SpringMVC空指針異常NullPointerException的緣由和解決方法

時間  2020-04-05
標籤 springmvc 指針 異常 nullpointerexception 緣由 解決 方法 欄目 Spring 简体版
原文   原文鏈接

前言

在寫單元測試的過程當中，出現過許屢次java.lang.NullPointerException，而這些空指針的錯誤又是不一樣緣由形成的，本文從實際代碼出發，研究一下空指針的產生緣由。html

一句話歸納：空指針異常，是在程序在調用某個對象的某個方法時，因爲該對象爲null產生的。java

因此若是出現此異常，大多數狀況要判斷測試中的對象是否被成功的注入，以及Mock方法是否生效。git

基礎

出現空指針異常的錯誤信息以下：github

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)

這其實是方法棧，就是在WorkServiceImplTest.java測試類的137行調用WorkServiceImpl.java被測試類的178行出現問題。dom

下面從兩個實例來具體分析。ide

實例

1

目的：測試服務層的一個用於更新做業的功能。單元測試

接口測試

/**
     * 更新做業分數
     * @param id
     * @param score
     * @return 
     */
    Work updateScore(Long id, int score);

接口實現：this

@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爲" + id + "的做業"));
        if (!this.isTeacher()) {
            throw new AccessDeniedException("無權斷定做業");
        }
        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;
    }

測試：spa

@Test
    public void updateScore() {
        Long id = this.random.nextLong();
        Work oldWork = new Work();
        oldWork.setStudent(this.currentStudent);
        oldWork.setItem(Mockito.spy(new Item()));
        int score = 100;

        Mockito.when(this.workRepository.findById(Mockito.eq(id)))
                .thenReturn(Optional.of(oldWork));

        Mockito.doReturn(true)
                .when(oldWork.getItem())
                .getActive();

        Work work = new Work();
        work.setScore(score);


        Work resultWork = new Work();
 203    Mockito.when(this.workRepository.save(Mockito.eq(oldWork)))
                .thenReturn(resultWork);

        Assertions.assertEquals(resultWork, this.workService.updateScore(id, score));
        Assertions.assertEquals(oldWork.getScore(), work.getScore());

    }

運行測試，出現空指針：
java.lang.NullPointerException

at club.yunzhi.workhome.service.WorkServiceImpl.isTeacher(WorkServiceImpl.java:130)
at club.yunzhi.workhome.service.WorkServiceImplTest.updateScore(WorkServiceImplTest.java:203)
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.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

問題出在功能代碼的第130行，能夠看到報錯的代碼根本不是要測試的方法，而是被調用的方法。
再看測試代碼的203行，測試時的原本目的是爲了Mock掉這個方法，但使用的是when().thenReturn方式。

圖片.png

對於Mock對象（徹底假的對象），使用when().thenReturn和doReturn().when的效果是同樣的，均可以製造一個假的返回值。
可是對於Spyk對象（半真半假的對象）就不同了，when().thenReturn會去執行真正的方法，再返回假的返回值，在這個執行真正方法的過程當中，就可能出現空指針錯誤。
而doReturn().when會直接返回假的數據，而根本不執行真正的方法。
參考連接：https://sangsoonam.github.io/...

因此把測試代碼的改爲：

-    Mockito.when(this.workService.isTeacher()).thenReturn(true);
   +    Mockito.doReturn(true).when(workService).isTeacher();

再次運行，就能經過測試。

2

目的：仍是測試以前的方法，只不過新增了功能。

接口

/**
     * 更新做業分數
     * @param id
     * @param score
     * @return 
     */
    Work updateScore(Long id, int score);

接口實現（在原有的儲存學生成績方法上新增了計算總分的功能）

@Override
    public Work updateScore(Long id, int score) {
        Work work = this.workRepository.findById(id)
                .orElseThrow(() -> new ObjectNotFoundException("未找到ID爲" + id + "的做業"));
        if (!this.isTeacher()) {
            throw new AccessDeniedException("無權斷定做業");
        }
        work.setScore(score);
        work.setReviewed(true);
        logger.info(String.valueOf(work.getScore()));

   +    //取出此學生的全部做業
   +    List<Work> currentStudentWorks = this.workRepository.findAllByStudent(work.getStudent());
   +    //取出此學生
   +    Student currentStudent = this.studentService.findById(work.getStudent().getId());
   +    currentStudent.setTotalScore(0);
   +    int viewed = 0;
   +
   +    for (Work awork : currentStudentWorks) {
   +        if (awork.getReviewed() == true) {
   +            viewed++;
   +            //計算總成績
   +            currentStudent.setTotalScore(currentStudent.getTotalScore()+awork.getScore());
   +            //計算平均成績
   +            currentStudent.setAverageScore(currentStudent.getTotalScore()/viewed);
   +        }
   +    }
   +
   +    studentRepository.save(currentStudent);
        return this.save(work);
    }

因爲出現了對學生倉庫studentRepository的調用，須要注入：

final WorkRepository workRepository;
    final StudentService studentService;
    final UserService userService;
    final ItemRepository itemRepository;
    final AttachmentService attachmentService;
   +final StudentRepository studentRepository;

