Here's a plugin to automatically decompile Java .class files as they're read in. Tweak the javap flags for what you want to see. I didn't post this as a script because it's too simple and it's really more useful for demonstrating how to read decompilable files (or other binary files that can be converted to text).

function s:ReadClass(dir, classname)
  execute "cd " . a:dir
  execute "0read !javap -c " . a:classname
  1
  setlocal readonly
  setlocal nomodified
endfunction

autocmd BufReadCmd *.class
  \ call <SID>ReadClass(expand("<afile>:p:h"), expand("<afile>:t:r"))

CommentsEdit

It doesn't work when package is used in java code.

Foo.java
package com.foo.test;
class Foo
{

}

The question is how to save package information?

For jad:

au BufReadCmd *.class setl readonly nomodified | %!jad -p <afile>