Question

我在页面加载时注册了典型的SSE：

客户端：

sseTest: function(){

var source = new EventSource('mySSE');
source.onopen = function(event){
console.log("eventsource opened!");
};

source.onmessage = function(event){
var data = event.data;
console.log(data);
document.getElementById('sse').innerHTML+=event.data + "<br />";
};
}

我的Javascript-Debugger说，“eventsource已经打开了！”成功了。

我的服务器代码是Servlet 3.0：

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Random;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(urlPatterns={"/mySSE"}, name = "hello-sse", asyncSupported=true)
public class MyServletSSE extends HttpServlet {

@Override
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

resp.setContentType("text/event-stream");
resp.setCharacterEncoding("UTF-8");

Random random = new Random();
PrintWriter out = resp.getWriter();

//AsyncContext aCtx = req.startAsync(req, resp);
//ServletRequest sReq = aCtx.getRequest();

String next = "data: " + String.valueOf(random.nextInt(100) + 1) + "\n\n";
//out.print("retry: 600000\n"); //set the timeout to 10 mins in milliseconds
out.write(next);
out.flush();
// do not close the stream as EventSource is listening
//out.close();
//super.doGet(req, resp);
}
}

代码有效！客户端代码每3秒触发一次doGet（） - Method并检索新数据。

问题： 但是，我想通过使用新的Servlet 3.0 Futures（例如Async-Support或asyncContext.addListener（asyncListener）或其他我不知道的东西）如何使代码更好。由于我从不关闭流，我想知道我的服务器将如何扩展？

理论上，最好的方法是在新数据存在时通过服务器端代码显式触发doGet（） - 方法，因此客户端不需要触发客户端“onmessage（）” - 方法因此服务器端“doGet（）” - 每3秒对新数据进行一次方法。

Answer 1

这是一个很好的问题，这是一个完整的工作示例（Servlet 3.0 / Java EE 6）

一些注意事项：

它通过out.checkError()处理“浏览器标签/窗口关闭”，同时调用flush()
我写得很快，所以我确信它可以改进，只是一个POC，不要在测试前用于生产

Servlet：（为简洁省略了导入，很快就会更新完整的要点）

@WebServlet(urlPatterns = {"/mySSE"}, asyncSupported = true)
public class MyServletSSE extends HttpServlet {

  private final Queue<AsyncContext> ongoingRequests = new ConcurrentLinkedQueue<>();
  private ScheduledExecutorService service;

  @Override
  public void init(ServletConfig config) throws ServletException {
    final Runnable notifier = new Runnable() {
      @Override
      public void run() {
        final Iterator<AsyncContext> iterator = ongoingRequests.iterator();
        //not using for : in to allow removing items while iterating
        while (iterator.hasNext()) {
          AsyncContext ac = iterator.next();
          Random random = new Random();
          final ServletResponse res = ac.getResponse();
          PrintWriter out;
          try {
            out = res.getWriter();
            String next = "data: " + String.valueOf(random.nextInt(100) + 1) + "num of clients = " + ongoingRequests.size() + "\n\n";
            out.write(next);
            if (out.checkError()) { //checkError calls flush, and flush() does not throw IOException
              iterator.remove();
            }
          } catch (IOException ignored) {
            iterator.remove();
          }
        }
      }
    };
    service = Executors.newScheduledThreadPool(10);
    service.scheduleAtFixedRate(notifier, 1, 1, TimeUnit.SECONDS);
  }

  @Override
  public void doGet(HttpServletRequest req, HttpServletResponse res) {
    res.setContentType("text/event-stream");
    res.setCharacterEncoding("UTF-8");

    final AsyncContext ac = req.startAsync();
    ac.setTimeout(60 * 1000);
    ac.addListener(new AsyncListener() {
      @Override public void onComplete(AsyncEvent event) throws IOException {ongoingRequests.remove(ac);}
      @Override public void onTimeout(AsyncEvent event) throws IOException {ongoingRequests.remove(ac);}
      @Override public void onError(AsyncEvent event) throws IOException {ongoingRequests.remove(ac);}
      @Override public void onStartAsync(AsyncEvent event) throws IOException {}
    });
    ongoingRequests.add(ac);
  }
}

<强> JSP：

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>JSP Page</title>
        <script>
            function test() {
                var source = new EventSource('mySSE');
                source.onopen = function(event) {
                    console.log("eventsource opened!");
                };

                source.onmessage = function(event) {
                    var data = event.data;
                    console.log(data);
                    document.getElementById('sse').innerHTML += event.data + "<br />";
                };
            }
            window.addEventListener("load", test);
        </script>
    </head>
    <body>
        <h1>Hello SSE!</h1>
        <div id="sse"></div>
    </body>
</html>

Answer 2

有用的例子。

对于startAsync（），人们可能会得到“IllegalStateException：不支持”，在这种情况下要么不要忘记：

@WebServlet(urlPatterns = "/Sse", asyncSupported=true)

或使用

request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true);

来自this帖子

SSE和Servlet 3.0

2 个答案: