Java Virtual Machine implementation in Java and C# for the "Szybkość programów w Javie" assignment.

This is an implementation of most of the JVM instruction set. The main branch contains a virtual machine written in Java, the branch csharp contains the same program reimplemented in C#.

Code structure

• Instructions are implemented as subclasses of the abstract class jvmception.instructions.Instruction and decoded in jvmception.InstructionDecoder.
• Class loading and the main method are implemented jvmception.objects.cp.JVMClassLoader.
• Stack management and method invocation are implemented in jvmeption.CallFrame.
• Heap allocations and garbage collection are implemented in jvmeption.objects.JVMInstance and jvmeption.jvmtypes.JVMReference (this functionality is rather untested though).

Execution times

Tested with the following program...

package test;

public class Test {

	public static int sField = 0;
	
	public static float testStaticMethod(int r) {
		return r*35 + 53.4f;
	}
	
	public static void main(String[] args) 
	{
		float t = 0.23f;
		
		for (int i = 0; i < 1000000; i++) {
			t += testStaticMethod(i);
			sField = (int)t + i;
		}
	}
}

...execution times are as follows:

• Java 17.0.4.1 (-Xmixed): 823ms
• C# 7.3: 1289ms
• Java 17.0.4.1 (-Xcomp): 1304ms
• Java 17.0.4.1 (-Xint): 27086ms

Unimplemented instructions:

• invokedynamic
• newarray
• anewarray
• arraylength
• athrow
• checkcast
• instanceof
• monitorenter
• monitorexit
• wide
• multianewarray
• ifnull
• ifnonnull
• goto_w
• jsr_w

Notable missing features:

• Exceptions
• Arrays
• Runtime type and access checking
• Native functions

The implementation is otherwise more or less complete and is able to execute simple Java programs. Due to native functions being unimplemented, no standard library can be loaded. The java.lang.Object is built into the class loader.