使用JDBC的Java - 连接太多了？

Question

我正在为酒吧写一个库存补货系统作为我的最后一年项目。 我可以从MYSQL数据库中检索信息，我可以一次滚动一个结果。

我正在尝试根据所选类别更改结果。 我已经设法使用组合框来实现这一点，但在类别之间移动时出现以下错误：

线程“main”中的异常com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException：数据源拒绝建立连接，来自服务器的消息：“连接太多”

两个单独文件的代码如下：

• RetrieveStockQuery的SQL查询

   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()); } } 

• 我的ViewStockGUI课程

    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); } } 

如果有人可以提供帮助，将不胜感激。

Answer 1

当您有太多打开的连接时，会出现此异常。
这是可配置的，但在您的情况下，问题出在您的代码中。

你发布的代码很奇怪（至少）。
您要么不关闭连接，除非您得到SQLException或在处理结果集期间关闭它！

在任何情况下，您都应该重构代码，以便在完成后立即关闭连接和结果集等。
例：

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(); 
}

通过将关闭置于finally您可以确保连接以正确的流程或异常关闭（为清楚起见，我省略了try-catch ）。

因此，通过这种方式修改代码以关闭连接。

为了获得更好的性能，您应该研究连接池

Answer 2

我想我在你的代码中看到了问题。 你的代码非常奇怪，但这就是我认为发生的事情：

在ViewStockQuery类中，您有retrieveStock()方法。 然后，此方法创建GUI类的实例ViewStockGUI 。 除了这是不好的做法之外，它会导致一个问题，因为在ViewStockGUI的构造函数中，您在此行的retrieveStock()方法中调用了它：

stockGUI = new ViewStockGUI();

然后再用这一行调用retrieveStock() ：

final ViewStockQuery stock = new ViewStockQuery();

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

这会导致递归问题，因为ViewStockGUI()构造函数调用retrieveStock()方法创建连接并再次调用ViewStockGUI()构造函数，该构造函数将再次调用未关闭连接并尝试打开的retrieveStock()方法不同的联系。 希望你能得到这张照片。

解决它的方法是清理代码。 不要在retrieveStock()调用构造函数。 查找为您的查询传递所选类别的其他方法。

编辑：

像你这样写ViewStockQuery ：

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 {
...

然后编写ViewStockGUI ，以便在选择类别时，即创建ViewStockQuery并传入所选字符串。

编辑：

就像我之前说的那样，你的代码最终需要改变很多东西。 但是，出于本期的目的，您可以在ViewStockGUI中执行以下操作：

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

第一行从组合框中收集选定的类别，第二行创建ViewStockQuery的实例，并将选定的ViewStockQuery传递给构造函数。 然后，这将初始化mSelected，就像我在上面为ViewStockQuery提出的构造函数中看到的ViewStockQuery 。

Answer 3

如果你想使用JDBC（而不是JPA），那么我建议使用Spring的JdbcTemplate或Java SE 7的自动资源管理（ARM）。（我还没有尝试使用JDBC的JDBC，但它应该可以工作。 ）

基本上，您需要在try-catch子句的finally块中关闭Closeables。