Update private static final int in base class for unittest

Question

I try to change the int value of a private static final int for unittesting. I looked at http://zarnekow.blogspot.de/2013/01/java-hacks-changing-final-fields.html and http://java-performance.info/updating-final-and-static-final-fields/ and other stackoverflow examples like: changing final variables through reflection, why difference between static and non-static final variable

30 Seconds is a default timer. i want to set it with setOverReflectionMax_SECONDS(3); to 3 seconds. But it does not work, any hints ?

My baseclass with

public class BaseClass {

    private static final int MAX_SECONDS = 30;

}

and other class

public final class MyClass extends BaseClass {

    public static List<Field> getFields(final Class<?> clazz){
        final List<Field> list = new ArrayList<>();
        list.addAll(Arrays.asList(clazz.getDeclaredFields()));

        if(clazz.getSuperclass() != null){
            list.addAll(getFields(clazz.getSuperclass()));
        }
        return list;
    }

    public static void setOverReflectionMax_SECONDS(final int newValue) {

        final List<Field> fields = getFields(MyClass.class);
        for (final Field field : fields) {
            if (field.getName().equals("MAX_SECONDS")) {

                field.setAccessible(true);   //EDIT 4 

                Field modifiersField;
                try {
                    modifiersField = Field.class.getDeclaredField("modifiers");
                    modifiersField.setAccessible(true);
                    modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
                    field.set(null, newValue);
                } catch (final Exception e2) {
                    e2.printStackTrace();
                }

            }
        }
    }

}

Edit1: wrong classname

Edit2: I cant change BaseClass (got only the .class file)

Edit3: new Exception

Class MyClass can not access a member of class BaseClass with modifiers "private static"

Edit4 : see code , fixes the exception, but it does not change the int

Why don't you just temporarily modify the variable by hand and rerun your unit test? This seems like a lot of work to simply change a final variable for testing. — tier1, Nov 19 '14 at 13:56
So if you can't change it, why would you want to change it for unit testing? — tier1, Nov 19 '14 at 13:59
Secondly, I'm assuming you get an exception from this? What does that look like? — tier1, Nov 19 '14 at 14:00
@tier1 He doesn't get an exception; the change simply makes no effect due to inlining of constants. — Marko Topolnik, Nov 19 '14 at 14:01
I try to run some negative tests. the MAX_SECONDS is a timeout, so each negative test, will take 30 seconds. Basically i try to find something that isnt there and i dont need to wait the whole 30 seconds — svenhornberg, Nov 19 '14 at 14:03
Sorry my bad, i forgot the exception, i am looking to long on the output — svenhornberg, Nov 19 '14 at 14:06
i forgot field.setAccessible(true); but now i got no exception and it does not change the value — svenhornberg, Nov 19 '14 at 14:17

score 0 · Accepted Answer · edited May 23 '17 at 11:56

This may not be possible even using reflection, because the value of the field may be inlined by the compiler (I believe the compiler is free to choose whether it does this or not). In that case, a copy of the value is hardcoded where it is actually used, and can be changed only by manipulating the byte code.

This is explained in more detail in this question: Change private static final field using Java reflection.

tl,dr: There is no way to reliably change a static final field using reflection. Your options are:

manipulate the byte code (a rather desperate option)
find another solution (subclass, wrapper, proxy...)
(bug the author to) change the source code, to make the value settable/configurable

thanks! i will wait until tomorrow if someone got it working, if not i will accept your answer — svenhornberg, Nov 19 '14 at 14:34

Update private static final int in base class for unittest

1 Answers1