--
Bjørn Vårdal
----- Original message -----
From: Brian Goetz <[email protected]>
Sent by: "valhalla-spec-experts"
<[email protected]>
To: [email protected]
Cc:
Subject: Nestmates
Date: Wed, Jan 20, 2016 2:57 PM
This topic is at the complete opposite end of the spectrum from
topics
we've been discussing so far. It's mostly an implementation
story, and
of particular interest to the compiler and VM implementers here.
Background
----------
Since Java 1.1, the rules for accessibility when inner classes are
involved at the language level are not fully aligned with those
at the
VM level. In particular, private and protected access from and
to inner
classes is stricter in the VM than in the language, meaning that in
these cases, the static compiler emits an access bridge (access$000)
which effectively downgrades the accessed member's accessibility to
package.
Access bridges have some disadvantages. They're ugly, but that's
not a
really big deal. They're imprecise; they allow wider-than-necessary
access to the member. Again, this is not a huge deal on its own.
But
the real problem is the complexity of the compiler implementation
when
we add generic specialization to the story.
Specialization adds a new category of cross-class accesses that are
allowed at the language level but not at the VM level, which would
dramatically increase the need for, and complexity of, accessibility
bridges. For example:
class Foo<any T> {
private T t;
void m(Foo<int> foo) {
int i = foo.t;
}
}
Now we execute:
Foo<long> fl = ...
Foo<int> fi = ...
fl.m(fi)
The spirit of the language rules clearly allow the access from
Foo<long>
to Foo<int>.t -- they are in the "same class". But at the VM level,
Foo<int> and Foo<long> are different classes, so the access from
Foo<long> to a private member of Foo<int> is disallowed.
One reason that this increases the complexity, and not just the
number,
of accessibility bridges is that bridges are (currently) static
methods;
if they represent instance methods, we pass the receiver as the first
argument. For access between inner classes, this is fine, but
when it
comes to access between specializations, this breeds new
complexity --
because the method signature of the accessor needs to be specialized
based on the type parameters of the receiver. This interaction means
the current static-accessor solution would need its own special,
ad-hoc
treatment in specialization, adding to the complexity of
specialization.
More generally, this situation arises in any case where a single
logical
unit of encapsulation at the source level is split into multiple
runtime
classes (inner classes, specialization classes, synthetic helper
classes.) We propose to address this problem more generally, by
providing a mechanism where language compilers can indicate that
multiple runtime classes live in the same unit of encapsulation.
We do
so by (a) adding metadata to classes to indicate which classes
belong in
the same encapsulation unit and (b) relaxing some VM
accessibility rules
to bring them more in alignment with the language level rules.
Overview
--------
Our proposed strategy is to reify the relationship between
classes that
are members of the same _nest_. Nestmate-ness can then be
considered in
access control decisions (JVMS 5.4.4).
Classes that derive from a common source class form a _nest_, and two
classes in the same nest are called _nestmates_. Nestmate-ness is an
equivalence relation (reflexive, symmetric, and transitive.)
Nestmates
of a class C include C's inner classes, synthetic classes
generated as
part of translating C, and specializations thereof.
Since nestmate-ness is an equivalence relation, it forms a partition
over classes, and we can nominate a canonical member for each
partition.
We nominate the "top" (outermost lexically enclosing) class in the
nest as the canonical member; this is the top-level source class from
which all other nestmates derive.
This makes it easy to calculate nestmate-ness for two classes C
and D; C
and D are nestmates if their "top" class is the same.
Example
-------
class Top<any T> {
class A<any U> { }
class B<V> { }
}
<any T> void genericMethod() { }
}
When we compile this, we get:
Top.class // Top
Top$A.class // Inner class Top.A
Top$A$B.class // Inner class Top.A.B
Top$Any.class // Wildcard interface for Top
Top$A$Any.class // Wildcard interface for Top.A
Top$genericMethod.class // Holder class for generic method
The explicit classes Top, Top.A, and Top.A.B, the synthetic $Any
classes, and the synthetic holder class for genericMethod, along with
all of their specializations, form a nest. The top member of
this nest
is Top.
