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 act

Answer 1

Check if you want something as the below code where I have passed the runnable to a Thread object in your inspect method.

    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) {
            Thread t = new Thread(runnable);
            t.start();

          }


        }

output will be:

How to access this?

Answer 2

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

Answer 3

With reflection, it is not possible to get local variables and method parameter values. Instead you can use AOP to intercept the method call and inspect the parameters.