# Specification - JVMRS

## Project Overview
JVMRS is a simplified Java Virtual Machine implementation in Rust, designed for educational purposes and as a foundation for more advanced JVM implementations.

## Goals

### Primary Goals
1. **Educational Value**: Provide a clear, understandable implementation of JVM internals
2. **Correctness**: Faithfully implement JVM specification where supported
3. **Modularity**: Create a clean, well-structured codebase that's easy to extend
4. **Documentation**: Comprehensive documentation of implementation decisions

### Secondary Goals
1. **Performance**: Reasonable execution speed for supported features
2. **Compatibility**: Run simple Java programs correctly
3. **Extensibility**: Easy to add new features and optimizations
4. **Testability**: Comprehensive test suite for all components

## Scope

### In Scope (Phase 1)
- [x] Basic class file parsing
- [x] Constant pool handling
- [x] Simple instruction interpreter
- [x] Stack-based execution
- [x] Integer and float arithmetic
- [x] Basic method invocation
- [x] Local variable management

### In Scope (Phase 2)
- [x] Object creation and manipulation
- [x] Array support
- [x] String operations
- [x] Exception handling
- [x] Class inheritance
- [x] Interface implementation
- [x] Garbage collection

### In Scope (Phase 3)
- [x] Just-In-Time compilation (Cranelift)
- [x] AOT compilation to native object files
- [x] LLVM IR export
- [x] WebAssembly backend

### Out of Scope (for now)
- JNI (Java Native Interface)
- Threading and concurrency
- Reflection API (basics planned)
- Security manager

## Technical Specifications

### 1. Class File Format Support
**Supported Versions**: Java 8 class file format (version 52.0)
**Required Features**:
- Magic number validation
- Version checking
- Constant pool parsing (types 1-18)
- Field and method information
- Code attributes
- Line number tables (basic)

**Optional Features**:
- Source file attributes
- Local variable tables
- Stack map tables
- Annotation processing

### 2. Instruction Set
**Core Opcodes (Required)**:
```
iconst, bipush, sipush, ldc
iload, istore, iinc
iadd, isub, imul, idiv, irem
ineg, ishl, ishr, iushr, iand, ior, ixor
i2l, i2f, i2d, l2i, f2i, d2i, i2b, i2c, i2s
lcmp, fcmpl, fcmpg, dcmpl, dcmpg
ifeq, ifne, iflt, ifge, ifgt, ifle
if_icmpeq, if_icmpne, if_icmplt, if_icmpge, if_icmpgt, if_icmple
goto, tableswitch, lookupswitch
ireturn, return
getstatic, putstatic, getfield, putfield
invokevirtual, invokespecial, invokestatic, invokeinterface
new, newarray, anewarray, arraylength
athrow, checkcast, instanceof
monitorenter, monitorexit
```

**Extended Opcodes (Phase 2)**:
```
wide, multianewarray
jsr, ret, jsr_w
breakpoint, impdep1, impdep2
```

### 3. Memory Model
**Heap Organization**:
- Object allocation with simple bump pointer
- Reference tracking for GC
- Array storage with bounds checking

**Stack Frames**:
- Local variables: up to 65535 slots
- Operand stack: up to 65535 entries
- Frame data: return address, exception handler

**Value Representation**:
```rust
enum Value {
    Int(i32),
    Long(i64),
    Float(f32),
    Double(f64),
    Reference(Option<ObjectRef>),
    ReturnAddress(usize),
}
```

### 4. Class Loading
**Bootstrap Class Loader**:
- Load from file system
- Parse class files
- Resolve constant pool entries
- Link methods and fields

**Class Resolution**:
- Superclass resolution
- Interface implementation checking
- Method signature validation
- Access control checking

### 5. Method Invocation
**Invocation Types**:
- `invokestatic`: Static method calls
- `invokevirtual`: Virtual method calls
- `invokespecial`: Constructor/super calls
- `invokeinterface`: Interface method calls

**Call Semantics**:
- Parameter passing
- Return value handling
- Exception propagation
- Stack frame management

## Implementation Requirements

### 1. Code Quality
- **Rust 2021 Edition**: Use modern Rust features
- **Error Handling**: Custom error types with context
- **Testing**: >80% code coverage
- **Documentation**: All public APIs documented
- **Performance**: No unnecessary allocations in hot paths

### 2. Architecture Constraints
- **Modular Design**: Clear separation between components
- **Minimal Dependencies**: Only essential external crates
- **No Unsafe Code**: Unless absolutely necessary and documented
- **Thread Safety**: Design for future threading support

### 3. Testing Requirements
- **Unit Tests**: Each function/method tested
- **Integration Tests**: End-to-end execution tests
- **Property Tests**: Random class file generation
- **Benchmarks**: Performance regression tests

## API Specification

### Public API (Phase 1)
```rust
// Main entry point
pub fn run_class(class_name: &str) -> Result<(), JvmError>;

// Class loading
pub fn load_class(class_file: &[u8]) -> Result<ClassFile, ParseError>;

// Execution control
pub struct Interpreter {
    pub fn new() -> Self;
    pub fn load_class(&mut self, path: &str) -> Result<(), LoadError>;
    pub fn run_main(&mut self, class_name: &str) -> Result<(), RuntimeError>;
}
```

