使用JSP / JAVA从mysql下拉列表

Question

我有两张桌子： 第一个表，有两列专业（profession_id，profession_name）。 第二个表，申请人有五列（applicant_id，profession_id，last_name，first_name，entrance_year）。 表中的profession_id表和表中的profession_id是MySQL中的相关字段。 我需要，而不是profession_id，在表申请人中获得可用职业的下拉列表。 我的桌子：

 CREATE TABLE PROFESSION (
  PROFESSION_ID   INT         NOT NULL AUTO_INCREMENT,
  PROFESSION_NAME VARCHAR(50) NOT NULL,
  PRIMARY KEY (PROFESSION_ID)
);

CREATE TABLE APPLICANT (
  APPLICANT_ID  INT         NOT NULL AUTO_INCREMENT,
  PROFESSION_ID INT         NOT NULL,
  LAST_NAME     VARCHAR(30) NOT NULL,
  FIRST_NAME    VARCHAR(30) NOT NULL,
  ENTRANCE_YEAR INT         NOT NULL,
  PRIMARY KEY (APPLICANT_ID),
  FOREIGN KEY (PROFESSION_ID) REFERENCES PROFESSION (PROFESSION_ID)
);

表申请人的方法：

public Applicant getApplicant(long applicantId) throws Exception {
    PreparedStatement preparedStatement = null;
    Applicant applicant = null;
    try {
        preparedStatement = connection.prepareStatement("SELECT * FROM applicant WHERE applicant_id=?");
        preparedStatement.setInt(1, (int) applicantId);

        ResultSet resultSet = preparedStatement.executeQuery();
        while (resultSet.next()) {
            applicant = new Applicant();
            applicant.setId(resultSet.getInt("applicant_id"));
            applicant.setFirstName(resultSet.getString("first_name"));
            applicant.setLastName(resultSet.getString("last_name"));
            applicant.setProfessionId(resultSet.getInt("profession_id"));
            applicant.setEntranceYear(resultSet.getInt("entrance_year"));
        }
    } catch (SQLException e) {
        e.printStackTrace();
    } finally {
        if (preparedStatement != null) {
            preparedStatement.close();
        }
    }

    return applicant;
}

public List<Applicant> getApplicants() throws Exception {
    Statement statement = null;
    List <Applicant> applicants = new ArrayList<>();

    try {
        statement = connection.createStatement();
        //ResultSet resultSet = statement.executeQuery("SELECT * FROM applicant");
        ResultSet resultSet = statement.executeQuery("select a.applicant_id, a.first_name, a.last_name, a.entrance_year, p.profession_name from applicant a join profession p on a.profession_id = p.profession_id");
        Applicant applicant = null;
        while (resultSet.next()) {
            applicant = new Applicant();
            applicant.setId(resultSet.getInt("applicant_id"));
            applicant.setFirstName(resultSet.getString("first_name"));
            applicant.setLastName(resultSet.getString("last_name"));
            //applicant.setProfessionId(resultSet.getInt("profession_id"));
            applicant.setProfessionName(resultSet.getString("profession_name"));
            applicant.setEntranceYear(resultSet.getInt("entrance_year"));
            applicants.add(applicant);

        }

    } catch (SQLException e) {
        throw new Exception(e);
    }

    return applicants;
}

public void saveApplicant(Applicant applicant) throws Exception {
    PreparedStatement preparedStatement = null;

    try {
        if (applicant.getId() == -1) {
            preparedStatement = connection.prepareStatement("INSERT INTO applicant (first_name, last_name, profession_id, entrance_year) VALUES (?,?,?,?)");

            preparedStatement.setString(1, applicant.getFirstName());
            preparedStatement.setString(2, applicant.getLastName());
            preparedStatement.setInt(3, (int)applicant.getProfessionId());
            preparedStatement.setInt(4, applicant.getEntranceYear());

