Spring MVC异步处理简介

摘要：异步处理简介地址相关系列文章异步处理详解分析本文讲到的所有特性皆是基于的，不是基于的。用于异步返回结果，使用自己的，使用负责处理它。配置执行异步操作需要用到，这个可以在用方法来提供相关文档。

Github地址

相关系列文章：

Servlet 3.0 异步处理详解

Servlet 3.1 Async IO分析

本文讲到的所有特性皆是基于Servlet 3.0 Async Processing的，不是基于Servlet 3.1 Async IO的。

A Callable can be returned when the application wants to produce the return value asynchronously in a thread managed by Spring MVC.

用于异步返回结果，使用的是Spring MVC的AsyncTaskExecutor，Spring MVC使用CallableMethodReturnValueHandler负责处理它。

下面是例子CallableController：

@RestController
public class CallableController {

  @RequestMapping("callable-hello")
  public Callable hello() {
    return () -> new SlowJob("CallableController").doWork();
  }
}

用浏览器访问：http://localhost:8080/callable-hello 查看返回结果。

A DeferredResult can be returned when the application wants to produce the return value from a thread of its own choosing.

用于异步返回结果，使用的是client code自己的thread，Spring MVC使用DeferredResultMethodReturnValueHandler负责处理它。

下面是例子DeferredResultController：

@RestController
public class DeferredResultController {

  @Autowired
  @Qualifier("customExecutorService")
  private ExecutorService executorService;

  @RequestMapping("deferred-result-hello")
  public DeferredResult hello() {
    DeferredResult deferredResult = new DeferredResult<>();
    executorService.submit(() -> {
      try {
        deferredResult.setResult(new SlowJob("DeferredResultController").doWork());
      } catch (Exception e) {
        deferredResult.setErrorResult(e);
      }

    });
    return deferredResult;
  }

}

在这个例子里使用了ExecutorService（见ExecutorServiceConfiguration），你也可以根据实际情况采用别的机制来给DeferredResult.setResult。

用浏览器访问：http://localhost:8080/deferred-result-hello 查看返回结果。

A ListenableFuture or CompletableFuture/CompletionStage can be returned when the application wants to produce the value from a thread pool submission.

用于异步返回结果，使用client code自己的thread pool，Spring MVC使用DeferredResultMethodReturnValueHandler负责处理它。

下面是例子ListenableFutureController：

@RestController
public class ListenableFutureController {

  @Autowired
  @Qualifier("customExecutorService")
  private ExecutorService executorService;

  @RequestMapping("listenable-future-hello")
  public ListenableFutureTask hello() {

    ListenableFutureTask listenableFutureTask = new ListenableFutureTask<>(
        () -> new SlowJob("ListenableFutureController").doWork());
    executorService.submit(listenableFutureTask);
    return listenableFutureTask;
  }

}

用浏览器访问：http://localhost:8080/listenable-future-hello 查看返回结果。

下面是例子CompletionFutureController

@RestController
public class CompletionFutureController {

  @RequestMapping("completable-future-hello")
  public CompletableFuture hello() {

    return CompletableFuture
        .supplyAsync(() -> new SlowJob("CompletionFutureController").doWork());
  }

}

用浏览器访问：http://localhost:8080/completable-future-hello 查看返回结果。

A ResponseBodyEmitter can be returned to write multiple objects to the response asynchronously; also supported as the body within a ResponseEntity.

用于异步的写入多个消息，使用的是client code自己的thread，Spring MVC使用ResponseBodyEmitterReturnValueHandler负责处理它。

下面是例子ResponseBodyEmitterController

@RestController
public class ResponseBodyEmitterController {

  @Autowired
  @Qualifier("customExecutorService")
  private ExecutorService executorService;

  @RequestMapping("response-body-emitter-hello")
  public ResponseBodyEmitter hello() {

    ResponseBodyEmitter emitter = new ResponseBodyEmitter();
    executorService.submit(() -> {
      try {
        for (int i = 0; i < 5; i++) {

          String hello = new SlowJob("ResponseBodyEmitterController").doWork();
          emitter.send("Count: " + (i + 1));
          emitter.send("
");
          emitter.send(hello);
          emitter.send("

");
        }
        emitter.complete();
      } catch (Exception e) {
        emitter.completeWithError(e);
      }

    });

    return emitter;
  }
}

用浏览器访问：http://localhost:8080/response-body-emitter-hello 查看返回结果。

An SseEmitter can be returned to write Server-Sent Events to the response asynchronously; also supported as the body within a ResponseEntity.

