https://www.baeldung.com/spring-deferred-result
https://docs.spring.io/spring/docs/current/spring-framework-reference/web.html#mvc-ann-async
https://spring.io/blog/2012/05/07/spring-mvc-3-2-preview-introducing-servlet-3-async-support
https://www.reactiveprogramming.be/spring-webflux-servlet-api/
https://www.javaworld.com/article/2077995/java-concurrency-asynchronous-processing-support-in-servlet-3-0.html
Asynchronous support was introduced in Servlet 3.0 and, simply put, it allows processing an HTTP request in another thread than the request receiver thread.
DeferredResult, available from Spring 3.2 onwards, assists in offloading a long-running computation from an http-worker thread to a separate thread.
Although the other thread will take some resources for computation, the worker threads are not blocked in the meantime and can handle incoming client requests.
The async request processing model is very useful as it helps scale an application well during high loads, especially for IO intensive operations.
来源：CSDN
作者：michaelforgood
链接：https://blog.csdn.net/michaelforgood/article/details/103681157