详解spring mvc对异步请求的处理

在spring mvc3.2及以上版本增加了对请求的异步处理，是在servlet3的基础上进行封装的。

1、修改web.xml

<?xml version="1.0" encoding="UTF-8"?><web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee"  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee   http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">...</web-app>

1.1、声明version="3.0"，声明web-app_3_0.xsd

1.2、为servlet或者filter设置启用异步支持： <async-supported>true</async-supported>，修改WEB应用的web.xml

<!-- spring mvc --><servlet><servlet-name>SpringMvc</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class><init-param><param-name>contextConfigLocation</param-name><param-value>...</param-value></init-param><load-on-startup>1</load-on-startup><async-supported>true</async-supported></servlet>

2、使controller类支持async

2.1、返回java.util.concurrent.Callable来完成异步处理

package org.springframework.samples.mvc.async; import java.util.concurrent.Callable; import org.springframework.stereotype.Controller;import org.springframework.ui.Model;import org.springframework.web.bind.annotation.ExceptionHandler;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.RequestParam;import org.springframework.web.bind.annotation.ResponseBody;import org.springframework.web.context.request.async.WebAsyncTask; @Controller@RequestMapping("/async/callable")public class CallableController {  @RequestMapping("/response-body")  public @ResponseBody Callable<String> callable() {     return new Callable<String>() {      @Override      public String call() throws Exception {        Thread.sleep(2000);        return "Callable result";      }    };  }   @RequestMapping("/view")  public Callable<String> callableWithView(final Model model) {     return new Callable<String>() {      @Override      public String call() throws Exception {        Thread.sleep(2000);        model.addAttribute("foo", "bar");        model.addAttribute("fruit", "apple");        return "views/html";      }    };  }   @RequestMapping("/exception")  public @ResponseBody Callable<String> callableWithException(      final @RequestParam(required=false, defaultValue="true") boolean handled) {     return new Callable<String>() {      @Override      public String call() throws Exception {        Thread.sleep(2000);        if (handled) {          // see handleException method further below          throw new IllegalStateException("Callable error");        }        else {          throw new IllegalArgumentException("Callable error");        }      }    };  }   @RequestMapping("/custom-timeout-handling")  public @ResponseBody WebAsyncTask<String> callableWithCustomTimeoutHandling() {     Callable<String> callable = new Callable<String>() {      @Override      public String call() throws Exception {        Thread.sleep(2000);        return "Callable result";      }    };     return new WebAsyncTask<String>(1000, callable);  }   @ExceptionHandler  @ResponseBody  public String handleException(IllegalStateException ex) {    return "Handled exception: " + ex.getMessage();  } }

2.2、在异步处理完成时返回org.springframework.web.context.request.async.DeferredResult其他线程，例如一个JMS或一个AMQP消息,Redis通知等等：

@RequestMapping("/quotes")@ResponseBodypublic DeferredResult<String> quotes() { DeferredResult<String> deferredResult = new DeferredResult<String>(); // Add deferredResult to a Queue or a Map... return deferredResult;}  // In some other thread...deferredResult.setResult(data);// Remove deferredResult from the Queue or Map

3、spring配置文件的修改

spring mvc的dtd的声明必须大于等于3.2

<mvc:annotation-driven><!-- 可不设置，使用默认的超时时间 -->  <mvc:async-support default-timeout="3000"/></mvc:annotation-driven>

实际使用示例：

function deferred(){  $.get('quotes.htm',function(data){    console.log(data);    deferred();//每次请求完成,再发一次请求,避免客户端定时刷新来获取数据  });}

这么做的好处避免web server的连接池被长期占用而引起性能问题，调用后生成一个非web的服务线程来处理，增加web服务器的吞吐量~~

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