chongwish / jjvm

Fewer than 20 files make a JVM! Support Java 8, Java 11 and Java 14!

A toy JVM written by Java

Instruction

Fewer than 20 files make a JVM for yourself! Now there are only 11 files!

Support Java 8, Java 11 and Java 14!

Purpose

This purpose is not only for me, but also for the other javaer.

1. For fun.
2. To know how JVM works.
3. To learn Java deeply.
4. Make it (JJVM) Simple, Clean and Funny.

Project structure

.                         # main project
├── build                 #  generated by gradle
├── build.gradle
├── demo                  # subproject
│   ├── build             #  generated by gradle
│   ├── build.gradle
│   └── src
│       └── main          #  some examples
├── gradle
├── gradlew
├── gradlew.bat
├── README.md
├── settings.gradle
└── src                   # gradle project
    ├── main              #  core code
    └── test              #  test case

The structure of the core code:

src/main/java/me/chongwish/jjvm/
├── Argument.java         # parse command line argument
├── Bytecode.java         # bytecode wrapper
├── Classfile.java        # parse the .class file
├── Classloader.java      # class loader
├── Classpath.java        # find class from [file,directory,jar,zip,jrt]
├── Frame.java            # java frame, is not jframe
├── Instruction.java      # jvm instruction
├── Interpreter.java      # jvm interpreter
├── NativeMethod.java     # jvm native method
├── RuntimeDataArea.java  # jvm runtime data area
└── Starter.java          # jjvm entry

How to work

Development Environment

Mine is that:

• emacs + lsp-java
• openjdk 14 & openjdk 11 & openjdk 8
• gradle version 6
• archlinux container on gentoo host & windows 10

I haven't tested other jdk version.

Build & Run

Build a jar first:

./gradlew clean build

And then a JVM which name is jjvm.jar will be generated in the build directory. Run it:

java -jar build/libs/jjvm.jar

Some message was displayed in your terminal:

Usage: jjvm [-?] [-version] [-cp=<classpaths>]... mainclass [args......]
A toy of jvm
      mainclass      main class
      [args......]   arguments following the main class
  -?, -help          display help message
      -cp, -classpath=<classpaths>
                     class search path of directories
      -version       display version info

There are some samples in the demo project. For example, there are three java file:

// file: demo/src/main/java/me/chongwish/jjvm/demo/Main.java
package me.chongwish.jjvm.demo;

import me.chongwish.jjvm.demo.sort.Bubble;
import me.chongwish.jjvm.demo.sort.Sort;

public class Main {
    public static void main(String[] args) {
        System.out.println("Bubble.sort");
        Integer[] array = new Integer[] { 5, 4, 1, 8, 12, 6 };
        System.out.print("before: ");
        for (int n : array) {
            System.out.print(n + " ");
        }
        System.out.println();
        Sort<Integer> fn = new Bubble<>();
        fn.sort(array);
        System.out.print("after: ");
        for (int n : array) {
            System.out.print(n + "");
        }
        System.out.println();
    }
}

// file: demo/src/main/java/me/chongwish/jjvm/demo/sort/Sort.java
package me.chongwish.jjvm.demo.sort;

public interface Sort<T extends Comparable<? super T>> {
    public T[] sort(T[] array);
}

// file: demo/src/main/java/me/chongwish/jjvm/demo/sort/Bubble.java
package me.chongwish.jjvm.demo.sort;

public class Bubble<T extends Comparable<? super T>> implements Sort<T> {
    @Override
    public T[] sort(T[] array) {
        for (int i = 0; i < array.length - 1; ++i) {
            for (int j = 0; j < array.length - 1 - i; ++j) {
                if (array[j].compareTo(array[j + 1]) > 0) {
                    T temp = array[j];
                    array[j] = array[j + 1];
                    array[j + 1] = temp;
                }
            }
        }
        return array;
    }
}

The class file was generated automatically when you run the command gradle run build.

demo/build/classes/java/main/me/chongwish/jjvm/demo/
├── Main.class
└── sort
    ├── Bubble.class
    └── Sort.class

To use our JVM to run the class which name is Main:

java -jar build/libs/jjvm.jar -cp demo/build/classes/java/main me.chongwish.jjvm.demo.Main

The output:

Bubble.sort
before: 5 4 1 8 12 6 
after: 1 4 5 6 8 12

Todo

• Unfinished Instruction (0/4)
• Multi Thread
• Reflection (A poorly implementation)
• Native Method
• Exception (A poorly implementation)
• Newer Feature come from Java 8+
• VM System Class Initialization

Reference

• The Java® Language Specification
• The Java® Virtual Machine Specification
• 《自己动手写 Java 虚拟机》