奇怪的Null指针异常Java Swing SQL

Question

有人可以解释一下为什么我在代码中得到空指针异常

 try
    {
        dao = new EmployeeDAO();
    }
    catch (Exception e)
    {
        JOptionPane.showMessageDialog(
                null,
                "Creating Dao Error: " + e);
    }

我收到对话消息，指出发生了空指针异常。

这是Swing类的完整代码

public class EmployeeSearchApp extends JFrame
{
      private JPanel mainPanel;
      private JPanel textLabelPanel;
      private JTextField lastNameTxtField;
      private JButton searchButton;
      private JLabel lastNameLabel;

      private EmployeeDAO dao;

public EmployeeSearchApp()
{
    try
    {
        dao = new EmployeeDAO();
    }
    catch (Exception e)
    {
        JOptionPane.showMessageDialog(
                null,
                "Creating Dao Error: " + e);
    }

    searchButton.addActionListener(e ->
    {

        try
        {
            String lastName = lastNameTxtField.getText();

            List<Employee> employees = null;

            if (lastName != null && lastName.trim().length() > 0)
            {
                employees = dao.searchEmployees(lastName);
            }
            else
            {
                employees = dao.getAllEmployee();
            }

            for (Employee emp : employees)
            {
                System.out.println(emp);
            }
        }
        catch (Exception exc)
        {
            JOptionPane.showMessageDialog(
                    null,
                    "addActionListener Error: " + exc);
        }

    });
}

private void createUIComponents()
{
    // TODO: place custom component creation code here
}

public static void main(String[] args)
{
    try
    {
        EmployeeSearchApp frame = new EmployeeSearchApp();
        frame.setVisible(true);
    }
    catch (Exception e)
    {
        e.printStackTrace();
    }
}

}

这是DAO类的代码

package DataAccesObject;

import Core.Employee;


import java.io.FileInputStream;
import java.sql.*;
import java.util.ArrayList;
import java.util.List;
import java.util.Properties;

public class EmployeeDAO

{     私人连接conn;

public EmployeeDAO()
{
    Properties props = null;

    //getting properties
    try
    {
        props = new Properties();
        props.load(new FileInputStream("props.properties"));

        conn = DriverManager.getConnection(
                props.getProperty("dbURL"),
                props.getProperty("user"),
                props.getProperty("password"));

    }
    catch (java.io.IOException e)
    {
        System.out.println("properties file is not found");
        System.exit(-2);
    }
    catch (java.sql.SQLException e)
    {
        System.out.println("couldn't connect to DATABASE");
        System.exit(-3);
    }

    System.out.println("Connection to DB successful");

}

public List<Employee> getAllEmployee() throws java.sql.SQLException
{
    List<Employee> list = new ArrayList<>();

    Statement stmt = null;
    ResultSet resSet = null;

    try
    {
        stmt = conn.createStatement();
        resSet = stmt.executeQuery("Select * from employees");

        while (resSet.next())
        {
            //list.add(convertToEmployee(resSet));
            Employee tmp = convertToEmployee(resSet);
            list.add(tmp);
        }

        return list;
    }
    finally//Execute ALWAYS after try block doesn't matter if try thrown exception
    {
        if (stmt != null)
            stmt.close();

        if (resSet != null)
            resSet.close();
    }
}

public List<Employee> searchEmployees(String lastName) throws Exception
{
    List<Employee> list = new ArrayList<>();

    PreparedStatement myStmt = null;
    ResultSet myRs = null;

    try
    {
        lastName = "%" + lastName + "%";
        myStmt = conn.prepareStatement("select * from employees where last_name like ?");

        myStmt.setString(1, lastName);

        myRs = myStmt.executeQuery();

        while (myRs.next())
        {
            Employee tempEmployee = convertToEmployee(myRs);
            list.add(tempEmployee);
        }

        return list;
    }
    finally
    {
        if (myStmt != null)
            myStmt.close();

        if (myRs != null)
            myRs.close();
    }
}

private Employee convertToEmployee(ResultSet res) throws java.sql.SQLException
{
    return new Employee(
            res.getString("first_name"),
            res.getString("last_name"),
            res.getString("email"),
            res.getBigDecimal("salary")
    );

}

public static void main(String[] args) throws Exception
{
    EmployeeDAO dao = new EmployeeDAO();
    System.out.println(dao.searchEmployees("thom").size());
}

}