filetype.vim

autocmd BufReadPost,BufNewFile *.class call s:detect_javaclass()

function! s:detect_javaclass()
  let file = expand('<afile>')
  if !filereadable(file)
    return
  endif
  let line = readfile(file, 'b', 1)[0]
  if 4 <= strlen(line) && line[:3] == "\xCA\xFE\xBA\xBE"
    setlocal filetype=javaclass
  endif
endfunction

javaclass.vim

if executable('jad')
  silent %!jad -t -clear -ff -nonlb -8 -p "%"
  runtime! syntax/java.vim
  setlocal readonly
endif