我正在嘗試將值插入MySQL數據庫，得到NullPointerException

Question

我正在嘗試將GUI表單捕獲的一些數據插入數據庫。 當我運行程序並嘗試添加供應商時，我得到了nullpointerexception，並且難以弄清原因。 任何指導，不勝感激。

Java方法：

private void addVendorToDbButtonActionPerformed(java.awt.event.ActionEvent evt) {                                                    

         addVendorPanel.setVisible(true);

         //ANDY:
         String vendorName = addVendorNameField.getText();
         String customerName = addCustomerNumField.getText();
         String vendorCode = addVendorCodeField1.getText();

     String sql = "INSERT INTO VENDORS (Vendor,Cust#,Vendorcode)"+"VALUES('"+vendorName+"','"+customerName+"','"+vendorCode+"')";
     try {
         openconnection();
         rs = statement.executeQuery(sql);
     } catch (SQLException ex) {
         Logger.getLogger(gui.class.getName()).log(Level.SEVERE, null, ex);
     }
}       

OpenConnection的：

public void openconnection(){
     Connection conn = null;
        try {
      conn = DriverManager.getConnection("jdbc:mysql://sql3.freemysqlhosting.net:3306/sql376856", "xxxx","xxxxx");
      System.out.println("Connected database successfully...");
        } catch (Exception e) {
            System.out.println(e);
        }
    try {
        Statement statement = conn.createStatement();
    } catch (SQLException ex) {
        Logger.getLogger(gui.class.getName()).log(Level.SEVERE, null, ex);
    }
}

聲明要使用的全局變量：

public class gui extends javax.swing.JFrame {


 Statement statement;  
 Connection conn;
 ResultSet rs;
/**
 * Creates new form gui
 */
public gui() {
    initComponents();
}

錯誤信息：

run:
Connected database successfully...
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at database.gui.addVendorToDbButtonActionPerformed(gui.java:1142)
at database.gui.access$1300(gui.java:23)
at database.gui$14.actionPerformed(gui.java:562)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at        javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6527)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6292)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4883)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4705)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2739)
at java.awt.Component.dispatchEvent(Component.java:4705)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:746)
at java.awt.EventQueue.access$400(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:697)
at java.awt.EventQueue$3.run(EventQueue.java:691)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:719)
at java.awt.EventQueue$4.run(EventQueue.java:717)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:716)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Answer 1

我認為您在方法addVendorToDbButtonActionPerformed中的語句變量為null，因此存在nullpointer異常。

Answer 2

public void openconnection(){
        try {
      conn = DriverManager.getConnection("jdbc:mysql://sql3.freemysqlhosting.net:3306/sql376856", "xxxx","xxxxx");
      System.out.println("Connected database successfully...");
        } catch (Exception e) {
            System.out.println(e);
        }
    try {
        statement = conn.createStatement();
    } catch (SQLException ex) {
        Logger.getLogger(gui.class.getName()).log(Level.SEVERE, null, ex);
    }
}

如果你走這條路。 確保使用其他方法關閉連接。

我還建議移動statement = conn.createStatement(); 到定義查詢的方法。 確保在檢索結果后關閉該方法中的語句和結果集。