Dynamically creating object instance of interface / abstract class

May 11, 2023

In Groovy or Java, assume I have an interface (or abstract class) with some unimplemented methods. I would like to generate an instance of this interface/abstract class dynamically in runtime, and have some behaviour for those unimplemented methods (e.g. based on annotation on those methods)

i.e. something like

interface Foo {
    @SomeAnnotation(...)
    foo();
}


....

Foo foo = someMagic(Foo.class, somethingToProvideLogicOfUnimplementedMethods);

Are there any library or technique that can achieve this?

>Solution :

In Groovy the following magic is called Map to Interface coercion and is available out of the box:

import static java.lang.annotation.ElementType.METHOD
import static java.lang.annotation.RetentionPolicy.RUNTIME
import java.lang.annotation.Retention
import java.lang.annotation.Target

@Retention( RUNTIME )
@Target( [ METHOD ] )
@interface Some {
  String value() default ''
}

interface Foo {
  @Some( 'AWESOME' )
  void foo()
}

Foo magicFoo = [ foo:{->
  String annotationValue = Foo.methods.first().annotations.first().value()
  println "the $annotationValue MAGIC printed!"
} ] as Foo

assert Foo.isAssignableFrom( magicFoo.getClass() )

magicFoo.foo()

prints in the console: