Question

我是J2EE应用程序的新手，在尝试通过学生ID（也是我的主键）搜索数据库后，尝试显示数据库中的记录。 jdbc / studentDB是我的JNDI名称在studentdao.java中我的语法@Resource（name =“jdbc / studentDB”）是否足够用于连接？如果是，问题是方法executeFetchQuery（int id）吗？我出错的地方请帮忙。

    //search.java(servlet)

    ResultSet rs = ***studentDao.executeFetchQuery(v);***
    String fName = null; 
    try{
          fName = rs.getString("firstname");
        while(rs.next()){ 

    fName = rs.getString("firstname");


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

        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head>");
        out.println("<title>Servlet getdata</title>");            
        out.println("</head>");
        out.println("<body>");
        out.println("fname: "+ fName);
        out.println("</body>");
        out.println("</html>");

// studentdao.java

 //required imports
 @Stateless
 public class StudentDao implements StudentDaoLocal {
 @Resource(name="jdbc/studentDB")
 private DataSource datasource;


@PersistenceContext
private EntityManager em;

@Override
public void addStudent(Student student) {
    em.persist(student);
}

@Override
public void editStudent(Student student) {
    em.merge(student);
}

@Override
public void deleteStudent(int studentId) {
    em.remove(getStudent(studentId));
}

@Override
public Student getStudent(int studentId) {
    return em.find(Student.class, studentId);
}

@Override
public List<Student> getAllStudents() {
    return em.createNamedQuery("Student.getAll").getResultList();
}

/**
 *
 * @param id
 * @return
 */
@Override
***public  ResultSet executeFetchQuery(int id){***
    ResultSet rs = null;
    try{ 
       Connection conn = datasource.getConnection();
       String sql = "SELECT * FROM STUDENT WHERE STUDENTID = '" + id +"'";
       rs = conn.createStatement().executeQuery(sql);
       conn.close();
   } catch (Exception e) {
       System.err.println(e.getMessage());
   }
  return rs;
  }

//输出数据库中的id

fname：null

Answer 1

~~以下始终会导致异常，因为您在rs.next（）~~之前调用rs.getString（）

<德尔> 尝试{ fName = rs.getString（＆＃34; firstname＆＃34;）; 而（rs.next（））{ fName = rs.getString（＆＃34; firstname＆＃34;）; } } <德尔> 删除第一个`fName = rs.getString（＆＃34; firstname＆＃34;）;`try块就像 <德尔> 尝试{ 而（rs.next（））{ fName = rs.getString（＆＃34; firstname＆＃34;）; } }

<强>更新

在StudentDao.executeFetchQuery(int)中，您将返回ResultSet，但用于获取Connection的{{1}}已关闭，因此ResultSet也将关闭。

这不是正确的方法。从[{1}}返回ResultSet所以它应该是。

firstname

并在StudentDao.executeFetchQuery(int)

public String executeFetchQuery(int id){
  String firstname = null;
  try{ 
    Connection conn = datasource.getConnection();
    String sql = "SELECT * FROM STUDENT WHERE STUDENTID = '" + id +"'";
    rs = conn.createStatement().executeQuery(sql);
    if(rs.next()){
      firstName = rs.getString("firstname");
    }
    conn.close();
  }catch (Exception e) {
    System.err.println(e.getMessage());
  }
  return firstname;
}

无法在servlet http响应上从trycatch块打印输出字符串变量

1 个答案: