的Java NullPointerException異常上的createStatement

-1

可能重複：
What can I do to avoid a NullPointerException?的Java NullPointerException異常上的createStatement

我開發一個餐廳的應用程序。當我嘗試初始化某個類時發生該錯誤。這是代碼。

public class DbItem { 

private static Connection conn = null; 

static { 
    conn = DBConnection.connect(); 
} 
public static ArrayList<Item> fetchAll() { 
    ArrayList<Item> items = new ArrayList<>(); 
    String sql = "select * from itens"; 
    try { 
     Statement st = conn.createStatement(); 
     ResultSet rs = st.executeQuery(sql); 
     while(rs.next()) { 
      items.add(new Item(rs.getString("nome"), rs.getString("descricao"), rs.getFloat("preco"))); 
     } 
     return items; 
    } catch (SQLException ex) { 
     Logger.getLogger(DbItem.class.getName()).log(Level.SEVERE, null, ex); 
    } 
    return null; 
} 
}

的連接是從該類

public class DBConnection { 

private static Connection conn; 

public static Connection connect() { 
    if(conn == null) { 
     String url = "jdbc:mysql://localhost:3306/"; 
     String dbName = "restaurante_comandas_jonnathan"; 
     String driver = "com.mysql.jdbc.Driver"; 
     String userName = "root"; 
     String password = "root"; 
     try { 
      Class.forName(driver).newInstance(); 
      conn = DriverManager.getConnection(url + dbName, userName, password); 
      System.out.println("Connected to " + dbName + " with " + userName); 
      JOptionPane.showMessageDialog(null, conn == null); 
      return conn; 
     } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | SQLException e) { 
      e.printStackTrace(); 
     } 
    } 
    return null; 
} 

}

我試着用彈出對話框調試拍攝。我在返回命令之前檢查連接是否爲空，並且發現不是。然後，我檢查了第一個腳本的調用之後，發現它確實是空的。現在，如果一個對象不在返回之前，對象在檢索後如何可以爲空？

編輯：

這是它。堆棧跟蹤

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
at model.data.DbItem.fetchAll(DbItem.java:48) 
at model.Item.getAll(Item.java:76) 
at view.PedidoForm.<init>(PedidoForm.java:22) 
at view.Login$2.run(Login.java:99) 
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251) 
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:721) 
at java.awt.EventQueue.access$200(EventQueue.java:103) 
at java.awt.EventQueue$3.run(EventQueue.java:682) 
at java.awt.EventQueue$3.run(EventQueue.java:680) 
at java.security.AccessController.doPrivileged(Native Method) 
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76) 
at java.awt.EventQueue.dispatchEvent(EventQueue.java:691) 
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)

來源

2013-01-19 Patrick Villela

你嘗試過用調試器附着？粘貼你的堆棧跟蹤... –

我其實不知道如何使用調試器。我的大部分時間都使用純文本編輯器。 –

現在可能是投入一些努力和學習的時候了。您可以免費下載IntelliJ IDE並調試您的代碼。它是一個Web應用程序或一個普通的Java程序？ –

我已修改您的代碼並進行了一些更改。請試試這個

public class DBConnection { 

private static Connection conn; 

public static Connection connect() { 
    if(conn == null) { 
    String url = "jdbc:mysql://localhost:3306/"; 
    String dbName = "restaurante_comandas_jonnathan"; 
    String driver = "com.mysql.jdbc.Driver"; 
    String userName = "root"; 
    String password = "root"; 
    try { 
     Class.forName(driver).newInstance(); 
     conn = DriverManager.getConnection(url + dbName, userName, password); 
     System.out.println("Connected to " + dbName + " with " + userName); 
     JOptionPane.showMessageDialog(null, conn == null);    
    } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | SQLException e) { 
     e.printStackTrace(); 
    } 
} 
return conn; 
}

}

來源

2013-01-19 07:27:38

我不確定我能否接受這個答案，因爲阿米爾是第一個指給我的人。 –

的Java NullPointerException異常上的createStatement

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