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Let's say you have a MANIFEST.MF file of the form

Manifest-Version: 1.0
SomeVar: SomeValue

In a directory such as src/main/resources/META-INF, in the case of War projects files in Gradle, then when your WAR file is compiled it'll be in WEB-INF/classes/META-INF/.

When your application starts you can start a listener that parses this file, e.g. in your web.xml file define a listener that starts a class, which should be the first listener incase any subsequent listeners want to use the parsed variables:

<web-app>
        ...
        <listener>
                <listener-class>com.example.ManifestVars</listener-class>
        </listener>
        ...
 </web-app>

In the ManifestVars class, which must implement ServletContextListener since it's a listener, you can then parse the MANIFEST.MF file, giving the rest of your app static access to the fields therein.

package com.example;

import java.io.InputStream;
import java.util.jar.Attributes;
import java.util.jar.Manifest;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

/** 
 * Reads the war's manifest from /META-INF/MANIFEST.MF on application startup.
 * 
 * Must be included as the first <listener></listener> in the web.xml file.
 */
public class ManifestVars implements ServletContextListener {

	private static Attributes sMainManifestAttributes;
	
	/**
	 * Read the manifest from /META-INF/MANIFEST.MF
	 */
	@Override
	public void contextInitialized(ServletContextEvent sce) {
		try {
			InputStream inputStream = getClass().getClassLoader().getResourceAsStream("META-INF/MANIFEST.MF");
			Manifest manifest = new Manifest(inputStream);
			sMainManifestAttributes = manifest.getMainAttributes();
		} catch (Exception e) {
			throw new RuntimeException(e);
		}	
	}

	@Override
	public void contextDestroyed(ServletContextEvent sce) {
		sMainManifestAttributes = null;
	}

	/**
	 * Generic querying of the manifest.
	 * @return The result, as run through String.trim()
	 */
	public static String getValue(String name) {
		return sMainManifestAttributes.getValue(name).trim();
	}

}

In this class we get an InputStream of META-INF/MANIFEST.MF, and since our WAR file classes's directory is at WEB-INF/classes, then in this context, we're accessing WEB-INF/classes/META-INF/MANIFEST.MF.

It's probably a good idea, in this class, to define new static variables for parts of your MANIFEST.MF file, for example a getter and setter for 'SomeVar' in the MANIFEST.MF file.

Now in rest of your application, you can simple call somthing like ManifestVars.getSomeVar(), should you have defined that variable