Thanks to everyone who replied!
It is clear that JVM can't be part of the kernel. Because of three main
reasons:
1. Java is not GPL'ed
2. the more code in the kernel the worse. Java is too big and not so stable.
Here is one possible design:
1. At boot time a wrapper is started in the user space as root. It may be
written in Java with some native code if needed.
Running the wrapper as root raises a serious security problem [1].
2. When we want to start a java program we make a request to the wrapper.
It loads/reloads the class from the requested a .class file in the user's
CLASSPATH and calls its Main(String args[]) in a new thread. Here there are
two problems:
- if the class is loaded and then changed what we need to reload it [2].
- if we reload the class and what happens to another application which uses
an old version ov the class? [3]
- we have to solve the problem of different CLASSPATHs of different
applications.[4]
[1] This can be solved with a good SecurityManager which checks all that the
kernel checks when running user applications: file permissions, ports < 1024.
The alternative is have a C wrapper which fork(), chuid() - now user security
applies and runs java. I have to say that I don't have experience with JNI.
Is there a really cheap way to start JVM from C or add a thread to running
JVM?
[2] Maybe ClassLoader.findClass(String) or . loadClass(String) can do the
trick?
[3] and [4] remain open questions
Of course there are surely more problems. These were the only I see but I
don't know the internals of the JVM.
Also thanks for pointing me to kissme(http://kissme.sourceforge.net/). It is
an VM with an interesting feature - persistence:
"Supports an extension to the JVM allowing persistent java, which means that
the JVM allows in-memory data structures to automagically persist to
permanent storage"
"The persistence abstraction allows the creation and manipulation of data in
a manner that is independent of its lifetime thereby integrating the database
view of information with the programming language view."
There is also a link to another persistent Java project -
PJama(http://www.dcs.glasgow.ac.uk/pjava/)
However I didn't see in the feature list something like JVM shared between
applications.
Yavor
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