5.0_04-b05 (Sun Microsystems Inc.)
Main-class: abc.xyz.yourclass

#ADVANCED

Name: java/util/
Specification-Title: Java Utility Classes
Specification-Version: 1.2
Specification-Vendor: Sun Microsystems, Inc.
Implementation-Title: java.util
Implementation-Version: build57
Implementation-Vendor: Sun Microsystems, Inc.
Sealed: true

How to add jars into manifest classpath?.

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

How to set executable class in jar file ?.

Main-Class: abc.xyz.yourclass

Why am getting NoClassDefFoundError ?

Check classpath included in manifest.mf file and jvm classpath included jar

Am I loading all the classes from the correct code base ?.

To confirm this we have to seal our package.

Example:
Name: myCompany/myPackage/
Sealed: true

References:
JAR Tutorial | Manifest file