C:>Help exit
Quits the CMD.EXE program (command interpreter) or the current batch
script.

EXIT [/B] [exitCode]

  /B          specifies to exit the current batch script instead of
              CMD.EXE.  If executed from outside a batch script, it
              will quit CMD.EXE

  exitCode    specifies a numeric number.  if /B is specified, sets
              ERRORLEVEL that number.  If quitting CMD.EXE, sets the process
              exit code with that number.
              
在一个batch 里面，如果用Runtime.exec(a.bat), 那么必须用 exit exitCode, 而不能用exit /b exitCode.
否则java得不到a.bat 的return code