What is the purpose of Manifest file

Question

https://docs.oracle.com/javase/tutorial/deployment/jar/manifestindex.html

What does a manifest file do? What is the purpose of it?

Manifest-Version: 1.0

Name: java/math/BigDecimal.class
SHA1-Digest: TD1GZt8G11dXY2p4olSZPc5Rj64=
MD5-Digest: z6z8xPj2AW/Q9AkRSPF0cg==

Name: java/math/BigInteger.class
SHA1-Digest: oBmrvIkBnSxdNZzPh5iLyF0S+bE=
MD5-Digest: wFymhDKjNreNZ4AzDWWg1Q==

Does it update these classes in the jar automatically if there any changes in the server?

Answer 1

The manifest is a special file that can contain information about the files packaged in a JAR file. By tailoring this "meta" information that the manifest contains, you enable the JAR file to serve a variety of purposes.

Mainfest file is mainly known for contains the name of the class that holds the main function, among various classes in the package...

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

Answer 2

What does a manifest file do? What is the purpose of it?

The purpose is to hold metadata about the JAR file and the classes that it contains. The metadata is used for a variety of things, including tracking the origin of the JAR, protecting against tampering, and providing the extra information needed for an executable JAR.

The link in your question is broken. Here are a couple of alternatives:

• Working with Manifest Files: The Basics
• JAR File Specification

Does it update these classes in the jar automatically if there any changes in the server?

No.

Those entries are part of a scheme for detecting tampering with JAR files; e.g. someone replacing those classes with modified versions. (The other parts of the scheme are a digital signature of the hashes, and stuff in the launcher that checks the signatures and hashes before launching.)