如何在Java中保存变量的值

debugcn 发表于 Dev

路西法

我有一个要求，我需要保存一个变量的值。我的问题是，我需要将网页中的值第一次发送到servlet，其中变量的值首次为null，但是当我从选择框中选择一个值时，它就进入了servlet，并且正在处理该值，但是我的问题是在这里我需要选择值后刷新页面。因此，现在当我执行该操作时，该值再次变为零且未发生该操作，可以从选择中选择一些值后保存该变量的值吗？

这是我的代码。

   <body>
    Select Country:
    <select id="country">
        <option>Select Country</option>
        <option value="1">1</option>
        <option value="2">2</option>
        <option value="3">3</option>

    </select>

    <input type="button" value="Reload page" onclick="reloadPage()">
</body>


<script>
    function reloadPage(){

        location.reload();
    }
</script>  



 <script>
        $(document).ready(function() {
            $('#country').change(function(event) {  
                var $country=$("select#country").val();
                $.get('JsonServlet',{countryname:$country},function(responseJson) {   
                    var $select = $('#states');                           
                    $select.find('option').remove();                          
                    $.each(responseJson, function(key, value) {               
                        $('<option>').val(key).text(value).appendTo($select);      
                    });
                });
            });
        });          
    </script>

这是我的servlet

public class JsonServlet extends HttpServlet {

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    PrintWriter out = response.getWriter();
    String value = request.getParameter("countryname");
    System.out.println("comes from ajax" + value);
    JsonGenerator generator = new JsonGenerator();
    Entry entry = null;
    if (value != null) {

        HttpSession session = request.getSession();

session.setAttribute("value", value);
        entry = generator.aMethod2Json(value);
        Gson g = new Gson();

        response.setContentType("application/json");
        response.setCharacterEncoding("UTF-8");
        response.getWriter().write(g.toJson(entry));


    } else {
        entry = generator.aMethod2Json("1");
        Gson g = new Gson();

        response.setContentType("application/json");
        response.setCharacterEncoding("UTF-8");
        response.getWriter().write(g.toJson(entry));


    }



}

// <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
/**
 * Handles the HTTP
 * <code>GET</code> method.
 *
 * @param request servlet request
 * @param response servlet response
 * @throws ServletException if a servlet-specific error occurs
 * @throws IOException if an I/O error occurs
 */
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    processRequest(request, response);
}

/**
 * Handles the HTTP
 * <code>POST</code> method.
 *
 * @param request servlet request
 * @param response servlet response
 * @throws ServletException if a servlet-specific error occurs
 * @throws IOException if an I/O error occurs
 */
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    processRequest(request, response);
}

/**
 * Returns a short description of the servlet.
 *
 * @return a String containing servlet description
 */
@Override
public String getServletInfo() {
    return "Short description";
}// </editor-fold>

}

进步

我希望一些代码示例可以为您提供帮助：

一个简单的柜台

为了演示servlet的生命周期，我们将从一个简单的示例开始。例3-1显示了一个servlet，它计算并显示访问它的次数。为了简单起见，它输出纯文本。

示例3-1 一个简单的柜台

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

public class SimpleCounter extends HttpServlet {

  int count = 0;

  public void doGet(HttpServletRequest req, HttpServletResponse res) 
                           throws ServletException, IOException {
    res.setContentType("text/plain");
    PrintWriter out = res.getWriter();
    count++;
    out.println("Since loading, this servlet has been accessed " +
            count + " times.");
  }
}

否则，如果您想要更高级的选择，一个好的选择是使用A Holistic Counter：

示例3-2 整体柜台

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

public class HolisticCounter extends HttpServlet {

  static int classCount = 0;  // shared by all instances
  int count = 0;              // separate for each servlet
  static Hashtable instances = new Hashtable();  // also shared

  public void doGet(HttpServletRequest req, HttpServletResponse res) 
                           throws ServletException, IOException {
    res.setContentType("text/plain");
    PrintWriter out = res.getWriter();

    count++;
    out.println("Since loading, this servlet instance has been accessed " +
            count + " times.");

    // Keep track of the instance count by putting a reference to this
    // instance in a Hashtable. Duplicate entries are ignored. 
    // The size() method returns the number of unique instances stored.
    instances.put(this, this);
    out.println("There are currently " + 
            instances.size() + " instances.");

    classCount++;
    out.println("Across all instances, this servlet class has been " +
            "accessed " + classCount + " times.");
  }
}

该HolisticCounter使用count实例变量跟踪自己的访问计数，使用classCount类变量跟踪共享计数，并使用实例哈希表（必须是类变量的另一个共享资源）跟踪实例的数量。

参考 Jason的Java Servlet编程

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