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

Question

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-simple-1.1.1.jar
 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 JarRsrcLoader - may be it requires some special syntax to address packages. I have also tried ".com/domain/...", "/com/domain/..." and ".../controller", and even "rsrc:./com/domain..." 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 getImplementationVersion() method.

Answer 1

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");

  System.out.println(implVersion);
} catch (IOException E) {
      // handle
}

Answer 2

Solved! At least for me :) You need to make sure, that the package in question is located only in one JAR and that JAR has the correct manifest.

Makes sense, since you query a package for a version and what should the JVM return, if that package is present in many JARs all with different or partially unset implementation versions?