Spring-MVC异步请求之Servlet异步处理

在Servlet3.0的规范中新增了对异步请求的支持，SpringMVC又在此基础上对异步请求提供了方便。

异步请求是在处理比较耗时的业务时先将request返回，然后另起线程处理耗时的业务，处理完后在返回给用户。

异步请求可以给我们带来很多方便，最直接的用法就是处理耗时的业务，比如，需要查询数据库，需要调用别的服务器来处理等情况下可以先将请求返回给客户端，然后启用新线程处理耗时业务。

如果我们合适的扩展可以实现订阅者模式的消息订阅功能，比如，当有异常情况发生时可以主动将相关信息发送给运维人员，还有现在的很多邮箱自动回复都是使用这种技术。

Http协议是单向的，只能客户端自己拉不能服务器主动推，Servlet对异步请求的支持并没有修改Http,而是对Http的巧妙利用。异步请求的核心原理主要分为两大类，一类是轮询，另一类是长连接。

轮询就是定时自动发起请求检查有没有需要返回的数据，这种对资源浪费比较大。长连接的原理是客户端发起请求，服务端处理并返回后并不结束连接，这样就可以在后面再次返回给客户端数据。

Servlet对异步请求的支持其实采用的是长连接的方式，也就是说，异步请求中在原始的请求返回的时候并没有关闭连接，关闭的只是处理请求的那个县城，只有在异步请求全部处理完之后才会关闭连接。

Servlet3.0对异步请求的支持

在Servlet3.0规范中使用异步处理请求非常简单，只需要在请求处理过程中调用request的startAsync返回AsyncContext。

什么是AsyncContext在异步请求中充当着非常重要的角色，可以称为异步请求上下文也可以称为异步请求容器。类似于ServletContext.我们多次调用startAsync都是返回的同一个AsyncContext。代码如下：

public interface AsyncContext {
  String ASYNC_REQUEST_URI = "javax.servlet.async.request_uri";
  String ASYNC_CONTEXT_PATH = "javax.servlet.async.context_path";
  String ASYNC_PATH_INFO = "javax.servlet.async.path_info";
  String ASYNC_SERVLET_PATH = "javax.servlet.async.servlet_path";
  String ASYNC_QUERY_STRING = "javax.servlet.async.query_string";
  ServletRequest getRequest();
  ServletResponse getResponse();
  boolean hasOriginalRequestAndResponse();
  void dispatch();
  void dispatch(String var1);
  void dispatch(ServletContext var1, String var2);
  void complete();
  void start(Runnable var1);
  void addListener(AsyncListener var1);
  void addListener(AsyncListener var1, ServletRequest var2, ServletResponse var3);
   T createListener(Class var1) throws ServletException;
  void setTimeout(long var1);
  long getTimeout();
}

getResponse() 用于获取response。dispatch用于分发新地址。complete用于通知容器已经处理完了，start方法用于启动实际处理线程，addListener用于添加监听器;setTimeout方法用于修改超时时间。

Servlet3.0处理异步请求实例

@WebServlet(
name = “WorkServlet”,
urlPatterns = “/work”,
asyncSupported = true
)
public class WorkServlet extends HttpServlet {
private static final long serialVersionUID =1L;

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
  this.doPost(req, resp);
}

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
  //设置ContentType,关闭缓存
  resp.setContentType("text/plain;charset=UTF-8");
  resp.setHeader("Cache-Control","private");
  resp.setHeader("Pragma","no-cache");
  final PrintWriter writer= resp.getWriter();
  writer.println("老师检查作业了");
  writer.flush();
  List zuoyes=new ArrayList();
  for (int i = 0; i < 10; i++) {
    zuoyes.add("zuoye"+i);;
  }
  final AsyncContext ac=req.startAsync();//开启异步请求
  doZuoye(ac,zuoyes);
  writer.println("老师布置作业");
  writer.flush();
}

private void doZuoye(final AsyncContext ac, final List zuoyes) {
  ac.setTimeout(1*60*60*1000L);
  ac.start(new Runnable() {
    @Override
    public void run() {
      //通过response获得字符输出流
      try {
        PrintWriter writer=ac.getResponse().getWriter();
        for (String zuoye:zuoyes) {
          writer.println("\""+zuoye+"\"请求处理中");
          Thread.sleep(1*1000L);
          writer.flush();
        }
        ac.complete();
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
  });
}

异步请求监听器

在上面的程序是我们最基本的异步请求，不过不够完善。老师是需要思考宏观问题，所以在写完作业之后需要给老师汇报哪些题难，哪些题目有问题或者自己的这次经验总结，不过这些事不应该由做作业的学生来做，应该由专门的学习汇报员来统计分析。所以就有了监听器。

public class TeacherListener implements AsyncListener {
  final SimpleDateFormat formatter=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
  @Override
  public void onComplete(AsyncEvent event) throws IOException {
    System.out.println("在"+formatter.format(new Date())+"工作处理完成");
  }

  @Override
  public void onTimeout(AsyncEvent event) throws IOException {
    System.out.println("在"+formatter.format(new Date())+"工作超时");
  }

  @Override
  public void onError(AsyncEvent event) throws IOException {
    System.out.println("在"+formatter.format(new Date())+"工作处理错误");
  }

  @Override
  public void onStartAsync(AsyncEvent event) throws IOException {
    System.out.println("在"+formatter.format(new Date())+"工作处理开始");
  }
}

所有代码具体参照github地址

https://github.com/lzggsimida123/ServletAsync

补充：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。

  springmvc
  org.springframework.web.servlet.DispatcherServlet
  
    contextConfigLocation
    /WEB-INF/applicationContext-mvc.xml
  
  1
  
  true


  springmvc
  /

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 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 forCallableWithTimeout(Model model) throws Exception {
    long timeout = 5 * 1000L;
    WebAsyncTask 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 forDeferredResult() throws Exception {
  DeferredResult 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 forDeferredResultWithTimeout() throws Exception {
  DeferredResult 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

可以通过的子元素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。

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
	 */
	 void beforeConcurrentHandling(NativeWebRequest request, Callable 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
	 */
	 void preProcess(NativeWebRequest request, Callable 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
	 */
	 void postProcess(NativeWebRequest request, Callable 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
	 */
	 Object handleTimeout(NativeWebRequest request, Callable 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
	 */
	 void afterCompletion(NativeWebRequest request, Callable task) throws Exception;
 
}

它的配置是通过配置的。

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
	 */
	 void beforeConcurrentHandling(NativeWebRequest request, DeferredResult 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
	 */
	 void preProcess(NativeWebRequest request, DeferredResult 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
	 */
	 void postProcess(NativeWebRequest request, DeferredResult 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
	 */
	 boolean handleTimeout(NativeWebRequest request, DeferredResult 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
	 */
	 void afterCompletion(NativeWebRequest request, DeferredResult deferredResult) throws Exception;
 
}

自定义的DeferredResultProcessingInterceptor是通过配置的。

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）

以上为个人经验，希望能给大家一个参考，也希望大家多多支持脚本之家。如有错误或未考虑完全的地方，望不吝赐教。