Calling a java program thru a trigger or a function in postgresql

Here's a high-level overview of what I want to do:

1. A record in a postgresql database is changed.
2. If the change causes the data record to meet certain conditions based on
Boolean logic, a database trigger will invoke a Java UDF.
3. This Java UDF uses the JavaMail API to send an e-mail to a specified
recipient.

The javamailing program is written separately and is running fine.

Can any one give me the following details :

1. Where to place the java or class file for access.
2. plz give me the exact create function statements.
3. plz give me the exact create trigger statments.

O R
Plz give me the code to satisfy my requirement.

Thanks in advance for ur help

venky