History

In 2009, during my vacation, I was experimenting with code and developed a pure Java JVM interpreter compatible with J2ME CLDC 1.0. This version didn’t support loading external classes, and the task was not purely academic. Within three days, I had successfully written the interpreter and produced a proof of concept. The result worked well and ran reasonably fast on a Nokia 6100. I then published the project as open source on my homepage under the name M-JVM.

In 2015, my homepage was redesigned and the old projects were removed. Since I believed this project could still be useful—at least for learning purposes—I refactored it and published the source code on GitHub under the name J-J-JVM. The updated version included several improvements: support for inner classes, support for double and long values, and dozens of additional tests.

The JVM interpreter doesn't contain any "Poka-yoke" (mistake-proofing) mechanism and verification of byte-code, it doesn't make any stack map verification and any requests or communication with Java security manager !

How to build

It is a regular maven project and doesn't have any magic, just build through mvn clean install in the root of the project.

Hello world

For instance, you can see below class writing just "Hello world!" on console.

package com.igormaznitsa.testjjjvm;

public class HelloWorld {
  public static void main(final String ... args){
    System.out.println("Hello world!");
  }
}

Let use Javassist to load the class but make some changes in its execution with J-J-JVM library.

JJJVMProvider provider = new JSEProviderImpl(){
  @Override
  public Object invoke(JJJVMClass caller, Object instance, String jvmFormattedClassName, String methodName, String methodSignature, Object[] arguments) throws Throwable {
    if (jvmFormattedClassName.equals("java/io/PrintStream") && methodName.equals("println") && methodSignature.equals("(Ljava/lang/String;)V")){
      System.out.println("<<"+arguments[0]+">>");
      return null;
    }
    return super.invoke(caller, instance, jvmFormattedClassName, methodName, methodSignature, arguments); //To change body of generated methods, choose Tools | Templates.
  }
};
JJJVMClassImpl jjjvmClass = new JJJVMClassImpl(new ByteArrayInputStream(javassist.ClassPool.getDefault().get("com.igormaznitsa.testjjjvm.HelloWorld").toBytecode()), provider);
jjjvmClass.findMethod("main", "([Ljava/lang/String;)V").invoke(null, null);

and the code above will print

<<Hello world!>>