authenticating your app (code has expired error)

Chan, You're getting this error because each "authorization code" that you exchange for the access token is short-lived (just a few minutes) and can only be used once, so you can only use each one once, soon after retrieving it. 

After that, you can just store and re-use the resulting access token that your app receives from the app authorization flow for each account. These access tokens don't expire automatically (though they can be manually revoked) so your app can re-use them without manual user intervention.

And yes, you can set this up to work without the manual/copy paste. The tutorial uses the flow without a redirect URI, but instead, for web apps, you'll want to use DbxWebAuth:

https://dropbox.github.io/dropbox-sdk-java/api-docs/v1.7.x/com/dropbox/core/DbxWebAuth.html