Mjolnir

Thor Hammer and secondarily a way to express invokedynamic in Java

Goal

Mjolnir is a Java class allowing to initialize a stable value by calling a bootstrap method once.

The implementation is optimized so the stable value is very cheap to get. A bytecode rewriter is provided to replace the access to the stable value by an invokedynamic making the call even cheaper (mostly free).

Mjolnir has the following properties:

• Mjolnir.get().invokeExact() is semantically equivalent to an invokedynamic
• initialize the constant with a bootstrap method no bootstrap call in the fast path
• no boxing of arguments
• no static analysis requires for the bytecode rewriter
  • crawling the bytecode is enough
• should work without the bytecode rewriter (for testing)

Video

How to build it

Mjolnir is built using pro which is my own build tool, you can download it from github (amazon S3) like this

sh get_pro.sh

(if you are not on linux, you have to build pro by yourself, sorry)

and build Mjolnir just by running pro.

pro/bin/pro

Examples

The following example implements the equivalent of the macro__LINE__ i.e. it returns the current line number like in C

  private static int boostrap(Lookup lookup) {
    String className = lookup.lookupClass().getName();
    int lineNumber = StackWalker.getInstance()
        .walk(s -> s.skip(1).filter(f -> f.getClassName().equals(className)).findFirst())
        .get()
        .getLineNumber();
    return lineNumber;
  }

  public static void main(String[] args) {
    int __LINE__ = Mjolnir.get(lookup -> boostrap(lookup));
  }

This mechanism can be used to express an invokedynamic in Java, the bootstrap method can return a MethodHandle that will be called with invokeExact.

  private static String hello(String name) {
    return "Hello " + name;
  }
  
  private static MethodHandle initHello(Lookup lookup) throws NoSuchMethodException, IllegalAccessException {
    Class<?> declaringClass = lookup.lookupClass();
    return lookup.findStatic(declaringClass, "hello", methodType(String.class, String.class));
  }

  public static void main(String[] args)  hello() throws Throwable {
    String result = (String)Mjolnir.get(lookup -> initHello(lookup)).invokeExact("Mjolnir");
    System.out.println(result);   // Hello Mjolnir
  }