Java 如何使两个servlet共享相同的项目列表

这是一个简单的CRUD网络应用程序

我有两个仆人

一个用于列出（

MyProjectListServlet.java

）页面上的所有项目

另一个处理项目信息的更新（

MyPprojectEditorServlet.java

）或将项目添加到列表中

目前，两个servlet都创建并维护自己的相同项列表。因此，列表Servlet始终显示其自己的未更改项列表，更新Servlet显示更新的项信息（如果编辑项，否则它将显示与列表Servlet相同的信息）

如何使两个servlet（

MyProjectListServlet

和

MyPprojectEditorServlet

）共享同一个

itemRepo

列表

MyProjectListServlet.java：

package com.myproject;

import java.io.IOException;

import javax.inject.Inject;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.myproject.MyProjectRepository;

@WebServlet("/project/")
public class MyProjectListServlet extends HttpServlet {

    @Inject
    private MyProjectRepositoryImpl itemsRepo = new MyProjectRepositoryImpl(); // creates brand new list of items

    public MyProjectListServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        request.setAttribute("items", itemsRepo.listItems());

        getServletContext().getRequestDispatcher("/WEB-INF/pages/item-list.jsp").forward(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
    }

}

package com.myproject;

import java.io.IOException;

import javax.inject.Inject;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.myproject.myitem;
import com.myproject.myprojectRepository;

@SuppressWarnings("serial")
@WebServlet("/project")
public class MyPprojectEditorServlet extends HttpServlet {

    @Inject
    private MyProjectRepositoryImpl itemRepo = new MyProjectRepositoryImpl(); //creates its own list of items

    public MyProjectEditorServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String id = request.getParameter("id");

        if(id != null && !id.isEmpty()){
            MyItem item = itemRepo.lookupitemById(id);
            request.setAttribute("item", item);

        }

        request.setAttribute("items", itemRepo.listMyItems());

        getServletContext().getRequestDispatcher("/WEB-INF/pages/item-form.jsp").forward(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub

        String str1 = request.getParameter("str1");
        String str2 = request.getParameter("str2");
        String str3 = request.getParameter("str3");
        String str4 = request.getParameter("str4");
        String str5 = request.getParameter("str5");

        String id = request.getParameter("id");
        if(id == null || id.isEmpty()){
            itemRepo.addItem(str1, str2, str3, str4, str5);
        }else{
            itemRepo.updateItem(id, str1, str2, str3, str4, str5);
        }
        response.sendRedirect(request.getContextPath() + "/project/");
    }

}

MyPprojectEditorServlet.java：

package com.myproject;

import java.io.IOException;

import javax.inject.Inject;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.myproject.MyProjectRepository;

@WebServlet("/project/")
public class MyProjectListServlet extends HttpServlet {

    @Inject
    private MyProjectRepositoryImpl itemsRepo = new MyProjectRepositoryImpl(); // creates brand new list of items

    public MyProjectListServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        request.setAttribute("items", itemsRepo.listItems());

        getServletContext().getRequestDispatcher("/WEB-INF/pages/item-list.jsp").forward(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
    }

}

package com.myproject;

import java.io.IOException;

import javax.inject.Inject;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.myproject.myitem;
import com.myproject.myprojectRepository;

@SuppressWarnings("serial")
@WebServlet("/project")
public class MyPprojectEditorServlet extends HttpServlet {

    @Inject
    private MyProjectRepositoryImpl itemRepo = new MyProjectRepositoryImpl(); //creates its own list of items

    public MyProjectEditorServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        String id = request.getParameter("id");

        if(id != null && !id.isEmpty()){
            MyItem item = itemRepo.lookupitemById(id);
            request.setAttribute("item", item);

        }

        request.setAttribute("items", itemRepo.listMyItems());

        getServletContext().getRequestDispatcher("/WEB-INF/pages/item-form.jsp").forward(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub

        String str1 = request.getParameter("str1");
        String str2 = request.getParameter("str2");
        String str3 = request.getParameter("str3");
        String str4 = request.getParameter("str4");
        String str5 = request.getParameter("str5");

        String id = request.getParameter("id");
        if(id == null || id.isEmpty()){
            itemRepo.addItem(str1, str2, str3, str4, str5);
        }else{
            itemRepo.updateItem(id, str1, str2, str3, str4, str5);
        }
        response.sendRedirect(request.getContextPath() + "/project/");
    }

}

---

您可以使用单例/应用程序或会话范围变量，或者对于更具可扩展性的解决方案，您可以将数据存储在类似JPA/Redis的格式中。

您有两个选项：

将列表另存为HttpSession对象（会话范围）的属性

将列表另存为ServletContext对象（应用程序范围）的属性

JPA（Java持久性API）允许您将运行时对象存储在数据库中，即使您重新启动容器或重新加载servlet，也可以访问它们。要使用JPA，类需要一些注释来定义对象在数据库中的存储方式

---

保存对象后，可以很容易地从其他servlet再次访问它，修改也会在基础数据库中更新。

但是我为MyProjectRepositoryImpl设置了@ApplicationScoped注释集，它实现了MyProjectRepository接口。这是否使它的应用程序范围像您所说的那样？是的，但是如果您有一个servlet从中读取，而另一个servlet可能同时更改列表的值，那么您需要通过添加同步包装器或其他方法来确保列表是线程安全的。但是，使用Redis，您不必担心线程安全，而且如前所述，更易于扩展。这实际上是一个非常好的用例CopyOnWriteArrayList，因为它是线程安全的，并且可以在视图中轻松迭代：