        } else {
            preparedStatement = connection.prepareStatement("UPDATE applicant SET first_name=?, last_name=?, profession_id=?, entrance_year=?  WHERE applicant_id=?");

            preparedStatement.setString(1, applicant.getFirstName());
            preparedStatement.setString(2, applicant.getLastName());
             preparedStatement.setInt(3, (int) applicant.getProfessionId());
            preparedStatement.setInt(4, applicant.getEntranceYear());
            preparedStatement.setInt(5, (int) applicant.getId());
        }
        preparedStatement.executeUpdate();
    } catch (SQLException e) {
        throw new Exception(e);
    } finally {
        if (preparedStatement != null) {
            preparedStatement.close();
        }
    }
}

在方法List getApplicants（）中，我实现了INNER JOIN，并且在结果表中，我有而不是applicant_id，专业名称。看：

ID  First Name  Last Name   Profession ID   Entrance Year   Actions
1   The         Rock        Java            2007            Delete Edit
2   CM          Punk        JS              2008            Delete Edit
3   Duke        Nukem       Java            2007            Delete Edit
Add new applicant

之前，它的方法显示ID，而不是专业名称

ID  First Name  Last Name   Profession ID   Entrance Year   Actions
1   The         Rock        1               2007            Delete Edit
2   CM          Punk        2               2008            Delete Edit
3   Duke        Nukem       1               2007            Delete Edit
Add new applicant

是的，我有一份专业清单，而不是职业ID。但是，我需要在方法saveApplicant中更改，以获取下拉列表。

我的类saveApplicant：

public class SaveApplicantCommand implements ICommand {

    private ApplicantDBProvider provider = ApplicantDBProvider.INSTANCE;

    @Override
    public String execute(HttpServletRequest request, HttpServletResponse resp) {

            Applicant applicant = new Applicant();

        applicant.setFirstName(request.getParameter("first_name"));
        applicant.setLastName(request.getParameter("last_name"));
        applicant.setProfessionId(Long.parseLong(request.getParameter("profession_id")));
        applicant.setEntranceYear(Integer.parseInt(request.getParameter("entrance_year")));

        if (request.getParameter("applicant_id") != null) {
            applicant.setId(Long.parseLong(request.getParameter("applicant_id")));
        }

        try {
            provider.saveApplicant(applicant);
        } catch (Exception e) {
            request.setAttribute("error", e);
            return "pages/error.jsp";
        }

        return "controller?command=applicants";
    }
}

我的jsp。 edit_applicant：

    <%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
  <title></title>
</head>
<body>
<h1>Add applicant</h1>

<form method="post" action="controller?command=saveApplicant">

  <table>
    <tr>
      <th>First Name</th>
      <th>Last Name</th>
      <th>Profession ID</th>
      <th>Entrance Year</th>
    </tr>
  </table>

  <c:choose>
    <c:when test="${applicant ne null}">
      <input type="text" name="first_name" value="${applicant.getFirstName()}"/>
      <input type="text" name="last_name" value="${applicant.getLastName()}"/>
      <input type="text" name="profession_id" value="${applicant.getProfessionId()}"/>
      <input type="text" name="entrance_year" value="${applicant.getEntranceYear()}"/>
      <input type="hidden" name="applicant_id" value="${applicant.getId()}"/>
    </c:when>
    <c:otherwise>
      <input type="text" name="first_name" value=""/>
      <input type="text" name="last_name" value=""/>
      <input type="text" name="profession_id" value=""/>
      <input type="text" name="entrance_year" value=""/>
    </c:otherwise>
  </c:choose>
  <input type=submit value=submit>
</form>
</body>
</html>

我听说，用标签实现的下拉列表。但是，如何做到这一点，我不知道。我需要，从第二张表中获取专业清单。在方法getApplicants（）中，我实现了它。但是，在方法getApplicants（）中，我使用INNER。在方法saveApplicant（）中，我使用了INSERT和UPDATE。以及如何从其他表中下载专业列表，我不知道。请帮忙。谢谢。抱歉，您需要阅读大量代码。