使用RequestDispatcher 調派需求

在網頁應用程式中，經常需要多個Servlet完成需求，像是將一個Servlet的請求包含進來，或轉發給其他的Servlet處理。這時候可以使用 HttpServletRequest 的 getRequestDispatcher() 取得 RequestDispatcher，在這接口中定義了兩個方法：include() 和 forward()

RequestDispatcher disp = request.getRequestDispatcher("one.view");

使用include()
RequestDispatcher 的 include()，可以將另外一個Servlet的執行流程包含在目前Servlet的執行流程中。如下:

One.java

@WebServlet("/one.view")
public class One extends HttpServlet {
 private static final long serialVersionUID = 1L;

    public One() {
      super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  
        PrintWriter out = response.getWriter();
        out.println("First view Start");
        RequestDispatcher disp = request.getRequestDispatcher("two.view");
        disp.include(request, response);
        out.println("First view End");
    }

}

Two.java

@WebServlet("/two.view")
public class Two extends HttpServlet {
    
    private static final long serialVersionUID = 1L;
    
    public Two() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        PrintWriter out = response.getWriter();
        out.println("This is second view");  
    }

}

在網頁看到的回應順序如下:

使用forward()

RequestDispatcher 有個forward()方法，呼叫的時候一樣傳入request和response兩個物件，將請求處理轉發給其他Servlet。

One.java

@WebServlet("/hello.do")
public class One extends HttpServlet {
 private static final long serialVersionUID = 1L;

    public One() {
    }
    
 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  
  HelloModel model = new HelloModel();
  String name = request.getParameter("name");
  String message = model.sayMessage(name);
  RequestDispatcher disp = request.getRequestDispatcher("message.view");
  request.setAttribute("message", message);
  disp.forward(request, response);
   
 }

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

}

HelloModel.java

public class HelloModel {
 
 private Map messages = new HashMap();
 
 public HelloModel() {
  messages.put("Eric","Hi");
  messages.put("Arch","Where are you?"); 
 }
 
 public String sayMessage(String user){
  return user+","+messages.get(user);
 }
}

Two.java

@WebServlet("/message.view")
public class Two extends HttpServlet {
 private static final long serialVersionUID = 1L;
       
    public Two() {
        super();
    }

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 
  String template = "

%s
"; 
  String html = String.format(template, (String)request.getAttribute("message"));
  response.getWriter().print(html); 
 }

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

執行時的結果

參考資料

1. Servlet&JSP 教學手冊