W3Schools上的說明:
server-sent event is when a web page automatically gets updates from a server.
This was also possible before, but the web page would have to ask if any updates were available. With server-sent events, the updates come automatically.
Examples: Facebook/Twitter updates, stock price updates, news feeds, sport results, etc.
過去HTTP 都是一個單一回合結束，如果要取得新的資料，必須重新發送一次
HTTP Request，而SSE則是可以持續接收Server資訊，無須主動發送Request ，
但Client端無法透過SSE與Server端進行溝通，這是與webSocket不同的地方。
以下有一個範例，利用Java Servlet 透過SSE的方式與Client端JavaScript溝通.
當下按Start Button後，JavaScript會開始接收由Server端傳送過來的SSE資料，並更新
在網頁上。
Client端程式碼
當下按Start Button後，JavaScript會開始接收由Server端傳送過來的SSE資料，並更新
在網頁上。
<!DOCTYPE HTML> <html> <head> <title>Server-Sent Events Servlet example</title> <script> function start() { var eventSource = new EventSource("Server"); eventSource.onmessage = function (event) { document.getElementById('myDiv').innerHTML = event.data; } } ; </script> <style> body { font-family: sans-serif; } </style> </head> <body> Time: <span id="myDiv"></span> <br><br> <button onclick="start()">Start</button> </body> </html>
Server端Servlet程式碼
/* * To change this license header, choose License Headers in Project Properties. * To change this template file, choose Tools | Templates * and open the template in the editor. */ import java.io.IOException; import java.io.PrintWriter; import java.text.SimpleDateFormat; import java.util.Calendar; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; /** * * @author boywhychen */ public class Server extends HttpServlet { /** * Processes requests for both HTTP <code>GET</code> and <code>POST</code> * methods. * * @param request servlet request * @param response servlet response * @throws ServletException if a servlet-specific error occurs * @throws IOException if an I/O error occurs */ protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/event-stream"); response.setCharacterEncoding("UTF-8"); PrintWriter writer = response.getWriter(); for (int i = 0; i < 20; i++) { writer.write("data: " + getTime() + "\n\n"); writer.flush(); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } writer.close(); } private String getTime() { Calendar cal = Calendar.getInstance(); SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss"); return sdf.format(cal.getTime()); } // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code."> /** * Handles the HTTP <code>GET</code> method. * * @param request servlet request * @param response servlet response * @throws ServletException if a servlet-specific error occurs * @throws IOException if an I/O error occurs */ @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } /** * Handles the HTTP <code>POST</code> method. * * @param request servlet request * @param response servlet response * @throws ServletException if a servlet-specific error occurs * @throws IOException if an I/O error occurs */ @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } /** * Returns a short description of the servlet. * * @return a String containing servlet description */ @Override public String getServletInfo() { return "Short description"; }// </editor-fold> }
參考資料來源：
https://gist.github.com/viralpatel/7007662
