调用MySql数据库时出现空指针异常

Question

我正在练习Java Netbeans程序，从MySql数据库中调用值，但这给了我Null指针异常。 我是新手，所以请帮助我解决这个问题。

java.lang.NullPointerException
at cooling.system.pkg1.pkg0.Calculation1.calculate(Calculation1.java:371)
at cooling.system.pkg1.pkg0.Calculation1.btn_setActionPerformed(Calculation1.java:1081)
at cooling.system.pkg1.pkg0.Calculation1.access$1000(Calculation1.java:22)
at cooling.system.pkg1.pkg0.Calculation1$11.actionPerformed(Calculation1.java:862)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
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:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

这是我的代码：

public void calculate() throws ClassNotFoundException
{
    if(!(chckbx_current_value.isSelected()))
    {
        JOptionPane.showMessageDialog(this, "Select Current value");
    }
    if(chckbx_big_close_value.isSelected() && chckbx_small_close_value.isSelected())
    {
        JOptionPane.showMessageDialog(this, "Select Only one closest value");
        chckbx_big_close_value.setSelected(false);
        chckbx_small_close_value.setSelected(false);
    }
    else if(!(chckbx_big_close_value.isSelected()) && !(chckbx_small_close_value.isSelected()))
    {
        JOptionPane.showMessageDialog(this, "Select Atleast one closest value");
    }
    Connection con=null;
    try 
    {
                Class.forName("com.mysql.jdbc.Driver");
                con = DriverManager.getConnection("jdbc:mysql://localhost/coolingSystem","root","password");
                String small = "select * from dryAir_properties where T = "+closest_small+"";
                PreparedStatement ps_small = con.prepareStatement(small);
                ResultSet rs_small = ps_small.executeQuery();


                String big = "select * from dryAir_properties where T = "+closest_big+"";
                PreparedStatement ps_big = con.prepareStatement(big);
                ResultSet rs_big = ps_big.executeQuery();
                while(rs_small.next() && rs_big.next())
                {
                    Cp_big = Float.valueOf(rs_big.getString(3));
                    k_big = Float.valueOf(rs_big.getString(4));
                    viscosity_big = Float.valueOf(rs_big.getString(5));
                    density_big = Float.valueOf(rs_big.getString(6));

                    Cp_small = Float.valueOf(rs_small.getString(3));
                    k_small = Float.valueOf(rs_small.getString(4));
                    viscosity_small = Float.valueOf(rs_small.getString(5));
                    density_small = Float.valueOf(rs_small.getString(6));
                } 
    }
    catch (NumberFormatException | SQLException  e) 
    {
        System.out.println("Error...... "+e);
    }

    float Cp = (((tma - closest_small)*(Cp_big - Cp_small))/(closest_big - closest_small)) + Cp_big;
    float k = (((tma - closest_small)*(k_big - k_small))/(closest_big - closest_small)) + k_big;
    float viscosity = (((tma - closest_small)*(viscosity_big - viscosity_small))/(closest_big - closest_small)) + viscosity_big;
    float density = (((tma - closest_small)*(density_big - density_small))/(closest_big - closest_small)) + density_big;
    txt_Cp_a.setText(String.valueOf(Cp));
    txt_Conductivity_a.setText(String.valueOf(k));
    txt_Viscosity_a.setText(String.valueOf(viscosity));
    txt_Density_a.setText(String.valueOf(density));
}

Answer 1

我可以看到2种可能的原因，为什么DriverManager.getConnection返回null 。

1. JDBC URL无效。 请检查您的MySQL服务器是否在默认端口（3306）的计算机上运行。 如果不是，请在URL中明确包含端口。 然后， coolingSystem应该是服务器上现有数据库的名称（而不是表或其他名称）。 最后，请仔细检查您的凭据。

2. 驱动程序不包含在运行时类路径中。 Class.forName是否会引发任何异常/错误？ 为此，请检查日志。

Answer 2

您的类路径中有mysql-connector吗？ 似乎Connection为空。 测试Connection是否为null并打印出来，以进行检查。