Java 使用arraylist保存servlet中的字符串，并在其他servlet中检索相同的arraylist以检索数据

大家好，我是java新手，我正在尝试通过一个通过html发送的servlet将表单数据保存到数组列表中，并使用第二个servlet检索相同的数组列表。 问题是，我可以使用ServletOne使用“is.setAl（str）；”将数据添加到列表中，也可以在ServletOne中检索数据。但当我尝试使用“arrList=is.getAl（）；”检索servlet 2中的数据时，它给出了null值

下面是我的servlet和目标代码

---

Servlet 1：

public class TestServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
public static InfoSave is = new InfoSave();
/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    doPost(request, response);
}

/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    BufferedReader br =request.getReader();

    String str = br.readLine();
    System.out.println("The String Recieved is:\n"+str);

    is.setAl(str);  


    System.out.println("----> "+is.getAl()+"\n");

    response.setContentType("application/json");
    response.getWriter().write(str);
}

...
InfoSave is = new InfoSave();
is.setAl(str);
request.getSession().setAttribute("IS", is);
...

---

Servlet 2：

public class SearchServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
private static InfoSave is = new InfoSave();
/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
 *      response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    // TODO Auto-generated method stub
    doPost(request, response);
}

/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
 *      response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    // TODO Auto-generated method stub
    BufferedReader br = request.getReader();

    String str = br.readLine();
    System.out.println(str);


     ArrayList<String> arrList = null, result = null;

      arrList = is.getAl();
      System.out.println("\n"+arrList);
      result = new ArrayList<>();
      for (Iterator iterator = arrList.iterator(); iterator.hasNext();)
      {
        String string = (String) iterator.next();
        if(string.contains(str)){
            result.add(string);
        }else{
            String message = "No Matching result Found";
            response.setContentType("application/json");
            response.getWriter().write(str);
        }

      }
      System.out.println("Size: "+result.size());
      String json = new Gson().toJson(result);


    response.setContentType("application/json");
    response.getWriter().write(json);
}
}

...
InfoSave is = (InfoSave) request.getSession().getAttribute("IS");
arrList = is.getAl();
...

公共类SearchServlet扩展了HttpServlet{
私有静态最终长serialVersionUID=1L；
private static InfoSave is=new InfoSave（）；
/**
*@参见HttpServlet#doGet（HttpServletRequest请求，HttpServletResponse
*（答复）
*/
受保护的void doGet（HttpServletRequest请求、HttpServletResponse响应）
抛出ServletException、IOException{
//TODO自动生成的方法存根
doPost（请求、响应）；
}
/**
*@参见HttpServlet#doPost（HttpServletRequest，HttpServletResponse
*（答复）
*/
受保护的void doPost（HttpServletRequest请求、HttpServletResponse响应）
抛出ServletException、IOException{
//TODO自动生成的方法存根
BufferedReader br=request.getReader（）；
字符串str=br.readLine（）；
系统输出打印项次（str）；
ArrayList arrList=null，result=null；
arrList=is.getAl（）；
System.out.println（“\n”+arrList）；
结果=新的ArrayList（）；
for（Iterator Iterator=arrList.Iterator（）；Iterator.hasNext（）；）
{
字符串=（字符串）迭代器。下一步（）；
if（string.contains（str））{
结果。添加（字符串）；
}否则{
String message=“未找到匹配结果”；
setContentType（“应用程序/json”）；
response.getWriter（）.write（str）；
}
}
System.out.println（“大小：+result.Size（））；
字符串json=new Gson（）.toJson（结果）；
setContentType（“应用程序/json”）；
response.getWriter（）.write（json）；
}
}

---

ArrayList对象：

public class InfoSave {
private ArrayList<String> al = new ArrayList<>();

public ArrayList<String> getAl() {
    return al;
}

public void setAl(String s) {
    al.add(s);
}
}

公共类信息保存{
private ArrayList al=new ArrayList（）；
公共数组列表getAl（）{
返回al；
}
公共无效集合（字符串s）{
等。添加（s）；
}
}

在Servlet 2中：

public class SearchServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
private static InfoSave is = new InfoSave();
/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
 *      response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    // TODO Auto-generated method stub
    doPost(request, response);
}

