ReflectUtils.java

import java.lang.reflect.Field;

public class ReflectUtils {

    /* Reflectively replaces instances of clazz on supplied instance with supplied mock object */
    public static String injectMockByClass(Class<?> clazz, Object instance, Object mock) {
        Field[] fields = instance.getClass().getDeclaredFields();
        String msg = "No instances of " + clazz.getCanonicalName() + " found";
        for (Field field : fields) {
            if (clazz.isAssignableFrom(field.getType())) {
                try {
                    field.setAccessible(true);
                    field.set(instance, mock);
                    msg = "Injected " + mock.getClass().getCanonicalName()
                          + " instance into field " + field.getName()
                          + " of " + instance.getClass().getName();
                } catch (IllegalArgumentException e) {
                    e.printStackTrace();
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
        }
        return msg;
    }
}