code: public final MappedByteBuffer force() {
          checkMapped();
          if ((address != 0) && (capacity() != 0)) {
              long offset = mappingOffset();
              force0(fd, mappingAddress(offset), mappingLength(offset));
          }
          return this;
      }
docs: Forces any changes made to this buffer's content to be written to the
  storage device containing the mapped file.
  
  <p> If the file mapped into this buffer resides on a local storage
  device then when this method returns it is guaranteed that all changes
  made to the buffer since it was created, or since this method was last
  invoked, will have been written to that device.
  
  <p> If the file does not reside on a local device then no such guarantee
  is made.
  
  <p> If this buffer was not mapped in read/write mode ({@link
  java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
  method has no effect. </p>
  
  @return This buffer