Troubleshooting C3P0 Connection Pool

While using C3P0 as your connection pool (either with Spring or otherwise), sometimes you might get an exception like this:

** BEGIN NESTED EXCEPTION **
com.mysql.jdbc.CommunicationsException
MESSAGE: Communications link failure due to underlying exception:

** BEGIN NESTED EXCEPTION **
java.io.EOFException

STACKTRACE:
java.io.EOFException at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:1956)

This is quite a common problem which shows up when C3P0 is configured incorrectly or by using the default settings. The problem occurs when C3P0 is trying to use a connection already established by the pool but was closed by the database server. When C3P0 tries to use this already-closed connection, the error is thrown.
In c3p0.properties, try experimenting with the following values:

c3p0.preferredTestQuery=SELECT 1 
c3p0.testConnectionOnCheckout=true

This will force C3P0 to test each connection before passing it to the application.
If you are configuring using spring/hibernate, do note that *only* these properties are configurable in the applicationContext.xml file:

hibernate.c3p0.acquire_increment
hibernate.c3p0.idle_test_period
hibernate.c3p0.timeout
hibernate.c3p0.max_size
hibernate.c3p0.max_statements
hibernate.c3p0.min_size
hibernate.c3p0.validate

The other properties should go to the c3p0.properties file located in the webapp class root folder.

If you get an exception such as:

java.sql.SQLException: com.mchange.v2.resourcepool.ResourcePoolException: Attempted to use a closed or broken resource pool 

Ensure that the timeout value configured for c3p0 does not not exceed the database server's wait timeout.