可以在jsf中显示的行数限制

时间:2013-02-20 10:11:10

标签: java mysql jsf

我开发了jsf应用程序。我有一个MySQL Student表,包含30-40列和600行。以下是StudentBean中的代码:

@ManagedBean(name = "student")
@SessionScoped
public class StudentBean implements Serializable{

private List<Student> studentlist = new ArrayList<Student>();
private Student student = new Student();
private HtmlDataTable datatable;
private HtmlInputHidden studentnpm = new HtmlInputHidden();
private String querybynpm;
private String querybyangkatan;
private String status;
private String result;

public String init() {

    try {
        Class.forName(JurusanConection.getDriver());
    } catch (ClassNotFoundException e) {
        System.out.println(e.getMessage());
    }


    try {
        Connection con = DriverManager.getConnection(JurusanConection.getUrl(),
                JurusanConection.getUser_name(), JurusanConection.getPass_word());

        String query = "SELECT * FROM student";
        Statement ps = con.createStatement();
        ResultSet rs = ps.executeQuery(query);

        while (rs.next()) {
            Student students = new Student();
            students.setNpm(rs.getString("npm"));
            students.setNama(rs.getString("nama").toString());
            //setter and getter 
            students.setTanggalregistrasi(rs.getString("tanggalregistrasi").toString());
            students.setDosenwali(rs.getString("dosenwali").toString());
            students.setEmail(rs.getString("email"));
            students.setKelas(rs.getString("kelas"));
            studentlist.add(students);
        }
        result="mahasiswa";
        ps.close();
        con.close();

    } catch (SQLException e) {
        setResult(null);
        setStatus(e.getMessage());
    }
    return result;
}

我没有通过构造函数填充数据库,而是调用init()方法。我收到一条错误消息javax.faces.el.E​​valuationException:java.lang.NullPointerException。但是当我修改查询以限制显示150行的数字时,没有错误。任何人都可以弄清楚如何显示这600行没有错误。谢谢......

1 个答案:

答案 0 :(得分:1)

您可能会在以下不安全的代码行中获取NullPointerExceptions:

rs.getString("nama").toString()
rs.getString("tanggalregistrasi").toString()
rs.getString("dosenwali").toString()

这假设从不是记录集中这三列的空值。您应该将此分配给变量并在使用.运算符之前进行空检查。

此外,你为什么在这里调用toString()? ResultSet.getString方法已经返回一个String对象。这完全没必要。