Java應用程序查詢數據庫返回nullpointerexception

Question

我在我的代碼在java中有問題。它應該將數據插入到數據庫中，但它返回一個nullpointerexception。這是我的代碼。

package client; 
import java.sql.*; 
import java.awt.Desktop; 
import java.awt.EventQueue; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 
import java.io.*; 
import java.util.*; 
import java.util.logging.Level; 
import java.util.logging.Logger; 
import javax.swing.JFrame; 
import javax.swing.JTextField; 
import javax.swing.JLabel; 
import javax.swing.JComboBox; 
import javax.swing.JButton; 
import javax.swing.JFileChooser; 
import javax.swing.JOptionPane; 

public class AddCon { 


    private Connection conn; 
private JFrame frmAddContract; 
private JTextField ContractNo; 
private JTextField EngagerName; 
private JTextField contNo; 
    private JTextField month; 
    private JTextField day; 
private JTextField year; 
private JTextField tme; 
private JTextField cladd; 
private JTextField eventadd; 
private JTextField textField_fp; 
    private JTextField refer; 


/** 
* Launch the application. 
*/ 
public static void main(String[] args) { 



     EventQueue.invokeLater(new Runnable() { 
     public void run() { 
      try { 
       AddCon window = new AddCon(); 
       window.frmAddContract.setVisible(true); 
      } catch (Exception e) { 
       e.printStackTrace(); 
      } 
     } 
    }); 
} 

/** 
* Create the application. 
*/ 
public AddCon() { 

     try{ 
      //Load database driver and connect 
       Class.forName("com.mysql.jdbc.Driver"); 
       Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/KusinaNiKambal","root","1234"); 
       Statement stmt = null; 
       stmt = conn.createStatement();} 

     catch (Exception e){ 
       JOptionPane.showMessageDialog(null, e);} 

/** 
* Initialize the contents of the frame. 
*/ 
    initialize(); 
} 
private void initialize() { 
    frmAddContract = new JFrame(); 
    frmAddContract.setResizable(false); 
    frmAddContract.setTitle("Add Contract"); 
    frmAddContract.setBounds(100, 100, 450, 640); 
    frmAddContract.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    frmAddContract.getContentPane().setLayout(null); 

    ContractNo = new JTextField(); 
    ContractNo.setBounds(38, 32, 152, 20); 
    frmAddContract.getContentPane().add(ContractNo); 
    ContractNo.setColumns(10); 

    JLabel lblContract = new JLabel("Contract #"); 
    lblContract.setBounds(38, 18, 76, 14); 
    frmAddContract.getContentPane().add(lblContract); 

    JLabel lblEngager = new JLabel("Engager"); 
    lblEngager.setBounds(38, 63, 48, 14); 
    frmAddContract.getContentPane().add(lblEngager); 

    EngagerName = new JTextField(); 
    EngagerName.setBounds(38, 78, 152, 20); 
    frmAddContract.getContentPane().add(EngagerName); 
    EngagerName.setColumns(10); 

    contNo = new JTextField(); 
    contNo.setBounds(38, 128, 152, 20); 
    frmAddContract.getContentPane().add(contNo); 
    contNo.setColumns(10); 

    JLabel lblNewLabel = new JLabel("Contact #"); 
    lblNewLabel.setBounds(38, 109, 86, 14); 
    frmAddContract.getContentPane().add(lblNewLabel); 

    month = new JTextField(); 
    month.setBounds(38, 255, 116, 20); 
    frmAddContract.getContentPane().add(month); 

    day = new JTextField(); 
    day.setBounds(181, 255, 81, 20); 
    frmAddContract.getContentPane().add(day); 

    year = new JTextField(); 
    year.setBounds(294, 255, 86, 20); 
    frmAddContract.getContentPane().add(year); 
    year.setColumns(10); 

    JLabel lblMonth = new JLabel("Month in number format"); 
    lblMonth.setBounds(38, 236, 100, 14); 
    frmAddContract.getContentPane().add(lblMonth); 

    JLabel lblDay = new JLabel("Day"); 
    lblDay.setBounds(181, 236, 48, 14); 
    frmAddContract.getContentPane().add(lblDay); 

    JLabel lblYear = new JLabel("Year"); 
    lblYear.setBounds(294, 236, 48, 14); 
    frmAddContract.getContentPane().add(lblYear); 

    JLabel lblEventDate = new JLabel("Event Date"); 
    lblEventDate.setBounds(38, 222, 76, 14); 
    frmAddContract.getContentPane().add(lblEventDate); 

    tme = new JTextField(); 
    tme.setBounds(38, 307, 59, 20); 
    frmAddContract.getContentPane().add(tme); 
    tme.setColumns(10); 

    cladd = new JTextField(); 
    cladd.setBounds(38, 180, 363, 31); 
    frmAddContract.getContentPane().add(cladd); 
    cladd.setColumns(10); 

