写文件 Region

public class DefaultFileRegion extends AbstractReferenceCounted implements FileRegion {   
 private FileChannel file;
  public long transferTo(WritableByteChannel target, long position) throws IOException {      
  long count = this.count - position;       
 if (count >= 0L && position >= 0L) {            
if (count == 0L) {               
 return 0L;          
  } else if (this.refCnt() == 0) {               
 throw new IllegalReferenceCountException(0);      
      } else {            
    this.open();            
    long written = this.file.transferTo(this.position + position, count, target);              
  if (written > 0L) {           
         this.transferred += written;           
     } else if (written == 0L) {            
        validate(this, position);      
          }
              
  return written;          
  }        } else {      
     throw new IllegalArgumentException("position out of range: " + position + " (expected: 0 - " + (this.count - 1L) + ')');        }    }        ...}