Add option to allow Spring Batch custom isolation levels

[tiborsulyan]

Fixes #28802

Create spring.batch.jdbc.isolation-level-for-create property with the following behavior:

• Default value is null
• If not set, Spring Batch with JPA will use ISOLATION_DEFAULT level, and if no JPA is detected, then the Spring Batch default (ISOLATION_SERIALIZABLE) is used. This is the current behavior
• If the property is set, it will be passed to Spring Batch and used as isolation level for create operations.

[philwebb]

Generally we prefer to keep member variables private. Is there a reason you changed these?

[tiborsulyan]

From the design of this class it looks like it it's open for extension (see protected methods). However, subclasses cannot access the members those protected methods operate on, even though it's passed to their constructor. I'd suggest either extend the visibility to protected or provide protected getters to these. Since the members are final I think it's safe and would improve extensibility. What do you think?

For this concrete member, the new JpaBatchConfigurer implementation uses the properties too in determineIsolationLevel

[wilkinsona]

Given that the fields don't need to be made protected to implement the configurable isolation level, I think they should be considered via a separate issue please. Even properties could remain private as JpaBatchConfigurer could capture what it needs in its constructor. This would minimise the changes to the public API.

[tiborsulyan]

Sorry for the delay. Sure, I'll roll this change back and will consider a separate PR for this.

[snicoll]

Thanks for the follow-up @tiborsulyan. Spring Batch has added an enum in the next major release so we'd like to create a local enum to improve user experience in 2.7.x. No need to act on it, we'll do it as part of merging it.

[snicoll]

@tiborsulyan thank you for making your first contribution to Spring Boot.