"Cannot dispatch without an AsyncContext" 提供文件时

Question

我需要在 "file download controller" 之上编写一个 "disposable file download" MVC 控制器。文件传输到客户端后，必须从服务器上删除它。

最初，编写代码是为了提供文件服务

进口org.springframework.core.io.Resource

@GetMapping("/get/{someParam}")
public ResponseEntity<Resource> downloadFile(Long someParam)
{

    Long fileId = identify(someParam);

    return super.downloadFile(fileId); //This uses a "File repository" service binding file IDs to physical paths

}

protected ResponseEntity<Resource> downloadFile(Long fileId){

    File theFile = resolve(fileId);

    return new FileSystemResource(theFile);

}

由于 ResponseEntity 是某种 "future" 实体，我不能删除 finally 块中的文件，因为它还没有被提供。

所以我先写了一个文件下载的异步版本，利用 Commons IO 来复制有效载荷。然后我利用回调来仅通过我的方法处理文件。

protected WebAsyncTask<Void> downloadFileAsync(Long fileId,HttpResponse response){ //API method for multiple uses across the application

    InputStream is = new FileInputStream(resolve(fileId));
    Callable<Void> ret = () -> {
        IOUtils.copy(is,response.getOutputStream());
        is.close();
        return null;
    };


    return ret;
}

@GetMapping("/get/{someParam}")
public WebAsyncTask<Void> downloadFile(Long someParam,HttpResponse response)
{
    Long fileId = identify(someParam);
    WebAsyncTask ret = downloadFileAsync(fileId,response);


    ret.onCompletion(()-> fileService.delete(fileId)); //Here I leverage the callback because this file, in this point, is disposable

    return ret;
}

当我运行第二个版本时，出现以下错误。服务器是 Tomcat 8.0.50

10-Sep-2018 12:20:37.551 AVVERTENZA [ajp-nio-8009-exec-3] org.apache.catalina.core.AsyncContextImpl.setErrorState onError() failed for listener of type [org.apache.catalina.core.AsyncListenerWrapper]
 java.lang.IllegalArgumentException: Cannot dispatch without an AsyncContext
    at org.springframework.util.Assert.notNull(Assert.java:134)
    at org.springframework.web.context.request.async.StandardServletAsyncWebRequest.dispatch(StandardServletAsyncWebRequest.java:128)
    at org.springframework.web.context.request.async.WebAsyncManager.setConcurrentResultAndDispatch(WebAsyncManager.java:369)
    at org.springframework.web.context.request.async.WebAsyncManager.access0(WebAsyncManager.java:60)
    at org.springframework.web.context.request.async.WebAsyncManager.handle(WebAsyncManager.java:311)
    at org.springframework.web.context.request.async.StandardServletAsyncWebRequest.onError(StandardServletAsyncWebRequest.java:144)
    at org.apache.catalina.core.AsyncListenerWrapper.fireOnError(AsyncListenerWrapper.java:49)
    at org.apache.catalina.core.AsyncContextImpl.setErrorState(AsyncContextImpl.java:421)
    at org.apache.catalina.connector.CoyoteAdapter.asyncDispatch(CoyoteAdapter.java:370)
    at org.apache.coyote.ajp.AbstractAjpProcessor.asyncDispatch(AbstractAjpProcessor.java:745)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:666)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1539)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1495)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)

我已经在 web.xml 中配置了所有 servlet 和过滤器以支持异步操作。我做了一些研究，this answer 没有帮助，因为我使用的是较新的 Tomcat 版本。

我的代码有什么问题？为了简单起见，我没有完全发布它，但在调试时我看到写入操作成功并具有正确的有效负载。

Answer 1

我遇到了同样的问题。就我而言，解决方案是配置 AsyncTaskExecutor:

@Configuration
public class WebConfig extends WebMvcConfigurerAdapter {

    @Override
    public void configureAsyncSupport(AsyncSupportConfigurer configurer) {
        configurer.setDefaultTimeout(-1);
        configurer.setTaskExecutor(asyncTaskExecutor());
    }

    @Bean
    public AsyncTaskExecutor asyncTaskExecutor() {
        // an implementaiton of AsyncTaskExecutor
        return new SimpleAsyncTaskExecutor("async");
    }

}

Answer 2

根据@MDenium 的评论

Don't use WebAsyncTask that is intended for internal use. Just use a CompletableFuture or return a Callable. If you put the try/finally inside your Callable it will work

WebAsyncTask 不是 API，因此当您从 MVC 方法 return 时，Spring 不知道如何处理它。这不是执行异步执行的正确方法。它仅在内部用于承载任务和上下文。

Spring MVC 支持：

延迟结果
可调用
可完成的未来

可能还有少数其他

"Cannot dispatch without an AsyncContext" 提供文件时

"Cannot dispatch without an AsyncContext" when serving files

java

asynchronous

spring-mvc

servlet-3.0