Java 8 - how to access object and method encapsulated as lambda

Question

In Java you can "capture" a "method call on object" as a Runnable, as in belows example.

Later, having access to this instance of Runnable, is it possible to actually access the "captured" object and the method parameters of a method which is called (if possible this probably needs to be done via reflection).

For example:

class SomePrintingClass {
  public void print(String myText) {
    System.out.println(myText);

  }
}


public class HowToAccess {
  public static void main(String[] args) throws Exception {
    final String myText = "How to access this?";

    final SomePrintingClass printer = new SomePrintingClass();

    Runnable r = () -> printer.print(myText); // capture as Runnable

    inspect(r);
  }


  private static void inspect(Runnable runnable) {
    // I have reference only to runnable... can I access "printer" here

  }


}

Is it possible in the "inspect" method to access (probably via reflection) "printer" object and "myText" which was passed as a parameter?

Answer 1

It is possible, because the captured references are translated into fields of the runnable (as with all anonymous classes). The names will be not be consistent however.

I found by testing that you need to make myText non-final, otherwise it will be seen as a compile time constant and in-lined (and will not be accessible as a field):

private static void inspect(Runnable runnable) throws Exception {       
    for(Field f : runnable.getClass().getDeclaredFields()) {
        f.setAccessible(true);
        System.out.println("name: " + f.getName());
        Object o = f.get(runnable);
        System.out.println("value: " + o);
        System.out.println("class: " + o.getClass());
        System.out.println();
    }
}

Prints:

name: arg$1
value: test.SomePrintingClass@1fb3ebeb
class: class test.SomePrintingClass

name: arg$2
value: How to access this?
class: class java.lang.String