java实体类set方法报空指针异常_SpringMVC空指针异常NullPointerException的原因和解决方法...

前言

在写单元测试的过程中，出现过许多次java.lang.NullPointerException，而这些空指针的错误又是不同原因造成的，本文从实际代码出发，研究一下空指针的产生原因。

一句话概括：空指针异常，是在程序在调用某个对象的某个方法时，由于该对象为null产生的。

所以如果出现此异常，大多数情况要判断测试中的对象是否被成功的注入，以及Mock方法是否生效。

基础

出现空指针异常的错误信息如下：

java.lang.NullPointerException

at club.yunzhi.workhome.service.WorkServiceImpl.updateOfCurrentStudent(WorkServiceImpl.java:178)

at club.yunzhi.workhome.service.WorkServiceImplTest.updateOfCurrentStudent(WorkServiceImplTest.java:137)

at java.base/java.util.ArrayList.forEach(ArrayList.java:1540)

at java.base/java.util.ArrayList.forEach(ArrayList.java:1540)

at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)

at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)

at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

这实际上是方法栈，就是在WorkServiceImplTest.java测试类的137行调用WorkServiceImpl.java被测试类的178行出现问题。

下面从两个实例来具体分析。

实例

(代码仅为了报错时方便分析，请勿仔细阅读，避免浪费时间)

1

目的：测试服务层的一个用于更新作业的功能。

接口

/**

* 更新作业分数

* @param id

* @param score

* @return

*/

Work updateScore(Long id, int score);

接口实现：

@Service

public class WorkServiceImpl implements WorkService {

private static final Logger logger = LoggerFactory.getLogger(WorkServiceImpl.class);

private static final String WORK_PATH = "work/";

final WorkRepository workRepository;

final StudentService studentService;

final UserService userService;

final ItemRepository itemRepository;

final AttachmentService attachmentService;

public WorkServiceImpl(WorkRepository workRepository, StudentService studentService, UserService userService, ItemRepository itemRepository, AttachmentService attachmentService) {

this.workRepository = workRepository;

this.studentService = studentService;

this.userService = userService;

this.itemRepository = itemRepository;

this.attachmentService = attachmentService;

}

...

@Override

public Work updateScore(Long id, int score) {

Work work = this.workRepository.findById(id)

.orElseThrow(() -> new ObjectNotFoundException("未找到ID为" + id + "的作业"));

if (!this.isTeacher()) {

throw new AccessDeniedException("无权判定作业");

}

work.setScore(score);

logger.info(String.valueOf(work.getScore()));

return this.save(work);

}

@Override

public boolean isTeacher() {

User user = this.userService.getCurrentLoginUser();

130 if (user.getRole() == 1) {

return false;

}

return true;

}

测试：

@Test

public void updateScore() {

Long id = this.random.nextLong();

Work oldWork = new Work();

oldWork.setStudent(this.c