Access to private inherited fields via reflection in Java

Question

This should demonstrate how to solve it:

import java.lang.reflect.Field;

class Super {
    private int i = 5;
}

public class B extends Super {
    public static void main(String[] args) throws Exception {
        B b = new B();
        Field f = b.getClass().getSuperclass().getDeclaredField("i");
        f.setAccessible(true);
        System.out.println(f.get(b));
    }
}

(Or Class.getDeclaredFields for an array of all fields.)

Output:

5

The best approach here is using the Visitor Pattern do find all fields in the class and all super classes and execute a callback action on them.

---

Implementation

Spring has a nice Utility class ReflectionUtils that does just that: it defines a method to loop over all fields of all super classes with a callback: ReflectionUtils.doWithFields()

Documentation:

Invoke the given callback on all fields in the target class, going up the class hierarchy to get all declared fields.

Parameters:
- clazz - the target class to analyze
- fc - the callback to invoke for each field
- ff - the filter that determines the fields to apply the callback to

Sample code:

ReflectionUtils.doWithFields(RoleUnresolvedList.class,
    new FieldCallback(){

        @Override
        public void doWith(final Field field) throws IllegalArgumentException,
            IllegalAccessException{

            System.out.println("Found field " + field + " in type "
                + field.getDeclaringClass());

        }
    },
    new FieldFilter(){

        @Override
        public boolean matches(final Field field){
            final int modifiers = field.getModifiers();
            // no static fields please
            return !Modifier.isStatic(modifiers);
        }
    });

Output:

Found field private transient boolean javax.management.relation.RoleUnresolvedList.typeSafe in type class javax.management.relation.RoleUnresolvedList
Found field private transient boolean javax.management.relation.RoleUnresolvedList.tainted in type class javax.management.relation.RoleUnresolvedList
Found field private transient java.lang.Object[] java.util.ArrayList.elementData in type class java.util.ArrayList
Found field private int java.util.ArrayList.size in type class java.util.ArrayList
Found field protected transient int java.util.AbstractList.modCount in type class java.util.AbstractList

This'll do it:

private List<Field> getInheritedPrivateFields(Class<?> type) {
    List<Field> result = new ArrayList<Field>();

    Class<?> i = type;
    while (i != null && i != Object.class) {
        Collections.addAll(result, i.getDeclaredFields());
        i = i.getSuperclass();
    }

    return result;
}

If you use a code coverage tool like EclEmma, you have to watch out: they add a hidden field to each of your classes. In the case of EclEmma, these fields are marked synthetic, and you can filter them out like this:

private List<Field> getInheritedPrivateFields(Class<?> type) {
    List<Field> result = new ArrayList<Field>();

    Class<?> i = type;
    while (i != null && i != Object.class) {
        for (Field field : i.getDeclaredFields()) {
            if (!field.isSynthetic()) {
                result.add(field);
            }
        }
        i = i.getSuperclass();
    }

    return result;
}

public static Field getField(Class<?> clazz, String fieldName) {
    Class<?> tmpClass = clazz;
    do {
        try {
            Field f = tmpClass.getDeclaredField(fieldName);
            return f;
        } catch (NoSuchFieldException e) {
            tmpClass = tmpClass.getSuperclass();
        }
    } while (tmpClass != null);

    throw new RuntimeException("Field '" + fieldName
            + "' not found on class " + clazz);
}

(based on this answer)

In fact i use a complex type hierachy so you solution is not complete. I need to make a recursive call to get all the private inherited fields. Here is my solution

 /**
 * Return the set of fields declared at all level of class hierachy
 */
public Vector<Field> getAllFields(Class clazz) {
    return getAllFieldsRec(clazz, new Vector<Field>());
}

private Vector<Field> getAllFieldsRec(Class clazz, Vector<Field> vector) {
    Class superClazz = clazz.getSuperclass();
    if(superClazz != null){
        getAllFieldsRec(superClazz, vector);
    }
    vector.addAll(toVector(clazz.getDeclaredFields()));
    return vector;
}