Java servlet通过事件驱动进行高性能长轮询详解

package com.hiwe.demo.event;
import javax.servlet.AsyncContext;
public class HttpEvent {
    /**
     * 可以是业务数据主键，这里用请求名称做个简单demo
     */
    private String requestName;
    private AsyncContext asyncContext;
    public HttpEvent(String requestName,AsyncContext asyncContext){
        this.requestName = requestName;
        this.asyncContext = asyncContext;
    }
    public String getRequestName() {
        return requestName;
    }
    public AsyncContext getAsyncContext() {
        return asyncContext;
    }
}

package com.hiwe.demo.event;
import javax.servlet.AsyncContext;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.Map;
public class EventManager {
    private final static Map<String,HttpEvent> subHttpEvents = new HashMap<>();
    /**
     * 新增事件订阅
     * @param event
     */
    public static void addHttpEvent(HttpEvent event){
        subHttpEvents.put(event.getRequestName(),event);
    }
    /**
     * 触发事件
     * @param requestName
     */
    public static void onEvent(String requestName){
        HttpEvent httpEvent = subHttpEvents.get(requestName);
        if(httpEvent==null){
            return;
        }
        AsyncContext asyncContext = httpEvent.getAsyncContext();
        try {
            PrintWriter writer = asyncContext.getResponse().getWriter();
            writer.print(requestName+" request success!");
            writer.flush();
            asyncContext.complete();
            subHttpEvents.remove(requestName);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

package com.hiwe.demo.listener;
import javax.servlet.AsyncContext;
import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebListener;
import java.io.IOException;
import java.io.PrintWriter;
@WebListener
public class AppAsyncListener implements AsyncListener {
    @Override
    public void onComplete(AsyncEvent asyncEvent) throws IOException {
        System.out.println("AppAsyncListener onComplete");
        // we can do resource cleanup activity here
    }
    @Override
    public void onError(AsyncEvent asyncEvent) throws IOException {
        System.out.println("AppAsyncListener onError");
        //we can return error response to client
    }
    @Override
    public void onStartAsync(AsyncEvent asyncEvent) throws IOException {
        System.out.println("AppAsyncListener onStartAsync");
        //we can log the event here
    }
    /**
     * 超时触发
     * @param asyncEvent
     * @throws IOException
     */
    @Override
    public void onTimeout(AsyncEvent asyncEvent) throws IOException {
        AsyncContext asyncContext = asyncEvent.getAsyncContext();
        ServletResponse response = asyncEvent.getAsyncContext().getResponse();
        PrintWriter out = response.getWriter();
        //返回code码，以便前端识别，并重建请求
        out.write(201+" longPolling timeout");
        out.flush();
        asyncContext.complete();
    }
}

package com.hiwe.demo.controller;
import com.hiwe.demo.listener.AppAsyncListener;
import com.hiwe.demo.event.EventManager;
import com.hiwe.demo.event.HttpEvent;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import javax.servlet.AsyncContext;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@RestController
@RequestMapping("/app")
public class AsyncController {
    /**
     * 长轮询接口
     * @param requestName
     * @param request
     * @param response
     */
    @GetMapping("/asyncGet")
    public void getDemo(@RequestParam(value = "requestName") String requestName, HttpServletRequest request, HttpServletResponse response){
        //开启异步支持
        request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true);
        AsyncContext asyncContext = request.startAsync();
        //添加监听器
        asyncContext.addListener(new AppAsyncListener());
        //设置超时时间
        asyncContext.setTimeout(30000);
        //添加到事件集合中去
        HttpEvent httpEvent = new HttpEvent(requestName, asyncContext);
        EventManager.addHttpEvent(httpEvent);
    }
    /**
     * 触发事件使用
     * @param requestName
     */
    @GetMapping("/trigger")
    public void triggerDemo(@RequestParam(value = "requestName") String requestName){
        EventManager.onEvent(requestName);
    }
}