我有一个实现动作监听器的注册对话框。如果用户输入名称并且它已经存在,我想在控制台上打印一条消息。如果用户名不存在,MySQL应将其添加到数据库中。不幸的是,这段代码不起作用:
private void regButtonActionPerformed(java.awt.event.ActionEvent evt) {
if(!userBox.getText().equals(""))
{
try
{
Class.forName("com.mysql.jdbc.Driver");
Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/genx", "root", "Warlock1989");
Statement stmt = con.createStatement();
String query = "SELECT name FROM accounts";
ResultSet rs = stmt.executeQuery(query);
while(rs.next())
{
String uname = rs.getString("name");
if(!uname.equals(userBox.getText()))
{
PreparedStatement pstmt = con.prepareStatement("INSERT INTO accounts(name) VALUES(?)");
pstmt.setString(1, userBox.getText());
pstmt.executeUpdate();
System.out.println("Username " + userBox.getText() + " has been registered.");
}
else
{
System.out.println("Username " + userBox.getText() + " already exists.");
}
}
}
catch(Exception ex)
{
ex.printStackTrace();
}
}
答案 0 :(得分:1)
您当前的方法从数据库加载所有记录,并尝试查找将导致内存异常的用户(如果数据库包含大量记录)。所以,
不要从数据库中获取所有记录而只是使用where name=?
运行查询来检查用户是否已存在于数据库中,如下所示:
PreparedStatement pstmt1 = null;
PreparedStatement pstmt2 = null;
ResultSet rs = null;
try {
String userInput = userBox.getText();
String query = "SELECT name FROM accounts where name=?";
pstmt1 = con.preparedStatement(query);
pstmt1.setString(1, userInput);
rs = pstmt1.executeQuery();
if(rs.next()) {
pstmt2 = con.prepareStatement("INSERT INTO accounts(name) VALUES(?)");
pstmt2.setString(1, userInput);
pstmt2.executeUpdate();
System.out.println("Username " + userInput + " has been registered.");
} else {
System.out.println("Username " + userInput + " already exists.");
}
} catch(SQLException sqlexe) {
//add logging
} finally {
if(pstmt1 != null)
pstmt1.close();
if(pstmt2 != null)
pstmt2.close();
if(rs != null)
rs.close();
}
您当前的代码未发布resultsset
& preparedstatement
个对象,因此请确保释放finally块中的资源。