如何解决此java.lang.NullPointerException？

问：

我正在尝试运行在HSQL和MySQL数据库上运行的应用程序，当我运行它时，出现此错误：

java.lang.NullPointerException: Cannot invoke "java.sql.Connection.prepareStatement(String)" because "this.cnn" is null
at DataBase.DBcontrol.creer_piece(DBcontrol.java:122)
at pdr.FrontController.initialize(FrontController.java:160)
at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2573)
at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2466)
at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3237)
at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3194)
at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3163)
at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3136)
at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3113)
at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:3106)
at pdr.Main.start(Main.java:13)
at javafx.graphics/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$9(LauncherImpl.java:846)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(PlatformImpl.java:455)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
at java.base/java.lang.Thread.run(Thread.java:832)

它在前几行中引用的代码是这样的：

  public void creer_piece() throws ClassNotFoundException {
                HConnexion dbc = new HConnexion();        cnn=dbc.connectDb();
    PreparedStatement pss;
    ResultSet s;
    try {
        req = "CREATE TABLE IF NOT EXISTS moussa.piece "
                + " (id VARCHAR(30), "
                + " atelier VARCHAR(30), "
                + " piece VARCHAR(30), "
                + " type VARCHAR(30) NULL, "
                + " pa VARCHAR(2) NULL, "
                + " ref1 VARCHAR(10) NULL , "
                + " ref2 VARCHAR(10) NULL  , "
                + " quant INTEGER,"
                + " emp VARCHAR(30),"
                + " pos VARCHAR(30),"
                +  " UNIQUE (id))";

        pss = cnn.prepareStatement(req);
        int e = pss.executeUpdate();
        if (e > 0) {
            JOptionPane.showMessageDialog(null, " erraurgg");
            
        }
    } catch (SQLException ex) {
        JOptionPane.showMessageDialog(null, ex);
    }

还有这个：

 public void initialize(URL url, ResourceBundle rb) {
 
    try {
       run_wamp(); 
             timer.schedule(new TimerTask() {

            @Override
            public void run() {
               
            }
        },10000); 
        dbc.creer_piece();dbc.creer_refs();dbc.creer_carte();dbc.creer_module();dbc.creer_eqip();
         dbc.creer_c8();dbc.creer_secteur();dbc.creer_unite();dbc.creer_eqip();
        dbc.creer_atelier();
        dbc.get_natelier("SELECT * FROM moussa.atelier",combo_piece1);
    } catch (Exception ex) {
        Logger.getLogger(FrontController.class.getName()).log(Level.SEVERE, null, ex);
    }

我认为这与我的数据库有关，要么应用程序没有创建表，要么我应该手动创建表并连接它们，虽然我只是在这里吐口水，但需要一些帮助。

编辑：所以在看了评论之后，你们告诉我，CNN因为connectDB而返回null，这里是：

    public class HConnexion {
     private static String msg ;
    Connection conn = null;Statement stm =null;
//=====================================================================================    
    public Connection connectDb() throws ClassNotFoundException {
        String s = System.getProperty("user.home");
        String JDBC_URL = "jdbc:hsqldb:Pdr_db;";
        run_wamp();
        String path= "jdbc:mysql://localhost:3306/mysql?zeroDateTimeBehavior=convertToNull";

        try {
           Class.forName("com.mysql.jdbc.Driver");
         Connection conn = DriverManager.getConnection(JDBC_URL, "root", ""); //hsql database
            conn = DriverManager.getConnection(path, "root", ""); //mysql database
           stm=conn.createStatement();
           stm.executeUpdate("create database IF NOT EXISTS moussa ");
            return conn;
        } catch (SQLException e) {
          
            JOptionPane.showMessageDialog(null, e);

            return null;
        }
    }

这个方法（如果我错了，请纠正我）应该将我连接到我的数据库（或者创建一个新数据库，以防万一），所以如果 CNN 返回 null，这是否意味着它没有创建其他方法可以通过 CNN 连接的数据库。