Tomcat 5.x - MySQL Bug: "Cannot get a connection, pool exhausted"

A web application has to explicetely close ResultSet's, Statement's, and Connection's. Failure of a web application to close these resources can result in them never being available again for reuse, a db connection pool "leak". This can eventually result in your web application db connections failing if there are no more available connections.

There is a solution to this problem. The Jakarta-Commons DBCP can be configured to track and recover these abandoned dB connections. Not only can it recover them, but also generate a stack trace for the code which opened these resources and never closed them.

To configure a DBCP DataSource so that abandoned dB connections are removed and recycled add the following attribute to the Resource configuration for your DBCP DataSource:

removeAbandoned="true"

When available db connections run low DBCP will recover and recyle any abandoned dB connections it finds. The default is false.

Use the removeAbandonedTimeout attribute to set the number of seconds a dB connection has been idle before it is considered abandoned.

removeAbandonedTimeout="60

The default timeout for removing abandoned connections is 300 seconds.

The logAbandoned attribute can be set to true if you want DBCP to log a stack trace of the code which abandoned the dB connection resources.

logAbandoned="true"

The default is false.

Abstract taken from
http://tomcat.apache.org/tomcat-5.5-doc/jndi-datasource-examples-howto

Additional information:
http://dev.mysql.com/doc/refman/4.1/en/connector-j-usagenotes-j2ee.html#connector-j-usagenotes-tomcat