DeferredResult常用来实现客户端长轮询，可以将异步处理的结果在特定时间内（如果设置了超时时间）返回给客户端。

java">@Slf4j
@RestController
@RequestMapping("/demo")
public class DemoDeferredResult {

    @GetMapping("/deferredResult")
    public DeferredResult<ApiResponse<String>> getDeferredResult(@RequestParam String param) {
        DeferredResult<ApiResponse<String>> deferredResult = new DeferredResult<>();
        deferredResult.onCompletion(() -> log.info("onCompletion"));
        deferredResult.onTimeout(() -> log.info("onTimeout"));

        new Thread(() -> {
            try {
                Thread.sleep(60000);
            } catch (InterruptedException e) {
                log.error("InterruptedException", e);
            }
            deferredResult.setResult(ApiResponse.success(param + UUID.randomUUID()));
        }).start();

        return deferredResult;
    }

}

流程：
1.请求发送到服务端
2.处理器处理controller方法，返回DeferredResult
3.返回值处理器根据DeferredResult调用 selectHandler 选择对应的处理器DeferredResultMethodReturnValueHandler
4.handleReturnValue方法里面通过WebAsyncUtils.getAsyncManager(webRequest).startDeferredResultProcessing(result, mavContainer) 处理异步请求
5.设置addTimeoutHandler，addErrorHandler，addCompletionHandler，setResultHandler等处理器，最关键的是setResultHandler，后续在deferredResult.setResult时会调用
6.异步结果处理完之后，也就是业务代码调用deferredResult.setResult时会执行resultHandler逻辑
7.最终会执行this.asyncWebRequest.dispatch();分发请求，返回给客户端