Reading my own Jar's Manifest

Question

I need to read the Manifest file, which delivered my class, but when I use:

getClass().getClassLoader().getResources(...)

I get the MANIFEST from the first .jar loaded into the Java Runtime.
My app will be running from an applet or a webstart,
so I will not have access to my own .jar file, I guess.

I actually want to read the Export-package attribute from the .jar which started the Felix OSGi, so I can expose those packages to Felix. Any ideas?

Solution 1

You can do one of two things:

1. Call getResources() and iterate through the returned collection of URLs, reading them as manifests until you find yours:

   Enumeration<URL> resources = getClass().getClassLoader()
     .getResources("META-INF/MANIFEST.MF");
   while (resources.hasMoreElements()) {
       try {
         Manifest manifest = new Manifest(resources.nextElement().openStream());
         // check that this is your manifest and do what you need or get the next one
         ...
       } catch (IOException E) {
         // handle
       }
   }
   

2. You can try checking whether getClass().getClassLoader() is an instance of java.net.URLClassLoader. Majority of Sun classloaders are, including AppletClassLoader. You can then cast it and call findResource() which has been known - for applets, at least - to return the needed manifest directly:

   URLClassLoader cl = (URLClassLoader) getClass().getClassLoader();
   try {
     URL url = cl.findResource("META-INF/MANIFEST.MF");
     Manifest manifest = new Manifest(url.openStream());
     // do stuff with it
     ...
   } catch (IOException E) {
     // handle
   }
   

Solution 2

You can find the URL for your class first. If it's a JAR, then you load the manifest from there. For example,

Class clazz = MyClass.class;
String className = clazz.getSimpleName() + ".class";
String classPath = clazz.getResource(className).toString();
if (!classPath.startsWith("jar")) {
  // Class not from JAR
  return;
}
String manifestPath = classPath.substring(0, classPath.lastIndexOf("!") + 1) + 
    "/META-INF/MANIFEST.MF";
Manifest manifest = new Manifest(new URL(manifestPath).openStream());
Attributes attr = manifest.getMainAttributes();
String value = attr.getValue("Manifest-Version");

Solution 3

You can use Manifests from jcabi-manifests and read any attribute from any of available MANIFEST.MF files with just one line:

String value = Manifests.read("My-Attribute");

The only dependency you need is:

<dependency>
  <groupId>com.jcabi</groupId>
  <artifactId>jcabi-manifests</artifactId>
  <version>0.7.5</version>
</dependency>

Also, see this blog post for more details: http://www.yegor256.com/2014/07/03/how-to-read-manifest-mf.html

Solution 4

I will admit up front that this answer does not answer the original question, that of generally being able to access the Manifest. However if what is really required is to read one of a number of "standard" Manifest attributes, the following solution is much simpler than those posted above. So I hope that the moderator will allow it. Note that this solution is in Kotlin, not Java, but I would expect that a port to Java would be trivial. (Although I admit I don't know the Java equivalent of ".`package`".

In my case I wanted to read the attribute "Implementation-Version" so I started with the solutions given above to obtain the stream and then read it to obtain the value. While this solution worked, a coworker reviewing my code showed me an easier way to do what I wanted. Note that this solution is in Kotlin, not Java.

val myPackage = MyApplication::class.java.`package`
val implementationVersion = myPackage.implementationVersion

Once again note that this does not answer the original question, in particular "Export-package" does not seem to be one of the supported attributes. That said, there is a myPackage.name that returns a value. Perhaps someone who understands this more than I can comment on whether that returns the value the original poster is requesting.

Solution 5

The easiest way is to use JarURLConnection class :

String className = getClass().getSimpleName() + ".class";
String classPath = getClass().getResource(className).toString();
if (!classPath.startsWith("jar")) {
    return DEFAULT_PROPERTY_VALUE;
}

URL url = new URL(classPath);
JarURLConnection jarConnection = (JarURLConnection) url.openConnection();
Manifest manifest = jarConnection.getManifest();
Attributes attributes = manifest.getMainAttributes();
return attributes.getValue(PROPERTY_NAME);

Because in some cases ...class.getProtectionDomain().getCodeSource().getLocation(); gives path with vfs:/, so this should be handled additionally.

View more solutions

Comments

[Chris Dolan]

I think the FrameworkUtil.getBundle() answer below is the best. It answers what you actually want to do (get the bundle's exports) rather than what you asked (read the manifest).

[ChssPly76]

If your class is named "com.mypackage.MyClass", calling class.getResource("myresource.txt") will try to load that resource from com/mypackage/myresource.txt. How exactly are you going to use this approach to get the manifest?

[Houtman]

Perfect! I never knew you could iterate through resources with the same name.

