To understand the `idGenerator` field usage in `MessageHeaders`

[mjd507]

The idGenerator static field in MessageHeaders looks always null, and no static setIdGenerator method provided.

but the static getGenerator method will check this null idGenerator first, then the default-idGenerator.

I see in spring integration, their IdGeneratorConfigurer will use reflection to set this idGenerator field.

I want to understand the design, its a little strange for me.

Originally posted by @mjd507 in spring-projects/spring-integration#11014 (comment)