/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
 *      response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    // TODO Auto-generated method stub
    BufferedReader br = request.getReader();

    String str = br.readLine();
    System.out.println(str);


     ArrayList<String> arrList = null, result = null;

      arrList = is.getAl();
      System.out.println("\n"+arrList);
      result = new ArrayList<>();
      for (Iterator iterator = arrList.iterator(); iterator.hasNext();)
      {
        String string = (String) iterator.next();
        if(string.contains(str)){
            result.add(string);
        }else{
            String message = "No Matching result Found";
            response.setContentType("application/json");
            response.getWriter().write(str);
        }

      }
      System.out.println("Size: "+result.size());
      String json = new Gson().toJson(result);


    response.setContentType("application/json");
    response.getWriter().write(json);
}
}

...
InfoSave is = (InfoSave) request.getSession().getAttribute("IS");
arrList = is.getAl();
...

arrList=is.getAl（）更改为
TestServlet.is.getAl（）

在Servlet 2中，
是
，Servlet 1是不同的对象。
在Servlet 2中，您访问的是Servlet 2的静态变量
是
，而不是Servlet 1的
是
变量；它们不引用相同的
InfoSave
对象。servlet 2中的变量未填充，当调用
时，返回的列表为空

在这种情况下，如果您想在这两个servlet之间共享信息，可以将对象放在servlet 1的会话中，并从servlet 2中的会话中检索

Servlet 1：

public class TestServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
public static InfoSave is = new InfoSave();
/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    doPost(request, response);
}

/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    BufferedReader br =request.getReader();

    String str = br.readLine();
    System.out.println("The String Recieved is:\n"+str);

    is.setAl(str);  


    System.out.println("----> "+is.getAl()+"\n");

    response.setContentType("application/json");
    response.getWriter().write(str);
}

...
InfoSave is = new InfoSave();
is.setAl(str);
request.getSession().setAttribute("IS", is);
...

Servlet 2：

public class SearchServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
private static InfoSave is = new InfoSave();
/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
 *      response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    // TODO Auto-generated method stub
    doPost(request, response);
}

/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
 *      response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    // TODO Auto-generated method stub
    BufferedReader br = request.getReader();

    String str = br.readLine();
    System.out.println(str);


     ArrayList<String> arrList = null, result = null;

      arrList = is.getAl();
      System.out.println("\n"+arrList);
      result = new ArrayList<>();
      for (Iterator iterator = arrList.iterator(); iterator.hasNext();)
      {
        String string = (String) iterator.next();
        if(string.contains(str)){
            result.add(string);
        }else{
            String message = "No Matching result Found";
            response.setContentType("application/json");
            response.getWriter().write(str);
        }

      }
      System.out.println("Size: "+result.size());
      String json = new Gson().toJson(result);


    response.setContentType("application/json");
    response.getWriter().write(json);
}
}

...
InfoSave is = (InfoSave) request.getSession().getAttribute("IS");
arrList = is.getAl();
...

这样，您就不需要在servlet中使用静态变量

有关servlet会话的更多信息：

嗯，甚至不要尝试这样做

Servlet旨在以可能的多线程方式从Servlet容器中为不同的客户端会话调用。这意味着它们应该是无状态对象。您可以在一个servlet中使用一个可公开访问的静态
InfoSave
对象，并从另一个servlet中使用它，但这不是javaee的方式

如果希望每个活动会话都有一个InfoSave，或者在servlet上下文中使用一个单独的共享列表，那么使用session属性是正确的。这样做的好处是，您可以更轻松地从一路切换到另一路

下面是使用会话的示例代码：

public class TestServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    ...    
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Get access to a session scoped InfoSave - local to keep the servlet stateless
        HttpSession session = request.getSession();
        InfoSave is = (InfoSave) session.getAttribute("INFOSAVE");
        if (is == null) { // no InfoSave here: create a brand new one
            is = new InfoSave();
            session.setAttribute("INFOSAVE", is); // store it into the session
        }
        // the remaining of the code can safely access the session scoped InfoSave
        ...
    }
}

顺便说一句，如果您想使用静态InfoSave或将其存储在servlet上下文属性中，您应该同步访问，因为多个线程可以同时执行一个servlet-但希望只有一个线程可以使用给定的会话…
您需要创建InfoSave对象以访问其方法