Change type and value of a final field in a Java/Groovy class

FieldReflection.groovy

static class TestJ3 {

    static final Integer f = 1

    static public void set(String str) {
        Field field = TestJ3.class.getDeclaredField("f")

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

        Field fieldType = Field.getDeclaredField("type")
        fieldType.setAccessible(true)
        fieldType.set(field, String.class)

        field.setAccessible(true)
        field.set(null, str)
    }

}


TestJ3.set("Hello")
assert TestJ3.f == "Hello" 

Courtesy of https://twitter.com/nicolas_frankel/status/655303856298065920