使用jetty的continuations实现"服务器推"

 在实际的开发中,我们可能会有这样的场景:许多客户端都连接到服务器端,当有某个客户端的消息的时候,服务器端会主动"推"消息给客户端,手机app的推送是一个典型的场景(IOS的推送都是要经过苹果的服务器的,一般是通过苹果的APNS服务来实现,不需要做过多的开发,安卓的推送就需要我们自己来实现了)

我们可选的技术方案实际上是很多的,使用netty这样的异步的网络通信框架或者servlet容器提供的异步的方案都是可以实现的,它们的理念都是一样的,异步和事件驱动,客户端请求服务器,当服务器没有需要推送的数据(或者是需要执行很长时间的IO操作)的时候,请求会被挂起,当服务器端的数据准备好的时候(例如需要向客户端推送一个消息的时候,或者是服务器端IO操作执行完毕了)请求会被重新激活,数据返回客户端.

使用jetty的continuations或者是netty来实现这两种是我觉得比较好的实现方案,今天介绍一下如何使用jetty的continuations来实现一个服务器推的原型,和正式环境中向安卓手机的推送的实现方法是完全一样的

continuations介绍:jetty的continuations是jetty实现的实现异步请求和事件驱动的组件,从jetty7起,continuations不止在jetty中可以使用,任何支持servlet3.0规范的servlet容器都可以使用continuations来实现异步和事件驱动,相比servlet3.0规范中的异步servlet,continuations提供了更加简化的编程模型.

目标:用浏览器请求服务器的一个URL(用浏览器来模拟我们的客户端),实现任何时候当服务器需要推送数据的时候,浏览器能够立即显示出来

我们需要提供两个接口:提供给客户端做长连接的接口,向客户端发送数据的接口

提供给客户端连接的servlet:

package com.jiaoyiping.websample.asyncServlet.jetty;
 /*
  * Created with Intellij IDEA
  * USER: 焦一平
  * Mail: jiaoyiping@gmail.com
  * Date: 2016/10/23
  * Time: 23:52
  * To change this template use File | Settings | Editor | File and Code Templates
 */

import org.eclipse.jetty.continuation.Continuation;
import org.eclipse.jetty.continuation.ContinuationListener;
import org.eclipse.jetty.continuation.ContinuationSupport;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.Writer;
import java.util.Map;

@WebServlet(urlPatterns = "/pull", asyncSupported = true)
public class ContinuationServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String user = req.getParameter("user");
        Map<String, PushAgent> pushAgentMap = (Map<String, PushAgent>) req.getServletContext().getAttribute("agentmap");
        if (pushAgentMap.containsKey(user)) {
            PushAgent pushAgent = pushAgentMap.get(user);
            Continuation continuation = ContinuationSupport.getContinuation(req);
            continuation.setTimeout(90000000);
            //第一次请求进来
            if (continuation.isInitial()) {
                resp.setContentType("text/evf;charset=utf-8");
                resp.setHeader("Connection", "keep-alive");
                resp.setHeader("Keep-Alive", "timeout=2000");
                PushAdapter pushAdapter = new PushAdapter(continuation, pushAgent);
                continuation.setAttribute("adapter", pushAdapter);
                continuation.addContinuationListener(new ContinuationListener() {
                    @Override
                    public void onComplete(Continuation continuation) {
                        PushAdapter adapter = (PushAdapter) continuation.getAttribute("adapter");
                        if (null != adapter) {
                            continuation.setAttribute("adapter", null);
                        }
                    }

                    @Override
                    public void onTimeout(Continuation continuation) {
                        onComplete(continuation);
                    }

                });
                resp.flushBuffer();
            }
            if (continuation.isExpired()) {
                return;
            }
            Writer writer = getWriter(resp);
            PushAdapter adapter = (PushAdapter) continuation.getAttribute("adapter");
            Message message;
            while (true) {
                message = adapter.getPushAgent().pull();
                if (null == message)
                    break;
                try {
                    writer.write(message.getContent());
                    writer.flush();
                    writer.write("
");
                    writer.flush();
                    resp.flushBuffer();
                } catch (Exception e) {
                    throw e;

                }
            }
            //若没有该客户端的消息,则请求被挂起
            continuation.suspend();
        }

    }

    private Writer getWriter(HttpServletResponse response) throws IOException {
        OutputStream os = response.getOutputStream();
        return new OutputStreamWriter(os, "UTF-8");
    }


}

向客户端推送消息的servlet:

package com.jiaoyiping.websample.asyncServlet.jetty;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Map;

/*
  * Created with Intellij IDEA
  * USER: 焦一平
  * Mail: jiaoyiping@gmail.com
  * Date: 2016/10/25
  * Time: 23:46
  * To change this template use File | Settings | Editor | File and Code Templates
 */
@WebServlet(urlPatterns = "/send")
public class MesssageSendServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //不要在自己实现的servlet中调用 super.doGet(0)或者是super.doPost()
        //因为在tomcat它们的默认实现是报405(HTTP1.1)或者400(其他版本的HTTP)
        //        super.doPost(req, resp);

        String target = req.getParameter("target");
        String messageStr = req.getParameter("message");

        Map<String, PushAgent> agentMap = (Map<String, PushAgent>) req.getServletContext().getAttribute("agentmap");
        if (agentMap.keySet().contains(target)) {
            Message message = new Message();
            message.setTarget(target);
            message.setContent(messageStr);
            if (agentMap.get(target).isInited()) {
                agentMap.get(target).onEvent(message);
            }
            agentMap.get(target).send(message);
            PrintWriter out = resp.getWriter();
            out.print("发送成功");
            out.flush();
            out.close();
        }


