从JDBC检索值并使用JSTL标记来调用方法 [英] Retrieve values from JDBC and use JSTL tags to call the methods

问题描述

下面是我为从数据库中检索值而编写的代码（我添加了整个代码，以便您更容易理解我在这里想说的内容）：

Below is the code which i have written to retrieve values from the database (I have added the whole code so it will be easier for you to understand what i am trying to say here):

package ipscheme;

import java.sql.*;

public class AddRecords {

Connection con = new DBConnection().getConnection();
ResultSet  resultSet = null;  

public String [] populateSelect() throws SQLException {

    Statement statement = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);

    resultSet = statement.executeQuery("SELECT * FROM ipsections");

    resultSet.last();
    int size = resultSet.getRow();
    resultSet.beforeFirst();

    String [] sectionName = new String[size];

    int j = 0;
    while (resultSet.next()){
        sectionName[j] = resultSet.getString("section_name");
        j = j + 1;
    }

    resultSet.close();
    statement.close();
    con.close();

    return sectionName;

}

}

在这里是我用来将数据库中的值填充到选择框中的JSP代码。

And down here is the JSP Code which i have used to populate values from the database into a selectbox.

<select name="sltSection">
 <% 
    AddRecords added = new AddRecords();
    String sectionNm  [] = added.populateSelect();
        for(int i=0; i<sectionNm.length; i++){ %>   
    <option>
        <% out.print(sectionNm[i]); %>
    </option>
 <% } %>
</select>

以上代码工作正常，没有任何编译错误。正如您所看到的，我在.jsp页面中使用了Java代码来完成任务，这是一件错误的事情。

The above code works fine without any compilation errors. As you can see i have used Java code in my .jsp page to fulfill the task, which is a wrong thing to do.

我需要根据MVC实现这段代码，因此我应该使用JSTL标签来实现.jsp的代码？如果是这样我怎么做？如何从类 AddRecords 中实例化Object并调用其方法？有人可以帮帮我吗。在此先感谢！

I need to implement this code according to the MVC and therefore i should use JSTL tags to implement the code to .jsp? if so how do i do that? How do i instantiate Object from the class AddRecords and call its methods? Can someone please help me. Thanks in advance!

推荐答案

因为看起来您是Java Web Programming的新手，您可以使用JSP实现完整的MVC场景（视图），Servlet（控制器）和实体，DAO和服务层（模型）。这就是你拥有的：

Since it looks you're new to Java Web Programming, you can implement a full MVC scenario using JSP (View), Servlet (Controller) and Entities, DAOs and Service layer (Model). This is what you have:

型号：

• DBConnection 作为数据库访问类（数据访问层）


• AddRecords 作为您的DAO类（数据）访问层）



• 没有类作为服务类（业务逻辑层）。对于这个例子，我们有一个 RecordService 类：

• DBConnection as a database access class (Data access layer)
• AddRecords as your DAO class (Data access layer)

• No class as Service class (Business logic layer). For this example, let's have a RecordService class for this:

public class RecordService {
    public RecordService() {
    }
    //since it has no real business logic, it will serve as facade
    public String[] getRecords() {
        AddRecords addRecords = new AddRecords();
        return addRecords.populateSelect();
    }
}

控制器：

• 尚未获得Controller类。由于我假设您使用的是纯Java EE，因此您将使用Servlet（基于 Servlets StackOverflow wiki 的实现）：

@WebServlet("/RecordServlet")
public class RecordsServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
        //it will fire on a GET request (like accessing directly to the URL
        //from the browser)
        //here you should load the data for the View (JSP)
        loadData(request);
        //forward to show the view
        request.getRequestDispatcher("hello.jsp").forward(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
        //it will fire on a POST request (like submitting the form using POST method)
        String selectedRecord = request.getParameter("selectedRecord");
        System.out.println(selectedRecord);
        request.setAttribute("selectedRecord", selectedRecord);
        loadData(request);
        //forward to show the view
        request.getRequestDispatcher("hello.jsp").forward(request, response);
    }

    //method to load the data available to select
    private void loadData(HttpServletRequest request) {
        RecordService recordService = new RecordService();
        String[] records = recordService.getRecords();
        //set the data as attribute on the request to be available on the View
        request.setAttribute("records", records);
    }
}

查看：

• 您已有JSP文件。由于您尚未发布该名称，我们称之为 hello.jsp 。我将使用 JSTL 调整您的实际JSP代码：

• You already have a JSP file. Since you haven't posted the name, let's call it hello.jsp. I will just adap your actual JSP code using JSTL:

<!-- at the beginning of the JSP, call the JSTL library -->
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<form action="RecordServlet" method="POST">
    Please select an element:
    <select id="selectedRecord" name="selectedRecord">
        <c:forEach items="${records}" var="record">
        <option value="${record}">${record}</option>
        </c:forEach>
    </select>
    <br />
    <input type="submit" value="Show selected record" />
    <c:if test="${not empty selectedRecord}">
        <br />
        You've selected ${selectedRecord}!
    </c:if>
</form>

现在，要运行该示例，请构建使用 http：// localhost：8080 / YourWebProjectName / RecordServlet 进行项目和访问。此外，您可以在 web.xml 文件中设置：

Now, to run the example, build the project and access to it using http://localhost:8080/YourWebProjectName/RecordServlet. Also, you can set in the web.xml file:

<welcome-file-list>
    <welcome-file>RecordServlet</welcome-file>
</welcome-file-list>

只需运行网络项目。

一些注释：

• 使用类/方法的相关名称，我使用 RecordXXX 因为你有这个 AddRecord 类（它应该是 RecordDAO 或对代码读者更有用的东西，比如 UserDAO 或 YourEntityDAO ）。


• 通过不同的包分发您的类。每个包应仅包含其范围的相关类，即DAO类的 edu.home.dao ， edu.home.service 用于服务/业务逻辑类，以及......

• Use relevant names for the class/methods, I used RecordXXX since you had this AddRecord class (it should be RecordDAO or something more useful to code readers like UserDAO or YourEntityDAO).
• Distribute your classes through different packages. Every package should contain only relevant classes to its scope i.e. edu.home.dao for DAO classes, edu.home.service for service/business logic classes, and on...

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