    JLabel lblAddress = new JLabel("Client Address"); 
    lblAddress.setBounds(38, 159, 98, 14); 
    frmAddContract.getContentPane().add(lblAddress); 

    JLabel lblTime = new JLabel("Time"); 
    lblTime.setBounds(38, 286, 60, 14); 
    frmAddContract.getContentPane().add(lblTime); 



    eventadd = new JTextField(); 
    eventadd.setBounds(38, 358, 373, 50); 
    frmAddContract.getContentPane().add(eventadd); 
    eventadd.setColumns(10); 

    JLabel lblEventAddress = new JLabel("Event Address"); 
    lblEventAddress.setBounds(38, 338, 116, 14); 
    frmAddContract.getContentPane().add(lblEventAddress); 

    textField_fp = new JTextField(); 
    textField_fp.setBounds(38, 454, 276, 20); 
    frmAddContract.getContentPane().add(textField_fp); 
    textField_fp.setColumns(10); 

    JLabel lblFilepathOfContract = new JLabel("Filepath of Contract - the PDF file"); 
    lblFilepathOfContract.setBounds(38, 431, 116, 14); 
    frmAddContract.getContentPane().add(lblFilepathOfContract); 

    JButton btnBrowse = new JButton("Browse..."); 
    btnBrowse.setBounds(324, 453, 89, 23); 
    frmAddContract.getContentPane().add(btnBrowse); 
      btnBrowse.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent arg0) { 
         final JFileChooser fc = new JFileChooser(); 
         int returnVal = fc.showOpenDialog(null); 
         String filepath = fc.getSelectedFile().getAbsolutePath(); 
         textField_fp.setText(filepath); 
        } 
    }); 

    refer = new JTextField(); 
    refer.setBounds(38, 510, 276, 20); 
    frmAddContract.getContentPane().add(refer); 
    refer.setColumns(10); 

    JLabel lblEncodedBy = new JLabel("Referred By"); 
    lblEncodedBy.setBounds(38, 485, 116, 14); 
    frmAddContract.getContentPane().add(lblEncodedBy); 




    JButton btnAdd = new JButton("Add"); 
    btnAdd.setBounds(101, 560, 89, 23); 
    frmAddContract.getContentPane().add(btnAdd); 
      btnAdd.addActionListener(new ActionListener() { 
       public void actionPerformed(ActionEvent ae) { 

      try{ 

       String cntr = ContractNo.getText(); 
       String en = EngagerName.getText(); 
       String cont = contNo.getText(); 
       String mo = month.getText(); 
       String d = day.getText(); 
       String yr = year.getText(); 
       String dte = yr + "-" + mo + "-" + d; 
       String cla = cladd.getText(); 
       String tm = tme.getText() + ":00"; 
       String evadd = eventadd.getText(); 
       String filepath = textField_fp.getText(); 
       String ref = refer.getText(); 


       String SQL = "insert into cis "+ 
        "values ('"+cntr+"','"+en+"','"+ 
        cont+"','"+cla+"' '"+dte+"','"+tm+"','"+ 
        evadd+"','"+filepath+"','"+ref+")"; 
       PreparedStatement stmt = conn.prepareStatement(SQL); 
       ResultSet rs = stmt.executeQuery(); 

      } 
      catch(Exception e){ 
       JOptionPane.showMessageDialog(null, e); 
      } 
       } 


      }); 

    JButton btnCancel = new JButton("Cancel"); 
    btnCancel.setBounds(225, 560, 89, 23); 
    frmAddContract.getContentPane().add(btnCancel); 
      btnCancel.addActionListener(new ActionListener() { 
       public void actionPerformed(ActionEvent ae) { 
        frmAddContract.setVisible(false); 
       } 



    }); 
} 
} 

對不起，我不是真的在Java編程經驗，這是我第一次拍碼查詢數據庫。

Answer 1

您從不初始化conn實例變量。這將導致一個NullPointerException（至少）在這一行：

PreparedStatement stmt = conn.prepareStatement(SQL); 
         ^------------ NEVER INITIALIZED 

---

public class AddCon { 
    private Connection conn; 

... 

// constructor 
public AddCon() { 
    try{ 
     //Load database driver and connect 
     Class.forName("com.mysql.jdbc.Driver"); 
     Connection conn = DriverManager.getConnection(<your parameters>); 
     ^----------- MISTAKE 
... 

在創建連接上面的代碼，但不是將其分配給實例變量，你只需要使用一個本地變量。替換該行以：

this.conn = DriverManager.getConnection(<your parameters>); 

編輯：請總是張貼堆棧跟蹤在例外情況下！