douchuan/jvm

---

douchuan/jvm.json

{
  "createdAt": "2019-12-12T05:47:17Z",
  "defaultBranch": "master",
  "description": "JVM in Rust",
  "fullName": "douchuan/jvm",
  "homepage": "",
  "language": "Rust",
  "name": "jvm",
  "pushedAt": "2026-04-23T01:11:06Z",
  "stargazersCount": 531,
  "topics": [
    "jit",
    "jvm",
    "rust"
  ],
  "updatedAt": "2026-07-04T03:17:32Z",
  "url": "https://github.com/douchuan/jvm"
}

jvm

A JVM implementation written in Rust with an LLVM-based JIT compiler.

Features

• Class loading — directory, JAR, and JImage (JDK 9+ lib/modules) sources
• Interpreter — all 202 JVM opcodes implemented (one file per opcode group)
• LLVM JIT — ~110 opcodes translated to LLVM IR via inkwell (int/long/float/double arithmetic, bitwise, stack ops, type conversions, branching). Falls back to interpreter for uncompiled methods
• Object model — slot-based heap (Oop::Ref(u32)), zero unsafe for object access. Safe Monitor via std::sync::{Mutex, Condvar}
• JNI — ~30 native method implementations covering java.lang.*, java.io.*, sun.misc.*, sun.reflect.*
• Threading — Java threads mapped to OS threads, with pool management

Workspace Structure

jvm/                     # Binary crate — CLI entry point
crates/
  classfile/             # JVM class file format type definitions (no dependencies)
  class-parser/          # Bytes → ClassFile parser (Cursor + Read)
  vm/                    # Core VM: interpreter, JIT, oop model, native methods
  class-verification/    # Class verification (skeleton)
tools/
  javap/                 # Class file disassembler (javap-style output)

Dependency Graph

jvm (binary) → vm → class-parser → classfile
                          ↘ classfile
javap (tool) → class-parser → classfile
                  ↘ classfile

All dependencies are centralized in the root Cargo.toml under [workspace.dependencies]. Sub-crates reference them with dep.workspace = true.

Quick Start

cargo build --workspace
cargo test --workspace
cargo run -p jvm -- --classpath /path/to/classes MyMainClass

See scripts/dev.sh for convenience commands.

Architecture

classfile

Pure type definitions matching the JVM class file specification. No parsing logic.

class-parser

Reads raw bytes and produces ClassFile. Uses std::io::Cursor + Read. Supports generics parsing via signature.rs.

vm

Module	Description
oop/	Object model — Oop enum, slot-based heap, instances, arrays, mirrors
runtime/interp/	Bytecode interpreter (per-opcode files, no macros)
runtime/jit/	LLVM JIT — bytecode → LLVM IR via stack-to-register conversion
runtime/invoke.rs	Method dispatch (JIT first, interpreter fallback)
runtime/class_path_manager.rs	Classpath: directories, JARs, JImage
runtime/class_loader.rs	Class loading + system dictionary
runtime/thread/	Thread model, thread pool, Java monitor
native/	JNI native method implementations (~30 classes)

jvm

CLI binary. Initializes VM (oop, runtime, native), resolves classpath, loads and runs the entry class.

javap

Standalone javap-style disassembler. Reads class files and outputs human-readable representations including constant pool, fields, methods, bytecode, line numbers, and stack maps. Uses handlebars templates for rendering.

Status

Component	State	Notes
Class file parser	Done	nom → Cursor+Read rewrite complete
Interpreter	Done	202/202 opcodes
Method invocation	Done	v_table, static, special, interface
LLVM JIT	Done	~110 opcodes, ~110 opcodes translated
Oop model	Done	Slot-based, zero unsafe
GC	Not implemented	Free-list allocation only, no collection
Class verification	Skeleton	No verification implemented
invokedynamic	Not implemented

Known Issues

• 5 class_path_manager tests fail due to missing test/ fixture directory
• No garbage collection — objects are allocated but never reclaimed
• invoke* bytecodes are not JIT-compiled; they fall back to the interpreter