On Tue, 15 Oct 2002, Paul French wrote:
> Date: Tue, 15 Oct 2002 12:30:19 +0100
> From: Paul French <[EMAIL PROTECTED]>
> Reply-To: Jakarta Commons Users List <[EMAIL PROTECTED]>
> To: "'[EMAIL PROTECTED]'" <[EMAIL PROTECTED]>
> Subject: dbcp question
>
> When a database connection pool is created using the usual lookup code
>
> Context initContext = new InitialContext();
> Context envContext = (Context)initContext.lookup("java:/comp/env");
> ds = (DataSource)envContext.lookup("jdbc/SID");
>
> Where should this be executed?
>
> Should it only be called once per webapp to obtain a pool of connections?
>
If you are taking the JNDI lookup approach, you will generally call the
code described above, plus a little bit more, every time you need a
Connection object to interact with the database, such as in a DAO object
that wants to execute a query, or persist its current state:
InitialContext initContext = new InitialContext();
Context envContext = (Context) initContext.lookup("java:comp/env");
DataSource ds = (DataSource) envContext.lookup("jdbc/SID");
Connection conn = ds.getConnection();
... perform your database operations ...
conn.close();
When used in a connection pool environment, the close() method does not
actually close the physical connection -- it simply returns the connection
to the pool.
You want to make the call every time, rather than caching and saving the
data source once, for at least the following reasons:
* If you call it only once, you have to pass a DataSource
or Connection parameter in to every method that needs one.
Using the recommended technique, any code that needs access
to the database is responsible for getting its own connection.
* In a high-availability environment, the server administrator
might have replaced the connection pool on the fly (for example,
if the database had to be restarted).
There's some examples of this in the Tomcat docs:
http://jakarta.apache.org/tomcat/tomcat-4.1-doc/jndi-resources-howto.html
http://jakarta.apache.org/tomcat/tomcat-4.1-doc/jndi-datasource-examples-howto.html
> If it is called repeatedly does tomcat simply return the same connection
> pool instance?
>
Tomcat returns the same DataSource instance, but other servers might not.
> Are the calls to getConnection() thread safe?
Yes, but the Connection object you get back isn't -- it should be used
only in the context of a single request processing thread. You also need
to make absolutely sure that you return the connection to the pool, no
matter what sort of exceptions happen. Using a "finally" block as shown
in the docs above is a good technique to manage this.
>
> You can see my lack of understanding is coming from the fact that I want to
> share the connection pool across multiple servlets. I also need to know if I
> have to wrap the getConnection() call in a synchronized bloke to avoid
> multiple threading issues.
Nope -- you'll be fine on this issue.
>
> If you can provide some insight that would be much appreciated
>
> Paul
>
>
Craig
--
To unsubscribe, e-mail: <mailto:[EMAIL PROTECTED]>
For additional commands, e-mail: <mailto:[EMAIL PROTECTED]>
