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Spring-MVC異步請(qǐng)求之Servlet異步處理

 更新時(shí)間:2021年01月20日 09:53:15   作者:li563868273  
這篇文章主要介紹了Spring-MVC異步請(qǐng)求之Servlet異步處理,具有很好的參考價(jià)值,希望對(duì)大家有所幫助。一起跟隨小編過(guò)來(lái)看看吧

在Servlet3.0的規(guī)范中新增了對(duì)異步請(qǐng)求的支持,SpringMVC又在此基礎(chǔ)上對(duì)異步請(qǐng)求提供了方便。

異步請(qǐng)求是在處理比較耗時(shí)的業(yè)務(wù)時(shí)先將request返回,然后另起線程處理耗時(shí)的業(yè)務(wù),處理完后在返回給用戶。

異步請(qǐng)求可以給我們帶來(lái)很多方便,最直接的用法就是處理耗時(shí)的業(yè)務(wù),比如,需要查詢數(shù)據(jù)庫(kù),需要調(diào)用別的服務(wù)器來(lái)處理等情況下可以先將請(qǐng)求返回給客戶端,然后啟用新線程處理耗時(shí)業(yè)務(wù)。

如果我們合適的擴(kuò)展可以實(shí)現(xiàn)訂閱者模式的消息訂閱功能,比如,當(dāng)有異常情況發(fā)生時(shí)可以主動(dòng)將相關(guān)信息發(fā)送給運(yùn)維人員,還有現(xiàn)在的很多郵箱自動(dòng)回復(fù)都是使用這種技術(shù)。

Http協(xié)議是單向的,只能客戶端自己拉不能服務(wù)器主動(dòng)推,Servlet對(duì)異步請(qǐng)求的支持并沒(méi)有修改Http,而是對(duì)Http的巧妙利用。異步請(qǐng)求的核心原理主要分為兩大類(lèi),一類(lèi)是輪詢,另一類(lèi)是長(zhǎng)連接。

輪詢就是定時(shí)自動(dòng)發(fā)起請(qǐng)求檢查有沒(méi)有需要返回的數(shù)據(jù),這種對(duì)資源浪費(fèi)比較大。長(zhǎng)連接的原理是客戶端發(fā)起請(qǐng)求,服務(wù)端處理并返回后并不結(jié)束連接,這樣就可以在后面再次返回給客戶端數(shù)據(jù)。

Servlet對(duì)異步請(qǐng)求的支持其實(shí)采用的是長(zhǎng)連接的方式,也就是說(shuō),異步請(qǐng)求中在原始的請(qǐng)求返回的時(shí)候并沒(méi)有關(guān)閉連接,關(guān)閉的只是處理請(qǐng)求的那個(gè)縣城,只有在異步請(qǐng)求全部處理完之后才會(huì)關(guān)閉連接。

Servlet3.0對(duì)異步請(qǐng)求的支持

在Servlet3.0規(guī)范中使用異步處理請(qǐng)求非常簡(jiǎn)單,只需要在請(qǐng)求處理過(guò)程中調(diào)用request的startAsync返回AsyncContext。

什么是AsyncContext在異步請(qǐng)求中充當(dāng)著非常重要的角色,可以稱(chēng)為異步請(qǐng)求上下文也可以稱(chēng)為異步請(qǐng)求容器。類(lèi)似于ServletContext.我們多次調(diào)用startAsync都是返回的同一個(gè)AsyncContext。代碼如下:

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 extends AsyncListener> T createListener(Class<T> var1) throws ServletException;
  void setTimeout(long var1);
  long getTimeout();
}

getResponse() 用于獲取response。dispatch用于分發(fā)新地址。complete用于通知容器已經(jīng)處理完了,start方法用于啟動(dòng)實(shí)際處理線程,addListener用于添加監(jiān)聽(tīng)器;setTimeout方法用于修改超時(shí)時(shí)間。

