ZK控制器中java空指针异常

Question

我在部署 ZK 时遇到一些错误

Jan 16, 2012 5:00:59 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [zkLoader] in context with path [/ZK509App6] threw exception
java.lang.NullPointerException
    at controller.jurnal.JournalController.<init>(JournalController.java:39)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at java.lang.Class.newInstance0(Class.java:355)
    at java.lang.Class.newInstance(Class.java:308)

这是我的 JournalController.java 类

public class JournalController extends GenericForwardComposer {

    private Listbox listlk;
    private Datebox datebox;
    private koneksi k;
    private Connection c;
    private SimpleDateFormat sdf;
    private Button pilih;
    private Date dates;
    private jurnalDAO jd = new jurnalDAO(k.getConnection(), dates);


    public JournalController() {
    }

    public void onClick$pilih(Event e) throws Exception, SQLException {
        try {
            Date d = datebox.getValue();
            d = dates;
            ForwardEvent forwardEvt = (ForwardEvent) e;
            Listbox Listlk = (Listbox) forwardEvt.getOrigin().getTarget();
        } catch (Exception a) {
            a.printStackTrace();
        }
    }
    public List<Jurnal_tbl> getAllEvents() throws SQLException, Exception {

        return jd.getLKeuangan();
    }
}

(JournalController.java:39) 行是 private jurnalDAO jd = new jurnalDAO(k.getConnection(), dates);

参考，这是 DAO 类

public class jurnalDAO {

    private Connection conn;
    private Date tanggal;

    public jurnalDAO(Connection conn, Date dates) {
        this.conn = conn;
        this.tanggal = dates;
    }


    public List<Jurnal_tbl> getLKeuangan() throws SQLException, Exception {
        PreparedStatement ps = null;

        try {
            List<Jurnal_tbl> llk = new ArrayList<Jurnal_tbl>();
            System.out.println("test1");


            String sql = "SELECT * from public.jurnal where tanggal = ?";
            ps = conn.prepareStatement(sql);
            ps.setDate(1, new java.sql.Date(tanggal.getTime()));
            System.out.println(ps.toString());;

            ResultSet rs = ps.executeQuery();
            Jurnal_tbl jt;
            while (rs.next()) {
                // llk.add(new Jurnal_tbl(rs.getString("kd_jurnal"), rs.getString("no_trans"), rs.getString("tanggal"), rs.getString("account"), rs.getString("descript"), rs.getString("dc"), rs.getDouble("amount"), rs.getString("no_item")));
                jt = new Jurnal_tbl();
                jt.setKode(rs.getString("kd_jurnal"));
                jt.setTransaksi(rs.getString("no_trans"));
                jt.setTanggal(rs.getString("tanggal"));
                jt.setAkun(rs.getString("akun"));
                jt.setDeskripsi(rs.getString("descript"));
                jt.setDC(rs.getString("dc"));
                jt.setAmount(rs.getDouble("amount"));
                jt.setItem(rs.getString("no_item"));
                llk.add(jt);
           }

            rs.close();
            return llk;
        } finally {
        }
    }
}

请帮助我，TT 这是我学士学位的最后一个项目 TT

Answer 1

您正在调用 k.getConnection()，但从未初始化 k，因此它将具有默认值 null。这肯定会抛出 NullPointerException 。您期望它在什么对象上调用getConnection()？

顺便说一句，值得注意的是名称：

• k对于实例变量来说是一个毫无意义的名称
• “journal”和“jurnal”之间不一致
• 类型名称koneksi< /code> 不遵循 Java 命名约定（大写）
• 变量名称 Listlk 不遵循 Java 命名约定（大写）
• 类型名称 Jurnal_tbl 不遵循 Java命名约定（下划线）

另外：

• 你几乎永远不应该捕获Exception
• 你应该关闭finally块中的资源

（我还没有对代码进行彻底的分析 - 这只是我的初步印象。）