Java programing for 64 bit JVM

Question

Few Questions :):

How to identify what JVM is currently installed ? (64 bit / 32 bit)

Do I need to do some considerations while programming for 64 bit JVM target Platform ?

Can my Java code work on 32 bit as well as 64 bit JVM ?

How to run my Java App on 64 bit JVM ?

How to identify what JVM is being used for my Java App ?

Answer 1

Normally a 64 bit jvm will identify itself as such.

32/64 bit Considerations I have seen:

1. address space - if you're not expecting to address more that 1.3Gb of memory then you won't see a difference
2. native libraries - if you're loading any JNI libs, then they will need to match your VM architecture. For example, don't try loading 32-bit native libraries from a 64-bit vm (without -d32 type of flags)

Yes, the same code will run on both JVMs.

System Property "sun.arch.data.model" has the 32/64 flag I think.

There's some helpful info here: http://www.oracle.com/technetwork/java/hotspotfaq-138619.html

Answer 2

In your own Java code, you don't have to do anything special with regard to 32- or 64-bit. Unlike for example C and C++, an int in Java is always 32 bits and a long is always 64 bits (in C and C++, the size of those types is system-dependent).

There are no separate 32-bit and 64-bit versions of Java bytecode; the bytecode is exactly the same, regardless of the fact that the JVM you might be running it on is 32-bit or 64-bit. You don't have to compile your Java source differently for 32-bit or 64-bit. With regard to functionality it does not matter for your Java application if it runs on a 32-bit or 64-bit JVM.

There might be some technical differences that jowierun already mentioned. There might also be performance differences; for example Oracle's 64-bit JVM for Windows is tuned differently than the 32-bit JVM, it does other JIT optimizations. I noticed this myself with a computation-intensive application that I wrote recently; on a 64-bit JVM it runs much faster than on a 32-bit JVM. (But that's only one example, don't take this as proof that any program runs much faster on a 64-bit JVM).

Answer 3

You have these questions back to front.

1. You don't have to do anything in your Java code, it will run on both 32-but and 64-bit.
2. Therefore you don't need to know whether it's a 64 bit JVM or not.

Answer 4

If you plan to write native code using the Java Native Interface (JNI), you have to be extra careful on writing proper C code that will run on both 64 and 32 bit machines. Make sure you use proper Java types when passing arguments to/from java code (JNI provides a set of typedefs for java types), especially when converting arrays. And when testing your native code, test on both architectures (-m32 will force a 32 bit arch on GNU gcc) with different JVMs.