使用JDBC的Java - 連接太多？

我正在寫一個酒吧的庫存補貨系統作爲我最後一年的項目。我可以從MYSQL數據庫中檢索信息，並且可以一次滾動查看一個結果。使用JDBC的Java - 連接太多？

我試圖根據所選類別更改結果。我已經成功地使用組合框，以達致這可是類別之間移動時，我收到以下錯誤：在線程

異常「主要」 com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException：數據源拒絕建立連接，消息從服務器：「有太多的聯繫」

爲兩個單獨的文件中的代碼如下：

的SQL查詢中RetrieveStockQuery

public JComboBox getComboBox() throws SQLException { 
    con = SQLConnect.getConnection(); 
    combo = new JComboBox(); 
    combo.removeAllItems(); 
    try { 
    stat = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE); 
    rs = stat.executeQuery("SELECT categoryName FROM Category"); 

    while (rs.next()) { 
     combo.addItem(rs.getString("categoryName")); 
     categoryName = rs.getString("categoryName"); 
    } 
    } 
    catch (SQLException sqle) { 
    System.out.println(sqle); 
    stat.close(); 
    con.close(); 
    } 
    return combo; 
} 

//---------------------------------------------------------------- 

public void retrieveStock() throws SQLException { 

    con = SQLConnect.getConnection(); 
    stockGUI = new ViewStockGUI(); // I THINK THIS IS WHAT IS CAUSING THE ERROR 

    String viewStock = "SELECT * FROM Stock where categoryName = '" + "'" + stockGUI.selected + "'"; 
    System.out.println(viewStock); 

    try { 
    stat = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE); 
    rs = stat.executeQuery(viewStock); 

    while(rs.next()){ 
     stockID = rs.getInt("stockID"); 
     stockName = rs.getString("stockName"); 
     stockDescription = rs.getString("stockDescription"); 
     stockPrice = rs.getFloat("stockPrice"); 
     stockQuantity = rs.getInt("stockQuantity"); 
     categoryName = rs.getString("categoryName"); 

     ID = Integer.toString(stockID); 
     price = Float.toString(stockPrice); 
     quantity = Double.toString(stockQuantity); 
     stat.close(); 
     con.close(); 

     System.out.println("Stock ID: " + stockID + " Stock Name: " + stockName + " Stock Description: " + stockDescription + " Stock Price: " + stockPrice + " Stock Quantity:" + stockQuantity + " Category: " + categoryName); 
    }  
    } 
    catch (SQLException err) { 
    System.out.println(err.getMessage()); 
    } 
}

我ViewStockGUI類

public class ViewStockGUI extends JPanel { 

    private static final long serialVersionUID = 1L; 
    final JFrame viewFrame; 
    ViewStockQuery stockQuery; 
    ViewStockQuery stockName; 
    JComboBox comboGUI; 
    String selected; 
    JComboBox combo; 

    public ViewStockGUI() throws SQLException { 

     final ViewStockQuery stock = new ViewStockQuery(); 

     comboGUI = stock.getComboBox(); 
     stock.retrieveStock(); 
     viewFrame = new JFrame("View Stock"); 

     JPanel p = new JPanel(); 
     p.setBorder (new TitledBorder(new LineBorder(Color.black, 1, true))); 
     p.setPreferredSize(new Dimension(500,400)); 

     JPanel p2 = new JPanel(); 
     p2.setBorder (new TitledBorder(new LineBorder(Color.black, 1, true))); 
     p2.setPreferredSize(new Dimension(500, 50)); 

     JPanel p3 = new JPanel(); 
     JPanel p4 = new JPanel(); 
     JPanel p5 = new JPanel(); 
     JPanel p6 = new JPanel(); 

     Box box = Box.createVerticalBox();   
     Box box2 = Box.createHorizontalBox(); 
     Box box3 = Box.createHorizontalBox(); 
     Box box4 = Box.createHorizontalBox(); 

     final JTextField textfieldStockName; 
     final JTextField textfieldStockID; 
     final JTextField textfieldStockDescription; 
     final JTextField textfieldStockPrice; 
     final JTextField textfieldStockQuantity; 
     final JTextField textfieldStockCategory; 

     final JLabel stockName = new JLabel("Name:"); 
     JLabel stockID = new JLabel("ID:"); 
     JLabel stockDescription = new JLabel("Description:"); 
     JLabel stockPrice = new JLabel("Price:"); 
     JLabel stockQuantity = new JLabel("Quantity:"); 
     JLabel categoryName = new JLabel("Category:"); 

