What are the advantages of using Virtual Machine compilation (eg. JVM) over natively compiled languages?

Question

I've heard that the advantage of java is that people can write code, compile it for the JVM, and run it anywhere. Each person just needs a JVM app for their platform.

Of course, it looks similar to the current situation, where everybody has a compiler specific for their platform. So the advantage isn't explained by that. But I think I see the explanation.. the issue must be that in the java situation, you can't or weren't meant to access the real machine directly in an OS specific way.

I suppose it'd mean that in other languages, the code itself has to be amended depending on what computer it is running on.

Can anybody provide short examples of this like a Hello World program that demonstrates this? No doubt it'd be in non-java e.g. C

Since it's not something that'd -normally- happen in a Hello World program or most i've seen since the books I used on java, they were unfortunately "how to program" style books, and all the stuff in them didn't demonstrate it(perhaps 'cos they couldn't or didn't want to use java to demonstrate it!). Though they trumpeted it as a big advantage. I'd like to see examples of it.

Answer 1

Besides the advantages of the JVM that will allow you to execute code independently of the CPU architecture with reasonable performance thanks to the JIT-compiler, one fundamental advantage of Java is that it's not just a programming language, but a runtime environment with a API common to all the underlying platforms on which it can run (there are some differences occasionally, but they tend to be bugs).

gcc (GNU Cross Compiler), for example, will let you compile C code for more or less any platform. That's fine in principle for anything that's limited to using calls in stdio.h and a few others. However, you'll run into trouble quite quickly as soon as you try to use something a bit more OS specific, which tends to appears quite quickly: GUI, some I/O, threading, processes, networking.

As soon as you get an #include <win32.h> or similar in your C code, you'll have to rewrite parts of the code to port it to a Linux/OSX platform, some of this work may not be obvious or directly possible.

The advantage of Java isn't just its virtual-machine and ability to read and run the same bytecode on any platform, it's also the availability of a rather large library as part of the JRE (for example J2SE) and a common threading and networking model.

Answer 2

... where everybody has a compiler specific for their platform. So the advantage isn't explained by that.

Porting code written in for example C or C++ is almost always much more involved than simply recompiling the code. It's certainly not something that an average, non-developer computer user can do easily. Code written in compiled languages is very often written against the API of a specific operating system (the Win32 API, for example) and so it cannot be compiled on other operating systems easily.

Java bytecode runs on any platform where there is a Java runtime environment available. The code doesn't need to be recompiled. Ofcourse you can write operating-system specific code in Java, but Java's standard library, and the many free libraries available on the web, provide a very rich cross-platform environment.

Besides portability, running on a virtual machine has other advantages. Java uses a JIT compiler to compile Java bytecode to native machine code at runtime. The JIT compiler can do sophisticated optimizations for the specific CPU that the program is running on and it can use profiling information that wouldn't be available to an ahead-of-time compiler - in principle, a JIT compiler can therefore produce more optimal code than a "normal" compiler.

Besides the Java VM, there are other virtual machines. For example, Microsoft .NET contains the CLR (Common Language Runtime) and there's also LLVM, which has front-ends for many different languages including C and C++ (and which is supposed to bring the advantages of JIT compilation also to C and C++).