Re: Accessibility of suBclass-fields from suPERclass (reflection)
On 8/14/2014 8:58 PM, Andreas Leitgeb wrote:
It is clear to me, that a class may need to protect its fields from direct
access by subclasses or unrelated classes, mostly because the developer of
a base class doesn't always know all derived or unrelated classes ahead of
time.
Otoh, someone who derives a class, or even does it in a statement block of
their own code, obviously can be expected to "know" and "trust" those
classes he derives or includes derivatives of.
E.g., someone writing this:
public class Outer extends Base {
static {
class Inner extends Base { ... }
}
}
had better know/trust "class Base", and have no need to have his code
protected from reflective examination by class Base.
Now, apparently, the designers of Java felt differently, and to examine (by
use of reflection) the fields of "Inner" from within Base's code, Base has to
jump an extra hoop (namely calling setAccessibility(true)), requesting access
from an eventually installed SecurityManager.
Surprisingly (or not), that extra hoop is not required to examine "Outer"'s
fields from within "Base". Also, it is not possible to mark an inner class
as public in an attempt to grant accessibility of "Inner" to "Base".
It makes no sense to me, why there is any defense-mechanism at all against
field access from code within a known Baseclass.
Can you produce an example showing the problem?
My first attempt:
package august;
import java.lang.reflect.Field;
public class AccessFun {
public static void main(String[] args) {
Outer o = new Outer();
o.test();
}
}
class Base {
private static void test(Base o) {
System.out.print(o.getClass().getName() + " : ");
try {
Field f = o.getClass().getField("v");
int v = f.getInt(o);
System.out.println(v);
} catch (NoSuchFieldException | SecurityException |
IllegalArgumentException | IllegalAccessException e) {
System.out.println("no access");
}
}
private static void testWith(Base o) {
System.out.print(o.getClass().getName() + " (with) : ");
try {
Field f = o.getClass().getDeclaredField("v");
f.setAccessible(true);
int v = f.getInt(o);
System.out.println(v);
} catch (NoSuchFieldException | SecurityException |
IllegalArgumentException | IllegalAccessException e) {
System.out.println("no access");
}
}
public void test() {
Outer o = (Outer)this;
test(o);
test(o.one);
test(o.two);
testWith(o);
testWith(o.one);
testWith(o.two);
}
}
class Outer extends Base {
private static Base hack;
public Base one;
public Base two;
private int v = 0;
static class InnerOne extends Base {
private int v = 1;
}
static {
class InnerTwo extends Base {
private int v = 2;
}
hack = new InnerTwo();
}
public Outer() {
one = new InnerOne();
two = hack;
}
}
gave:
august.Outer : no access
august.Outer$InnerOne : no access
august.Outer$1InnerTwo : no access
august.Outer (with) : 0
august.Outer$InnerOne (with) : 1
august.Outer$1InnerTwo (with) : 2
which looks consistent to me.
Arne