code: public synchronized DownloadResponse getJarDiffEntry( ResourceCatalog catalog, DownloadRequest dreq,
                                                            JnlpResource res )
      {
          if ( dreq.getCurrentVersionId() == null )
          {
              return null;
          }
  
          // check whether the request is from javaws 1.0/1.0.1
          // do not generate minimal jardiff if it is from 1.0/1.0.1
          boolean doJarDiffWorkAround = isJavawsVersion( dreq, "1.0*" );
  
          // First do a lookup to find a match
          JarDiffKey key =
                  new JarDiffKey( res.getName(), dreq.getCurrentVersionId(), res.getReturnVersionId(), !doJarDiffWorkAround );
  
          JarDiffEntry entry = (JarDiffEntry) _jarDiffEntries.get( key );
          // If entry is not found, then the querty has not been made.
          if ( entry == null )
          {
              if ( _log.isInformationalLevel() )
              {
                  _log.addInformational( "servlet.log.info.jardiff.gen", res.getName(), dreq.getCurrentVersionId(),
                                         res.getReturnVersionId() );
              }
              File f = generateJarDiff( catalog, dreq, res, doJarDiffWorkAround );
              if ( f == null )
              {
                  _log.addWarning( "servlet.log.warning.jardiff.failed", res.getName(), dreq.getCurrentVersionId(),
                                   res.getReturnVersionId() );
              }
              // Store entry in table
              entry = new JarDiffEntry( f );
              _jarDiffEntries.put( key, entry );
          }
  
          // Check for no JarDiff to return
          if ( entry.getJarDiffFile() == null )
          {
              return null;
          }
          else
          {
              return DownloadResponse.getFileDownloadResponse( entry.getJarDiffFile(), _jarDiffMimeType,
                                                               entry.getJarDiffFile().lastModified(),
                                                               res.getReturnVersionId() );
          }
      }
docs: Returns a JarDiff for the given request
  
  @param catalog TODO
  @param dreq    TODO
  @param res     TODO
  @return TODO