"Provides" and "with" type relationships

[Alex Buckley]

// Ignore last mail (mail client did a surprising thing)

A 'provides' clause specifies two things: a service interface and a 
service implementation. Using those terms helps to avoid confusion.

A service interface does not have to be an interface; it can be an 
abstract class or even (not recommended) a concrete class.

A service implementation must not be an interface, or an abstract class; 
it must be a concrete class.

Therefore, it's legal (but not recommended) for a concrete class to be 
specified as both service interface and service implementation. It's 
illegal for an interface (or abstract class) to be specified as both 
service interface and service implementation. JCK will be writing tests 
for edge cases like this.

Alex

On 3/15/2016 12:39 PM, Paul Benedict wrote:
> Thanks for your response Alex. If I am understanding you correctly,
> "provides" is "not constrained to be an interface" because it can be "a
> single interface or abstract class". So shouldn't my concrete class for
> "provides" be rejected by the compiler? And is it okay that both types
> were identical?
>
> Cheers,
> Paul
>
> On Tue, Mar 15, 2016 at 2:26 PM, Alex Buckley <alex.buckley at oracle.com
> <mailto:alex.buckley at oracle.com>> wrote:
>
>     The first operand to 'provides' (the "service interface") is not
>     constrained to be an interface by "Modules in the Java Language and
>     JVM". This is because the spec of j.u.ServiceLoader ("a service is
>     represented by a single type, that is, a single interface or
>     abstract class").
>
>     The second operand to 'provides' (the "service implementation") is
>     constrained not to be an interface or an abstract class by "Modules
>     in the Java Language and JVM". This is also because of the spec of
>     j.u.ServiceLoader ("provider classes must have a zero-argument
>     constructor so that they can be instantiated during loading").
>
>     Bear in mind that the JCK team can easily set up abstract test cases
>     like this. What they can't do is check whether YOUR application runs
>     on JDK-9-with-Jigsaw, or whether arbitrary JARs on YOUR classpath
>     work as automatic modules.
>
>     Alex
>
>
>     On 3/15/2016 12:07 PM, Paul Benedict wrote:
>
>         module z {
>               exports z;
>               provides z.Main with z.Main;
>         }
>
>         The SOTM says "Service-provider declarations can be further
>         interpreted to
>         ensure that providers (e.g., com.mysql.jdbc.Driver) actually do
>         implement
>         their declared service interfaces" (section 4, para. 8).
>
>         I see javac checking that they are related types, but javac is
>         not checking
>         that "provides" is an interface type. That is what I was
>         expecting based on
>         the reading material.
>
>         The other unexpected outcome was that provides/with allows the
>         identical
>         type. I don't know if that's intended, but please advise.
>
>         PS: I did go through the open tickets this time (thanks Alan)
>         and do not
>         see any similar reports. If I missed it, I apologize; just
>         trying not to
>         waste your time by reporting a duplicate.
>
>         Cheers,
>         Paul
>
>