Servlet共享變量Context，Session，或Request的作用範圍

在Servlet中進行變量的共享可硬通過Servlet容器中存在的ServletContext，HttpSession和HttpServletRequest的執行個體來實作。

在三中方式共享變量的方法是使用Context，Session，Request類型的執行個體調用serAttribute（"varName","obj"）方法将需要共享的變量存儲到對象當中。然後在需要使用該共享變量的地方在通過getAttribute（"varName"）方法來獲得變量。

/*提供共享變量的Servlet類：ShareSharingVars.java*/
package javaee.servlet;
import ......
@WebServlet("/Share")
public class ShareSharingVars extends HttpServlet{
  ......
  protected void doGet(HttpServletRequest request,HttpServletResponse response)
    throws ServletException,IOException{
    //使用ServletContext共享變量
    ServletContext sc = this.getServletContext();
    sc.setAttribute("sharingvar_sc", "context");
    //使用httpSession共性變量
    HttpSession session = request.getSession();
    session.setAttribute("sharingvar_se", "session");
    //使用HttpServletRequest共性變量
    request.setAttribute("sharingvar_req", "request");
    
    //在同一頁面中讀取共享變量
    String sc_value = (String)sc.getAttribute("sharingvar_sc");
    String session_value = (String) session.getAttribute("sharingvar_se");
    String request_value = (String) request.getAttribute("sharingvar_req");
    //在對用戶端的響應中顯示讀取的共享變量值
    response.setContentType("text/html; charset=UTF-8");
    PrintWriter out = response.getWriter();
    ......
    out.println("Context:" + sc_value + "<br>");
    out.println("Session:" + session_value + "<br>");
    out.println("Request:" + request_value + "<br>");
    out.println("<a href=/HelloWorld/GetSharingVars>");
    out.println("使用超連結跳轉到使用共享變量頁面");
    out.println("</a>");
    ......
  }
}      

/*提供共享變量的Servlet類：GetSharingVars.java*/
package javaee.servlet;
import ......
@WebServlet("/GetSharingVars")
public class GetSharingVars extends HttpServlet{
  ......
  protected void doGet(HttpServletRequest request,HttpServletResponse response)
    throws ServletException,IOException{
    //讀取共享變量的值
    ServletContext sc = this.getServletContext();
    HttpSession session = request.getSession();
    String sc_value = (String) sc.getAttribute("sharingvar_sc");
    String session_value = (String) session.getAttribute("sharingvar_se");
    String request_value = (String) request.getAttribute("sharingvar_req");  
    //在對用戶端的響應中顯示讀取的共享變量
    ......
    out.println("GetSharingVars輸出頁面<br>");
    out.println("在同一個頁面中讀取共享變量<br>");
    out.println("Context:" + sc_value + "<br>");
    out.println("Session:" + session_value + "<br>");
    out.println("Request:" + request_value + "<br>");
    .......
}      

/*提供共享變量的Servlet類：ShareSharingVarsDispatch.java*/
package javaee.servlet;
import ......
@WebServlet("/ShareAndDispatch")
public class ShareSharingVarsDispatch extends HttpServlet{
  ......
  protected void doGet(HttpServletRequest request, HttpServletResponse  response)
      throws ServletException,IOException{
    this.doPost(request, response);
  }
  protected void doPost(HttpServletRequest request,HttpServletResponse response)
      throws ServletException,IOException{
    //使用ServletContext共享變量
    ServletContext sc = this.getServletContext();
    sc.setAttribute("sharingvar_sc", "context");
    //使用HttpSession共享變量
    HttpSession session = request.getSession();
    session.setAttribute("sharingvar_se", "session");
    //使用HttpServletRequest共享變量
    request.setAttribute("sharingvar_req", "request");
    //使用轉發跳轉處理
    request.getRequestDispatcher("/GetSharingVars").forward(request, response);
  }
}      

/*提供共享變量的Servlet類：ShareSharingVarsAndRedirect.java*/
package javaee.servlet;
import ......
@WebServlet("/SharAndRedirect")
public class ShareSharingVarsAndRedirect extends HttpServlet{
  ......
  protected void doGet(HttpServletRequest request, HttpServletResponse  response)
      throws ServletException,IOException{
    this.doPost(request, response);
  }
  protected void doPost(HttpServletRequest request,HttpServletResponse response)
      throws ServletException,IOException{
    //使用ServletContext共享變量
    ServletContext sc = this.getServletContext();
    sc.setAttribute("sharingvar_sc", "context");
    //使用HttpSession共享變量
    HttpSession session = request.getSession();
    session.setAttribute("sharingvar_se", "session");
    //使用HttpServletRequest共享變量
    request.setAttribute("sharingvar_req", "request");
    //使用重定向跳轉處理
    response.sendRedirect("/HelloWorld/GetSharingVars");
  }
}      

同一個頁面内：

點選跳轉後(超連結)：

采用轉發機制處理（request.getRequestDispatch）：

采用重定向（response.sendRedirect）：

關閉後再打開浏覽器（http://localhost:8080/HelloWorld/GetSharingVars）:

PS：同一個頁面，或者采用轉發機制進行處理，ServletContext、HttpSession、HttpServletRequest的執行個體都不會發生改變。