Spring Expression Language (SpEL)
https://docs.spring.io/spring-integration/docs/5.3.0.RELEASE/reference/html/spel.html
Spring Expression Language (SpEL) You can configure many Spring Integration components by using expressions written in the Spring Expression Language. In most cases, the #root object is the Message, which has two properties ( headers and payload) that allow such expressions as payload, payload.thing, headers ['my.header'], and so on.
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