Java 尝试从Mysql库中选择数据时出现NullPointerException

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

单身人士类别：

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;
}

}

类以选择数据： 公共类艺术作品{ 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;
}
}

---

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。但该方法从未在代码中调用。因此调用

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

将返回

连接

对象的空实例

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

这应该能解决你的问题

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

---

谢谢你，现在可以了

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


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

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

---

请发布异常的堆栈跟踪引发NullPointerException的位置？您应该在某处调用

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

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


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

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