Why am I able to execute JARs directly from bash?

Question

I'm a longtime Java guy, and know that the way to run a JAR with a main class referenced in a MANIFEST.MF file within the Jar is easy:

java -jar theJar.jar

I was using this to start up the Fabric3 server (contained in bin/server.jar in its standard distribution). I noticed that when I unpacked it from the distribution tarball, it was flagged as executable. On a whim, I tried

./server.jar

from my bash command line (bash version 4.1.5 in Ubuntu 10.10), and lo and behold, the server started up as if I had typed the normal java -jar ... command. The JAR is structured like a normal JAR; I did a head on it, and there was no #! command in the first few bytes, so bash shouldn't magically know to start a Java VM with it, right? Or has this version of bash grown the ability to start JARs with a proper manifest correctly? Inquiring minds want to know...

Answer 1

This may not be a feature of bash at all. If you are running linux (as you have tagged) you may have the binfmt_misc kernel module, which does a little bit of magic, when you try to execute a program. Basically what this does is it extends the binary formats that linux can execute from just elf to also include some user defined formats, with enough instructions to run them.