Thank you, pgrimard. Sometime after posting, I learned what you've said. The problem is failure to set the "prompt" property in the driver to "false."
Ideally, I'd be able to set connectionProperties from the application context thus:
Unfortunately, the version of Apache Commons I'm using doesn't have a setConnectionProperties method. Fortunately, I'm already subclassing BasicDataSource, so I just hard-coded a call to addConnectionProperty.
<property name="connectionProperties" value="prompt=false" />