When should you use JNI with native code and when should you just let the JIT handle things?


If I'm starting a new JVM project and need a particular section of
code to be really fast, should I write it in a naively compiled
language or just let the JIT handle things?



What is even the point of the href="http://docs.oracle.com/javase/7/docs/technotes/guides/jni/"
rel="nofollow">JNI if the href="http://docs.oracle.com/cd/E15289_01/doc.40/e15058/underst_jit.htm"
rel="nofollow">JIT compiler exists?

I have created a Java file "Dock.java" which uses JNI to
access a C function called "setStruts." The C function needs
two header files, Xlib.h and Xatom.h.
Whenever I try to run
Dock's main function, which just calls setStruts, I get the following
error:
Exception in thread "main"
java.lang.UnsatisfiedLinkError: Dock.setStruts(III)V
at
Dock.setStruts(Native Method)
at
Dock.main(Dock.java:12)
I am assuming this is because
System.loadLibrary is not loading the right library. I know that
loadLibrary's string parameter doesn't need to have the directory path
or the ".so" extension,
Ubuntu

How to load native libraries using JNI



I tried to load but i am getting error as



Native code library failed to load.
java.lang.UnsatisfiedLinkError:

Web Development

I understand the gist of how JIT compilation works (after reading
such resources as href="http://stackoverflow.com/questions/95635/what-does-a-just-in-time-jit-compiler-do">this
SO question). However, I am still wondering how does it actually
execute the machine code at runtime?



I don't have a deep background in operating systems or compiler
optimizations, and haven't done anything with machine code directly,
but am starting to explore it. I have started playing around in
assembly, and see how something like NASM can take your assembly code
and compile it to machine code (the executable), and then you can
"invoke" it from the command line like
./my-executable.



But how is a JIT compiler actually doing that at runtime? Is it
like streaming machine code into stdin or something, or how does it
work? If you could provide an example or some pseudocode of how some
assembly (or something along those lines, not as high level as C
though) might look to demonstrate the basic flow, that would be
amazing too.

Web Development

I am working with Firefox on a research project. Firefox makes uses
of lots of JIT'ed code during run time.



I instrumented Firefox using a custom PIN tool to find
out locations(address) of some things I as looking for. The issue is
that those location are in JIT'ed code. I want to know
what is actually happening over there in the code.



To do this I dumped the corresponding memory region and used
objdump to disassemble the dump.
I used objdump -D -b binary -mi386 file.dump to see the
instructions that would have been executed. To my surprise the only
section listed is .data section (a very big one).



Either i am incorrectly disassembling it or something else is wrong
with my understanding. I expect to see more sections like
.text where actual executable instructions should be
present and .data section should not be executable.



Am I correct in my understanding here?



Also If some one can please advise me on how to properly know what
is happening in Jit'ed code.



Machine
Linux 3.13.0-24-generic #47-Ubuntu SMP x86_64



Thanks

Web Development

I am having problems with relative file paths that native functions
use. When I call native C function from Java code, I get segmentation
fault due to null file pointer. The only thing that works is to change
these paths into absolute file paths, which is not solution for me. Is
there any way to set root directory for native functions or to use
Java project root folder to navigate through directories, or the
absolute path is the only way?

Web Development

If I'm starting a new JVM project and need a particular section of
code to be really fast, should I write it in a naively compiled
language or just let the JIT handle things?



What is even the point of the href="http://docs.oracle.com/javase/7/docs/technotes/guides/jni/"
rel="nofollow">JNI if the href="http://docs.oracle.com/cd/E15289_01/doc.40/e15058/underst_jit.htm"
rel="nofollow">JIT compiler exists?

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