如何使用 Package 类的 getImplementationVersion() 方法从 JAR 的清单中获取包版本

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, eg 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?

Answer 3

Please make sure you have a newline at the end of the manifest. If there is no newline character at the end, getImplementationVersion() will return null .

Reference : http://docs.oracle.com/javase/7/docs/technotes/guides/jar/jar.html

Answer 4

I had the same issue in eclipse. The problem comes from the Eclipse jar-in-jar class loader. Simply put don't use the package your external libs in the jar method to generate the jar, use the extract external libs into your jar instead method and it should work.

Answer 5

I found this issue in my IDE (Eclipse) because the run from within the IDE wasn't using the generated Manifest in the jar. To see the correct behavior, I just went to the command line of the computer to run the jar (since I had an appropriate main class set). But put another way, even if it doesn't work within the IDE, it should work in deployment.