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In a servlet where should you create a JDBC connection?

Posted on 2002-07-30
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I've got a couple of textbooks about JDBC and servlets. They differ in where they create a connection.
One holds the connection as an instance variable; creates the connection in the init method; closes it in the doGet method and also closes it in the destroy method and in a finally clause.
The other author creates and closes the connection inside the try clause, no instance variable.
What are the advantages and disadvantages of each method?

Colin
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Question by:cjmackenzie
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twh270 earned 50 total points
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A database connection is "expensive" to set up and tear down.  Most people use a connection pool to cache connections.  There are a lot of them out there, everything from free stuff to high-end expensive kits.  The servlet acquires a connection from the pool via something like "connectionPool.acquireConnection()" and releases it through e.g. "connectionPool.releaseConnection()".

To directly answer your question: Setting up the connection in init() means you create a single connection, only once, for the lifetime of each servlet instance.  This means you don't have the overhead of creating a database connection each time the servlet services a request, but it means only one request at a time can use the connection, so you'll need to synchronize access to it.  Creating a connection in try/catch means every request creates a new connection.  This is expensive.  However, it allows you to service N requests simultaneously.  (N is usually limited by the number of simultaneous connections your database allows.)

Both of these problems are mitigated through a connection pool (though not necessarily eliminated).

Keep in mind: textbook example code shows you how to write code that works.  It isn't intended to demonstrate good design.

-Thomas
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by:cjmackenzie
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Thanks Thomas, I was finding it hard to get my head round pooled connections and threads and your answer has helped clarify my options.
Within my application I have a static class that takes a connection and a query as parameters, then puts the results into a List of Javabeans for use in a JSP.
So I think I could very quickly improve the performance by creating the connection in the init method, then make the static method synchronized.
I am up against a deadline and I have a small number of potential users users, anyway (10 at the most). I don't think I've got time to look at pooled connections although that is obviously better still.


Colin
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by:twh270
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Glad to help.

One thing I forgot to mention is that if the connection is unused for a long time it may timeout -- get closed by the database.  You can call Connection.isClosed() before using the connection.

-Thomas
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