jakarta-ee - 我怎样才能改进我的 DAO？ - Java EE

Question

我想访问我的申请人数据库，这就是我为其创建 DAO 类的原因。

我觉得我有很多代码味道，因为我一直在重复一些代码。那么我该怎么做才能使我的代码更简单以实现更少的代码味道呢？我违反了哪些规则？我怎样才能改进我的代码？谢谢。

我的代码如下:

public class ApplicantDAO {

private static ApplicantDAO me = null;

private ApplicantDAO(){};

public static synchronized ApplicantDAO getInstance() {
    if(me == null) {
        me = new ApplicantDAO();
    }
    return me;
}

public Applicant getApplicant(int applicantNumber) throws SQLException {
    Applicant applicant = null;

    Connection conn = null; 
    Statement statement= null;
    String query = null;
    ResultSet rs = null;

    try {
        conn = ConnectionManager.getConnection();
        statement = conn.createStatement();
        query = "SELECT * FROM applicant WHERE applicant_no = '" + applicantNumber +"'";    //check applicant_number
        rs = statement.executeQuery(query);

        while(rs.next()){
            applicant = new Applicant();

            applicant.setApplicantNumber(rs.getInt("applicant_no"));
            applicant.setApplicationDate(rs.getString("applicant_date")); 
            applicant.setfName(rs.getString("first_name"));
            applicant.setlName(rs.getString("last_name"));
            applicant.setmName(rs.getString("middle_name"));
            applicant.setAge(rs.getInt("age"));
            applicant.setGender(rs.getString("gender"));
            applicant.setEmail(rs.getString("email_address"));
            applicant.setContactNumber(rs.getString("contact_no"));
            applicant.setCity(rs.getString("city"));
            applicant.setSchool(rs.getString("school"));
            applicant.setCourse(rs.getString("course"));
            applicant.setYearGraduated(rs.getInt("year_graduated"));
            applicant.setYearWorkExp(rs.getInt("year_work_exp"));
            applicant.setSourceChannel(rs.getString("source_channel"));
            applicant.setStatus_id(rs.getInt("status_id"));

        }

    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }finally {
        if (rs != null) try { rs.close(); } catch (SQLException logOrIgnore) {}
        if (statement != null) try { statement.close(); } catch (SQLException logOrIgnore) {}
        if (conn!= null) try { conn.close(); } catch (SQLException logOrIgnore) {}
    }


    return applicant;
}

public ArrayList<Applicant> getApplicants() throws SQLException{
    ArrayList<Applicant> applicantList = null;
    Applicant applicant = null;

    Connection conn = null; 
    Statement statement= null;
    String query = null;
    ResultSet rs = null;

    try {
        conn = ConnectionManager.getConnection();
        statement = conn.createStatement();
        query = "select * from applicant";
        rs = statement.executeQuery(query);

        while(rs.next()){
            if(applicantList == null){
                applicantList = new ArrayList<Applicant>();
            }
            applicant = new Applicant();

            applicant.setApplicantNumber(rs.getInt("applicant_no"));
            applicant.setApplicationDate(rs.getString("applicant_date")); 
            applicant.setfName(rs.getString("first_name"));
            applicant.setlName(rs.getString("last_name"));
            applicant.setmName(rs.getString("middle_name"));
            applicant.setAge(rs.getInt("age"));
            applicant.setGender(rs.getString("gender"));
            applicant.setEmail(rs.getString("email_address"));
            applicant.setContactNumber(rs.getString("contact_no"));
            applicant.setCity(rs.getString("city"));
            applicant.setSchool(rs.getString("school"));
            applicant.setCourse(rs.getString("course"));
            applicant.setYearGraduated(rs.getInt("year_graduated"));
            applicant.setYearWorkExp(rs.getInt("year_work_exp"));
            applicant.setSourceChannel(rs.getString("source_channel"));
            applicant.setStatus_id(rs.getInt("status_id"));

            applicantList.add(applicant);
        }

    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }finally{
         if (rs != null) try { rs.close(); } catch (SQLException logOrIgnore) {}
         if (statement != null) try { statement.close(); } catch (SQLException logOrIgnore) {}
         if (conn!= null) try { conn.close(); } catch (SQLException logOrIgnore) {}
    }
    return applicantList;
}

Answer 1

我看到的巨大、明显的问题:

1. DAO 是一个单例。为什么？
2. 你没有使用 EntityManager .还有，为什么？

如果您实际上使用的是 Java EE(正如问题所标记的那样)而不是 J2EE(这是一个围绕 Java 1.4 构建的悲惨规范)，那么 DAO 模式就完全没有必要了。 EntityManager 是新的 DAO。

看看 The Java EE 6 Tutorial - Persistence 的前几节.

---

we are required to use J2ee.. :(

好的，所以您仍然需要修复单例实现。没有理由只创建该对象的一个​​实例，因为它不存储任何内部状态。有一个简单的解决方法:

• 完全删除 private static ApplicantDAO me = null;

• 将 getInstance() 实现更改为

  public static ApplicantDAO getInstance() {
      return new ApplicantDAO();
  }
  

其他气味:

• 您几乎总是希望声明 List 而不是 ArrayList，因此请更改声明，如

  public ArrayList<Applicant> getApplicants() throws SQLException
  // to
  public List<Applicant> getApplicants() throws SQLException
  
  // and
  
  ArrayList<Applicant> applicantList = null;
  // to
  List<Applicant> applicantList = null;