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.
???????????? ?
在一個batch 里面，如果用Runtime.exec(a.bat), 那么必須用 exit exitCode, 而不能用exit /b exitCode.
否則java得不到a.bat 的return code