[Jay]

Okay, I have to backtrack. That's what comes of not testing. I was thinking that you could say this.getClass().getResource("../../META-INF/MANIFEST.MF") (However many ".."'s are necessary given your package name.) But while that works for class files in a directory to work your way up a directory tree, it apparently doesn't work for JARs. I don't see why not, but that's how it is. Nor does this.getClass().getResource("/META-INF/MANIFEST.MF") work -- that gets me the manifest for rt.jar. (To be continued ...)

[Jay]

What you can do is use getResource to find the path to your own class file, then strip off everything after the "!" to get the path to the jar, then append "/META-INF/MANIFEST.MF". Like Zhihong suggested, so I'm voting his up.

[Jay]

I like this solution as it gets your own manifest directly rather than having to search for it.

[Jason S]

How do you know the classloader is only aware of a single .jar file? (true in many cases I suppose) I would much rather use something associated directly with the class in question.

[Alba Mendez]

it's a good practice to make separate answers for each one, instead of including the 2 fixes in one answer. Separate answers can be voted independently.

[Gregor]

just a note: I needed something similar but I'm inside a WAR on JBoss, so the second approach didn't work for me. I ended up with a variant of stackoverflow.com/a/1283496/160799

[Jay]

can be improved a little bit by removing condition check classPath.replace("org/example/MyClass.class", "META-INF/MANIFEST.MF"

[Petr Gladkikh]

@chris-dolan Gave the correct answer to this question (see comment above).

[assylias]

Very nice libraries. Is there a way to control the log level?

[yegor256]

All jcabi libs log through SLF4J. You can dispatch log messages using any facility you wish, for example log4j or logback

[ceving]

Who closes the stream?

[ceving]

This does not work in inner classes, because getSimpleName removes the outer class name. This will work for inner classes: clazz.getName().replace (".", "/") + ".class".

[ceving]

getCodeSource may return null. What are the criteria, that this will work? The documentation does not explain this.

[robermann]

+1; BTW, I've just found that EAR/META-INF/... is not included into the runtime classpath (on Weblogic 10.3.5), so it cannot be inspected as a resource.

[kevinarpe]

Thanks to add the wiring around java.util.jar.Manifest. I didn't know about that class before I read that post. I probably would have dumbly / manually parsed the Manifest file myself...

[BrianT.]

You need to close the stream, the manifest constructor does not.

[floer_m]

if using logback.xml the line you need to add is like <logger name="com.jcabi.manifests" level="OFF"/>

[Gordon]

Manifest is giving me a rather useless empty map, so I still have to parse it manually. Still useful code to get the resource at first, though.

[Jolta]

Where is DataUtilities imported from? It doesn't seem to be in the JDK.

[Jolta]

The first option didn't work for me. I got the manifests of my 62 dependency jars, but not the one where the current class was defined...

[Robert]

This answer uses a very complex and error prone way of loading the Manifest. the much simpler solution is to use cl.getResourceAsStream("META-INF/MANIFEST.MF").

[Alex Konshin]

Did you try it? What jar manifest it will get if you have multiple jars in classpath? It will take the first one that is not what you need. My code solves this problem and it really works.

[Robert]

I did not criticize the way how you us e the classloader for loading a specific resource. I was pointing out that all the code between classLoader.getResource(..) and url.openStream() is totally irrelevant and error prone as it tries to do the same as classLoader.getResourceAsStream(..) does.

[Alex Konshin]

Nope. It is different. My code takes manifest from the specific jar where the class is located rather than from the the first jar in the classpath.

[Robert]

Your "jar specific loading code" is equivalent to the following two lines: ClassLoader classLoader = cl.getClassLoader(); return new Manifest(classLoader.getResourceAsStream("/META-INF/MANIFEST‌​.MF"));

[aprodan]

indeed this solution can be refactored but it works. The two liners solution proposed by Robert ends-up with NullPointerException in spring boot applications.

[Chris Janicki]

Note that functions attr.containsKey("Manifest-Version") and attr.get("Manifest-Version") will return null since those functions require arguments that are of the inner class Attribute.Name. Unfortunately the functions accept Object... so there's no compile-time error when giving it a String, but it will return a null value. So stick with Attribute.getValue() as show in the example above. Attributes.getValue() explicitly accepts a String, and then wraps it in a new Attributes.Name for you. (I know this is because Attributes implements Map, but it's just not safe API coding in my opinion.)

[basin]

can result of clz.getResource(resource).toString() have backslashes?

[guai]

Multiple manifests from the same classloader overlap and overwrite each other

[Ian Robertson]

Indeed, the java port is straightforward: String implementationVersion = MyApplication.class.getPackage().getImplementationVersion();

[Aleksander Stelmaczonek]

It fact this is what I was looking for. I'm also happy that Java has also an equivalent.

[walen]

This is, by far, the easiest and cleanest way to do it.

[Matthew]

Option 2 appears to have stopped working as-of java 11 (maybe java 9).

[Greg Brown]

This doesn't answer the original question specifically, but it is exactly what I was looking for.