How does the Java Runtime Environment compare with the .NET framework in terms of compilation process?

Question

I am learning about the conversion of source code to machine code via the .NET and JRE Frameworks. To start off I did some research comparing the two processes and created this diagram. I need some help in criticizing its correctness, and more importantly adding any serious things I missed out to better understand the compilation pathway.

Answer 1

Both .NET and Java compile down to bytecode, that is an intermediate language which contains instructions for a virtual machine. It's not machine code because it cannot run directly on a physical machine. What happens instead (today at least; Java has a darker history in this regard) is that at runtime a just-in-time compiler is run which translates the VM instructions into native code that is then run directly. This has a major performance benefit over only interpreting it.

They differ in this regard a little. Oracle's Java implementation (Hotspot) uses a clever mix of interpretation, measuring and JIT compiling just the parts that are heavily used and interpreting otherwise. This is to reduce initial impact by the JIT compiler (which needs to run upfront otherwise, lengthening process startup time) while still allowing good performance where needed. .NET on the other hand always JIT-compiles all code that is used (unused code is not compiled, though).

Edit (2019): By now .NET also has tiered compilation where depending on what code runs a lot, that code will be optimized further.

As for a question you mentioned in your comments: Yes, the CLR and the JVM are the platforms such programs are run on. A virtual machine is a machine too, just less hardware-y. They both are tightly integrated with a corresponding framework, the Base Class Library for .NET and the Java class library for Java. Those are frameworks.