java.class.path doesn't bring Manifest.mf Class-Path property

Question

I'm trying to get my application classpath.

I have a jar (named application.jar) and it have in its Manifest.mf other jar files, like Class-Path: a.jar b.jar.

Why when I use System.getProperty("java.class.path") my jars a.jar and b.jar are not listed?

Answer 1

It possibly has to do with the fact that the java.class.path is a system property that is set from the classpath environment variable ($CLASSPATH or -classpath). These are ignored with the -jar option is used.

As per the java -jar documentation, when that jar option is used to run an application, only the manifest Class-Path is considered and other settings are ignored. From http://docs.oracle.com/javase/1.5.0/docs/tooldocs/windows/java.html:

-jar

Execute a program encapsulated in a JAR file. The first argument is the name of a JAR file instead of a startup class name. In order for this option to work, the manifest of the JAR file must contain a line of the form Main-Class: classname. Here, classname identifies the class having the public static void main(String[] args) method that serves as your application's starting point. See the Jar tool reference page and the Jar trail of the Java Tutorial for information about working with Jar files and Jar-file manifests.

When you use this option, the JAR file is the source of all user classes, and other user class path settings are ignored.

Answer 2

That's excactly the question I came along as well. Even if when using java -cp ..;myTest.jar test2.Needer I get only "..;myTest.jar" as a result for java.class.path property.

Note: Even when using the -cp parameter the given class-path in the MANIFEST.MF is searched! (Couldn't find this information on google and tested myself)

So i don't think that is has something to do with the -jar parameter. In Link you can find

Expansion of wildcards is done early, prior to the invocation of a program's main method, rather than late, during the class-loading process itself.

Interestingly i found out during my tests: The classpath in MANFIFEST.MF is searched recursivly. So if there is an given test.jar File in the classpath in MANIFEST.MF of myTest.jar, the class-path in the MANIFEST.MF of the test.jar will be looked up as well (when using java -cp "myTest.jar" test2.Needer).

As a result, when the java.class.path property would support showing the the classpath of the MANIFEST.MF, it should also show the classpathes of all subsequently depending .jar files. Since the classpath is only searched until classes are found, this wouldn t refer nicely to the lazy loading mechanism.

TL;DR: I think that this has nothing to do with the -jar Parameter (-cp is concerned as well). In my explanation, the support for showing the classpath from the MANIFEST.MF would only come with additional, senseless recursive search costs (because there needn't to be an actual dependency, respectivly what is used from the .jar). And since this sensless search would delay the program start (since the recursive search could be really deep), it is not implemented.