if a java app has to read that data from direct buffer, does it have to copy to a byte[] ? Or can it directly manipulate the ByteBuffer? You can read/write individual bytes from a direct byte buffer, or copy chunks to / from regular old byte arrays. What is the need for this when we have byte[] ? Even when it's just wrapping a standard byte[], ByteBuffer provides some convenience methods which can be helpful (e.g. 'get' and 'put' which keep track of the current position within the buffer, and methods which allow reading and writing of other multi-byte data types to/from the buffer). It also fits into a larger collection of classes used for IO. Having an interface like this allows those classes to interoperate together, and different buffer implementations to be switched out which have different underlying storage. Something more concrete and specific like byte[] cannot be used as flexibly. More on reddit.com

You mean off managed heap. Because Marshal still allocates from the heap, just the unmanaged one. stackalloc will allocate space on the stack. Also, a value type. I think you're question is unclear. More on reddit.com

Knowing nothing about the problem, I’d probably expose a rust function and call it from Java using FFI. https://docs.oracle.com/en/java/javase/21/core/calling-c-library-function-foreign-function-and-memory-api.html#GUID-E7255CE9-5A95-437C-B37A-276B6C9B5F4D https://doc.rust-lang.org/nomicon/ffi.html 2. You don’t, it’ll be garbage collected. 3. You don’t. If rust needs to keep the data post function, copy it. More on reddit.com

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