遇到的问题描述：Hibernate中JUnit版本导致空指针异常问题

首先我说明一下我遇到的问题：初次使用Hibernate时，没有仔细听项目负责人介绍Hibernate，而使用了Hibernate中的JUnit5，以至于后面一直出现空指针异常，排
除也排除不了，代码找不到错误。
下面是我的数据库、工程和代码：
数据库：

图片.png

图片.png

工程：

1.png

import org.hibernate.Session;
import com.qianfeng.model.User;
import com.qianfeng.tools.DBUtil;

public class UserDao {
public User selectById(Integer id) {
Session session=DBUtil.findsession();
User user = (User) session.get(User.class,id);
return user;
}
}

User.java类：
package com.qianfeng.model;

import java.io.Serializable;
public class User implements Serializable {
private Integer id;
private String username;
private String password;
private Integer gender;
private Integer age;
private String address;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public Integer getGender() {
return gender;
}
public void setGender(Integer gender) {
this.gender = gender;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public User() {
super();
}
}

User.hbm.xml文件：
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
<class name="com.qianfeng.model.User" table="JAVATEST">

<id name="id" column="ID"></id>

<property name="username" column="USERNAME"></property>
<property name="password" column="PASSWORD"></property>
<property name="gender" column="GENDER"></property>
<property name="age" column="AGE"></property>
<property name="address" column="ADDRESS"></property>
</class>
</hibernate-mapping>

HibernateTest.java类：
package com.qianfeng.test;

//import org.junit.Before;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.Test;
import com.qianfeng.dao.UserDao;
import com.qianfeng.model.User;

public class HibernateTest {
//UserDao dao;
/@Before
public static void init() {
dao=new UserDao();
}/
static UserDao dao;
@BeforeAll
public static void init() {
dao=new UserDao();
}
@Test
public void test() {
System.out.println(dao);
User user=dao.selectById(1);
System.out.println(user); System.out.println(user.getId()+","+user.getUsername()+","+user.getPassword());
}
}

DBUtil.java类：
package com.qianfeng.tools;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
public class DBUtil {
static SessionFactory sf;
static {
Configuration cfg=new Configuration().configure("hibernate.cfg.xml");
sf = cfg.buildSessionFactory();
}
public static Session findsession() {
return sf.openSession();
}
public static void main(String[] args) {
//初始化Hibernaete.cfg.xml文件
Configuration cfg=new Configuration().configure("hibernate.cfg.xml");
//初始化SessionFactory工厂
SessionFactory sf = cfg.buildSessionFactory();
System.out.println(sf.openSession());
}
}

hibernate.cfg.xml文件：
<?xml version="1.0"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>

<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="connection.url">jdbc:mysql://localhost:3306/javaee</property>
<property name="connection.username">root</property>
<property name="connection.password">123</property>
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>

<property name="show_sql">true</property>

<property name="format_sql">true</property>

<mapping resource="com/qianfeng/model/User.hbm.xml"/>
</session-factory>
</hibernate-configuration>

其中测试类中我使用的是JUnit5，因为负责人是JUnit4，所有我仔细审核了代码，发现代码是没有异样的，这里注意我使用的是被注释的HibernateTest类的@Before，故测试结果一直报User user=dao.selectById(1);这句空指针异常，我很奇怪，在这一句前面加了输出测试System.out.println(dao);发现dao为null；后面了解到原来是JUnit5中，初始化dao出错了，不应该用@Before，而应该用@BeforeAll，并且通过看@BeforeAll源码可以了解到该方法必须为静态，这就是JUnit5和JUnit4的区别之一。
所有我改掉之后
//UserDao dao;
/@Before
public static void init() {
dao=new UserDao();
}/
static UserDao dao;
@BeforeAll
public static void init() {
dao=new UserDao();
}
结果就出来了：
Hibernate:
select
user0_.ID as ID0_0_,
user0_.USERNAME as USERNAME0_0_,
user0_.PASSWORD as PASSWORD0_0_,
user0_.GENDER as GENDER0_0_,
user0_.AGE as AGE0_0_,
user0_.ADDRESS as ADDRESS0_0_
from
JAVATEST user0_
where
user0_.ID=?
com.qianfeng.model.User@2638011
1,XXX,123