Field

   protected boolean isHeadersCommitted;

should be declared as volatile to ensure that

   public void commit()
   {
      if (isHeadersCommitted) return;
      isHeadersCommitted = true;
      headers.commit();
   }

works as intended also if called by different threads.

There are JDK classes which decorate/adapt OutputStreams which invokes OutputStream.close() in the finalize() method (e.g. javax.imageio.stream.MemoryCacheImageOutputStream ). Such classes cannot be (easily) used by Entity Providers without such fix.