SpringMVC 长轮询

修改 web.xml 让其支持异步请求

    <filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
        <init-param>
            <param-name>encoding</param-name>
            <param-value>UTF-8</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
        <url-pattern>/*</url-pattern>

        <!--配置异步支持-->
        <dispatcher>ASYNC</dispatcher>
    </filter-mapping>

    <servlet>
        <servlet-name>Test</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:spring/springmvc.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
        <!--配置异步支持-->
        <async-supported>true</async-supported>
    </servlet>
    <servlet-mapping>
        <servlet-name>Test</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>
</web-app>

需要在 servlet 和 filter 都配置异步支持

Java 代码

@RestController
@RequestMapping("/polling")
public class PollingController extends BaseController {

    // 存放监听某个Id的长轮询集合
    // 线程同步结构
    public static Multimap<String, DeferredResult<String>> watchRequests = Multimaps.synchronizedMultimap(HashMultimap.create());
    private Long TIME_OUT = 30 * 1000L;

    /**
     * 设置监听
     */
    @RequestMapping(path = "watch/{id}")
    public DeferredResult<String> watch(@PathVariable String id) {
        // 延迟对象设置超时时间
        DeferredResult<String> deferredResult = new DeferredResult<>(TIME_OUT);
        // 异步请求完成时移除 key，防止内存溢出
        deferredResult.onCompletion(() -> {
            watchRequests.remove(id, deferredResult);
        });
        // 注册长轮询请求
        watchRequests.put(id, deferredResult);
        return deferredResult;
    }

    /**
     * 变更数据
     */
    @RequestMapping(path = "publish/{id}")
    @ResponseBody
    public String publish(@PathVariable String id) {
        // 数据变更 取出监听ID的所有长轮询请求，并一一响应处理
        if (watchRequests.containsKey(id)) {
            Collection<DeferredResult<String>> deferredResults = watchRequests.get(id);
            for (DeferredResult<String> deferredResult : deferredResults) {
                deferredResult.setResult("ok " + DateUtil.now());
            }
        }
        return "success";
    }
}

效果

浏览器请求 /polling/watch/1 后，浏览器进入 30s 的等待中

30s 内请求打开新的 tab 访问 /polling/publish/1 后，访问 /polling/watch/1 的 tab 得到相应，并输出 ok