JEP 370 - text example leads to exception

[Paul Sandoz]

Thanks for pointing out the inconsistencies.

I modified the JEP with updated code snippets that compile against the latest API in JDK 14 [*].

The handle created “withStride” requires an additional coordinate the is an offset from the base address.

You may find Maurizio’s recent talk at Fosdem 2020 helpful and informative:

  https://fosdem.org/2020/schedule/event/bytebuffers/ <https://fosdem.org/2020/schedule/event/bytebuffers/>

Hth,
Paul.

[*] I wish there was a way to automate the compile and test of such snippets without duplication.


> On Feb 7, 2020, at 7:23 PM, Chris T <tech.meshter at gmail.com> wrote:
> 
> I tried to build an example on top of this code snippet (from the JEP text):
> 
> VarHandle intHandle = MemoryHandles.varHandle(int.class);
> VarHandle intElemHandle = MemoryHandles.withStride(intHandle, 4);
> 
> try (MemorySegment segment = MemorySegment.allocateNative(100)) {
>   MemoryAddress base = segment.baseAddress();
>   for (int i = 0 ; i < 25 ; i++) {
>        intElemHandle.set(base, (long)i);
>   }
> }
> 
> The first issue was that the API for the first line need to get the
> ByteOrder parameter (that's not a big deal). I ended up with this:
> 
>    VarHandle intHandle = MemoryHandles.varHandle(int.class, order);
>    VarHandle intElemHandle = MemoryHandles.withStride(intHandle, 4);
> 
>    try (MemorySegment segment = MemorySegment.allocateNative(100)) {
>      MemoryAddress base = segment.baseAddress();
>      for (int i = 0; i < 25; i++) {
>        // this is the line where the app crashes:
>        intElemHandle.set(base, (long) i);
>      }
>    }
> 
> The issue that I have is that the code crashes with:
> java.lang.invoke.WrongMethodTypeException: cannot convert
> MethodHandle(VarHandle,MemoryAddressProxy,long,int)void to
> (VarHandle,MemoryAddress,long)void
> at
> java.base/java.lang.invoke.MethodHandle.asTypeUncached(MethodHandle.java:880)
> at java.base/java.lang.invoke.MethodHandle.asType(MethodHandle.java:865)
> at
> java.base/java.lang.invoke.VarHandleGuards.guard_LJ_V(VarHandleGuards.java:184)
> at
> com.github.kbnt.java14.fma.ForeignMemoryAccessExamples.exampleXXStrides(ForeignMemoryAccessExamples.java:65)
> at
> com.github.kbnt.java14.fma.ForeignMemoryAccessExamples.main(ForeignMemoryAccessExamples.java:25)
> 
> Any idea why this happens (and more importantly how the code can be
> changed)?
> 
> Thanks!
>  Chris T