孙卫琴《Tomcat与Java Web开发技术详解》(第2版)P149请求转发一节有一个问题不大懂：

下面是相关代码，实现了CheckServlet将请求转发给OutputServlet。疑惑的地方在，请求转发前后CheckServlet写入的数据都没有发给客户端，这是为什么呢？

（1）源Servlet：

public class CheckServlet extends GenericServlet {

  public void service(ServletRequest request,ServletResponse response)
    throws ServletException, IOException {

    //省略部分代码
    
    request.setAttribute("msg", message);
 
    ServletContext context = getServletContext();
    RequestDispatcher dispatcher =context.getRequestDispatcher("/output");  
 
    PrintWriter out=response.getWriter();

    // 转发前源Servlet的output
    out.println("Output from CheckServlet before forwarding request."); 
    System.out.println("Output from CheckServlet before forwarding request.");
    
    dispatcher.forward(request, response);

    // 转发后源Servlet的output
    out.println("Output from CheckServlet after forwarding request."); 
    System.out.println("Output from CheckServlet after forwarding request.");
  }
}

（2）目标Servlet

public class OutputServlet extends GenericServlet {

  public void service(ServletRequest request,ServletResponse response)
    throws ServletException, IOException {

    String message = (String)request.getAttribute("msg");
    PrintWriter out=response.getWriter();

    out.println(message); 
    out.close();
  }
}