如果sql查询什么都没找到，如何处理？ 在java中使用resultset
[英]How to handle if a sql query finds nothing? Using resultset in java
问题描述
I have found a few answers to this question on this site, but haven't been able to apply it correctly. 
我在这个网站上找到了这个问题的一些答案，但是无法正确应用它。 I'm trying to take a user's input for user name and password and check it against a database to see if its correct. 我正在尝试获取用户输入的用户名和密码，并根据数据库检查它是否正确。 Everything works fine unless the username entered is not in the database. 除非输入的用户名不在数据库中，否则一切正常。 In that case the query doesn't find any rows. 在这种情况下，查询找不到任何行。 I'm doing this with derby in netbeans. 我在netbeans中用德比做这个。 So I think I'm just having trouble detecting when the query is sent with an unknown username. 因此，我认为我在查找使用未知用户名发送查询时遇到问题。 Any Suggestions? 有什么建议么？
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.*;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(name = "MyServlet", urlPatterns ={"/echo"})
public class LoginAction extends HttpServlet {
private ResultSet resultSet;
private Connection con;
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html; charset=UTF-8");
PrintWriter out = response.getWriter();
try {
out.println("<!DOCTYPE html>");
out.println("<html><head>");
out.println("<meta http-equiv='Content-Type' content='text/html; charset=UTF-8'>");
String userName = request.getParameter("username");
String password = request.getParameter("password");
if (userName.length()>0 & password.length()>0){
try{
String url = "jdbc:derby:c:/users/project/.netbeans-derby/";
String db = "loginpass";
String driver = "org.apache.derby.jdbc.EmbeddedDriver";
String user = "user";
String pass = "pass";
Class.forName(driver).newInstance();
con = DriverManager.getConnection(url+db, user, pass);
String statement = "SELECT PASSWORD FROM LOGINPASS WHERE LOGIN = '"+userName+"'";
PreparedStatement prepStatement = con.prepareStatement(statement);
resultSet = prepStatement.executeQuery();
while (resultSet.next()){
if (resultSet.getString("password").equals(password)){
out.println("Login attempt successful!");
out.println("<br><a href='index.jsp'>Back to Login Page</a>");
} else {
out.println("Password incorrect");
out.println("<br><a href='index.jsp'>Back to Login Page</a>");
}
}
con.close();
}catch(Exception e){
out.println("caught...");
e.printStackTrace();
}
} else {
out.println("Login attempt failed");
out.println("<a href='index.jsp'>Back to Login Page</a>");
}
} finally {
out.close();
}
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
3 个解决方案
解决方案1
2 已采纳 2013-07-06 20:31:58
As you expect only one row, you may write: 正如您所期望的只有一行，您可以写道：
if (resultSet.next()) {
if (resultSet.getString("password").equals(password)){
out.println("Login attempt successful!");
out.println("<br><a href='index.jsp'>Back to Login Page</a>");
} else {
out.println("Password incorrect");
out.println("<br><a href='index.jsp'>Back to Login Page</a>");
}
} else {
out.println("User doesn't exist");
out.println("<br><a href='index.jsp'>Back to Login Page</a>");
}
And you'd better use parameters to avoid sql injections: 并且你最好使用参数来避免sql注入：
String statement = "SELECT PASSWORD FROM LOGINPASS WHERE LOGIN = ?";
PreparedStatement prepStatement = con.prepareStatement(statement);
prepStatement.setString(1, userName);
resultSet = prepStatement.executeQuery();
解决方案2
1 2013-07-06 20:25:10
You can check that from the ResultSet object itself . 您可以从ResultSet对象本身检查它。 ResultSet#isBeforeFirst() : ResultSet＃isBeforeFirst（） ：
Retrieves whether the cursor is before the first row in this ResultSet object.
Note: Support for the isBeforeFirst method is optional for ResultSets with a result set type of TYPE_FORWARD_ONLY
Returns:
true if the cursor is before the first row;
if (!resultSet.isBeforeFirst()) {
out.println("no rows fetched");
out.println("User with this password not found");
out.println("<br><a href='index.jsp'>Back to Login Page</a>");
}
解决方案3
0 2013-07-06 20:28:23
在While（resultSet.next（））之前，键入if（resultSet！= null）。如果查询没有结果，则resultSet将为null。
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