Clinker | Gene Fusion Visualiser

I thought that one of the major advantages of Project Panama would be that native code could access on-heap memory, to avoid having to copy.

Actually despite the major advance in usability coming with project Panama, this won't be possible, for multiple reasons.

• GC moves things around in the Java heap memory, hence the address of any object (including tables) may/will change over time. However the native code is given a pointer to a memory address which of course won't be updated after a GC cycle (not even mentioning accessing that memory in the middle of a cycle).
• JNI had APIs to actually prevent GC from happening while in the middle of native code through Get*Critical sections. Unfortunately preventing the GC may have significant impact on application performance.

In fact Project Panama is exactly trying to avoid blocking the GC. This is why there's a clear separation of the accessed memory and why it is necessary to copy to/from native memory.

This shouldn't be much of an issue, unless this is hot code path (ie it's called very very often), or the code deals with very big data. In such case the code may want to do most of the work off-heap. If the data is in a file, maybe access that file from native code, or use a panama memory mapped file.