What is the difference between getFields and getDeclaredFields in Java reflection

Question

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 1

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. It may be null if you DO want the Object fields.

To clarify, Lists.newArrayList comes from Guava.

Update

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

Answer 2

From Java Reflection tutorials:

Answer 3

As already mentioned, Class.getDeclaredField(String) only looks at the fields from the Class in which you call it.

If you want to search a Field in the Class hierarchy, you can use this simple function:

/**
 * Returns the first {@link Field} in the hierarchy for the specified name
 */
public static Field getField(Class<?> clazz, String name) {
    Field field = null;
    while (clazz != null && field == null) {
        try {
            field = clazz.getDeclaredField(name);
        } catch (Exception e) {
        }
        clazz = clazz.getSuperclass();
    }
    return field;
}

This is useful to find a private field from a superclass, for example. Also, if you want to modify its value, you can use it like this:

/**
 * Sets {@code value} to the first {@link Field} in the {@code object} hierarchy, for the specified name
 */
public static void setField(Object object, String fieldName, Object value) throws Exception {
    Field field = getField(object.getClass(), fieldName);
    field.setAccessible(true);
    field.set(object, value);
}

Answer 4

public Field[] getFields() throws SecurityException

Returns an array containing Field objects reflecting all the accessible public fields of the class or interface represented by this Class object. The elements in the array returned are not sorted and are not in any particular order. This method returns an array of length 0 if the class or interface has no accessible public fields, or if it represents an array class, a primitive type, or void.

Specifically, if this Class object represents a class, this method returns the public fields of this class and of all its superclasses. If this Class object represents an interface, this method returns the fields of this interface and of all its superinterfaces.

The implicit length field for array class is not reflected by this method. User code should use the methods of class Array to manipulate arrays.

public Field[] getDeclaredFields() throws SecurityException

Returns an array of Field objects reflecting all the fields declared by the class or interface represented by this Class object. This includes public, protected, default (package) access, and private fields, but excludes inherited fields. The elements in the array returned are not sorted and are not in any particular order. This method returns an array of length 0 if the class or interface declares no fields, or if this Class object represents a primitive type, an array class, or void.

And what if I need all fields from all parent classes? Some code is needed, e.g. from https://stackoverflow.com/a/35103361/755804:

public static List<Field> getAllModelFields(Class aClass) {
    List<Field> fields = new ArrayList<>();
    do {
        Collections.addAll(fields, aClass.getDeclaredFields());
        aClass = aClass.getSuperclass();
    } while (aClass != null);
    return fields;
}