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)

[rstoyanchev]

MessageHeaders was originally in Spring Integration and moved into the spring-messaging module in Spring Framework 4.0, and looks like it came with that idGenerator already in place as is. This is the commit that added it in the Spring Integration 2.0.x branch spring-projects/spring-integration@0441d52 and the issue spring-projects/spring-integration#5889 with the description.