使用executeQuery时出现NullPointerException

Question

我的代码中有一个空指针异常

ResultSet rs = aStatement.executeQuery(Query); // it can't be executed 

，如下所示：

 public static boolean testLogin(String user, String password) throws SQLException {
    String Query = "select * from TBL_Users where userName = '" + user + "' and  passWord = '" + password + "' ";
    ResultSet rs = aStatement.executeQuery(Query);

    while (rs.next()) {

        info.Id = rs.getInt("ID");
        info.userName = rs.getString("userName");
        info.Name = rs.getString("User_Name");
        info.Password = rs.getString("passWord");
        info.isAdmin = rs.getBoolean("Admin");
        return true;
    }
    return false;
}

}

Answer 1

aStatement 很可能为 null。

Answer 2

听起来您认为 aStatement 不应该为 null，但事实确实如此。

这是糟糕的 JDBC 代码，原因有很多：

1. 没有清理资源。
2. 不使用PreparedStatement
3. 不断地一遍又一遍地创建查询字符串，而不是使用静态变量
4. 不遵循Java编码标准（“Query”应该是“query”）

这是另一种编写方式。从接口开始：

package persistence;

import java.sql.SQLException;

public interface CredentialDao
{
    boolean isValidUser(String username, String password) throws SQLException;
}

编写实现：

package persistence;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class CredentialDaoImpl implements CredentialDao
{
    private static final String CREDENTIAL_QUERY = "SELECT COUNT() FROM USER WHERE USERNAME = ? AND PASSWORD = ?";

    private Connection connection;

    public CredentialDaoImpl(Connection connection)
    {
        this.connection = connection;
    }

    public boolean isValidUser(String username, String password) throws SQLException
    {
        boolean isValidUser = false;

        PreparedStatement ps = null;
        ResultSet rs = null;

        try
        {
            ps = this.connection.prepareStatement(CREDENTIAL_QUERY);
            ps.setString(1, username);
            ps.setString(2, password);
            rs = ps.executeQuery();
            while (rs.next())
            {
                int count = rs.getInt(1);
                isValidUser = (count > 0);
            }
        }
        finally
        {
            DatabaseUtils.close(rs);
            DatabaseUtils.close(ps);
        }

        return isValidUser;
    }
}

Answer 3

aStatement 变量显然为 null，请验证其设置是否正确。您应该考虑阅读 Java 命名约定 并确保使用较低的变量和 java bean 约定的驼峰式大小写。

对于stackoverflow中的代码片段，如果它们不言自明，则应遵守SSCCE，这将帮助您获得更多更好的答案。您还应该提供发生异常的堆栈跟踪。

Answer 4

使用准备好的语句。

    Connection con = ...; // obtain connection here
    PreparedStatement pstmt = con.prepareStatement("select * from TBL_Users where userName = ?'");
    pstmt.setInt(1, userName);

    ResultSet rs = pstmt .executeQuery();
...
// do clean up here

Answer 5

 while (rs.next()) {

    info.Id = rs.getInt("ID");
    info.userName = rs.getString("userName");
    info.Name = rs.getString("User_Name");
    info.Password = rs.getString("passWord");
    info.isAdmin = rs.getBoolean("Admin");
    return true;       //                                Huh? What?
}

info 指的是什么以及为什么在赋值之后立即有一个 return ？