timeout for a transaction

I have a connection opened, and begin a transaction.  If tomcat or web
server dies for no reasons, I found that in the backend, there is a
postgresql  server process waiting for the transaction to complete.  It
blocks all my database calls because of this waiting process.  I have to
manually kill that waiting process to unlock the database.

Is there any way of setting a timeout for the transaction?  How come the
postgresql  server doesn't detect the client dies, rollback and release
the resources?

I want the system be able to do it automatically.   Any suggestions?

Thanks,
Percy