在web應用之間轉發請求

在web應用之間轉發請求

1. 步驟
   
   • 由於每個web應用都配備了一個大管家—-ServletContext，我們可以獲得這個web應用的管家來轉發請求到這個web應用的某個Servlet、JSP、HTML文檔。
   • 由於從一個web應用訪問另一個web應用資源存在安全問題，所以默認是不能訪問的。我們可以在web應用的目錄META-INF下創建一個context.xml文件，指定crossContext的值爲true，即可訪問。它的值默認是false。
   • 具體到代碼，要獲得一個web應用的ServletContext，可以使用ServletContext對象的getContext(String uripath)方法，uripath指定其他web應用的URL入口。

2. 代碼

   • 使用eclipse新建兩個Dynamin Web Project項目，名稱是TestServlet、Dispatcher,分別新建兩個類CrossServlet、OutputServlet,都繼承HttpServlet前者將用戶請求轉發到Dispatcher應用下的outputServlet。類代碼如下:

     CrossServlet類:

     package com.lovo.visitother;
     
     import java.io.IOException;
     import java.io.PrintWriter;
     
     import javax.servlet.RequestDispatcher;
     import javax.servlet.ServletContext;
     import javax.servlet.ServletException;
     import javax.servlet.annotation.WebServlet;
     import javax.servlet.http.HttpServlet;
     import javax.servlet.http.HttpServletRequest;
     import javax.servlet.http.HttpServletResponse;
     
     public class CrossServlet extends HttpServlet {
         private static final long serialVersionUID = 1L;
     
         public CrossServlet() {
             super();
         }
     
         protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
             String name = request.getParameter("username");
             response.setContentType("text/plain;charset=utf-8");
             if (name == null) {
                 name = "Please Input Username!";
             } else {
                 name = "Hello! " + name;
             }
             PrintWriter out = response.getWriter();
             request.setAttribute("username", name);
             ServletContext context = this.getServletContext();
             //獲取web應用Dispatcher的ServletContext
             ServletContext crossContext = context.getContext("/Dispatcher");
             //獲取請求轉發的對象
             RequestDispatcher patcher = crossContext.getRequestDispatcher("/OutputServlet");
             patcher.forward(request, response);
         }
     
         protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
             doGet(request, response);
         }
     }
     

     OutputServlet類:

   package com.lovo.dispatcher;
   
   import java.io.IOException;
   import java.io.PrintWriter;
   
   import javax.servlet.ServletException;
   import javax.servlet.annotation.WebServlet;
   import javax.servlet.http.HttpServlet;
   import javax.servlet.http.HttpServletRequest;
   import javax.servlet.http.HttpServletResponse;
   
   public class OutputServlet extends HttpServlet {
       private static final long serialVersionUID = 1L;
   
       public OutputServlet() {
           super();
       }
   
       protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
           request.setCharacterEncoding("utf-8");
           String msg = (String) request.getAttribute("username");
           PrintWriter out = response.getWriter();
           out.println(msg);
           out.write("===================");
           out.close();
       }
   
       protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
           doGet(request, response);
       }
   }
   

   • 兩個Servlet的web.xml文件

   CrossServlet類的web.xml

   <?xml version="1.0" encoding="UTF-8"?>
   <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
       <display-name>TestServlet</display-name>
       <servlet>
           <servlet-name>CrossServlet</servlet-name>
           <servlet-class>com.lovo.visitother.CrossServlet</servlet-class>
       </servlet>
       <servlet-mapping>
               <servlet-name>CrossServlet</servlet-name>
               <url-pattern>/TestServlet/CrossServlet</url-pattern>
       </servlet-mapping>
   </web-app>

   OutputServlet類的web.xml

   <?xml version="1.0" encoding="UTF-8"?>
   <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
       <display-name>Dispatcher</display-name>
       <servlet>
           <servlet-name>OutputServlet</servlet-name>
           <servlet-class>com.lovo.dispatcher.OutputServlet</servlet-class>
       </servlet>
       <servlet-mapping>
               <servlet-name>OutputServlet</servlet-name>
               <url-pattern>/Dispatcher/OutputServlet</url-pattern>
       </servlet-mapping>
   </web-app>

   • 配置CrossServlet的context.xml文件

     • 在web應用TestServlet的webContent下的META_INF目錄創建context.xml文件，文件內容如下：

     <?xml version="1.0" encoding="UTF-8"?>
     <context reloadable="true" crossContext="true"></context>

   • reloadable="true"的意思是讓Tomcat監測文件是否改動，如果有改動，自動重新加載。

   • crossContext="true"表示可以訪問另一個web資源
   • 所有弄好之後，項目的文件如下圖所示:
     
3. 運行
   
   • 在eclipse運行Tomcat在瀏覽器輸入localhost:8080/TestServlet/CrossServlet，即可訪.