### Extension API (Phase 2)
```rust
// Custom class loaders
pub trait ClassLoader {
    fn load_class(&self, name: &str) -> Result<ClassFile, LoadError>;
}

// Memory management hooks
pub trait GarbageCollector {
    fn collect(&mut self, roots: &[ObjectRef]) -> Result<(), GcError>;
}

// Debugging interface
pub trait Debugger {
    fn breakpoint(&mut self, frame: &StackFrame) -> Result<(), DebugError>;
}
```

## Performance Targets

### Execution Speed
- **Baseline**: 10-100x slower than HotSpot JVM
- **Target**: < 50x slower than HotSpot for supported features
- **Stretch**: < 20x slower with optimizations

### Memory Usage
- **Baseline**: 2-4x more memory than class file size
- **Target**: < 2x class file size
- **Stretch**: < 1.5x with efficient data structures

### Startup Time
- **Baseline**: < 100ms for simple classes
- **Target**: < 50ms with lazy initialization
- **Stretch**: < 20ms with precomputed data

## Compatibility Requirements

### Java Language Features
**Supported**:
- Primitive types and operations
- Control structures (if, for, while)
- Method calls and returns
- Basic class hierarchy
- Simple exception handling

**Not Supported (Phase 1)**:
- Generics (type erasure handled)
- Annotations
- Lambda expressions
- Try-with-resources
- Module system

### Library Compatibility
**Basic Support**:
- `java.lang.Object` methods
- `java.lang.String` (basic)
- `java.lang.System` (out/err)

**Extended Support (Phase 2)**:
- Collections framework (basic)
- I/O streams (simple)
- Math utilities

## Security Considerations

### Class File Validation
- Magic number verification
- Version compatibility checking
- Constant pool integrity
- Bytecode verification (basic)

### Execution Safety
- Stack overflow prevention
- Array bounds checking
- Null pointer detection
- Type safety enforcement

### Resource Limits
- Memory usage limits
- Execution time limits
- Class loading limits
- Recursion depth limits

## Development Milestones

### Milestone 1: Foundation (Complete)
- [x] Class file parser
- [x] Basic interpreter loop
- [x] Integer arithmetic
- [x] Simple examples working

### Milestone 2: Core Features (Complete)
- [x] Object support
- [x] Arrays
- [x] Strings
- [x] Exception handling
- [x] Garbage collection

### Milestone 3: Advanced Features
- [x] Inheritance and polymorphism
- [x] Interfaces
- [x] Reflection basics
- [x] Native method interface

### Milestone 4: Optimization (Partial)
- [x] JIT compilation (Cranelift, tiered)
- [x] AOT compilation (cranelift-object)
- [ ] Memory optimizations
- [ ] Performance tuning
- [x] Benchmark suite

## Testing Strategy

### Test Categories
1. **Unit Tests**: Individual component testing
2. **Integration Tests**: Cross-component testing
3. **Compatibility Tests**: Java program execution
4. **Performance Tests**: Speed and memory usage
5. **Fuzz Tests**: Random input validation

### Test Data
- Simple Java examples (provided)
- Generated test cases
- Real-world Java programs (simplified)
- Edge case scenarios

### Test Automation
- CI/CD pipeline integration
- Automated regression testing
- Performance regression detection
- Coverage reporting

## Documentation Requirements

### User Documentation
- Installation guide
- Usage examples
- Troubleshooting guide
- API reference

### Developer Documentation
- Architecture overview
- Code organization
- Extension guide
- Contribution guidelines

### Internal Documentation
- Design decisions
- Implementation notes
- Performance characteristics
- Known limitations

## Deployment and Distribution

### Packaging
- Cargo crate publication
- Binary releases
- Docker images
- WebAssembly module

### Distribution Channels
- crates.io
- GitHub Releases
- Package managers (Homebrew, apt, etc.)
- Online demo

### Versioning
- Semantic versioning (SemVer)
- API stability guarantees
- Migration guides
- Deprecation policies

## Community and Contribution

### Contribution Guidelines
- Code style requirements
- Testing requirements
- Documentation requirements
- Review process

### Community Support
- Issue tracking
- Discussion forums
- Chat channels
- Regular updates

### Governance
- Maintainer team
- Decision process
- Release management
- Security response

## Success Metrics

### Technical Metrics
- Test coverage percentage
- Performance benchmarks
- Memory usage statistics
- Bug count and resolution time

### Usage Metrics
- Number of users
- Example programs running
- Community contributions
- External integrations

### Quality Metrics
- Code review feedback
- Documentation completeness
- API stability
- Security audit results

## Risk Management

### Technical Risks
- Performance bottlenecks
- Memory leaks
- Specification misinterpretation
- Compatibility issues

### Mitigation Strategies
- Early prototyping
- Comprehensive testing
- Code review process
- Performance profiling

### Contingency Plans
- Feature prioritization
- Architecture refactoring
- Alternative implementations
- Fallback mechanisms

## Future Directions

### Short-term (6 months)
- Complete core JVM features
- Improve performance
- Add comprehensive testing
- Enhance documentation

### Medium-term (1 year)
- Expand JIT bytecode coverage
- Threading support
- Advanced optimizations
- Tooling integration

### Long-term (2+ years)
- Production readiness
- Enterprise features
- Cloud deployment
- Research applications