SpringMVC对异步请求的支持

不念不忘少年蓝@ 2022-02-18 16:53 324阅读 0赞

# SpringMVC对Servlet3异步请求的支持 #

SpringMVC对Servlet3异步请求的支持有两种方式，分别是通过处理器方法返回Callable和DeferredResult。按照Servlet3的规范，支持异步请求时需要配置对应的Servlet和Filter支持异步请求，为了使SpringMVC支持异步请求的处理，需要在定义DispatcherServlet时配置其支持异步请求，在DispatcherServlet之前定义的Filter也需要配置支持异步请求。

<servlet>
 <servlet-name>springmvc</servlet-name>
 <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
 <init-param>
 <param-name>contextConfigLocation</param-name>
 <param-value>/WEB-INF/applicationContext-mvc.xml</param-value>
 </init-param>
 <load-on-startup>1</load-on-startup>
 
 <async-supported>true</async-supported>
 </servlet>
 <servlet-mapping>
 <servlet-name>springmvc</servlet-name>
 <url-pattern>/</url-pattern>
 </servlet-mapping>

## 返回Callable ##

当处理器的返回方法是Callable类型时会默认发起异步请求，并使用一个TaskExecutor来调用返回的Callable，之后的处理就跟正常的SpringMVC请求是一样的。Callable的返回结果也跟正常请求SpringMVC的一样，可以返回Model、ModelAndView、String、Object等，也可以结合`@ResponseBody`使用，具体可以参考`CallableMethodReturnValueHandler`的`handleReturnValue()`。

@RequestMapping("/callable")
 public Callable<String> forCallable(Model model) throws Exception { 
 return () -> { 
 TimeUnit.SECONDS.sleep(1);//睡眠1秒，模仿某些业务操作
 model.addAttribute("a", "aaaaaaa");
 return "async_request_callable";
 };
 }

如果需要针对于单个Callable请求指定超时时间，我们可以把Callable用一个WebAsyncTask包裹起来。然后还可以指定超时回调和正常处理完成的回调。

@RequestMapping("/callable/timeout")
 public WebAsyncTask<String> forCallableWithTimeout(Model model) throws Exception { 
 long timeout = 5 * 1000L;
 WebAsyncTask<String> asyncTask = new WebAsyncTask<>(timeout, () -> { 
 TimeUnit.MILLISECONDS.sleep(timeout + 10);
 model.addAttribute("a", "aaaaaaa");
 return "async_request_callable";
 });
 asyncTask.onTimeout(() -> { 
 System.out.println("响应超时回调");
 return "async_request_callable_timeout";
 });
 asyncTask.onCompletion(() -> { 
 System.out.println("响应callable调用完成的回调");
 });
 return asyncTask;
 }

## 返回DeferredResult ##

使用DeferredResult的返回结果的编程通常是在处理器方法中创建一个DeferredResult实例，把它保存起来后再进行返回，比如保存到一个队列中，然后在另外的一个线程中会从这个队列中拿到相应的DeferredResult对象进行相应的业务处理后会往DeferredResult中设置对应的返回值。返回了DeferredResult后SpringMVC将创建一个DeferredResultHandler用于监听DeferredResult，一旦DeferredResult中设置了返回值后，DeferredResultHandler就将对返回值进行处理。DeferredResult的处理过程见`DeferredResultMethodReturnValueHandler`的`handleReturnValue()`。

@RequestMapping("/deferredresult")
 public DeferredResult<String> forDeferredResult() throws Exception { 
 DeferredResult<String> result = new DeferredResult<>();
 new Thread(() -> { 
 try { 
 TimeUnit.SECONDS.sleep(2);
 } catch (InterruptedException e) { 
 e.printStackTrace();
 }
 result.setResult("async_request_deferredresult");
 }).start();
 return result;
 }

