当我尝试来自 Mysql base 的 Select 数据时出现 NullPointerException

Question

我有问题。当我尝试从数据库中获取 select 数据时，我看到错误 NullPointerException。要连接到数据库，我使用单例模式。

单例 class :

public class Database {

private static Database instance = new Database();

private Connection con;

private Database() {

}

public static Database getInstance() {
    return instance;
}

public Connection getConnection(){
    return con;
}
public void connect() throws Exception {

    if (con != null)
        return;

    try {
        Class.forName("com.mysql.jdbc.Driver");
    } catch (ClassNotFoundException e) {
        throw new Exception(e);
    }

    String url = String.format("jdbc:mysql://127.0.0.1:3306/newssystem");

    con = DriverManager.getConnection(url, "root","");
}

public void disconnect() {
    if (con != null) {
        try {
            con.close();
        } catch (SQLException e) {
            System.out.println("Nie można zakończyć połączenia");
        }
    }

    con = null;
}

}

Class 到 select 数据： public class ArtOpcje { public ArrayList getArtykul() 抛出 SQLException {

    ArrayList<Artykul> news = new ArrayList<Artykul>();

    Connection conn = Database.getInstance().getConnection();

    String sql = "SELECT id, title, article FROM news";

    Statement pa = conn.createStatement();

    ResultSet results = pa.executeQuery(sql);

    while(results.next()) {                                     
        int id = results.getInt("id");
        String title = results.getString("title");
        String article = results.getString("article");
        Artykul artykul = new Artykul(id, title, article);  
        news.add(artykul);
    }
    results.close();
    pa.close();
    return news;
}
}

以及当我在 Combobox 中使用此 Select 时的代码片段：

    combo.setBounds(150, 250, 350, 30);
    combo.removeAllItems();
    System.out.println("poza  pętlą");
    this.add(combo);

    ArrayList<Artykul> artykul = new ArrayList<Artykul>();
    artykul = art.getArtykul();


    String[] aa = new String[artykul.size()];
    for(int i =0; i<artykul.size(); i++){
         aa[i] = artykul.get(i).getTitle();
    }


Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
 at model.ArtOpcje.getArtykul(ArtOpcje.java:40)
 at view.View.<init>(View.java:82)
 at aplication.Aplication.runApp(Aplication.java:34)
 at aplication.Aplication.run(Aplication.java:21)
 at java.awt.event.InvocationEvent.dispatch(Unknown Source)
 at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
 at java.awt.EventQueue.access0(Unknown Source)
 at java.awt.EventQueue.run(Unknown Source)
 at java.awt.EventQueue.run(Unknown Source)
 at java.security.AccessController.doPrivileged(Native Method)
 at java.security.ProtectionDomain.doIntersectionPrivilege(Unknown Source)
 at java.awt.EventQueue.dispatchEvent(Unknown Source)
 at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
 at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
 at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
 at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
 at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
 at java.awt.EventDispatchThread.run(Unknown Source)

NullPointerException 在这里：

Statement pa = conn.createStatement();

Answer 1

如果仔细查看代码，您会发现 connection 对象是在 connect 方法中初始化的。但是您的代码中永远不会调用该方法。因此调用

Connection conn = Database.getInstance().getConnection();

将 return Connection 对象的空实例。相反，您应该使用以下代码

Database db = Database.getInstance();
db.connect();
db.getConnection();

这应该可以解决您的问题。

Note: In ideal condition, you should make use of Connection pool or data source instead of writing your own JDBC code.

Answer 2

谢谢。现在开始工作了。

    Database db;
    db = Database.getInstance();
    db.connect();
    db.getConnection();


    String sql = "SELECT id, title, article FROM news";

    PreparedStatement pa = db.getConnection().prepareStatement(sql);

当我尝试来自 Mysql base 的 Select 数据时出现 NullPointerException

NullPointerException when i try Select data from Mysql base

java

mysql

model-view-controller

jcombobox