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Spring MVC Async IO

在前一篇文章里介绍了Spring MVC对于Servlet 3.0 Async Processing的支持。 同时也提到了Spring MVC并没有提供对Servlet 3.1 Async IO的直接支持，本文介绍一些在Spring MVC中使用到Servlet 3.1 Async Processing的方法。

在开始讲解前要先了解Spring MVC是如何做Async Processing的，这里涉及到一个关键类WebAsyncManager。

WebAsyncManager

在前一篇文章里提到过以下几种AsyncHandlerMethodReturnValueHandler：

  1. CallableMethodReturnValueHandler
  2. DeferredResultMethodReturnValueHandler
  3. ResponseBodyEmitterReturnValueHandler
  4. StreamingResponseBodyReturnValueHandler

仔细看这些Handler的代码会发现最终都使用WebAsyncManager执行异步处理。

WebAsyncManager执行的大致动作有这么几个：

  1. 调用ServletRequest.startAsync()
  2. 使用AsyncTaskExecutor开启另一个线程执行任务
  3. 调用各种Interceptor的回调方法，比如CallableProcessingInterceptor和DeferredResultProcessingInterceptor

下面以Callable<?>为例，详细讲解一下WebAsyncManager的执行过程（下面的T-*代表不同的线程）：

  1. T-http-1: WebAsyncManager.startCallableProcessing
  2. T-http-1: CallableInterceptorChain.applyBeforeConcurrentHandling -> CallableProcessingInterceptor.beforeConcurrentHandling
  3. T-http-1: WebAsyncManager.startAsyncProcessing -> AsyncWebRequest.startAsync -> ServletRequest.startAsync
  4. T-http-1: AsyncTaskExecutor.submit(Callable)
  5. T-mvc-async: CallableInterceptorChain.applyPreProcess -> CallableProcessingInterceptor.preProcess
  6. T-mvc-async: Callable.run，等待执行完毕
  7. T-mvc-async: CallableInterceptorChain.applyPostProcess -> CallableProcessingInterceptor.postProcess
  8. T-mvc-async: WebAsyncManager.setConcurrentResultAndDispatch -> AsyncWebRequest.dispatch -> ServletRequest.dispatch
  9. T-http-2: DispatchServlet ...
  10. T-http-2: AsyncWebRequest.onComplete -> CallableInterceptorChain.triggerAfterCompletion -> CallableProcessingInterceptor.afterCompletion

上面的结果可以通过观察源代码、访问http://localhost:8080/callable-trace查看日志，下面是日志样例（注意观察CallableProcessingLogger）：

