编程中国 - Java Servlet和JSP教程

　　 10.2.3 在会话对象中保存数据

　　如上节所述，读取保存在会话中的信息使用的是getValue方法（或，对于2.2版的Servlet规范，使用getAttribute）。保存数据使用putValue（或setAttribute）方法，并指定键和相应的值。注意putValue将替换任何已有的值。有时候这正是我们所需要的（如下例中的referringPage），但有时我们却需要提取原来的值并扩充它（如下例previousItems）。示例代码如下：
  HttpSession session = request.getSession(true);
  session.putValue("referringPage", request.getHeader("Referer"));
  ShoppingCart previousItems =
    (ShoppingCart)session.getValue("previousItems");
  if (previousItems == null) {
    previousItems = new ShoppingCart(...);
  }
  String itemID = request.getParameter("itemID");
  previousItems.addEntry(Catalog.getEntry(itemID));

  session.putValue("previousItems", previousItems);

10.3 实例：显示会话信息

　　下面这个例子生成一个Web页面，并在该页面中显示有关当前会话的信息。
package hall;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.net.*;
import java.util.*;

public class ShowSession extends HttpServlet {
  public void doGet(HttpServletRequest request,
                 HttpServletResponse response)
      throws ServletException, IOException {
    HttpSession session = request.getSession(true);
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    String title = "Searching the Web";
    String heading;
    Integer accessCount = new Integer(0);;
    if (session.isNew()) {
      heading = "Welcome, Newcomer";
    } else {
      heading = "Welcome Back";
      Integer oldAccessCount =
        // 在Servlet API 2.2中使用getAttribute而不是getValue
        (Integer)session.getValue("accessCount");
      if (oldAccessCount != null) {
        accessCount =
          new Integer(oldAccessCount.intValue() + 1);
      }
    }
    // 在Servlet API 2.2中使用putAttribute
    session.putValue("accessCount", accessCount);

    out.println(ServletUtilities.headWithTitle(title) +
                "＜BODY BGCOLOR=＼"#FDF5E6＼"＞＼n" +
                "＜H1 ALIGN=＼"CENTER＼"＞" + heading + "＜/H1＞＼n" +
                "＜H2＞Information on Your Session:＜/H2＞＼n" +
                "＜TABLE BORDER=1 ALIGN=CENTER＞＼n" +
                "＜TR BGCOLOR=＼"#FFAD00＼"＞＼n" +
                "  ＜TH＞Info Type＜TH＞Value＼n" +
                "＜TR＞＼n" +
                "  ＜TD＞ID＼n" +
                "  ＜TD＞" + session.getId() + "＼n" +
                "＜TR＞＼n" +
                "  ＜TD＞Creation Time＼n" +
                "  ＜TD＞" + new Date(session.getCreationTime()) + "＼n" +
                "＜TR＞＼n" +
                "  ＜TD＞Time of Last Access＼n" +
                "  ＜TD＞" + new Date(session.getLastAccessedTime()) + "＼n" +
                "＜TR＞＼n" +
                "  ＜TD＞Number of Previous Accesses＼n" +
                "  ＜TD＞" + accessCount + "＼n" +
                "＜/TABLE＞＼n" +
                "＜/BODY＞＜/HTML＞");
  }
  public void doPost(HttpServletRequest request,
                 HttpServletResponse response)
      throws ServletException, IOException {
    doGet(request, response);
  }
}

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