Hello,
You can create a jar directly from eclipse following these steps:
http://www.fsl.cs.sunysb.edu/~dquigley/cse219/index.php?it=eclipse&tt=jar&pf=y
Or you can use the command line:
http://neptune.netcomp.monash.edu.au/JavaHelp/howto/jar.htm
If you use ant to build your projects there is a jar task that let you to do so:
http://ant.apache.org/manual/CoreTasks/jar.html
HTH