运行StudentDaoImplTest的时候报了空指针异常。。。。大神帮忙看一下

package com.imooc.dao;

import com.imooc.domain.Student;
import com.imooc.util.JDBCUtil;

import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

/**
 * StudentDao访问接口实现类
 */
public class StudentDaoImpl implements StudentDao{
    @Override
    public List<Student> query() {

        List<Student> students = new ArrayList<Student>();

        Connection connection = null;
        PreparedStatement preparedStatement = null;
        ResultSet resultSet = null;
        String sql  = "select id , name , age from student";
        try {
            connection = JDBCUtil.getConnection();
            preparedStatement = connection.prepareStatement(sql);
            resultSet = preparedStatement.executeQuery();


            //迭代resultSet
            while (resultSet.next()){
                int id = resultSet.getInt("id");
                String name = resultSet.getString("name");
                int age = resultSet.getInt("age");

                Student student = new Student();
                student.setId(id);
                student.setAge(age);
                student.setName(name);

                students.add(student);
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            JDBCUtil.release(resultSet,preparedStatement,connection);
        }

        return null;
    }
}

package com.imooc.dao;

import com.imooc.domain.Student;
import org.junit.Test;

import java.util.List;


public class StudentDaoImplTest {

    @Test
    public void testQuery(){

        StudentDao studentDao = new StudentDaoImpl();
        List<Student> students = studentDao.query();
        for(Student student : students){
            System.out.println("id:"+student.getId()
                    +",name:"+student.getName()
                    +",age:"+student.getAge());
        }
    }
}