    }

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

推送代理:就是可以拿到客户端相关信息,并且维护客户端消息队列的类,在这个推送代理中,我们可以加入一个监听器,当有数据需要推送的时候,激活请求

package com.jiaoyiping.websample.asyncServlet.jetty;
 /*
  * Created with Intellij IDEA
  * USER: 焦一平
  * Mail: jiaoyiping@gmail.com
  * Date: 2016/10/25
  * Time: 22:56
  * To change this template use File | Settings | Editor | File and Code Templates
 */

public interface PushAgent {

    Terminal getTerminal();

    String getAddress();

    String getToken();

    Message send(Message message);

    Message pull();

    void addListener(MessageListener messageListener);

    void onEvent(Message message);

    boolean isInited();
}

默认实现(每个需要接受推送的用户对应一个PushAgent,和用户端保持长连接的线程从queue里读取mesage对象,向某个用户推送的时候将message对象放到该用户对应的PushAgent的queue里,这里是一个生产者-消费者模式):

package com.jiaoyiping.websample.asyncServlet.jetty;
 /*
  * Created with Intellij IDEA
  * USER: 焦一平
  * Mail: jiaoyiping@gmail.com
  * Date: 2016/10/25
  * Time: 23:17
  * To change this template use File | Settings | Editor | File and Code Templates
 */

import java.util.PriorityQueue;
import java.util.Queue;

public class DefaultPushAgent implements PushAgent {

    private Terminal terminal;
    //客户端通过长连接连接到服务器时,服务器不断地从该队列poll(),若果拿到新的消息,则返回给客户端
    private Queue<Message> messages = new PriorityQueue<>();
    private MessageListener messageListener;

    @Override
    public Terminal getTerminal() {
        return this.terminal;
    }

    @Override
    public String getAddress() {
        return null;
    }

    @Override
    public String getToken() {
        return null;
    }

    @Override
    public Message send(Message message) {
        synchronized (message) {
            messages.add(message);
        }

        return message;
    }

    @Override
    public Message pull() {
        synchronized (messages) {
            return messages.poll();
        }

    }

    @Override
    public void addListener(MessageListener messageListener) {
        this.messageListener = messageListener;
    }

    @Override
    public void onEvent(Message message) {
        this.messageListener.onMessage(message);
    }

    @Override
    public boolean isInited() {
        return this.messageListener != null;
    }


    public DefaultPushAgent(Terminal terminal) {
        this.terminal = terminal;
    }
}

PushAdapter的实现(用户将Continuation和PushAgent关联起来):

package com.jiaoyiping.websample.asyncServlet.jetty;
 /*
  * Created with Intellij IDEA
  * USER: 焦一平
  * Mail: jiaoyiping@gmail.com
  * Date: 2016/10/25
  * Time: 23:37
  * To change this template use File | Settings | Editor | File and Code Templates
 */

import org.eclipse.jetty.continuation.Continuation;

public class PushAdapter {
    private Continuation continuation;
    private PushAgent pushAgent;

    public PushAdapter(Continuation continuation, PushAgent pushAgent) {
        this.continuation = continuation;
        this.pushAgent = pushAgent;
        this.pushAgent.addListener(message -> {
            if (PushAdapter.this.continuation.isSuspended()) {
                PushAdapter.this.continuation.resume();
            }
        });
    }

    public Continuation getContinuation() {
        return continuation;
    }

    public void setContinuation(Continuation continuation) {
        this.continuation = continuation;
    }

    public PushAgent getPushAgent() {
        return pushAgent;
    }


    public void setPushAgent(PushAgent pushAgent) {
        this.pushAgent = pushAgent;
    }
}

MessageListener的实现(监听需要推送消息的事件,这里为了做演示,并没有实现一个完整的观察者模式,只是在需要推送消息的时候,手工调用 onMessage()):

package com.jiaoyiping.websample.asyncServlet.jetty;
 /*
  * Created with Intellij IDEA
  * USER: 焦一平
  * Mail: jiaoyiping@gmail.com
  * Date: 2016/10/26
  * Time: 2:03
  * To change this template use File | Settings | Editor | File and Code Templates
 */

public interface MessageListener {
    void onMessage(Message message);
}    

测试数据:使用一个listener在应用初始化的时候,初始化一些数据做为测试数据

package com.jiaoyiping.websample.asyncServlet.jetty;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import java.util.HashMap;
import java.util.Map;

/*
  * Created with Intellij IDEA
  * USER: 焦一平
  * Mail: jiaoyiping@gmail.com
  * Date: 2016/10/25
  * Time: 23:55
  * To change this template use File | Settings | Editor | File and Code Templates
 */
@WebListener
public class PushListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        Map<String, PushAgent> agentMap = new HashMap<>();
        agentMap.put("zhangsan", new DefaultPushAgent(new Terminal() {{
            setAddress("zhangsan");
            setToken("zhangsan_token");
        }}));
        agentMap.put("lisi", new DefaultPushAgent(new Terminal() {{
            setAddress("lisi");
            setToken("lisi_token");
        }}));

        sce.getServletContext().setAttribute("agentmap",agentMap);
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {

    }
}

最终的效果是这样的,我截了一个git图: