package com.rgagnon;

public class Hello {
 public static void main(String[] args) {
    new Hello().say("Hello World!");
    }
 
 public void say(String s) {
    System.out.println(s);
 }
}

Manifest-Version: 1.0
Main-Class: com.rgagnon.Hello

>jar cvfm hello.jar MANIFEST.MF com\rgagnon\Hello.class

>java -jar hello.jar
Hello World!

Manifest-Version: 1.0
Main-Class: com.rgagnon.Hello
Specification-Version: 2.1
Implementation-Version: 1.1

package com.rgagnon;

public class Hello {
 public static void main(String[] args) {
    new Hello().say("Hello World!");
    }
 
 Hello() {
   Package p = this.getClass().getPackage();
   System.out.println("Hello Specification Version : " 
                         + p.getSpecificationVersion());
   System.out.println("Hello Implementation Version : " 
                         + p.getImplementationVersion());
 }
 
 public void say(String s) {
    System.out.println(s);
 }
}

> jar cvfm hello.jar MANIFEST.MF com\rgagnon\Hello.class

> java -jar hello.jar
Specification Version : 2.1
Implementation Version : 1.1
Hello World!

package com.rgagnon.howto;

import java.util.jar.*;

public class JarUtils {
  public static String getJarImplementationVersion(String jar)
       throws java.io.IOException
  {
    JarFile jarfile = new JarFile(jar);
    Manifest manifest = jarfile.getManifest();
    Attributes att = manifest.getMainAttributes();
    return att.getValue("Implementation-Version");
  }

  public static String getJarSpecificationVersion(String jar)
       throws java.io.IOException
  {
    JarFile jarfile = new JarFile(jar);
    Manifest manifest = jarfile.getManifest();
    Attributes att = manifest.getMainAttributes();
    return att.getValue("Specification-Version");
  }
  public static void main(String[] args) throws java.io.IOException{
    String javaMailJar = "C:/Program Files/Java/jre1.5.0/lib/mail.jar";

    System.out.println("Specification-Version: " 
        + JarUtils.getJarSpecificationVersion(javaMailJar));
    System.out.println("Implementation-Version: " 
        + JarUtils.getJarImplementationVersion(javaMailJar));
    /*
     * output :
     *      Specification-Version: 1.3
     *      Implementation-Version: 1.3.1
     */
  }
}