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From James Grahn <grahn...@gmail.com>
Subject Re: correctness, generics, and spaces
Date Wed, 01 Jun 2011 17:31:37 GMT
On Tue, May 31, 2011 at 10:51 PM, Shay Hassidim <shay@gigaspaces.com> wrote:
> James,
> In case this helps - After similar debates over the years we at GigaSpaces end up with:
> <T> T read(T template)
> <T> T take(T template)
> write(T entry)

Quite interesting.   This is exists presently in the experimental
generics branch, except the type is <T extends Entry>.

I take this to mean you did not address the corner case that worried
many on this list?   Does it simply prove to not be an issue in your
experience?

> See more here:
> http://www.gigaspaces.com/docs/JavaDoc8.0/org/openspaces/core/GigaSpace.html
>
> Please note we support POJO as well with our implementation as space classes and not
just Classes implementing the Entry interface.
> This works very well for us.

Entry, of course, is just a marker interface.   When you say "POJO",
though you mean Java objects without any requirements upon them?
(e.g. no requirement of a public, no-arg constructor, no need for
public fields, etc.)

The inclusion of asynchronous calls using Futures is also quite
noteworthy, as is your treatment of "snapshot".   I was about to start
poking the mailing list about snapshot myself, albeit with a different
proposal than yours.

I am now wondering: is the Gigaspaces code available?   Under what
license?   And is anyone there interested in contributing?

> Shay Hassidim
> Deputy CTO

By the way, welcome!   If nothing else, your feedback in discussions
will prove valuable, I'm sure.

james

> -----Original Message-----
> From: James Grahn [mailto:grahnian@gmail.com]
> Sent: Tuesday, May 31, 2011 7:05 PM
> To: dev@river.apache.org
> Subject: correctness, generics, and spaces
>
> Hello all,
> I've been doing some pondering on the question of generics in spaces again.
>
> ==Background==
>
> To refresh those who've forgotten prior discussion, the proposal was to replace methods
like this:
> Entry read(Entry)
>
> With this:
> <T extends Entry> T read(T)
>
> This change was desired because:
> 1) it reduces boilerplate code (mandatory casting of objects returned from space)
> 2) it is a more precise statement of the contract by which these methods operate: i.e.
the return type _should be_ the same as the type of the template.
>
> Some opposed this change because they felt any use of generics would be seen an implicit
guarantee that there are no corner cases with
> generics.   Meanwhile, it was hoped, a lack of generics in the
> interface would be seen as an implicit warning that no guarantees were
> made about interactions with generics.   Regardless of the API, the
> current specification does contain corner cases wherein the behavior is potentially surprising.
>
> Specifically, this corner case was brought up: If an Entry implementation determines
the type of one of its members through a type variable, the returned generic type from a read/take
operation would not be guaranteed to be correct if that variable were wildcarded using "null"
in a javaspace template search.
>
> Given:
> class Box<A extends Serializable> implements Entry{
>   public A member;
> }
>
> With the generic implementation of spaces:
> Box<Integer> value = read(new Box<Integer>(5)); //correct Box<Integer>
value2 = read(new Box<Integer>(null)); //potentially incorrect, inconsistent failures.
> Box<Object> value3 = read(new Box<Object>(null)); //correct, but unhelpful.
>
> With the non-generic implementation of spaces:
> Box<Integer> value = (Box<Integer>) read(new Box<Integer>(5)); //correct
Box<Integer> value2 = (Box<Integer>) read(new Box<Integer>(null)); //potentially
incorrect, inconsistent failures.
> Box<?> value3 = (Box<?>) read(new Box<Integer>(null)); //correct, but
unhelpful.
>
> Note that regardless of whether the implementation includes generic methods, both cases
require careful thought in the usage of generics to avoid errors. Because of this, a recommendation
was made that Entry objects should probably not have a parameterized types in general.
>
> == Proposal ==
>
> What I finally realized: we can actually require Entries to not contain any unassigned
type parameters.
>
> Because it is a bad practice regardless, why not do that?
>
> Costs:
> 1) People who are familiar with generics and would always properly handle corner cases
will not be able to do exactly as they please; they would need to provide a fully specified
wrapper class for what would otherwise be their Entry class.
> 2) An additional runtime check of an Entry's class would slow the execution time of Javaspace
methods (slightly).
>
> Benefits:
> 1) If this clears the opposition for using generics in the Javaspace interface, see the
above mentioned benefits.
> 2) Prevents violation of the principle of least surprise in using generics with space
(regardless of implementation).
>
> == Implementation Detail ==
>
> At the interface level, we would need to add a line to the documentation of Entry, stating
that any class implementing Entry may not include type parameters. Entry implementations already
have several requirements which can't be enforced at compile time, so adding another is not
the worst thing in the world.
>
> At the implementation level, we'd add enforcement in outrigger's EntryRep.ensureValidClass,
checking to confirm that the Entry that Outrigger is given has no type parameters via reflection.
>
> Shouldn't take long to edit the existing generics branch to do this.
>
> == Questions ==
>
> So here are my questions:
> 1) Is there a problem transforming this from a bad practice to a forbidden practice?
> 2) Does this address concerns with the generics? Are there any corner cases this will
not cover?
> 3) Any other concerns? Anything I'm not seeing?
>
> jamesG
>
>
>

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