m. vokhm m. vokhm - 11 months ago 56
Java Question

How to obtain a package version from the JAR's manifest, using the getImplementationVersion() method of the Package class

When I prepare my program for deployment, I pack it into a JAR, along with the Eclipse jar-in-jar class loader. When my program runs from the JAR, I need to know a package's version, but I can not obtain it from the jar's manifest a simple and "honest" way.
The manifest looks like this:

Manifest-Version: 1.0
Created-By: 1.8.0_73-b02 (Oracle Corporation)
Main-Class: org.eclipse.jdt.internal.jarinjarloader.JarRsrcLoader
Rsrc-Main-Class: com.domain.sp2.controller.Controller
Rsrc-Class-Path: ./ jar-in-jar-loader.zip javahelp-2.0.05.jar json-sim
Class-Path: .

Name: com/domain/sp2/controller/
Implementation-Version: 19

To obtain the package's implementation version, I try to use the simplest and straight-forward way:

package com.domain.sp2.controller;
public class Controller {
public static String getBuildNumber() throws IOException {
Package pckg = Controller.class.getPackage();
pr(pckg.getName()); // prints "com.domain.sp2.controller", as expected
return pckg.getImplementationVersion(); // returns null

According to http://docs.oracle.com/javase/tutorial/deployment/jar/packageman.html and http://docs.oracle.com/javase/8/docs/api/java/lang/Package.html#getImplementationVersion-- (and other sources), it should return "19", but it returns null. For the packages of the JRE libraries, it returns correct values. Perhaps I have missed a detail about how to name the package in the manifest, or the reason pertains to the
- may be it requires some special syntax to address packages. I have also tried
, and even
as the package name in the manifest - all without success. I could use other ways, e.g. to load the manifest as stream and parse it with the Manifest class, yet I would like to understand what is the correct way to use the


You can read the manifest file from the classLoader and get the value you need as below:

URLClassLoader cl = (URLClassLoader) YOUR_CLASS.class.getClassLoader();
try {
  URL url = cl.findResource("META-INF/MANIFEST.MF");
  Manifest manifest = new Manifest(url.openStream());
  Attributes mainAttributes = manifest.getMainAttributes();
  String implVersion = mainAttributes.getValue("Implementation-Version");

} catch (IOException E) {
      // handle