Servlet service() method

Question

Why we open the database connection in the servlet�s doPost() method, instead of the init() method, although opening database connection could be considered a one-time activity ?
But what happens if you declare the connection as an instance variable, and open it in the init() method, and close it in the destroy() method?
Can somebody give me more detail examples about this.
What�s the meaning of :
The servlet instance can throw a UnavailableException to indicate that it is temporarily or permananetly available, and about temporary or permanent failures?
Thank You very much.

Answer 1

A single connection can only be used by one thread at once, so declaring only one connection will make your application very unfriendly when used by multiple users.
But you are on the right track though, you should not be opening and closing connections for each statement you execute. What you need to look into is connection pooling, and that can be handled by most popular java webservers. If you are using tomcat for example, you can look here on how to set it up and use it:
http://tomcat.apache.org/tomcat-5.5-doc/jndi-datasource-examples-howto.html
The only thing I have to add to that document is that you need to put your database driver jar in common/lib for this to work.