Question

我正在尝试使用swing事件处理来访问数据库表记录，但是在按下按钮后它显示异常。我不知道出了什么问题。这是我的代码：

private void DisplayAllButtonActionPerformed(java.awt.event.Act ionEvent evt) {

    try { 

        String SQL = "SELECT * FROM OCCUPANTS";
        rs = stmt.executeQuery( SQL ); //line no 264

        while( rs.next( )) {
            int id_col = rs.getInt("ID");
            String id=Integer.toString(id_col);
            String room =rs.getString("ROOM");
            String occupant = rs.getString("OCCUPANT");

            DisplayArea.setText( id + " " + room + " " + occupant);
        }
   } 
   catch ( SQLException err ) {
        System.out.println( err.getMessage( ) );
   }
}

Fallow是个例外：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at wumpus.WumpusGUI.DisplayAllButtonActionPerformed(W umpusGUI.java:264)
at wumpus.WumpusGUI.access$300(WumpusGUI.java:16)
at wumpus.WumpusGUI$4.actionPerformed(WumpusGUI.java: 164)
at javax.swing.AbstractButton.fireActionPerformed(Abs tractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed (AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed (DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultB uttonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseRe leased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.jav a:6505)
at javax.swing.JComponent.processMouseEvent(JComponen t.java:3321)
at java.awt.Component.processEvent(Component.java:627 0)
at java.awt.Container.processEvent(Container.java:222 9)
at java.awt.Component.dispatchEventImpl(Component.jav a:4861)
at java.awt.Container.dispatchEventImpl(Container.jav a:2287)
at java.awt.Component.dispatchEvent(Component.java:46 87)
at java.awt.LightweightDispatcher.retargetMouseEvent( Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(C ontainer.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Conta iner.java:4422)
at java.awt.Container.dispatchEventImpl(Container.jav a:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719 )
at java.awt.Component.dispatchEvent(Component.java:46 87)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.j ava:703)
at java.awt.EventQueue.access$000(EventQueue.java:102 )
at java.awt.EventQueue$3.run(EventQueue.java:662)
at java.awt.EventQueue$3.run(EventQueue.java:660)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:676)
at java.awt.EventQueue$4.run(EventQueue.java:674)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 673)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:244)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:151)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:147)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:139)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:97)
BUILD SUCCESSFUL (total time: 28 seconds)

以下是stmt

的全局初始化

public class WumpusGUI extends javax.swing.JFrame {

    Connection con;
    Statement stmt;
    ResultSet rs;
    /**
     * Creates new form WumpusGUI
     */
    public WumpusGUI() {
        initComponents();
        DBConnect();
    }

    public void DBConnect() {
        try {
            String host = "jdbc:derby://localhost:1527/occupants";
            String uName = "ravi";
            String uPass= "ravi";
            Connection con = DriverManager.getConnection( host, uName, uPass );
            Statement stmt = con.createStatement();
        }  
        catch (Exception e) {
            e.printStackTrace();
        }
    }
}

感谢。

Answer 1

嗯，你还没有显示stmt被宣布的地方，但听起来它可能是空的。您期望将其设置为非空值？

关于这一点有两点：

每次执行查询时，您应该使用单独的PreparedStatement
您不应该在UI线程中执行数据库访问

（另外，您的方法名称违反了Java命名约定。）

编辑：从你的评论中，你得到了：

public class WumpusGUI ... {
    Statement stmt;

    public void DBConnect() {
        ...
        Statement stmt = con.createStatement();
        ...
    }
}

您的方法的那一行声明了一个新的本地变量 - 它没有为实例变量赋值，因此实例变量保持为空。

最小的“修复”是将该行更改为：

stmt = con.createStatement();

...但正如我上面所说，那不是正确的修复。理想情况下，您应该首先从GUI类中获取所有此数据库代码，但至少每次要执行它时都应该创建一个新语句。（然后在最后一个块中关闭它。）

编辑：此外，如果您在方法中遇到异常，那么您只需将其打印出来并快速继续...所以您的stmt变量仍然为空。 一般不捕获Exception，不捕获您真正无法继续的异常。

Answer 2

非常简单：stmt尚未初始化。

Answer 3

您的stamt变量可能已使用null进行全局初始化，但您无法分配必须从连接获取的值。检查你的代码。

Answer 4

在使用之前，必须调用DBConnect方法初始化stmt变量。

我无法解决这个“NullPointerException”

4 个答案: