我尝试从Mysql base选择数据时出现NullPointerException

Question

我有问题。 当我尝试从数据库中选择数据时，我看到错误NullPointerException。 要连接到数据库，我使用Singleton模式。

单例课程：

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;
}

}

选择数据的类：公共类ArtOpcje {公共ArrayList getArtykul（）引发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;
}
}

和代码片段，当我在Combobox中使用此Select时：

    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$1.run(Aplication.java:21)
 at java.awt.event.InvocationEvent.dispatch(Unknown Source)
 at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
 at java.awt.EventQueue.access$500(Unknown Source)
 at java.awt.EventQueue$3.run(Unknown Source)
 at java.awt.EventQueue$3.run(Unknown Source)
 at java.security.AccessController.doPrivileged(Native Method)
 at java.security.ProtectionDomain$1.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

如果仔细看一下代码，那么您会发现connection对象是在connect方法中初始化的。 但是该方法永远不会在您的代码中调用。 因此打电话

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

将返回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。

注意：在理想情况下，应该使用连接池或数据源，而不是编写自己的JDBC代码。

Answer 2

谢谢。 现在它的工作。

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


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

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