作用和ResponseBodyEmitter类似，也是异步的写入多个消息，使用的是client code自己的thread，区别在于它使用的是Server-Sent Events。Spring MVC使用ResponseBodyEmitterReturnValueHandler负责处理它。

下面是例子SseEmitterController

@RestController
public class SseEmitterController {

  @Autowired
  @Qualifier("customExecutorService")
  private ExecutorService executorService;

  @RequestMapping("sse-emitter-hello")
  public ResponseBodyEmitter hello() {

    SseEmitter emitter = new SseEmitter();
    executorService.submit(() -> {
      try {
        for (int i = 0; i < 5; i++) {

          String hello = new SlowJob("SseEmitterController").doWork();
          StringBuilder sb = new StringBuilder();
          sb.append("Count: " + (i + 1)).append(". ").append(hello.replace("
", ""));
          emitter.send(sb.toString());
        }
        emitter.complete();
      } catch (Exception e) {
        emitter.completeWithError(e);
      }

    });

    return emitter;
  }
}

用浏览器访问：http://localhost:8080/sse-emitter-hello 查看返回结果。

A StreamingResponseBody can be returned to write to the response OutputStream asynchronously; also supported as the body within a ResponseEntity.

用于异步write outputStream，使用的是Spring MVC的AsyncTaskExecutor，Spring MVC使用StreamingResponseBodyReturnValueHandler负责处理它。要注意，Spring MVC并没有使用Servlet 3.1 Async IO（[Read|Write]Listener）。

下面是例子StreamingResponseBodyController

@RestController
public class StreamingResponseBodyController {

  @RequestMapping("streaming-response-body-hello")
  public StreamingResponseBody hello() {

    return outputStream -> {
      String hello = new SlowJob("CallableController").doWork();
      outputStream.write(hello.getBytes());
      outputStream.flush();
    };

  }
}

用浏览器访问：http://localhost:8080/streaming-response-body-hello 查看返回结果。

Spring MVC执行异步操作需要用到AsyncTaskExecutor，这个可以在用WebMvcConfigurer.configureAsyncSupport方法来提供（相关文档）。
如果不提供，则使用SimpleAsyncTaskExecutor，SimpleAsyncTaskExecutor不使用thread pool，因此推荐提供自定义的AsyncTaskExecutor。

需要注意的是@EnableAsync也需要用到AsyncTaskExecutor，不过Spring MVC和它用的不是同一个。
顺带一提，EnableAsync默认也使用SimpleAsyncTaskExecutor，可以使用AsyncConfigurer.getAsyncExecutor方法来提供一个自定义的AsyncTaskExecutor。

例子见：MvcAsyncTaskExecutorConfigurer。

AsyncHandlerInterceptor，使用WebMvcConfigurer.addInterceptors注册

CallableProcessingInterceptor[Adapter]，使用WebMvcConfigurer.configureAsyncSupport注册

DeferredResultProcessingInterceptor[Adapter]，使用WebMvcConfigurer.configureAsyncSupport注册

官方文档：Intercepting Async Requests

WebAsyncManager是Spring MVC管理async processing的中心类，如果你可以阅读它的源码来更多了解Spring MVC对于async processing的底层机制。

Spring Web MVC Doc - Supported method return values

Spring Web MVC Doc - Asynchronous Request Processing

Spring Web MVC Doc - Configuring Asynchronous Request Processing

Configuring Spring MVC Async Threads

Spring MVC 3.2 Preview: Introducing Servlet 3, Async Support

Spring MVC 3.2 Preview: Techniques for Real-time Updates

Spring MVC 3.2 Preview: Making a Controller Method Asynchronous

Spring MVC 3.2 Preview: Adding Long Polling to an Existing Web Application

Spring MVC 3.2 Preview: Chat Sample