JJVM

JJVM is a toy JVM implementation which is written in Java, aiming for self-hosting.

Project Status

Just a toy project.

• -help can be self-hosted, but self-hosting of interpretation is WIP.
• Its boot is so slow for running System.initializeSystemClass() call.
• Not tested with any real-world application. Yours won't work either. See also: test/

Implementation scope

• Bytecode interpreter
• Native method
  • ... in Java 🤔
• Reading rt.jar of host JVM
• Java classfile parser
  • Not relying on com.sun.tools.classfile.* 😉

Demo

$ java -cp build/classes/java/main com.github.k0kubun.jjvm.JJVM -help
Usage: jjvm [-options] class [args...]
where options include
    -cp <class search path of directories and zip/jar files>
    -classpath <class search path of directories and zip/jar files>
                  A : separated list of directories, JAR archives,
                  and ZIP archives to search for class files.
    -help         print this help message

$ java -cp build/classes/java/main com.github.k0kubun.jjvm.JJVM \
>      -cp build/classes/java/main com.github.k0kubun.jjvm.JJVM -help
Usage: jjvm [-options] class [args...]
where options include
    -cp <class search path of directories and zip/jar files>
    -classpath <class search path of directories and zip/jar files>
                  A : separated list of directories, JAR archives,
                  and ZIP archives to search for class files.
    -help         print this help message

$ java -cp build/classes/java/main com.github.k0kubun.jjvm.JJVM -Xjjvmtrace \
>      -cp build/classes/java/main com.github.k0kubun.jjvm.JJVM -help
java/lang/System.<clinit>
  java/lang/System.registerNatives
java/lang/Class.<clinit>
  java/lang/Class.registerNatives
java/lang/System.initializeSystemClass
  ...(snip)...
  java/lang/System.newPrintStream
    java/io/BufferedOutputStream.<init>
      java/io/FilterOutputStream.<init>
        java/io/OutputStream.<init>
          java/lang/Object.<init>
    java/io/PrintStream.<init>
      ..(snip)...
  java/lang/System.setErr0
  ...(snip)...
com/github/k0kubun/jjvm/JJVM.main
  com/github/k0kubun/jjvm/JJVM.parseOptions
    ...(snip)...
    com/github/k0kubun/jjvm/JJVM.printHelp
      java/io/PrintStream.print
        java/io/PrintStream.write
          java/io/PrintStream.ensureOpen
          java/io/Writer.write
            java/lang/String.length
            java/io/BufferedWriter.write
              java/io/BufferedWriter.ensureOpen
              java/io/BufferedWriter.min
              java/lang/String.getChars
                java/lang/System.arraycopy
          java/io/BufferedWriter.flushBuffer
            java/io/BufferedWriter.ensureOpen
            java/io/OutputStreamWriter.write
              sun/nio/cs/StreamEncoder.write
                sun/nio/cs/StreamEncoder.ensureOpen
                sun/nio/cs/StreamEncoder.implWrite
Usage: jjvm [-options] class [args...]
where options include
    -cp <class search path of directories and zip/jar files>
    -classpath <class search path of directories and zip/jar files>
                  A : separated list of directories, JAR archives,
                  and ZIP archives to search for class files.
    -help         print this help message
                ...(snip)...
    java/lang/System.exit
      ...(snip)...
      java/lang/Runtime.getRuntime
      java/lang/Runtime.exit
        ...(snip)...
        java/lang/Shutdown.exit
          java/lang/Shutdown.sequence
            java/lang/Shutdown.runHooks
          java/lang/Shutdown.halt
            java/lang/Shutdown.halt0

You may notice the help output seems not buffered.
Charset is not working and we ended up stubbing sun/nio/cs/StreamEncoder.implWrite for now.

Complete call tree: https://gist.github.com/k0kubun/8a0b27ae48a5239c8b25af804e8d251f

How to run

JDK 8 should be used. Others are not tested.

VM

./gradlew jjvm -Pargs="-cp test Hello"

Disassemble

./gradlew jjvmp -Pargs="test/Hello.class"

Implementation overview

• JJVM.main: com/github/k0kubun/jjvm/JJVM.java
  • ClossLoader.setupBootstrapSearchPath: com/github/k0kubun/jjvm/virtualmachine/ClassLoader.java
  • ClassFileParser.parse: com/github/k0kubun/jjvm/classfile/ClassFileParser.java
  • VirtualMachine.executeMethod: com/github/k0kubun/jjvm/virtualmachine/VirtualMachine.java
    • BytecodeInterpreter.execute: com/github/k0kubun/jjvm/virtualmachine/BytecodeInterpreter.java
    • NativeMethod.dispatch: com/github/k0kubun/jjvm/virtualmachine/NativeMethod.java

JDK References

• Chapter 2. The Structure of the Java Virtual Machine
• Chapter 4. The class File Format
• Chapter 6. The Java Virtual Machine Instruction Set

License

MIT License