Michael T. Nygard warns in “Release it”  about the lack of timeouts caused by misconfigured database drivers and connection pools. He argues that if a connection pool gets exhausted when none of its calls return than all others threads get blocked as soon as they require a connection too. His advice is:
Scrutinize Resource Pools
Safe resource pools always limit the time a thread can wait to check out a resource.
(Michael T. Nygard, “Release it”, P. 50)
So I wondered, whether the connection pool HikariCP as well as the JDBC-driver of IBM DB2 and MySQL already ships with default timeouts (and how to adjust them).