从 Jmenu 中选择项目并单击按钮时出现 java.lang.NullPointerException

Question

这是一个从数据库获取数据的Jmenu。错误是，当按下查看定义按钮时，相应的事件没有被触发。理想情况下，当用户从 Jmenu 中选择一个项目并按下此按钮时，数据库将再次连接，以根据从 Jmenu 中选择的项目从数据库中获取相应的值。然而声明

ResultSet rs=st.executeQuery("select Metric_Name from Metrics where Metrics.Metric_Name='NAA'"); 

似乎从未执行并抛出错误

datajava.lang.NullPointerException

导致此问题的错误是什么以及如何解决此问题？这是我的代码:-

public class ListModelExample extends JPanel {

  JList list;

  DefaultListModel model;

  public ListModelExample() {
    setLayout(new BorderLayout());
    model = new DefaultListModel();
    list = new JList(model);
    JScrollPane pane = new JScrollPane(list);
    JButton addButton = new JButton("View Definition");

     Connection con;
      ResultSet rs;
      Statement st = null;

    try {  

        File dbFile = new File("executive_db.accdb");
        String path = dbFile.getAbsolutePath();
        con = DriverManager.getConnection("jdbc:odbc:Driver={Microsoft Access Driver (*.mdb, *.accdb)}; DBQ= " + path);
        Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
       st = con.createStatement();


       } 
    catch (Exception e)
    {  
JOptionPane.showMessageDialog(null,"Failed to Connect to Database","Error Connection", JOptionPane.WARNING_MESSAGE);  
System.exit(0);  
   }  


 try {  
     model.clear();  
     rs=st.executeQuery("select Metric_Name from Metrics");  
     while (rs.next())
     {  

         model.addElement(rs.getString("Metric_Name"));  
     }  

    } 
 catch (Exception e)
    {  
     System.out.println("Error in fetching data");  
    }
   addButton.addActionListener(new ActionListener() {
      Statement st;

    public void actionPerformed(ActionEvent e) {

          String def = ""; 


          try {  

                 model.clear();  
                 st = null;
                 System.out.println("came here");
                 ResultSet rs=st.executeQuery("select Metric_Name from Metrics where Metrics.Metric_Name='NAA'");  
                 System.out.println("came here too");
                 while (rs.next())
                 {  

                     def = rs.getString("Metric_Name");

                     System.out.println(def);
                   //  model.addElement(rs.getString("Metric_Name"));  
                 }  

                } 
             catch (Exception e1)
                {  
                 System.out.println("Error in fetching data"+e1);  
                }
          System.out.println(def);
          JOptionPane.showMessageDialog(null, def, "Error", JOptionPane.ERROR_MESSAGE);
      /*  model.addElement("Element " + counter);
        counter++;*/
      }
    });

    add(pane, BorderLayout.NORTH);
    add(addButton, BorderLayout.WEST);

  }

  public static void main(String s[]) {
    JFrame frame = new JFrame("List of Metrics used");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setContentPane(new ListModelExample());
    frame.setSize(260, 200);
    frame.setVisible(true);
  }
}

这是我执行 e1.printStackTrace(); 时遇到的错误我的代码中的第 94 行是

ResultSet rs=st.executeQuery("select Metric_Name from Metrics where Metrics.Metric_Name='NAA'"); 

<小时/>

java.lang.NullPointerException
    at ListModelExample$1.actionPerformed(ListModelExample.java:94)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$400(Unknown Source)
    at java.awt.EventQueue$2.run(Unknown Source)
    at java.awt.EventQueue$2.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)Error in fetching datajava.lang.NullPointerException


    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

看看你的代码:

st = null;
ResultSet rs = st.executeQuery(...);

如果没有获得 NullPointerException，则无法对 null 引用调用任何方法。您的 st 变量需要初始化为非空语句。