上QQ阅读APP看书，第一时间看更新

How to do it...

To complete this recipe, please perform the following steps:

First, we need to create UserServlet, which will call user.jsp:

@WebServlet(name = "UserServlet", urlPatterns = {"/UserServlet"})
public class UserServlet extends HttpServlet {

  protected void doRequest(HttpServletRequest request, 
                          HttpServletResponse response)
         throws ServletException, IOException {
     request.getRequestDispatcher("/user.jsp").forward(request, response);
     System.out.println("Redirected to user.jsp");
  } 

  @Override
  protected void doGet(HttpServletRequest request, 
                      HttpServletResponse response)
         throws ServletException, IOException {
     doRequest(request, response);
  }

  @Override
  protected void doPost(HttpServletRequest request, 
                       HttpServletResponse response)
         throws ServletException, IOException {
     doRequest(request, response);
  }
}

We need to do the same with ProfileServlet, but call profile.jsp:

@WebServlet(name = "ProfileServlet", urlPatterns = {"/ProfileServlet"})
public class ProfileServlet extends HttpServlet {

    protected void doRequest(HttpServletRequest request, 
                             HttpServletResponse response)
            throws ServletException, IOException {
        request.getRequestDispatcher("/profile.jsp").
        forward(request, response);
        System.out.println("Redirected to profile.jsp");
    }

    @Override
    protected void doGet(HttpServletRequest request, 
                         HttpServletResponse response)
            throws ServletException, IOException {
        doRequest(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, 
                          HttpServletResponse response)
            throws ServletException, IOException {
        doRequest(request, response);
    }
}

Now, we need to create a filter that will be executed on every request (urlPatterns = {"/*"}):

@WebFilter(filterName = "PushFilter", urlPatterns = {"/*"})
public class PushFilter implements Filter {
    
    @Override
    public void doFilter(ServletRequest request, 
    ServletResponse response,
            FilterChain chain)
            throws IOException, ServletException {
        
        HttpServletRequest httpReq = (HttpServletRequest)request;
        PushBuilder builder = httpReq.newPushBuilder();
        
        if (builder != null){
            builder
                .path("resources/javaee-logo.png")
                .path("resources/style.css")
                .path("resources/functions.js")
                .push(); 
            System.out.println("Resources pushed");
        }

        chain.doFilter(request, response);
        
    }   
}

Next, we need to create a page so that we can call our servlets:

<body>
 <a href="UserServlet">User</a>
 <br/>
 <a href="ProfileServlet">Profile</a>
</body>

Here are the pages that will be called by the servlets. First, there's the user.jsp page:

    <head>
        <meta http-equiv="Content-Type" content="text/html; 
         charset=UTF-8">
        <link rel="stylesheet" type="text/css"   
         href="resources/style.css">
        <script src="resources/functions.js"></script>
        <title>User Push</title>
    </head>

    <body>
        <h1>User styled</h1>
        <img src="resources/javaee-logo.png">
        <br />
        <button onclick="message()">Message</button>
        <br />
        <a href="javascript:window.history.back();">Back</a>
    </body>

Second, the profile.jsp page is called:

    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <link rel="stylesheet" type="text/css" href="resources/style.css">
        <script src="resources/functions.js"></script>
        <title>User Push</title>
    </head>

    <body>
        <h1>Profile styled</h1>
        <img src="resources/javaee-logo.png">
        <br />
        <button onclick="message()">Message</button>
        <br />
        <a href="javascript:window.history.back();">Back</a>
    </body>

When you run the preceding code, make sure to use the HTTPS port as it only works under this protocol; for example, https://localhost:4848/ch02-serverpush-1.0.