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());
        }
    }
}



package com.imooc.dao;
import com.imooc.domain.Student;

import java.util.List;

/*
StudengDAO访问接口
*/
public interface StudentDAO {
     /*
    查询所有学生
     */
    public List<Student> query();
}



package com.imooc.dao;

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

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
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 * from student";
        try {
            connection = JDBCUtil.getConnection();
            preparedStatement = connection.prepareStatement(sql);
            resultSet = preparedStatement.executeQuery();

            Student student = null;
            while (resultSet.next()) {
                int id = resultSet.getInt("id");
                String name = resultSet.getString("name");
                int age = resultSet.getInt("age");

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

                students.add(student);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            JDBCUtil.release(resultSet,preparedStatement,connection);
        }
        return null;
    }
}

java.lang.NullPointerException	at com.imooc.dao.StudentDAOImplTest.testQuery(StudentDAOImplTest.java:14)	
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)	
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)	
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)	
at java.lang.reflect.Method.invoke(Method.java:498)	
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)	
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)	
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)	
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)	
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)	
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)	
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)	
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)	
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)	
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)	
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)	
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)	
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)	
at org.junit.runner.JUnitCore.run(JUnitCore.java:160)	
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)	
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)	
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)	
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

报错的是第一段代码的for循环，怀疑是赋值没赋进去，有没有大佬帮我看一下