Using the DeferredResult Class in Spring (Non-blocking Java Servlet Controller Methods in Spring)

DeferredResultInSpring.java

GetMapping("/async-deferredresult")
public DeferredResult<ResponseEntity<?>> handleReqDefResult(Model model) {
    LOG.info("Received async-deferredresult request");
    DeferredResult<ResponseEntity<?>> output = new DeferredResult<>(); //just a declaration
     
    ForkJoinPool.commonPool().submit(() -> {        //we pass a service method to an ExecutorService / thread pool
        LOG.info("Processing in separate thread");
        try {
            Thread.sleep(6000);
        } catch (InterruptedException e) {
        }
        output.setResult(ResponseEntity.ok("ok"));  //this service deals with the DeferredResult object declared earlier
    });
     
    LOG.info("servlet thread freed"); //we free the servlet thread
    return output;                    //we send a "promise" of the expected output from the service called in a separate thread
}

//We can additionally register some callbacks:

deferredResult.onCompletion(() -> LOG.info("Processing complete"));

deferredResult.onTimeout(new Runnable() {
			@Override
			public void run() {
				deferredResult.setErrorResult(
						ResponseEntity.status(HttpStatus.REQUEST_TIMEOUT).body("Request timeout occurred."));
			}
});

deferredResult.onError((Throwable t) -> {
    deferredResult.setErrorResult(
      ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
        .body("An error occurred."));
});

//Takeaway:
/*
In above example, the request is processed in a separate thread. This approach carries the following advantages.

    Http worker threads released immediately by passing on the work to the non HTTP thread.
    The worker threads are able to handle additional HTTP request without waiting for the long-running process to finish the work.
    
*/
/*
The DeferredResult class provide the following three callbacks

    onComletion – block on execute when the request completes.
    onTimeout – Custom code to invoke once timeout occurs.
    onError – Execute code on error.
    
*/
//vide: https://www.baeldung.com/spring-deferred-result
//vide: https://www.javadevjournal.com/spring-mvc/spring-deferred-result/