对于DeferredResult也是可以单独指定超时时间和超时后的回调的，它的超时时间可以直接通过构造函数传递，单位是毫秒。

@RequestMapping("/deferredresult/timeout")
 public DeferredResult<String> forDeferredResultWithTimeout() throws Exception { 
 DeferredResult<String> result = new DeferredResult<>(10 * 1000);
 new Thread(() -> { 
 try { 
 TimeUnit.SECONDS.sleep(31);
 } catch (InterruptedException e) { 
 e.printStackTrace();
 }
 result.setResult("async_request_deferredresult");
 }).start();
 
 result.onTimeout(() -> { 
 System.out.println("响应超时回调函数");
 });
 
 result.onCompletion(() -> { 
 System.out.println("响应完成的回调函数");
 });
 
 return result;
 }

## 配置 ##

可以通过`<mvc:annotation-driven/>`的子元素`<mvc:async-support/>`来定义处理异步请求默认的超时时间和需要使用的TaskExecutor。如果不指定默认超时时间则默认会使用容器的异步请求超时时间，如果不指定需要使用的TaskExecutor，则默认会使用一个SimpleAsyncTaskExecutor。在下面的配置中我们就配置了默认的超时时间是15秒，且处理异步请求的TaskExecutor是bean容器中名为asyncTaskExecutor的TaskExecutor。

<mvc:annotation-driven>
 <mvc:async-support default-timeout="15000" task-executor="asyncTaskExecutor"/>
 </mvc:annotation-driven>

### 拦截器 ###

返回Callable类型的请求可以通过实现`CallableProcessingInterceptor`接口自定义一个拦截器来拦截，也可以通过继承`CallableProcessingInterceptorAdapter`抽象类来定义拦截器，这样就只需要选择自己感兴趣的方法进行实现。`CallableProcessingInterceptor`接口定义如下：

public interface CallableProcessingInterceptor { 
 
 	static final Object RESULT_NONE = new Object();
 
 	static final Object RESPONSE_HANDLED = new Object();
 
 	/** * Invoked before the start of concurrent handling in the original * thread in which the {@code Callable} is submitted for concurrent handling. * * * This is useful for capturing the state of the current thread just prior to * invoking the {@link Callable}. Once the state is captured, it can then be * transferred to the new {@link Thread} in * {@link #preProcess(NativeWebRequest, Callable)}. Capturing the state of * Spring Security's SecurityContextHolder and migrating it to the new Thread * is a concrete example of where this is useful. * * * @param request the current request * @param task the task for the current async request * @throws Exception in case of errors */
 	<T> void beforeConcurrentHandling(NativeWebRequest request, Callable<T> task) throws Exception;
 
 	/** * Invoked after the start of concurrent handling in the async * thread in which the {@code Callable} is executed and before the * actual invocation of the {@code Callable}. * * @param request the current request * @param task the task for the current async request * @throws Exception in case of errors */
 	<T> void preProcess(NativeWebRequest request, Callable<T> task) throws Exception;
 
 	/** * Invoked after the {@code Callable} has produced a result in the * async thread in which the {@code Callable} is executed. This method may * be invoked later than {@code afterTimeout} or {@code afterCompletion} * depending on when the {@code Callable} finishes processing. * * @param request the current request * @param task the task for the current async request * @param concurrentResult the result of concurrent processing, which could * be a {@link Throwable} if the {@code Callable} raised an exception * @throws Exception in case of errors */
 	<T> void postProcess(NativeWebRequest request, Callable<T> task, Object concurrentResult) throws Exception;
 
 	/** * Invoked from a container thread when the async request times out before * the {@code Callable} task completes. Implementations may return a value, * including an {@link Exception}, to use instead of the value the * {@link Callable} did not return in time. * * @param request the current request * @param task the task for the current async request * @return a concurrent result value; if the value is anything other than * {@link #RESULT_NONE} or {@link #RESPONSE_HANDLED}, concurrent processing * is resumed and subsequent interceptors are not invoked * @throws Exception in case of errors */
 	<T> Object handleTimeout(NativeWebRequest request, Callable<T> task) throws Exception;
 
 	/** * Invoked from a container thread when async processing completes for any * reason including timeout or network error. * * @param request the current request * @param task the task for the current async request * @throws Exception in case of errors */
 	<T> void afterCompletion(NativeWebRequest request, Callable<T> task) throws Exception;
 