Since nestmates all derive from a common top-level class, they are by
definition in the same package and module. A class can be in
only one
nest at once.
Runtime Representation
----------------------
We represent nestmate-ness with two new attributes -- one in the top
member, which describes all the members of the nest, and one in each
member, which requests access to the nest.
NestTop {
u2 name_index;
u4 length;
u2 child_count;
u2 childClazz[child_count];
}
NestChild {
u2 name_index;
u4 length;
u2 topClazz;
}
If a class has a NestTop attribute, its nest top is itself. If a
class
has a NestChild attribute, its nest top is the class named via
topClazz.
If a class is a specialization of another class, its nest top is the
nest top of the class for which it is a specialization.
When loading a class with a NestChild attribute, the VM can
verify that
the requested nest permits it as a member, and reject the class
if the
child and top do not agree.
The NestTop attribute can enumerate all inner classes and synthetic
classes, but cannot enumerate all specializations thereof. When
creating
a specialization of a class, the VM records the specialization as
being
a member of whatever nest the template class was a member of.
Semantics
---------
The accessibility rules here are strictly additions; nestmate-ness
creates additional accessibility over and above the existing rules.
Informally:
- A class can access the private members of its nestmates;
- A class can access protected members inherited by its nestmates.
This is slightly broader than the language semantics (but still less
broad than what we do today with access bridges.) The static
compiler
can continue to enforce the same rules, and the VM will allow these
accesses without bridges. (We could make the proposal match the
language semantics more closely at the cost of additional complexity,
but its not clear this is worthwhile.)
For private access, we can add the following to 5.4.4:
- A class C may access a private member D.R if C and D are
nestmates.
The rules for protected members are more complicated.
5.4.3.{2,3} first
resolve the true owner of the member, and feed that to 5.4.4; this
process throws away some needed information. We would augment
5.4.3.{2,3} as follows:
- When performing member resolution from class C on member D.R, we
remember both D (the target class) and E (the resolved class) and
make
them both available to 5.4.4.
We then adjust 5.4.4 accordingly, by adding:
- If R is protected, and C and D are nestmates, and E is
accessible to
D, then access is allowed.
Examples
--------
For private fields, we generate access bridges whenever an inner
class
accesses a private member (field or method) of the enclosing
class, or
of another inner class in the same nest.
In the classes below, the accesses shown are all permitted by the
language spec (child to parent, sibling to sibling, sibling to
child of
sibling, etc), and the ones requiring access bridges are noted.
class Foo {
public static Foo aFoo;
public static Inner1 aInner1;
public static Inner1.Inner2 aInner2;
public static Inner3 aInner3;
private int foo;
class Inner1 {
private int inner1;
class Inner2 {
private int inner2;
}
void m() {
int i = aFoo.foo // bridge
+ aInner1.inner1
+ aInner2.inner2 // bridge
+ aInner3.inner3; // bridge
}
}
class Inner3 {
private int inner3;
void m() {
int i = aFoo.foo // bridge
+ aInner1.inner1 // bridge
+ aInner2.inner2 // bridge
+ aInner3.inner3;
}
}
}
For protected members, the situation is more subtle.
/* package p1 */
public class Sup {
protected int pro;
}
/* package p2 */
public class Sub extends p1.Sup {
void test() {
... pro ... //no bridge (invokespecial)
}
class Inner {
void test() {
... sub.pro ... // bridge generated in Sub
}
}
}
Here, the VM rules allow Sub to access protected members of Sup,
but for
accesses from Sub.Inner or Sibling to Sub.pro to succeed, Sub
provides
an access bridge (which effectively makes Sub.pro package-visible
throughout package p2.)
The rules outlined eliminate access bridges in all of these cases.
Interaction with defineAnonymousClass
-------------------------------------
Nestmate-ness also potentially connects nicely with
Unsafe.defineAnonymousClass. The intuitive notion of dAC is,
when you
load anonymous class C with a host class of H, that C is being
"injected
into" H -- access control decisions for C are made using H's
credentials. With a formal notion of nestmateness, we can bring
additional predictability to dAC by saying that C is injected
into H's
nest.
