Two months ago, I was discussing with a friend about dynamic languages and JSR 292 when he mentions DLR Expression Trees. At that time, I had a vague memory, mostly because I was under the effect of a persisting jet lag, when I attend the talk of Mads Torgersen about it at last year JVM Summit.
By the way, I will attend the next JVM Summit in September, I have some good results with JSR 292 backport.
I had finished the debate with my friend saying that an expression tree like that can be easily implemented in one hour on top of JSR 292 API.
In fact, it takes me a little more than one day to just sketch a prototype. And Yes, I have some free times, I'm in vacation and my wife allow me to took my laptop with me :)

An Expression Tree

The main idea is to be able to create an AST and to transform it into runnable code at runtime.
The following example create a kind of anonymous method (a lambda, etc) that takes two integers and returns their sum.
The class ExprBuilder use a stack based notation like by example the PostScript language because I'm too lenient to implement a real AST (with automatic type conversions).

  MethodType methodType = MethodType.make(int.class, int.class, int.class);
    
  ExprBuilder builder = new ExprBuilder(methodType);
  Var a = builder.parameter(0);
  Var b = builder.parameter(1);
  builder.
    load(a).
    load(b).
    add();
    
  MethodHandle mh = builder.createMethodHandle(Main.class);
  System.out.println(mh.invoke(2, 3));

Also notice that createMethodhandle takes a class as argument because the anonymous method will be able to access to all fields, methods, etc even if they are declared private (thanks to the anonymous class loader) like a classical member of this class.

If you want to play with it, the code is available here and works with current jdk7 binaries (b67).
Run it with a command line like this one

/usr/jdk/jdk1.7.0/bin/java -cp classes:lib/asm-all-3.2.jar -XX:+EnableInvokeDynamic Main

Cheers,
Rémi