Fallback to platform classes for Database Reader's isolated ClassLoader

[paul-hristea]

Picking up from an older issue regarding some trouble with class loading from within isolated contexts such as Database Readers (see here).

Basically, in these contexts there is a separate ClassLoader built specifically based on the Resources assigned to those Channels/Connectors. This would allow different versions of different custom drivers to be used in parallel (ClassLoader instantiated here, more info in comments here).

Example steps to reproduce:

1. Download ojdbc17.jar from Oracle
2. Place it somewhere accessible to OIE but outside of server-lib or custom-lib
3. In OIE Settings, create a Resource containing only ojdbc17.jar
4. Create a Channel, assign the Resource to its Dependencies
5. Configure source as Database Reader with settings as follows:

• Driver: Oracle
• URL: jdbc:oracle:thin:@
• Username: username
• Password: password
• SQL: select

6. Attempt to deploy the channel
7. There should be a DeployException caused by NoClassDefFoundError: java/sql/Driver

Note that attempting to deploy the channel without having the Resource assigned should result in a different error (StartException: Failed to start channel fallback_test), which shows that the default server-lib ojdbc class loads successfully.

With the official move to Java 9+, this could now be solved by having the isolated ClassLoader fall back to ClassLoader.getPlatformClassLoader() as its parent, effectively replicating the behavior previously seen with null parent on Java 8.