Panama and Swing/AWT support

[Maurizio Cimadamore]

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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