theoretically every method is breakable
(even developing special hardware)
you can only use methods to make it harder for crackers
following are steps you can take:
1) use code obfuscator
2) native compilation
3) your own JVM
4) dedicated hardware designed for your own native code!

you can find this tool on netbeans project properites
this tool renaming class as a,b,c

The routine approach is Obfuscator, which most expert Obfuscator
use some techniques for dramatically complex your source  code from
decompilation , but the more advanced approach is excrypting your
source code after compilation and developing custom classloader for
decryting your sourcecode to execute on JVM.
so there is nothing to decompile or reverse engineering.

> decryting your sourcecode to execute on JVM. so there is nothing to decompile or reverse engineering.

one can put a layer on JVM to store the loaded classes on file and later reverse engineer them.

that'r right,
i think everything can happen in real world ;)
in fact for an action there is an reaction,
that's mean we can not reach to pure secure software,
but we try to reach safer level,
for instance Obfuscator can complex your source  code from
other but deObfuscator can revers them. (this is not except)

'so there is nothing to decompile or reverse engineering'
that's incorrect, sorry my friend

Alireza could you help me further on it,i didnt get exactly what you meant:
"the more advanced approach is excrypting your
source code after compilation and developing custom classloader for
decryting your sourcecode to execute on JVM.
so there is nothing to decompile or reverse engineering."
is that possible to introduce me some references

i dont have real Experience with this,
but i found some good reference:

www.artima.com/underthehood/classloadersP.html
www.javaworld.com/.../01-qa-0509-jcrypt.html