Servlet3.0處理異步請(qǐng)求實(shí)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 {
  //設(shè)置ContentType,關(guān)閉緩存
  resp.setContentType("text/plain;charset=UTF-8");
  resp.setHeader("Cache-Control","private");
  resp.setHeader("Pragma","no-cache");
  final PrintWriter writer= resp.getWriter();
  writer.println("老師檢查作業(yè)了");
  writer.flush();
  List<String> zuoyes=new ArrayList<String>();
  for (int i = 0; i < 10; i++) {
    zuoyes.add("zuoye"+i);;
  }
  final AsyncContext ac=req.startAsync();//開(kāi)啟異步請(qǐng)求
  doZuoye(ac,zuoyes);
  writer.println("老師布置作業(yè)");
  writer.flush();
}

private void doZuoye(final AsyncContext ac, final List<String> zuoyes) {
  ac.setTimeout(1*60*60*1000L);
  ac.start(new Runnable() {
    @Override
    public void run() {
      //通過(guò)response獲得字符輸出流
      try {
        PrintWriter writer=ac.getResponse().getWriter();
        for (String zuoye:zuoyes) {
          writer.println("\""+zuoye+"\"請(qǐng)求處理中");
          Thread.sleep(1*1000L);
          writer.flush();
        }
        ac.complete();
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
  });
}

異步請(qǐng)求監(jiān)聽(tīng)器

在上面的程序是我們最基本的異步請(qǐng)求,不過(guò)不夠完善。老師是需要思考宏觀問(wèn)題,所以在寫(xiě)完作業(yè)之后需要給老師匯報(bào)哪些題難,哪些題目有問(wèn)題或者自己的這次經(jīng)驗(yàn)總結(jié),不過(guò)這些事不應(yīng)該由做作業(yè)的學(xué)生來(lái)做,應(yīng)該由專(zhuān)門(mén)的學(xué)習(xí)匯報(bào)員來(lái)統(tǒng)計(jì)分析。所以就有了監(jiān)聽(tīng)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())+"工作處理完成");
  }

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

  @Override
  public void onError(AsyncEvent event) throws IOException {
    System.out.println("在"+formatter.format(new Date())+"工作處理錯(cuò)誤");
  }

  @Override
  public void onStartAsync(AsyncEvent event) throws IOException {
    System.out.println("在"+formatter.format(new Date())+"工作處理開(kāi)始");
  }
}

所有代碼具體參照github地址

https://github.com/lzggsimida123/ServletAsync

補(bǔ)充:SpringMVC對(duì)Servlet3異步請(qǐng)求的支持

SpringMVC對(duì)Servlet3異步請(qǐng)求的支持有兩種方式,分別是通過(guò)處理器方法返回Callable和DeferredResult。

按照Servlet3的規(guī)范,支持異步請(qǐng)求時(shí)需要配置對(duì)應(yīng)的Servlet和Filter支持異步請(qǐng)求,為了使SpringMVC支持異步請(qǐng)求的處理,需要在定義DispatcherServlet時(shí)配置其支持異步請(qǐng)求,在DispatcherServlet之前定義的Filter也需要配置支持異步請(qǐng)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>
  <!-- 啟用異步支持 -->
  <async-supported>true</async-supported>
</servlet>
<servlet-mapping>
  <servlet-name>springmvc</servlet-name>
  <url-pattern>/</url-pattern>
</servlet-mapping>

返回Callable

當(dāng)處理器的返回方法是Callable類(lèi)型時(shí)會(huì)默認(rèn)發(fā)起異步請(qǐng)求,并使用一個(gè)TaskExecutor來(lái)調(diào)用返回的Callable,之后的處理就跟正常的SpringMVC請(qǐng)求是一樣的。Callable的返回結(jié)果也跟正常請(qǐng)求SpringMVC的一樣,可以返回Model、ModelAndView、String、Object等,也可以結(jié)合@ResponseBody使用,具體可以參考CallableMethodReturnValueHandler的handleReturnValue()。

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

如果需要針對(duì)于單個(gè)Callable請(qǐng)求指定超時(shí)時(shí)間,我們可以把Callable用一個(gè)WebAsyncTask包裹起來(lái)。然后還可以指定超時(shí)回調(diào)和正常處理完成的回調(diào)。

 @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("響應(yīng)超時(shí)回調(diào)");
      return "async_request_callable_timeout";
    });
    asyncTask.onCompletion(() -> {
      System.out.println("響應(yīng)callable調(diào)用完成的回調(diào)");
    });
    return asyncTask;
  }

