Spring - Asynchronous Request

用法

@GetMapping("/ddd")
public Callable<String> process() { 

    return () -> {
        Thread.sleep(1000L);
        return "call";
    };
}


@GetMapping("/ddd")
public DeferredResult<String> quotes() {
    DeferredResult<String> deferredResult = new DeferredResult<String>();
     
    return deferredResult;
}

// In some other thread...
deferredResult.setResult(data);

返回Callable和DeferredResult區別在於the return value of DeferredResult will also be produced from any thread, i.e. one that is not managed by Spring MVC.

運行時

1. 客戶端請求，DispatcherType：REQUEST 即與同步請求同樣
2. Controller返回Callable
3. HandlerMethodReturnValueHandlerComposite選出CallableMethodReturnValueHandler來處理Callable
4. WebAsyncManager#startCallableProcessing -> #startAsyncProcessing -> StandardServletAsyncWebRequest#startAsync
5. 因爲spring的StandardServletAsyncWebRequest內部託管J2EE容器的Request（該Request實現ServletRequest），因此調用ServletRequest#startAsync(ServletRequest, ServletResponse)生成AsyncContext託管於StandardServletAsyncWebRequest內部，該AsyncContext保存ServletRequest和ServletResponse，以便以後的DispatcherType：ASYNC請求能取出對應的ServletResponse
6. WebAsyncManager#startCallableProcessing 調用完畢後，向taskExecutor線程池中提交任務來執行Callable#call，注意該任務最後一定會執行WebAsyncManager#setConcurrentResultAndDispatch，即執行StandardServletAsyncWebRequest#dispatch，就是執行前面生成的AsyncContext#dispatch
7. DispatcherType：ASYNC 這是由AsyncContext#dispatch發起的。
8. 注意這時RequestMappingHandlerAdapter#invokeHandlerMethod(HttpServletRequest, HttpServletResponse, HandlerMethod)中if (asyncManager.hasConcurrentResult())結果爲true，即將原來的ServletInvocableHandlerMethod替換成ConcurrentResultHandlerMethod，因此invocableMethod.invokeAndHandle(webRequest, mavContainer);直接返回taskExecutor線程池執行的結果。
9. HandlerMethodReturnValueHandlerComposite選出你結果類型對應的ReturnValueHandler來處理你的結果。
10. 返回客戶端。

提醒：RequestMappingHandlerAdapter#invokeHandlerMethod(HttpServletRequest, HttpServletResponse, HandlerMethod)中的invocableMethod.invokeAndHandle(webRequest, mavContainer);是你業務代碼執行的步驟。

The call to request.startAsync() returns AsyncContext which can be used for further control over async processing. For example it provides the method dispatch, that is similar to a forward from the Servlet API except it allows an application to resume request processing on a Servlet container thread.

運行時流程有點繞，沒功夫畫時序圖。

Exception Handling

when a Callable raises an Exception Spring MVC dispatches to the Servlet container with the Exception as the result and that leads to resume request processing with the Exception instead of a controller method return value. When using a DeferredResult you have a choice whether to call setResult or setErrorResult with an Exception instance.
因此異步異常處理和同步相同，在DispatcherType：ASYNC這段請求中處理。

Intercepting Async Requests

• The DeferredResult type also provides methods such as onTimeout(Runnable) and onCompletion(Runnable).
• When using a Callable you can wrap it with an instance of WebAsyncTask which also provides registration methods for timeout and completion.

AsyncHandlerInterceptor比HandlerInterceptor多#afterConcurrentHandlingStarted，調用時間即DispatcherType：ASYNC發起以前。

HTTP Streaming

正是請求被異步化，從而使得能long polling，即HTTP Streaming。

能夠用ResponseBodyEmitter來建立邏輯上的Streaming，注意ResponseBodyEmitter#send的內容都會經過對應的HttpMessageConverter來轉化：

@RequestMapping("/events")
public ResponseBodyEmitter handle() {
    ResponseBodyEmitter emitter = new ResponseBodyEmitter();
    // Save the emitter somewhere..
    return emitter;
}

// In some other thread
emitter.send("Hello once");

// and again later on
emitter.send("Hello again");

// and done at some point
emitter.complete();

固然 ResponseBodyEmitter can also be used as the body in a ResponseEntity in order to customize the status and headers of the response.