Re: Accessibility of suBclass-fields from suPERclass (reflection)
On 8/16/2014 4:46 AM, Andreas Leitgeb wrote:
Arne Vajh??j <arne@vajhoej.dk> wrote:
Can you produce an example showing the problem?
My 'problem' is understanding, why setAccessible(true) is even necessary
My first attempt:
[...]
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 |
[throws...]
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 |
[throws not]
which looks consistent to me.
Consistent maybe, but why cannot Base access these fields by default?
I'm not complaining about that extra setAccessibility(true), I just
would like to see an example of where a Base-class accessing its children
could be a security issue. I'm not talking of some arbitrary class
attempting access, but about the target class's parent (or grandparent)
class.
I'm probably unaware of some risk, and learning about it was my reason
for the posting.
The docs for setAccessible says:
<quote>
Set the accessible flag for this object to the indicated boolean value.
A value of true indicates that the reflected object should suppress Java
language access checking when it is used. A value of false indicates
that the reflected object should enforce Java language access checks.
First, if there is a security manager, its checkPermission method is
called with a ReflectPermission("suppressAccessChecks") permission.
</quote>
I read the first paragraph as:
call with false (and default) => access will only be allowed if access
would have been allowed without the use of reflection
call with true => allow access (if call succeed)
Since the access we are discussing would not be possible without
reflection, then with that interpretation of the docs the call to
setAccessible is indeed necessary.
The second paragraphs explains that a security manager may prevent
this form for access.
I guess this is done to prevent code in restricted context
(typical Java applet) from corrupting the state of instances
of the Java runtime classes.
Like switching a flag somewhere changing context from
restricted to non-restricted.
I doubt that there are any special case for base class
access to subclass.
Arne