catch (MySqlException ex) { }
时间: 2024-02-10 08:57:01 浏览: 15
This is a try-catch block in C# language that catches an exception of type MySqlException. The code inside the catch block will be executed if a MySqlException is thrown in the try block. In this case, the catch block is empty, which means that no action will be taken when the exception is caught. However, it is always recommended to handle exceptions appropriately by logging the error, notifying the user, or taking other corrective actions.
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public void actionPerformed(ActionEvent e) { String username=uField.getText().trim(); String password =pField.getText().trim(); String phonenum=tField.getText().trim(); String gender=bg.isSelected(maleBtn.getModel())?maleBtn.getText():femaleBtn.getText(); String Driver = "com.mysql.cj.jdbc.Driver"; String url = "jdbc:mysql://localhost:3306/data2?characterEncoding=UTF8&useSSL=false"; Connection conn = null; try{ Class.forName(Driver); conn = DriverManager.getConnection(url, "root", "yxh9497"); Statement stmt=conn.createStatement(); ResultSet rs = stmt.executeQuery("SELECT * FROM userinformation"); while(rs.next()) { if(username==rs.getString(username)) { JOptionPane.showMessageDialog(null, "用户名已被注册"); break; } } String sql1="insert into userinformation values('"+ username+ "','"+password+"','"+phonenum+"','"+gender+"')"; int a= stmt.executeUpdate(sql1); }catch(Exception ex){ ex.printStackTrace(); } try{ //关闭数据库联接类 conn.close(); }catch(Exception ex){ ex.printStackTrace(); } JOptionPane.showMessageDialog(null, "注册成功"); jf.setVisible(false); new GUI().init();}我想在当用户名重复时,不再执行String sql1="insert into userinformation values('"+ username+ "','"+password+"','"+phonenum+"','"+gender+"')"; int a= stmt.executeUpdate(sql1); }catch(Exception ex){ ex.printStackTrace(); } try{ //关闭数据库联接类 conn.close(); }catch(Exception ex){ ex.printStackTrace(); } JOptionPane.showMessageDialog(null, "注册成功"); jf.setVisible(false); new GUI().init();}这些语句,该怎么办
你可以在 while 循环中加一个标志变量,如 isExist,初始值为 false。如果找到重复的用户名,将该变量设为 true,然后在 while 循环结束后根据 isExist 的值来判断是否执行插入语句和注册成功的提示框。修改后的代码如下:
```
public void actionPerformed(ActionEvent e) {
String username = uField.getText().trim();
String password = pField.getText().trim();
String phonenum = tField.getText().trim();
String gender = bg.isSelected(maleBtn.getModel()) ? maleBtn.getText() : femaleBtn.getText();
String Driver = "com.mysql.cj.jdbc.Driver";
String url = "jdbc:mysql://localhost:3306/data2?characterEncoding=UTF8&useSSL=false";
Connection conn = null;
boolean isExist = false; // 标志变量
try {
Class.forName(Driver);
conn = DriverManager.getConnection(url, "root", "yxh9497");
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT * FROM userinformation");
while (rs.next()) {
if (username.equals(rs.getString("username"))) { // 修改此处
JOptionPane.showMessageDialog(null, "用户名已被注册");
isExist = true; // 修改标志变量的值
break;
}
}
if (!isExist) { // 如果用户名不存在,才执行以下语句
String sql1 = "insert into userinformation values('" + username + "','" + password + "','" + phonenum + "','" + gender + "')";
int a = stmt.executeUpdate(sql1);
JOptionPane.showMessageDialog(null, "注册成功");
jf.setVisible(false);
new GUI().init();
}
} catch (Exception ex) {
ex.printStackTrace();
}
try {
conn.close();
} catch (Exception ex) {
ex.printStackTrace();
}
}
```
public class dbConn { public dbConn() { try { jbInit(); } catch (Exception e) { e.printStackTrace(); } } private Statement conn() { try { // 加载驱动 Class.forName("com.mysql.jdbc.Driver"); // 数据库名称,管理员账号、密码 String url = "jdbc:mysql://localhost:3306/test"; String user = "root"; String pwd = "123456789"; // 连接 Connection con = null; con = DriverManager.getConnection(url, user, pwd); Statement stat = con.createStatement(); return stat; } catch (ClassNotFoundException ex) { return null; } catch (SQLException ex1) { return null; } } // 查询数据库 public ResultSet getRs(String sql) { try { Statement stat = conn(); ResultSet rs = stat.executeQuery(sql); System.out.println(rs); return rs; } catch (SQLException ex) { System.err.println("------------" + ex.getMessage()); return null; } } // 更新数据库 public int getUpdate(String sql) { try { Statement stat = conn(); int i = stat.executeUpdate(sql); return i; } catch (Exception ex) { System.out.println(">>>>>>>>" + ex.getMessage()); return -1; } } private void jbInit() throws Exception { conn(); } }代码报错java.lang.NullPointerException: Cannot invoke "java.sql.Statement.executeQuery(String)" because "stat" is null,帮我修改一下
你好,根据你提供的代码及错误信息,很可能是在执行查询数据库的方法 `getRs()` 时,`conn()` 方法返回的 `stat` 为 `null`,导致后续调用 `stat` 对象的方法时出现空指针异常。可以在 `conn()` 方法中加入判断语句,当获取连接失败时返回 `null`,并在调用 `getRs()` 方法时加入判断 `stat` 是否为 `null` 的语句。修改后的代码如下:
```java
public class dbConn {
public dbConn() {
try {
jbInit();
} catch (Exception e) {
e.printStackTrace();
}
}
private Statement conn() {
try {
// 加载驱动
Class.forName("com.mysql.jdbc.Driver");
// 数据库名称,管理员账号、密码
String url = "jdbc:mysql://localhost:3306/test";
String user = "root";
String pwd = "123456789";
// 连接
Connection con = null;
con = DriverManager.getConnection(url, user, pwd);
Statement stat = con.createStatement();
return stat;
} catch (ClassNotFoundException ex) {
return null;
} catch (SQLException ex1) {
return null;
}
}
// 查询数据库
public ResultSet getRs(String sql) {
try {
Statement stat = conn();
if (stat == null) { // 判断连接是否成功
return null;
}
ResultSet rs = stat.executeQuery(sql);
System.out.println(rs);
return rs;
} catch (SQLException ex) {
System.err.println("------------" + ex.getMessage());
return null;
}
}
// 更新数据库
public int getUpdate(String sql) {
try {
Statement stat = conn();
if (stat == null) { // 判断连接是否成功
return -1;
}
int i = stat.executeUpdate(sql);
return i;
} catch (Exception ex) {
System.out.println(">>>>>>>>" + ex.getMessage());
return -1;
}
}
private void jbInit() throws Exception {
conn();
}
}
```
希望能帮到你。