Executable jar files and manifest file

Question

Hi,
I have the following files in a folder named: Test
a.jar
b.jar
c.jar
Driver.class
Driver.mf
Here, MainClass is the main executable class that uses a,band c.jar files. This is how my MainClass.mf file looks:
Manifest-Version: 1.0
Main-Class: Driver
Class-Path: a.jar b.jar c.jar
Now I use the following command to make one executable jar file:
jar cmf Driver.mf DriverMain.jar *
It creates an executable jar file named : DriverMain.jar
Now I copy the executable jar file (DriverMain.jar) into a different folder named: RunTest
and double click it, doesn't work.
my question is:
what am I doing wrong? Any special characters needed in my Driver.mf file (space/newline/etc)?
What I am trying to get is: One execuatble jar file so I can just double click to run it, and that single executable jar file will have all the necessary jars in it (i.e. a.jar, b.jar, c.jar in this case)
Anyone please help!
Thanks
-Ron

Answer 1

Rony,
Sorry to disappoint, but you can not use embeded jar/zip files within an executable jar. The JDK sadly for some reason decided this was not a useful idea, so developers like me who want to distribute plugins with thier own dependencies have one of two choices. You either have to unzip the jar file that contains the embeded jars to a directory, then run your primary exectuable jar, OR you have to write a custom classloader that your "launcher" creates then loads the embeded jar files.
The best thing to do is either jar up a, b, c and driver into one jar, so that it works, if you can legally do this. A lot of 3rd party libraries may not allow this per their license. Otherwise, another choice is to use a free installer or buy an installer that allows you to distribute a single exectuable installer program that will properly create the dir structure you need.
As for the way it works, if you declare:
Class-Path: a.jar b.jar c.jar
the JAR loader code in the JDK looks in the root dir where your application was started for a/b/c jar files. They have to be on disk.
If you want to place them in /lib, for example, you would havd a jar file like:
Driver.class
Driver.mf
lib/a.jar
lib/b.jar
lib/c.jar
When unzipped, your "root" dir would look exactly like the above, and your manifest would have Class-Path using ./lib/a.jar ./lib/b.jar ./lib/c.jar
Anyway, there thus far isn't any way around this issue of embeding jar files in an executable jar file. You MIGHT be able to not specify any classpath, then in your Driver.class, create a new custom classloader that dervies from URLClassLoader, but in the findClass(), you get a ref to the .jar file that the classloader class is inside of, and from that use it to find the lib/*.jar files and add them to the classpath. For this to work, however, your Driver.class code should ALSO be contained in an embeded jar file that is loaded by this custom loader. The only thing Driver.class would contain (and I would rename it to something like Launcher) is the code to create the custom classloader.
It's fun stuff, but a little bit of work to make it work. You can infact make it work! I may yet one day take our plugin engine custom loader and create a way for this to work!