 }

它的配置是通过`<mvc:callable-interceptors/>`配置的。

<mvc:annotation-driven>
 <mvc:async-support default-timeout="15000" task-executor="asyncTaskExecutor">
 <mvc:callable-interceptors>
 <bean class="YourCallableProcessingInterceptor"/>
 </mvc:callable-interceptors>
 </mvc:async-support>
 </mvc:annotation-driven>

返回DeferredResult的也可以进行拦截，这需要我们实现`DeferredResultProcessingInterceptor`接口或者继承自`DeferredResultProcessingInterceptorAdapter`。`DeferredResultProcessingInterceptor`接口定义如下：

public interface DeferredResultProcessingInterceptor { 
 
 	/** * Invoked immediately before the start of concurrent handling, in the same * thread that started it. This method may be used to capture state just prior * to the start of concurrent processing with the given {@code DeferredResult}. * * @param request the current request * @param deferredResult the DeferredResult for the current request * @throws Exception in case of errors */
 	<T> void beforeConcurrentHandling(NativeWebRequest request, DeferredResult<T> deferredResult) throws Exception;
 
 	/** * Invoked immediately after the start of concurrent handling, in the same * thread that started it. This method may be used to detect the start of * concurrent processing with the given {@code DeferredResult}. * * The {@code DeferredResult} may have already been set, for example at * the time of its creation or by another thread. * * @param request the current request * @param deferredResult the DeferredResult for the current request * @throws Exception in case of errors */
 	<T> void preProcess(NativeWebRequest request, DeferredResult<T> deferredResult) throws Exception;
 
 	/** * Invoked after a {@code DeferredResult} has been set, via * {@link DeferredResult#setResult(Object)} or * {@link DeferredResult#setErrorResult(Object)}, and is also ready to * handle the concurrent result. * * This method may also be invoked after a timeout when the * {@code DeferredResult} was created with a constructor accepting a default * timeout result. * * @param request the current request * @param deferredResult the DeferredResult for the current request * @param concurrentResult the result to which the {@code DeferredResult} * @throws Exception in case of errors */
 	<T> void postProcess(NativeWebRequest request, DeferredResult<T> deferredResult, Object concurrentResult) throws Exception;
 
 	/** * Invoked from a container thread when an async request times out before * the {@code DeferredResult} has been set. Implementations may invoke * {@link DeferredResult#setResult(Object) setResult} or * {@link DeferredResult#setErrorResult(Object) setErrorResult} to resume processing. * * @param request the current request * @param deferredResult the DeferredResult for the current request; if the * {@code DeferredResult} is set, then concurrent processing is resumed and * subsequent interceptors are not invoked * @return {@code true} if processing should continue, or {@code false} if * other interceptors should not be invoked * @throws Exception in case of errors */
 	<T> boolean handleTimeout(NativeWebRequest request, DeferredResult<T> deferredResult) throws Exception;
 
 	/** * Invoked from a container thread when an async request completed for any * reason including timeout and network error. This method is useful for * detecting that a {@code DeferredResult} instance is no longer usable. * * @param request the current request * @param deferredResult the DeferredResult for the current request * @throws Exception in case of errors */
 	<T> void afterCompletion(NativeWebRequest request, DeferredResult<T> deferredResult) throws Exception;
 
 }

自定义的DeferredResultProcessingInterceptor是通过`<mvc:deferred-result-interceptors>`配置的。

<mvc:annotation-driven>
 <mvc:async-support default-timeout="15000" task-executor="asyncTaskExecutor">
 <mvc:deferred-result-interceptors>
 <bean class="YourDeferredResultProcessingInterceptor"/>
 </mvc:deferred-result-interceptors>
 </mvc:async-support>
 </mvc:annotation-driven>

> 当发起异步请求时，SpringMVC传统的HandlerInterceptor的postHandle()和afterCompletion()不会执行，但是等异步请求结束后它们还是会执行的。如果需要在异步处理完成之后做一些事情，也可以选择实现AsyncHandlerInterceptor接口的afterConcurrentHandlingStarted()，AsyncHandlerInterceptor接口继承了HandlerInterceptor。

（注：本文是基于Spring4.1.0所写）