2017-12-25 16:58:09.632 DEBUG 19794 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : DispatcherServlet with name 'dispatcherServlet' processing GET request for [/callable-trace]
2017-12-25 16:58:09.632 DEBUG 19794 --- [nio-8080-exec-1] s.w.s.m.m.a.RequestMappingHandlerMapping : Looking up handler method for path /callable-trace
2017-12-25 16:58:09.632 DEBUG 19794 --- [nio-8080-exec-1] s.w.s.m.m.a.RequestMappingHandlerMapping : Returning handler method [public java.util.concurrent.Callable<java.lang.String> me.chanjar.learning.CallableTraceController.hello()]
2017-12-25 16:58:09.632 DEBUG 19794 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Last-Modified value for [/callable-trace] is: -1
2017-12-25 16:58:09.633  INFO 19794 --- [nio-8080-exec-1] orConfiguration$CallableProcessingLogger : beforeConcurrentHandling
2017-12-25 16:58:09.633 DEBUG 19794 --- [nio-8080-exec-1] o.s.w.c.request.async.WebAsyncManager    : Concurrent handling starting for GET [/callable-trace]
2017-12-25 16:58:09.633 DEBUG 19794 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Leaving response open for concurrent processing
2017-12-25 16:58:09.633  INFO 19794 --- [ my-mvc-async-1] orConfiguration$CallableProcessingLogger : preProcess
2017-12-25 16:58:10.636  INFO 19794 --- [ my-mvc-async-1] orConfiguration$CallableProcessingLogger : postProcess
2017-12-25 16:58:10.636 DEBUG 19794 --- [ my-mvc-async-1] o.s.w.c.request.async.WebAsyncManager    : Concurrent result value [Hi from CallableTraceController. Current Thread: my-mvc-async-2] - dispatching request to resume processing
2017-12-25 16:58:10.636 DEBUG 19794 --- [nio-8080-exec-2] o.s.web.servlet.DispatcherServlet        : DispatcherServlet with name 'dispatcherServlet' resumed processing GET request for [/callable-trace]
2017-12-25 16:58:10.637 DEBUG 19794 --- [nio-8080-exec-2] s.w.s.m.m.a.RequestMappingHandlerMapping : Looking up handler method for path /callable-trace
2017-12-25 16:58:10.637 DEBUG 19794 --- [nio-8080-exec-2] s.w.s.m.m.a.RequestMappingHandlerMapping : Returning handler method [public java.util.concurrent.Callable<java.lang.String> me.chanjar.learning.CallableTraceController.hello()]
2017-12-25 16:58:10.637 DEBUG 19794 --- [nio-8080-exec-2] o.s.web.servlet.DispatcherServlet        : Last-Modified value for [/callable-trace] is: -1
2017-12-25 16:58:10.637 DEBUG 19794 --- [nio-8080-exec-2] s.w.s.m.m.a.RequestMappingHandlerAdapter : Found concurrent result value [Hi from CallableTraceController. Current Thread: my-mvc-async-2]
2017-12-25 16:58:10.638 DEBUG 19794 --- [nio-8080-exec-2] m.m.a.RequestResponseBodyMethodProcessor : Written [Hi from CallableTraceController. Current Thread: my-mvc-async-2] as "text/plain" using [org.springframework.http.converter.StringHttpMessageConverter@70444987]
2017-12-25 16:58:10.639 DEBUG 19794 --- [nio-8080-exec-2] o.s.web.servlet.DispatcherServlet        : Null ModelAndView returned to DispatcherServlet with name 'dispatcherServlet': assuming HandlerAdapter completed request handling
2017-12-25 16:58:10.639 DEBUG 19794 --- [nio-8080-exec-2] o.s.web.servlet.DispatcherServlet        : Successfully completed request
2017-12-25 16:58:10.639  INFO 19794 --- [nio-8080-exec-2] orConfiguration$CallableProcessingLogger : afterCompletion

将ReadListener加到MVC异步处理过程

先来回顾一下使用ReadListener的大致步骤：

protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

  AsyncContext asyncCtx = req.startAsync();
  ServletInputStream is = req.getInputStream();
  is.setReadListener(new ReadListener() {

    @Override
    public void onDataAvailable() {

        while (is.isReady() && !is.isFinished()) {
          int length = is.read(buffer);
          if (length == -1 && is.isFinished()) {
            asyncCtx.complete();
            return;
          }
          // 处理buffer
        }

    }

    @Override
    public void onAllDataRead() {
      // ...
    }

    @Override
    public void onError(Throwable t) {
      // ...
    }
  });

}

所以如果要使用ReadListener，那么能够切入的点有两个：第5步和第7步。

如果在第5步CallableProcessingInterceptor.preProcess里使用ReadListener则：

  • 在ReadListener.onAllDataRead或者onError之前，Callable<?>必须阻塞。
  • ReadListener和Callable<?>需要有通信机制。
  • Callable<?>可以根据不同情况返回不同的结果。

如果在第7步CallableProcessingInterceptor.postProcess里使用ReadListener则：

  • Callable<?>事实上变成事先返回结果，而不是等ReadListener.onAllDataRead或者onError。
  • CallableProcessingInterceptor.postProcess必须阻塞
  • ReadListener和Callable<?>无法存在通信机制。

spring.http.multipart.resolve-lazily=true，否则被Multipart***事先消费掉inputStream中的内容。

curl -X POST -F "bigfile=@src/main/resources/bigfile" --limit-rate 10k http://localhost:8080/async-io-read-hook-pre-process

Tomcat 8.5.24及之前存在BUG 61932，会导致出现。。。。。。TODO。。。。。。

将WriteListener加到MVC异步处理过程

TODO

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