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

中的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班级

  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，我们将不胜感激。

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。

您发布的代码很奇怪（至少）您要么不关闭连接，除非您获得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示例：

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(); 
}

通过将关闭放在finally中，您确定连接在正确的流程或异常中关闭（为清楚起见，我省略了try-catch）。

因此，以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，但这就是我认为正在发生的事情：

在ViewStockQuery课程中，您拥有retrieveStock()方法。然后，此方法将创建GUI类的实例ViewStockGUI。除了这是不好的做法之外，它会导致问题，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();

然后再次使用此行调用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retrieveStock()方法尚未关闭连接并尝试打开其他连接。希望你能得到这张照片。

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

编辑：

像这样写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 {
...

然后以这样的方式编写您的ViewStockGUI：当您选择某个类别时，即创建ViewStockQuery并传入所选字符串时。

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，出于此问题的目的，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，并将选定的内容传递给构造函数。然后，这将初始化mSelected，就像我在ViewStockQuery上面的构造函数中看到的那样。

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）。（我没有尝试使用JDBC的JDBC但是，它应该工作。）

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

使用JDBC的Java - 连接太多了？

3 个答案: