空指針異常雖然調用MySql數據庫

我正在練習一個Java Netbeans程序，我從MySql數據庫調用值，但它給了我這個空指針的例外。我是新手，所以請幫我解決這個問題。空指針異常雖然調用MySql數據庫

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)

，這是我的代碼：

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)); 
}

來源

2014-03-01 Ravi Longia

警告...... SQL注入... – fge

你在說什麼這不是問題 –

如果數據庫中有任何值爲空，則'Float.valueOf'會給出該異常。 '如果s爲null，則拋出NullPointerException。' –

我可以看到2個可能的原因爲何DriverManager.getConnection回報null。

JDBC URL無效。請檢查您的MySQL服務器是否在您的機器上的默認端口（3306）上運行。如果沒有，請在URL中明確包含端口。然後，coolingSystem應該是服務器上現有數據庫的名稱（不是表格或其他內容）。最後，請仔細檢查您的憑證。
該驅動程序未包含在運行時類路徑中。 Class.forName是否會拋出異常/錯誤？請檢查日誌用於此目的。

來源

2014-03-01 13:24:36

您的回答非常好，但這不是問題，但是，我得到了一個非常奇怪的解決方案，在我試圖執行的問題中一個線性插值公式，我把它移動到try塊中，現在它的工作......這不應該是答案，但它現在的工作很好奇怪的........ !!! Whyy –

你的類路徑中是否有mysql連接器？它看起來像Connection是空的。測試連接是否爲空並打印出來，只是爲了檢查。

來源

2014-03-01 13:25:51

沒有它的好處和問題以非常奇怪的方式解決......謝謝你的幫助:) –

空指針異常雖然調用MySql數據庫

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