Can multiple threads see writes on a direct mapped ByteBuffer in Java?

Question

I\'m working on something that uses ByteBuffers built from memory-mapped files (via FileChannel.map()) as well as in-memory direct ByteBuffers. I am trying to understand th

Answer 1

I would assume that direct memory provides the same guarantees or lack of them as heap memory. If you modify a ByteBuffer which shares an underlying array or direct memory address, a second ByteBuffer is another thread can see the changes, but is not guaranteed to do so.

I suspect even if you use synchronized or volatile, it is still not guaranteed to work, however it may well do so depending on the platform.

A simple way to change data between threads is to use an Exchanger

Based on the example,

class FillAndEmpty {
   final Exchanger exchanger = new Exchanger();
   ByteBuffer initialEmptyBuffer = ... a made-up type
   ByteBuffer initialFullBuffer = ...

   class FillingLoop implements Runnable {
     public void run() {
       ByteBuffer currentBuffer = initialEmptyBuffer;
       try {
         while (currentBuffer != null) {
           addToBuffer(currentBuffer);
           if (currentBuffer.remaining() == 0)
             currentBuffer = exchanger.exchange(currentBuffer);
         }
       } catch (InterruptedException ex) { ... handle ... }
     }
   }

   class EmptyingLoop implements Runnable {
     public void run() {
       ByteBuffer currentBuffer = initialFullBuffer;
       try {
         while (currentBuffer != null) {
           takeFromBuffer(currentBuffer);
           if (currentBuffer.remaining() == 0)
             currentBuffer = exchanger.exchange(currentBuffer);
         }
       } catch (InterruptedException ex) { ... handle ...}
     }
   }

   void start() {
     new Thread(new FillingLoop()).start();
     new Thread(new EmptyingLoop()).start();
   }
 }