     box.add(Box.createVerticalGlue()); 
     box.add(stockName); 
     box.add(textfieldStockName = new JTextField("")); 
     textfieldStockName.setText(stock.getStockName()); 
     textfieldStockName.setEditable(false); 

     box.add(stockID); 
     box.add(textfieldStockID = new JTextField("")); 
     textfieldStockID.setText(stock.getStockID()); 
     textfieldStockID.setEditable(false); 

     box.add(stockDescription); 
     box.add(textfieldStockDescription = new JTextField("")); 
     textfieldStockDescription.setText(stock.getStockDescription()); 
     textfieldStockDescription.setEditable(false); 

     box.add(stockPrice); 
     box.add(textfieldStockPrice = new JTextField("")); 
     textfieldStockPrice.setText(stock.getStockPrice()); 
     textfieldStockPrice.setEditable(false); 

     box.add(stockQuantity); 
     box.add(textfieldStockQuantity = new JTextField("")); 
     textfieldStockQuantity.setText(stock.getStockQuantity()); 
     textfieldStockQuantity.setEditable(false); 

     box.add(categoryName); 
     box.add(textfieldStockCategory = new JTextField("")); 
     textfieldStockCategory.setText(stock.getStockCategory()); 
     textfieldStockCategory.setEditable(false); 
     box.add(Box.createVerticalGlue()); 

     JButton next = new JButton("Next"); 
     next.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent e) { 
      stock.doNext(); 
      textfieldStockName.setText(stock.getStockName()); 
      textfieldStockID.setText(stock.getStockID()); 
      textfieldStockDescription.setText(stock.getStockDescription()); 
      textfieldStockPrice.setText(stock.getStockPrice()); 
      textfieldStockQuantity.setText(stock.getStockQuantity()); 
      textfieldStockCategory.setText(stock.getStockCategory()); 
     } 
     }); 

     JButton previous = new JButton("Previous"); 
     previous.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent e) { 
      stock.doPrevious(); 
      textfieldStockName.setText(stock.getStockName()); 
      textfieldStockID.setText(stock.getStockID()); 
      textfieldStockDescription.setText(stock.getStockDescription()); 
      textfieldStockPrice.setText(stock.getStockPrice()); 
      textfieldStockQuantity.setText(stock.getStockQuantity()); 
      textfieldStockCategory.setText(stock.getStockCategory()); 
     } 
     }); 

     final Counter counter = new Counter(); 
     final JLabel text = new JLabel(counter.getValue1()); 

     JButton plus = new JButton("+"); 
     plus.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent e) { 
      counter.increment(); 
      text.setText(counter.getValue1()); 
     } 
     }); 

    JButton minus = new JButton("-"); 
    minus.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent e) { 
     counter.decrease(); 
     text.setText(counter.getValue1()); 
     } 
    }); 

    JButton update = new JButton("Update"); 
    update.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent e) { 
     try { 
      stock.updateStock(counter.getValue1()); 
     } 
     catch (SQLException e1) { 
      e1.printStackTrace(); 
     } 
     finally { // doesn't update yet; will work on this later 
      textfieldStockQuantity.setText(stock.getStockQuantity()); 
     } 
     } 
    }); 

    comboGUI.addActionListener(new ActionListener() { 
     public void actionPerformed(ActionEvent e) { 
     try { 
      combo = (JComboBox) e.getSource(); 
      selected = (String)combo.getSelectedItem(); 
      textfieldStockName.setText(stock.getStockName()); 
      textfieldStockID.setText(stock.getStockID()); 
      textfieldStockDescription.setText(stock.getStockDescription()); 
      textfieldStockPrice.setText(stock.getStockPrice()); 
      textfieldStockQuantity.setText(stock.getStockQuantity()); 
      textfieldStockCategory.setText(stock.getStockCategory()); 
      stockQuery.con.close(); 
     } 
     catch (SQLException e1) { 
      e1.printStackTrace(); 
     } 
     } 
    }); 


    box.add(comboGUI);  
    box2.add(previous); 
    box2.add(next); 
    box3.add(minus); 
    box3.add(text); 
    box3.add(plus); 
    box4.add(update); 

    p.add(box2); 
    p.add(box); 
    p.add(box3); 
    p.add(box4); 

    this.add(p, BorderLayout.SOUTH); 
    } 
}

如果有人可以幫助我們將不勝感激。

來源

2012-04-16 Liam Bull

你能告訴我們'SQLConnect.getConnection（）;'的代碼嗎？ – dash1e 2012-04-16 15:24:50

可能的重複[數據源拒絕建立連接，來自服務器的消息：「連接太多」]（http://stackoverflow.com/questions/3203673/data-source-rejected-establishment-of-connection-message-from -server-too-many） – duffymo 2012-04-16 15:25:01

