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[英]Why am I getting a NullPointerException when I try to get data from a getter?
[英]NullPointerException when i try Select data from Mysql base
我有問題。 當我嘗試從數據庫中選擇數據時,我看到錯誤NullPointerException。 要連接到數據庫,我使用Singleton模式。
單例課程:
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;
}
}
選擇數據的類:公共類ArtOpcje {公共ArrayList getArtykul()引發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;
}
}
和代碼片段,當我在Combobox中使用此Select時:
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在這里:
Statement pa = conn.createStatement();
如果仔細看一下代碼,那么您會發現connection
對象是在connect方法中初始化的。 但是該方法永遠不會在您的代碼中調用。 因此打電話
Connection conn = Database.getInstance().getConnection();
將返回Connection
對象的空實例。 相反,您應該使用以下代碼
Database db = Database.getInstance();
db.connect();
db.getConnection();
這應該可以解決您的問題。
注意:在理想情況下,應該使用連接池或數據源,而不是編寫自己的JDBC代碼。
謝謝。 現在它的工作。
Database db;
db = Database.getInstance();
db.connect();
db.getConnection();
String sql = "SELECT id, title, article FROM news";
PreparedStatement pa = db.getConnection().prepareStatement(sql);
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