使用 JDBC 的 Java - 连接太多？

Question

作为我最后一年的项目，我正在为一家酒吧编写一个库存补充系统。我可以从 MYSQL 数据库中检索信息，并且一次可以滚动浏览一个结果。

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

两个独立文件的代码如下：

• 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，将不胜感激。

Answer 1

当您打开的连接过多时会出现此异常。
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）。
除非您收到SQLException，否则您要么不关闭连接，要么在处理结果集期间关闭它！

在任何情况下，您都应该在完成后重构代码以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，您可以确定连接以正确的流程或异常关闭（为了清楚起见，我省略了try-catch）。

所以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

我想我在您的代码中看到了问题。您的代码非常奇怪，但这是我认为正在发生的事情：

在您的ViewStockQuery 类中，您有retrieveStock() 方法。然后，此方法会创建您的 GUI 类的实例 ViewStockGUI。除了这种不好的做法之外，它还会导致一个问题，因为在 ViewStockGUI 的构造函数中，您在这一行的 retrieveStock() 方法中调用了它：

stockGUI = new ViewStockGUI();

然后您再次使用此行致电retrieveStock()：

final ViewStockQuery stock = new ViewStockQuery();

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

这会导致递归问题，因为您的ViewStockGUI() 构造函数调用创建连接的retrieveStock() 方法并再次调用ViewStockGUI() 构造函数，该构造函数将再次调用尚未关闭连接的retrieveStock() 方法，并且尝试打开不同的连接。希望你能得到照片。

解决此问题的一种方法是清理您的代码。不要调用retrieveStock() 中的构造函数。找到一种不同的方式为您的查询传递所选类别。

编辑：

像这样写给你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，即当您选择类别时，即创建ViewStockQuery 并传入选定的字符串。

编辑：

正如我之前所说，您最终需要在代码中更改很多内容。但是，就本期而言，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所选类别，第二行创建ViewStockQuery 的实例并将所选类别传递给构造函数。然后，这将初始化 mSelected，正如您在上面为 ViewStockQuery 放置的构造函数中看到的那样。

Answer 3

如果您想使用 JDBC（而不是 JPA），那么我建议您使用 Spring 中的 JdbcTemplate 或 Java SE 7 的自动资源管理 (ARM)。（我还没有尝试使用 ARM 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。