Panama and Swing/AWT support

[Clayton Walker]

> Are you worried about the cost of creating global references? (Trying to
understand more about the use case)

Not worried, just wondering. I'm certainly not a JNI expert. Thanks for the
clarification.

> So exposing a method which returns a segment (or a ByteBuffer) would look
a bit odd, I think.

Sounds good.

On Wed, May 24, 2023 at 3:08 PM Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

>
> On 24/05/2023 20:36, Clayton Walker wrote:
>
> Hi Maurizio,
> thanks for the detailed response!
>
> I believe I understood this for the most part, but a quick question
> related to the jni references. Do we need to use NewGlobalRef so that we
> can exit the native jni function and not have their jni reference freed?
>
> Yes, the global ref is required given that we go in and out of JNI
> functions. Are you worried about the cost of creating global references?
> (Trying to understand more about the use case)
>
>
> Also would it make more sense to go this route than to expose a new
> function called (e.g.) getDrawingSurface method to the awt Component class
> which returns a MemorySegment that we can free? Presumably to avoid
> cluttering up awt/swing with additional native function calls, or perhaps
> avoid leaking implementation details to the java standard library?
>
> I'm not an expert in AWT/Swing - but it would seem that, at a glance, the
> Component class doesn't have much in terms of low-level capabilities. So
> exposing a method which returns a segment (or a ByteBuffer) would look a
> bit odd, I think.
>
> Cheers
> Maurizio
>
>
> On Wed, May 24, 2023 at 12:22 PM Maurizio Cimadamore <
> maurizio.cimadamore at oracle.com> wrote:
>
>> Hi Clayton
>> interacting with JNI code has few issues:
>>
>> 1. you need a JNIEnv* segment
>> 2. you need to be able to wrap Java objects as JNI references (so that
>> they can be passed as pointers to native calls)
>> 3. expose some way to map method names into JNI mangled names (so that we
>> can look those up using a symbol lookup)
>>
>> This is something on our radar and we will like to address this use case
>> at some point (but it is possible this might be addressed _after_ the FFM
>> API is finalized). Two possible approaches are:
>>
>> * a set of static helper functions which allow to obtain JNIEnv segment,
>> as well as create, dereference and destroy JNI refs. We might even expose
>> JNI functions here (think of a JNIUtils class)
>> * a true JNILinker <: Linker - this might allow a tighter coupling with
>> JNI functions (as the JNIEnv will be passed automatically)
>>
>> I think the former approach is quite attractive in terms of bang for
>> bucks. It doesn't add a lot of complexity to the API, and adds the minimum
>> functionality that is required in order to allow the native linker to deal
>> with JNI functions as well (even though developers will need to take some
>> extra care when doing so).
>>
>> In the meantime, I think it might be possible to define a small JNI
>> library which takes care of the missing functionalities - e.g. it could
>> return/accept JNI references as "jlong" (an hack, I know) so, something
>> like this:
>>
>> native long makeGlobalRef(Object o) // call NewGlobalRef, cast result to
>> jlong and return
>> native void destroyGlobalRef(long ref) // cast to jobject, then call
>> DeleteGlobalRef
>> native Object readGlobalRef(long ref) // cast to jobject, return
>> native long getEnv(); // cast env parameter to jlong and return
>>
>> I've tried something along those lines and it seems to work as expected.
>> But I agree that it would be convenient if this "just worked" out of the
>> box.
>>
>> Cheers
>> Maurizio
>>
>>
>>
>> On 24/05/2023 18:28, Clayton Walker wrote:
>>
>> I am working on a project with one of the goals being able to use a swing
>> component as a render target. From the documentation here
>> <https://docs.oracle.com/javase/8/docs/technotes/guides/awt/AWT_Native_Interface.html> I
>> assume we need jni in order to access the native PlatformInfo struct. My
>> question is the same as asked in this stackoverflow question
>> <https://urldefense.com/v3/__https://stackoverflow.com/questions/75620948/how-do-i-get-the-hwnd-of-a-canvas-using-panama__;!!ACWV5N9M2RV99hQ!Pq0tFxFIxoDHgX_vezwj24Kuk4wsYvPXLMS09T7SYv_UyIpO7nN_Sc1jmNWMzwSORL6P3_hbxJIXfK4h2FNcd_LAkn4CRaOc8w$>,
>> is it possible to use panama to get the HWND of a swing window?
>>
>>
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