BlackHatSamurai BlackHatSamurai - 5 months ago 28
Java Question

What is the difference between getFields and getDeclaredFields in Java reflection

I'm a little confused about the difference between the

getFields
method and the
getDeclaredFields
method when using Java reflection.

I read that
getDeclaredFields
gives you access to all the fields of the class and that
getFields
only returns public fields. If this is the case, why wouldn't you just always use
getDeclaredFields
?

Can someone please elaborate on this, and explain the difference between the two methods, and when/why you would want to use one over the other?

Answer

getFields()

All the public fields up the entire class hierarchy.

getDeclaredFields()

All the fields, regardless of their accessibility but only for the current class, not any base classes that the current class might be inheriting from.

To get all the fields up the hierarchy

I have written the following function:

public static Iterable<Field> getFieldsUpTo(@Nonnull Class<?> startClass, 
                                   @Nullable Class<?> exclusiveParent) {

   List<Field> currentClassFields = Lists.newArrayList(startClass.getDeclaredFields());
   Class<?> parentClass = startClass.getSuperclass();

   if (parentClass != null && 
          (exclusiveParent == null || !(parentClass.equals(exclusiveParent)))) {
     List<Field> parentClassFields = 
         (List<Field>) getFieldsUpTo(parentClass, exclusiveParent);
     currentClassFields.addAll(parentClassFields);
   }

   return currentClassFields;
}

The exclusiveParent class is provided to prevent the retrieval of fields from Object. If may be null if you DO want the Object fields.

To claify, Lists.newArrayList comes from Guava.

Update

FYI, the above code is published on GitHub in my LibEx project in ReflectionUtils.