爲什麼連接在while循環內關閉？它應該在try catch的finally塊中。 – 2012-04-16 15:26:51

當你有太多打開的連接時，你會得到這個異常。
這是可配置的，但在你的情況下，問題出現在你的代碼中。

您發佈的代碼很奇怪（至少）。
除非得到SQLException或者在處理結果集期間關閉連接，否則您不要關閉連接！

在任何情況下，您應該重構您的代碼，以便在完成後立即關閉連接和結果集等。
例子：

try { 

    stat = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE); 
    rs = stat.executeQuery("SELECT categoryName FROM Category"); 

    while (rs.next()) { 
     combo.addItem(rs.getString("categoryName")); 
     categoryName = rs.getString("categoryName"); 


    } 
} catch (SQLException sqle) { 
    System.out.println(sqle); 
} 
finally{ 
if(stat != null) stat.close(); 
if(con != null) con.close(); 
}

通過投入finally收盤您確信該連接是無論是在正確的流量或異常關閉（我省略try-catch爲清楚起見）。

所以用這種方法修改代碼來關閉連接。

爲了獲得更好的性能，你應該看看連接池

來源

2012-04-16 15:27:44 Cratylus

如果你想使用JDBC（而不是JPA），那麼我建議要麼使用JdbcTemplate從春天或Java的自動資源管理（ARM）SE 7 （我還沒有用ARM試過JDBC，但它應該可以工作。）

基本上，您需要關閉try-catch子句的finally塊中的Closeables。

來源

2012-04-16 15:28:36 Puce

我想我在你的代碼中看到了問題。你的代碼非常奇怪，但這裏是我認爲正在發生的事情：

在你的ViewStockQuery類中，你有retrieveStock()方法。這個方法然後創建一個GUI類的實例，ViewStockGUI。除了這是不好的做法，它會導致一個問題就在這裏，因爲在ViewStockGUI的構造函數，你在retrieveStock()方法在這條線叫：

stockGUI = new ViewStockGUI();

你然後再調用retrieveStock()這一行：

final ViewStockQuery stock = new ViewStockQuery(); 

comboGUI = stock.getComboBox(); 
stock.retrieveStock();

這導致遞歸問題，因爲你的ViewStockGUI()構造函數調用retrieveStock()方法，它創建一個連接並再次調用ViewStockGUI()構造函數將再次調用已不是CL的retrieveStock()方法提出連接並嘗試打開不同的連接。希望你得到的照片。

解決它的一種方法是清理你的代碼。不要在retrieveStock()中調用構造函數。找到一種不同的方式爲您的查詢傳遞選定的類別。

編輯：

寫你ViewStockQuery像這樣：

private String mSelected; 

public ViewStockQuery(String selectedCategory) { 
    mSelected = selectedCategory; 
} 

... 

public void retrieveStock() throws SQLException { 

    con = SQLConnect.getConnection(); 


    String viewStock = "SELECT * FROM Stock where categoryName = '" + "'" + mSelected + "'"; 
    System.out.println(viewStock); 


    try { 
...

然後再編寫ViewStockGUI中，當一個類已經選擇這樣一種方式，那就是當你創建ViewStockQuery並傳入所選字符串。

編輯：

就像我之前說的，也有相當多的事情，你最終將需要在你的代碼改變。但是，對於這個問題的目的，你可以做的是在你的ViewStockGUI，做到：

String selected = (String)combo.getSelectedItem(); 
final ViewStockQuery stock = new ViewStockQuery(selected);

第一行從您的組合框中收集所選擇的類別和第二行創建的ViewStockQuery一個實例，並將所選到構造函數。然後這將初始化mSelected，如您在上面爲ViewStockQuery所提供的構造函數中所看到的那樣。

來源

2012-04-16 15:43:48 ugo

好吧，看着這個。 – 2012-04-17 12:37:30

你把私人mSelected; 這是什麼意思？一個字符串？有點困惑。 – 2012-04-17 14:46:06

@LiamBull，哎呀。對不起，它意味着是一個字符串。糾正。 – ugo 2012-04-18 17:26:25

使用JDBC的Java - 連接太多？

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