Using Spock to mock private static final variables in Java

Question

I'm trying to write some Spock tests with Groovy to test some Java code (specifically a servlet Filter). I have some private static and private static final variables that I would like to mock, but I can't determine if there is a way to do this. I know metaClass is available for methods, is there anything similar for variables?

For instance, I have:

public class MyFilter implements Filter {
  private static WebResource RESOURCE;
  private static final String CACHE_KEY = "key-to-be-used-for-cache";
  ... actual methods, etc ...
}

I've tried using Mock(MyFilter), as well as using Java reflection to change the value (based on this question and answer Change private static final field using Java reflection).

I would like to do this without adding something like Mockito or other frameworks, if that's possible, just use plain Groovy and Spock.

Thanks for any ideas!

UPDATE 1

At least for private static variables I did get the following to work:

Field field = MyFilter.class.getDeclaredField("CACHE_KEY")
field.setAccessible(true)
field.set(null, "new-key-value")

But I still haven't been able to get around the final aspect.

UPDATE 2

Thanks to Xv. I can now set this with the following:

Field field = MyFilter.class.getDeclaredField("CACHE_KEY")
field.setAccessible(true)

Field modifiersField = Field.class.getDeclaredField("modifiers")
modifiersField.setAccessible(true);
modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);

field.set(null, "new-key-value")

Answer 1

Based on what I learned from https://stackoverflow.com/a/25031713/239408, this works for me in spock

import java.lang.reflect.Field
import java.lang.reflect.Modifier

...

    def setup() {

        Field field = BackendCredentials.getDeclaredField("logger")
        field.setAccessible(true);

        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);
        modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);

        field.set(null, Mock(Logger))
    }

Looks like you are missing the unsetting of the Modifier.FINAL flag.