Java向MySQL插入值并从MySQL返回值

Question

我正在创建一个员工登录验证类，我可以将他们的信息插入到 mysql 数据库中，然后为了进行验证，我尝试仅选择该行的密码。这是我的代码:

     import java.sql.*;
        import java.sql.PreparedStatement;
        import java.sql.Connection;


    public class Employee 
    {

    private PreparedStatement statement;
    private Connection con;
    private String pass;
    public Employee()
    {   

    }
    public Employee(String firstName, String lastName, String userID, String userPassword, String role )
    {
        try
        {
            Class.forName("com.mysql.jdbc.Driver");
            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/test","root","");
            statement = con.prepareStatement("Insert into employee(firstName, lastName, userID, " +
                    "password, role)values(?, ?, ?, ?, ?)");    

            statement.setString(1, firstName);
            statement.setString(2, lastName);
            statement.setString(3, userID);
            statement.setString(4, userPassword);
            statement.setString(5, role);
            statement.executeUpdate();
            con.close(); 
        }
        catch(Exception e)
        {

        }
    }
    public void setPassword(String userID)
    {
        try
        {
            statement = con.prepareStatement("SELECT * from employee WHERE userID = ?");
            statement.setString(1, userID);
            ResultSet rs = statement.executeQuery();
            while(rs.next()){
                pass = rs.getString(4);
            }
        }
        catch(Exception e)
        {
            System.out.print(e);
        }
    }
    public String getPassword()
    {
        return pass;
    }


    }
and here is my main

        public class TestMain 
        {
        public static void main(String argsp[])
    {
        Employee emp = new Employee();
        emp.setPassword("ecka12");  
    }
    }

我遇到了空指针异常，我不知道为什么。

Answer 1

您正在使用以下方式创建对象:-

   Employee emp = new Employee();

在 0-arg 构造函数中，您尚未初始化连接

public Employee() {

}

因此，当您在 setPassword() 方法中使用 con 时，您将得到 NullPointerException : -

    // con is not initialized
    statement = con.prepareStatement("SELECT * from employee WHERE userID = ?");
    statement.setString(1, userID);

要纠正此问题，您需要将以下代码移至 0-arg 构造函数。并在您的其他构造函数中: - 添加 this() 作为您的第一个声明..

public Employee() {
    try {
         Class.forName("com.mysql.jdbc.Driver");
         con = DriverManager.getConnection(
                        "jdbc:mysql://localhost:3306/test","root","");
    } catch (Exception e) {
         e.printStackTrace();
    }
}



public Employee(String firstName, String lastName) {
     this(); // Initialize con..

     try {
        // You don't need to initialize your con here again..
        // Your remaining code.. 
     }
}