No no! I am not going to tell you what a jar file is and neither I am going to tell you how to make a jar file and use it properly. Actually there is a very small piece of information every jar carries with itself which I want to share with you all. I assume most of you might be knowing this already but I was not knowing this unless I discovered it myself in one of my mission critical moments in work. Actually there were same jar files compiled on different versions of java and I needed to differentiate between them. And for some time I just kept thinking about amazing ways to do that, never mind :). So coming to the point, if you want to know about similar information about a jar file viz. compiler version, product version et cetera you can peep into *.jar/META-INF/MANIFEST.MF file. META-INF is a special sub directory of the jar file.
The manifest file contains the information about the jar file and it’s contents in form of name value pair. The contents of the manifest are group into sections and the first section is called the main section. The entries in the main section are global to the jar file. Different sections in the manifest are separated by blank lines. Some of the attributes you may often see are Manifest-Version, Created-By, Implementation-Vendor, Implementation-URL et cetera.
To add/edit an update entries into the manifest file in a jar the cfm and ufm options of jar command can be used respectively.
For more information on jar and manifest visit http://java.sun.com/javase/6/docs/technotes/guides/jar