Java-MySQL：来自另一个类的命令

关于在Java中使用MySQL，我有两个问题。我需要在我的程序的不同类中使用不同的值，所以我的第一个问题是，我是否需要在这些类中的每一个类中建立连接，或者我可以在我的主要类中只做一次？其次，我不明白如何从另一个类调用SQL命令。我试过做一个准备陈述，但似乎不起作用

我试着做以下的事情。首先，我创建了一个DBConnection类（我已经删除了真实的用户和密码值）：

然后，我从我的主菜单中调用：

DBConnection dbConn = new DBConnection();
dbConn.connect();

然后，在我的LoginController中，我想通过将输入的密码和用户名与数据库中的值进行比较来验证输入的密码和用户名。我尝试通过以下操作来回答这里关于堆栈溢出的另一个问题：

    Connection conn = null;
    PreparedStatement pstmt = null;

try{
        String SQL  = "SELECT `username`,`password` FROM `Clinician` WHERE `username` =? AND `password` = ?;";
        pstmt = conn.prepareStatement(SQL);
        pstmt.setString(1, txtUsername.getText());
        pstmt.setString(2, String.valueOf(txtPassword.getText())); 
        ResultSet result = pstmt.executeQuery();

        if (result.next()){
                lblMessage.setText("Access confirmed!");                
            }else{
                lblMessage.setText("Access denied!");
                 }   
        }

    catch(SQLException e){

    }

    finally{
           pstmt.close();
    }

但它不起作用。我确实通过main建立了连接，但它没有验证登录。我是错过了什么，还是做错了什么

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1774)
at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Node.fireEvent(Node.java:8413)
at javafx.scene.control.Button.fire(Button.java:185)
at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3757)
at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:381)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$354(GlassViewEventHandler.java:417)
at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:416)
at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
at com.sun.glass.ui.View.notifyMouse(View.java:937)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.Trampoline.invoke(MethodUtil.java:71)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at sun.reflect.misc.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1769)
    ... 48 more
Caused by: java.lang.NullPointerException
    at Multimodality.view.LoginController.btnLoginAction(LoginController.java:77)
    ... 58 more

---

提前谢谢

有两个变量名为

conn

。第一个仅在

connect（）

方法中可用，第二个是

null

。因此，在

pstmt=conn.prepareStatement（SQL）行中应该有一个NullPointerException

您必须将preparedStatement内容移动到
DBConnection
类中，或者您的
connect（）
方法必须返回连接，以便您可以使用
connection conn=dbConn.connect（）在控制器类中

更新：

public Connection connect()...
     ....
     Connection conn = DriverManager.getConnection(connectionUrl, user, password);
    System.out.println("Connection established");
    return conn;
}

在控制类中：

DBConnection dbConn = new DBConnection();
Connection conn = dbConn.connect();

PreparedStatement pstmt = null;

try{
        String SQL  = "SELECT `username`,`password` FROM `Clinician` WHERE `username` =? AND `password` = ?;";
        pstmt = conn.prepareStatement(SQL);

如果不处理异常，就永远不要捕获它。在您的例子中，我认为您得到了一个异常，它被catch块忽略。更改为：
catch（SQLException e）{e.printStacktrace（）；}
并查看是否获得异常删除
在SQL语句的末尾statement@Jens我不允许我移除。当我运行代码时，它会说：“线程中的异常”JavaFX应用程序线程“java.lang.RuntimeException:java.lang.reflect.InvocationTargetException”，后跟30多行“at…”请使用编辑按钮将完整的堆栈跟踪添加到您的问题中function@Jens已经添加了！：-）谢谢，我没注意到我叫他们同样的名字！我不确定是否可以将preparedStatement内容移动到DBConnection类中，因为我需要运行许多不同的SQL命令才能使整个程序正常工作—登录只是第一个。除非我能让它变得非常通用，并在每个类中更改参数。如何使用connect（）返回连接？对不起，我对Java相当陌生。
DBConnection dbConn = new DBConnection();
Connection conn = dbConn.connect();

PreparedStatement pstmt = null;

try{
        String SQL  = "SELECT `username`,`password` FROM `Clinician` WHERE `username` =? AND `password` = ?;";
        pstmt = conn.prepareStatement(SQL);