返回DeferredResult

使用DeferredResult的返回結(jié)果的編程通常是在處理器方法中創(chuàng)建一個(gè)DeferredResult實(shí)例,把它保存起來(lái)后再進(jìn)行返回,比如保存到一個(gè)隊(duì)列中,然后在另外的一個(gè)線程中會(huì)從這個(gè)隊(duì)列中拿到相應(yīng)的DeferredResult對(duì)象進(jìn)行相應(yīng)的業(yè)務(wù)處理后會(huì)往DeferredResult中設(shè)置對(duì)應(yīng)的返回值。返回了DeferredResult后SpringMVC將創(chuàng)建一個(gè)DeferredResultHandler用于監(jiān)聽(tīng)DeferredResult,一旦DeferredResult中設(shè)置了返回值后,DeferredResultHandler就將對(duì)返回值進(jìn)行處理。DeferredResult的處理過(guò)程見(jiàn)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;
}

對(duì)于DeferredResult也是可以單獨(dú)指定超時(shí)時(shí)間和超時(shí)后的回調(diào)的,它的超時(shí)時(shí)間可以直接通過(guò)構(gòu)造函數(shù)傳遞,單位是毫秒。

@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("響應(yīng)超時(shí)回調(diào)函數(shù)");
  });
  
  result.onCompletion(() -> {
    System.out.println("響應(yīng)完成的回調(diào)函數(shù)");
  });
  
  return result;
}

配置

可以通過(guò)<mvc:annotation-driven/>的子元素<mvc:async-support/>來(lái)定義處理異步請(qǐng)求默認(rèn)的超時(shí)時(shí)間和需要使用的TaskExecutor。如果不指定默認(rèn)超時(shí)時(shí)間則默認(rèn)會(huì)使用容器的異步請(qǐng)求超時(shí)時(shí)間,如果不指定需要使用的TaskExecutor,則默認(rèn)會(huì)使用一個(gè)SimpleAsyncTaskExecutor。在下面的配置中我們就配置了默認(rèn)的超時(shí)時(shí)間是15秒,且處理異步請(qǐng)求的TaskExecutor是bean容器中名為asyncTaskExecutor的TaskExecutor。

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

攔截器

返回Callable類(lèi)型的請(qǐng)求可以通過(guò)實(shí)現(xiàn)CallableProcessingInterceptor接口自定義一個(gè)攔截器來(lái)攔截,也可以通過(guò)繼承CallableProcessingInterceptorAdapter抽象類(lèi)來(lái)定義攔截器,這樣就只需要選擇自己感興趣的方法進(jìn)行實(shí)現(xiàn)。CallableProcessingInterceptor接口定義如下:

public interface CallableProcessingInterceptor { 
	static final Object RESULT_NONE = new Object(); 
	static final Object RESPONSE_HANDLED = new Object();
 
	/**
	 * Invoked <em>before</em> the start of concurrent handling in the original
	 * thread in which the {@code Callable} is submitted for concurrent handling.
	 *
	 * <p>
	 * 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.
	 * </p>
	 *
	 * @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 <em>after</em> the start of concurrent handling in the async
	 * thread in which the {@code Callable} is executed and <em>before</em> 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 <em>after</em> 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;
 
}

它的配置是通過(guò)<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的也可以進(jìn)行攔截,這需要我們實(shí)現(xiàn)DeferredResultProcessingInterceptor接口或者繼承自DeferredResultProcessingInterceptorAdapter。DeferredResultProcessingInterceptor接口定義如下:

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}.
	 *
	 * <p>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.
	 *
	 * <p>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;
 
}

自定義的DeferredResultProcessingInterceptor是通過(guò)<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>

當(dāng)發(fā)起異步請(qǐng)求時(shí),SpringMVC傳統(tǒng)的HandlerInterceptor的postHandle()和afterCompletion()不會(huì)執(zhí)行,但是等異步請(qǐng)求結(jié)束后它們還是會(huì)執(zhí)行的。如果需要在異步處理完成之后做一些事情,也可以選擇實(shí)現(xiàn)AsyncHandlerInterceptor接口的afterConcurrentHandlingStarted(),AsyncHandlerInterceptor接口繼承了HandlerInterceptor。

(注:本文是基于Spring4.1.0所寫(xiě))

以上為個(gè)人經(jīng)驗(yàn),希望能給大家一個(gè)參考,也希望大家多多支持腳本之家。如有錯(cuò)誤或未考慮完全的地方,望不吝賜教。

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