Java HotSpot(TM) Client sharing vs no sharing

Question

I was looking at my JDK installations and I saw a difference between two installations:

java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) Client VM (build 25.121-b13, mixed mode)

vs

java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) Client VM (build 25.121-b13, mixed mode, sharing)

And I was wondering what the difference between the sharing and not sharing state is.

I couldn't find a good answer other than "a speed difference", but no explanations why/what this is about.

Answer 1

That's called Class Data Sharing, as described here; this only works for a Client VM.

Basically it is a way to speed-up the start-up time of the VM - the parsed classes are stored in a file and when the VM starts, it just reads that file (with already compiled classes), it also can be shared with other running VMs.

The shared build will contain a file jre\bin\client\classes.jsa and the other one will not.

Since java-8 you can add your own classes to that file - if I remember correctly and the native code (not the byte code) will be there for grabs.

I have not used Client VM's with this support - just read about it.