Why does the JRE use so much memory?

Question

I know this question has been asked a million times in one form or another but I have not seen a complete answer yet. When I compile and run the following code under WindowsXP pro, it appears that the JRE is using 3.1MB or RAM. When I run a similar program using C++ the program only uses 630k.
public class HelloWorld
public static void main(String args[])
System.out.println("Hello World");
Why 3MB to show "HelloWorld"? I love Java, it is much easier and more enjoyable to develop with but our applications appear to use large amounts of memory. I realize that making calls to the runtime object to check the amount of "heap" memory we are using will probably show much less memory being used but Task Manager shows 3MB. It is sometimes hard to convince my fellow developers of he benefits of Java when there are strange issues like this.
Thanks in advance

Answer 1

Java uses that much memory because every time you start a Java program, even this small, it loads into RAM the whole Java Virtual Machine. Which is itself a very large C++ program - on the order of several hundred thousand lines. Compare that to your C++ HelloWorld program - and you see the reason for a difference...
So a real question is, is it possible to write a modern Java VM such that it uses space proportional to the size/complexity of the Java program that it executes. Partially that's possible - for example I understand that say native GUI libraries are separate from the main JVM dll, so when you don't use AWT/Swing, you don't waste space with related native code. However, for the core JVM it's probably very difficult to separate parts of functionality such as the interpreter, the dynamic compiler, the runtime system, the garbage collector, the serviceability stuff, etc. They are very interdependent, and thus, probably unless you have this goal from the beginning (and ready to sacrifice some performance/maintainability for it), you can't do that.
There exist, however, a bunch of other JVMs of all sizes, say those used in mobile phones (or even smart cards!), that consume a lot less memory. They, however, typically implement only a subset of the full Java functionality. On the other hand, the HotSpot VM that comes with Sun's JDK, that you probably use, is a full-fledged JVM that implements everything, can be used for huge server apps, etc. - no wonder its minimum memory consumption is a few megs.