Re: Connector/J "Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost."

[Josh Jacobs]

I was seeing the same problem reported in my java application while using C3p0. I resolved the problem by removing this line from my my.cnf

#wait_timeout=30

I think the problem is the result of a mismatch of application connections properties and db configurations. I think that the database had a short wait_timeout than the application connection properties. In my case I had c3p0 set test connections every 60 seconds (<property name="hibernate.c3p0.idle_test_period">60</property>), while at the same time the db was expiring connections every 30 seconds. The error occurred because the database closes the connections while the application thinks they should still be guaranteed to be alive.

I will now try increasing the wait_timeout to a value of 90 to see if the problem arises again. That way I won't have to rely on a default in MySQL which could change.

I hope this helps others since this is a particularly excruciating bug to resolve!