java - 将sql语句从java推送到数据库，executeUpdate()返回NullPointerException

Question

所以我试图将信息从java应用程序存储到数据库中，但我收到此错误:“”AWT-EventQueue-0“java.lang.NullPointerException”并且我一生都找不到原因。我尝试更改变量的名称，并且也尝试使用“PreparedStatment”。但到目前为止还没有任何效果。

public class Db {

public static final String DRIVER ="com.mysql.jbdc.Driver";
public static final String URL = "jdbc:mysql://localhost:3306/oving8";
public static final String USER = "root";
public static final String PW = "";
private static Connection kobling;

private static Statement settOppStatment(){
    try {
        Class.forName(DRIVER);
        Connection tilkobling = DriverManager.getConnection(URL, USER, PW);
        return tilkobling.createStatement();
    } catch (ClassNotFoundException e) {
        System.out.println("Can not find the class in");
    } catch (SQLException e) {
        System.out.println("Can not connect to database");
        }
    return null;
}
public static void skrivEnhetTilDB(Enhet enhet){
Statement st = settOppStatment();

String ID = Enhet.getId();
String Navn = Enhet.getNavn();
String sqlEnhet = null;


if (enhet != null){
    sqlEnhet = "INSERT INTO enhet VALUES('"+ ID + "','" +  Navn + "');";
};
System.out.println(sqlEnhet);

try {
    st.executeUpdate(sqlEnhet);  <---- Line 123
} catch (SQLException e) {
    System.out.println("This unit is already registerd");
}
try {
    st.getConnection().close();
} catch (SQLException e) {
    e.printStackTrace();
}

};

和堆栈跟踪:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at fil.Db.skrivEnhetTilDB(Db.java:123)
at main.Jframe$9.actionPerformed(Jframe.java:385)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
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:6535)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6300)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4891)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4713)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
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

如果您遇到任何这些异常，以下代码将返回 null。在这种情况下，由于您有 sysout，您应该在控制台或日志文件中看到这些消息。

您还可以使用调试器逐行检查正在发生的情况。

private static Statement settOppStatment(){
    try {
        Class.forName(DRIVER);
        Connection tilkobling = DriverManager.getConnection(URL, USER, PW);
        return tilkobling.createStatement();
    } catch (ClassNotFoundException e) {
        System.out.println("Can not find the class in");
    } catch (SQLException e) {
        System.out.println("Can not connect to database");
        }
    return null;
}