Didactic JVM Implementation

This project implements a simplified Java Virtual Machine (JVM) in Java. The custom JVM can execute a subset of Java bytecode instructions, supporting basic operations such as arithmetic, control flow, and method invocations, as well as handling arrays and objects.

Limitations

• Only int and reference types are supported
• No constructors, so objects currently can only be constructed and need to be initialized manually since
• No garbage collection
• Only integer arrays
• No inheritance

Overview

The custom JVM represents all values as integers, using the following interpretation:

• If the expected type is an integer, the integer value is the value itself.
• If the expected type is a reference, the integer value is the heap address.

The VM consists of:

• A frame representing the current execution context.
• A call stack of frames for managing method invocations.
• A heap for storing objects and arrays.
• A class table mapping class IDs to class definitions.

Instruction Set

For a documentation of instructions you can inspect the actual JVM specification (Some older version here).

The currently supported instruction set includes:

Simple Stack Machine Instructions

• IConst(int value): Push an int constant onto the operand stack.
• IAdd(): Add two integers.
• ISub(): Subtract two integers.
• IMul(): Multiply two integers.
• Pop(): Pop the top value from the operand stack.
• Dup(): Duplicate the top value on the operand stack.
• Swap(): Swap the two top elements on the operand stack.

Local Control Flow

• IfEq(int offset): Branch if int equals zero.
• IfNe(int offset): Branch if int not equal to zero.
• Goto(int offset): Unconditional branch.

Simple Register Machine

• ILoad(int index): Load an int value from a local variable.
• IStore(int index): Store an int value into a local variable.
• ALoad(int index): Load a reference from a local variable.
• AStore(int index): Store a reference into a local variable.

Static Methods

• InvokeStatic(Integer classId, String methodName): Invoke a static method.
• Return(): Return from a method.

Arrays

• NewArray(): Create a new array of int.
• IALoad(): Load an int from an array.
• IAStore(): Store an int into an array.
• ArrayLength(): Get the length of an array.

Objects

• New(Integer classId): Create a new object.
• InvokeVirtual(String methodName, int numberOfArguments): Invoke an instance method.
• GetField(String fieldName): Fetch field from object.
• PutField(String fieldName): Set field in object.

Example

Consider the following Java code:

var sum = 0;
sum = sum + 5;
sum = sum + 3;

This code can be represented using the instruction set as:

Instruction[] instructions = {
  new IConst(0),
  new IStore(0),
  new ILoad(0),
  new IConst(5),
  new IAdd(),
  new IStore(0),
  new ILoad(0),
  new IConst(3),
  new IAdd(),
  new IStore(0),
  new Return()
};

Potential Extensions

Students can extend the custom JVM in various ways, including but not limited to:

1. More Simple Instructions:

   • Implement more bytecode instructions such as if_icmpge.

2. Garbage Collection (GC):

   • Implement a simple garbage collector, such as mark-and-sweep, which will require changes to the memory layout.

3. Additional Types:

   • Support other types. Be aware that types like long and double will require two registers and two slots on the heap.

4. Constructors:

   • Implement support for constructors to initialize objects.

5. Inheritance:

   • Add support for inheritance, allowing class hierarchies and method overriding.

6. Translation of Actual Bytecode:

   • Implement a translation layer to convert actual Java bytecode into the internal representation of the custom JVM (a challenging task).

7. Exception Handling:

   • Add support for exception handling using instructions like try, catch, throw.

...