結果集只在while循環中迭代一次

Question

所以我在我的java代碼中使用Statement來訪問mySql數據庫來執行我的查詢並將返回的ResultSet保存在ResultSet對象中然后使用相同的ResultSet對象，我迭代結果中的行並簡單地打印出每行的數據。

這是代碼：

public class DBConnect {
    private Connection conn;
    private Statement st;
    private ResultSet rs;

    public DBConnect(){
        try{
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/unal_grades", "root", "");
            st = conn.createStatement();
        }catch(Exception ex){
            System.out.println("Error: " + ex);
        }
    }

public ArrayList<Semester> getSemesters(){
    try{
        ArrayList<Semester> semesters = new ArrayList<Semester>();
        String query = "Select * from semester";
        rs = st.executeQuery(query);
        System.out.println("semesters");
        while(rs.next()){
            int id = rs.getInt("id");
            String name = rs.getString("name");
            float average = rs.getFloat("average");
            boolean active = rs.getBoolean("active");

            System.out.println(name+" / "+average+" / "+active);
            Semester semester = new Semester(id, name, average, active);
            semester.setClasses(getClasses(semester));
            semesters.add(semester);
        }
        System.out.println();
        return semesters;
    }catch(Exception ex){
        System.out.println("Error: " + ex);
    }
    return null;
}

我的問題是在while循環中， rs.next()語句僅在第一次迭代時返回true。 換句話說，它只在實際擁有許多行時打印數據庫中我表的第一行。

在調試時，如果我在實際運行它之前檢查rs.next() ，當我在第一次迭代時實際運行它時，它進入循環並打印出我表的第二行。

非常感謝您的幫助。

Answer 1

我的問題是在getClasses()方法中我也修改了rs變量，所以在while循環中變量不一樣所以它返回false。

解決方案是在每個方法中使ResultSet rs變量本地化。

像這樣：

public class DBConnect {
    private Connection conn;

    public DBConnect(){
        try{
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/unal_grades", "root", "");
        }catch(Exception ex){
             System.out.println("Error: " + ex);
        }
    }

    public ArrayList<Semester> getSemesters(){
        ResultSet rs;
        try{
            Statement st = conn.createStatement();
            ArrayList<Semester> semesters = new ArrayList<Semester>();
            String query = "Select * from semester";
            rs = st.executeQuery(query);
            System.out.println("semesters");
            while(rs.next()){
                int id = rs.getInt("id");
                String name = rs.getString("name");
                float average = rs.getFloat("average");
                boolean active = rs.getBoolean("active");

                System.out.println(name+" / "+average+" / "+active);
                Semester semester = new Semester(id, name, average, active);
                semester.setClasses(getClasses(semester));
                semesters.add(semester);
            }
            System.out.println();
            return semesters;
        }catch(Exception ex){
            System.out.println("Error: " + ex);
        }
        return null;
    }
}

Answer 2

Statement st;
ResultSet rs;
try{
  String query = "Select * from semester";
  st = con.createStatement();
  rs = st.executeQuery(query);
  System.out.println("semesters");

  while(rs.next()){
    int id = rs.getInt("id");
    String name = rs.getString("name");
    float average = rs.getFloat("average");
   boolean active = rs.getBoolean("active");

    System.out.println(name+" / "+average+" / "+active);
}

System.out.println();
} catch(Exception ex){
   System.out.println("Error: " + ex);
}