java.sql.DriverManager supports the service provider mechanism. This feature is that all the available JDBC drivers that announce themselves by providing a META-INF/services/java.sql.Driver file are automatically discovered, loaded and registered, relieving you from the need to load the database driver explicitly before you create a JDBC connection. However, the implementation is fundamentally broken in all Java versions for a servlet container environment. The problem is that java.sql.DriverManager will scan for the drivers only once.
The JRE Memory Leak Prevention Listener that is included with Apache Tomcat solves this by triggering the drivers scan during Tomcat startup. This is enabled by default. It means that only libraries visible to the listener such as the ones in $CATALINA_BASE/lib will be scanned for database drivers. If you are considering disabling this feature, note that the scan would be triggered by the first web application that is using JDBC, leading to failures when this web application is reloaded and for other web applications that rely on this feature.
Thus, the web applications that have database drivers in their WEB-INF/lib directory cannot rely on the service provider mechanism and should register the drivers explicitly.
The list of drivers in java.sql.DriverManager is also a known source of memory leaks. Any Drivers registered by a web application must be deregistered when the web application stops. Tomcat will attempt to automatically discover and deregister any JDBC drivers loaded by the web application class loader when the web application stops. However, it is expected that applications do this for themselves via a ServletContextListener.
