当我尝试从 Mysql 库中选择数据时出现 NullPointerException答案

【问题标题】：NullPointerException when i try Select data from Mysql base当我尝试从 Mysql 库中选择数据时出现 NullPointerException
【发布时间】：2015-05-10 14:56:36
【问题描述】：

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，我使用单例模式。

单例类：

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 { 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;
}
}

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$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();

【问题讨论】：

请贴出异常的stacktrace
NullPointerException 是在哪里引发的？
你应该在某个地方打电话给connect()...

标签： java mysql model-view-controller jcombobox

【解决方案1】：

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);

【讨论】：

【解决方案2】：

如果你仔细看代码，你会发现connection对象是在connect方法里面初始化的。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，您应该使用连接池或数据源，而不是编写自己的 JDBC 代码。

【讨论】：