   -public WorkServiceImpl(WorkRepository workRepository, StudentService studentService, UserService userService, ItemRepository itemRepository, AttachmentService attachmentService) {
   +public WorkServiceImpl(WorkRepository workRepository, StudentService studentService, UserService userService, ItemRepository itemRepository, AttachmentService attachmentService, StudentRepository studentRepository) {
        this.workRepository = workRepository;
        this.studentService = studentService;
        this.userService = userService;
        this.itemRepository = itemRepository;
        this.attachmentService = attachmentService;
   +    this.studentRepository = studentRepository;
    }

而後是測試代碼

class WorkServiceImplTest extends ServiceTest {
    private static final Logger logger = LoggerFactory.getLogger(WorkServiceImplTest.class);

    WorkRepository workRepository;
    UserService userService;
    ItemRepository itemRepository;
    ItemService itemService;
    WorkServiceImpl workService;
    AttachmentService attachmentService;
   +StudentService studentService;
   +StudentRepository studentRepository;
    @Autowired
    private ResourceLoader loader;

    @BeforeEach
    public void beforeEach() {
        super.beforeEach();
        this.itemService = Mockito.mock(ItemService.class);
        this.workRepository = Mockito.mock(WorkRepository.class);
        this.userService = Mockito.mock(UserService.class);
        this.itemRepository = Mockito.mock(ItemRepository.class);
        this.studentService = Mockito.mock(StudentService.class);
        this.studentRepository = Mockito.mock(StudentRepository.class);
        this.workService = Mockito.spy(new WorkServiceImpl(this.workRepository, this.studentService,
   +            this.userService, this.itemRepository, this.attachmentService, this.studentRepository));
    }

    ...

    @Test
    public void updateScore() {
        Long id = this.random.nextLong();

        Work oldWork = new Work();
        oldWork.setScore(0);
        oldWork.setStudent(this.currentStudent);
        oldWork.setItem(Mockito.spy(new Item()));

   +    Work testWork = new Work();
   +    testWork.setScore(0);
   +    testWork.setReviewed(true);
   +    testWork.setStudent(this.currentStudent);
   +    testWork.setItem(Mockito.spy(new Item()));
    
        int score = 100;
   +    List<Work> works= Arrays.asList(oldWork, testWork);
   +
   +    Mockito.doReturn(Optional.of(oldWork))
   +            .when(this.workRepository)
   +            .findById(Mockito.eq(id));
   +    Mockito.doReturn(works)
   +            .when(this.workRepository)
   +            .findAllByStudent(oldWork.getStudent());
        Mockito.doReturn(true)
                .when(oldWork.getItem())
                .getActive();
   +    Mockito.doReturn(this.currentStudent)
   +            .when(this.studentService)
                .findById(oldWork.getStudent().getId());

        Work work = new Work();
        work.setScore(score);
        work.setReviewed(true);

        Work resultWork = new Work();
        Mockito.when(this.workRepository.save(Mockito.eq(oldWork)))
                .thenReturn(resultWork);

        Mockito.doReturn(true).when(workService).isTeacher();
        Assertions.assertEquals(resultWork, this.workService.updateScore(id, score));
        Assertions.assertEquals(oldWork.getScore(), work.getScore());

        Assertions.assertEquals(oldWork.getReviewed(),work.getReviewed());
   +    Assertions.assertEquals(oldWork.getStudent().getTotalScore(), 100);
   +    Assertions.assertEquals(oldWork.getStudent().getAverageScore(), 50);
    }
    
    ...
    
}

順利經過測試，看似沒什麼問題，但是一跑全局單元測試，就崩了。

[ERROR] Failures: 
492[ERROR]   WorkServiceImplTest.saveWorkByItemIdOfCurrentStudent:105 expected: <club.yunzhi.workhome.entity.Student@1eb207c3> but was: <null>
493[ERROR] Errors: 
494[ERROR]   WorkServiceImplTest.getByItemIdOfCurrentStudent:73 » NullPointer
495[ERROR]   WorkServiceImplTest.updateOfCurrentStudent:138 » NullPointer
496[INFO] 
497[ERROR] Tests run: 18, Failures: 1, Errors: 2, Skipped: 0

一個斷言錯誤，兩個空指針錯誤。
但是這些三個功能我根本就沒有改，並且是以前已經經過測試的功能，爲何會出錯呢？

拿出一個具體的錯誤，從本地跑一下測試：
測試代碼

@Test
    public void updateOfCurrentStudent() {
        Long id = this.random.nextLong();
        Work oldWork = new Work();
        oldWork.setStudent(this.currentStudent);
        oldWork.setItem(Mockito.spy(new Item()));

        Mockito.when(this.workRepository.findById(Mockito.eq(id)))
                .thenReturn(Optional.of(oldWork));
        //Mockito.when(this.studentService.getCurrentStudent()).thenReturn(this.currentStudent);
        Mockito.doReturn(true)
                .when(oldWork.getItem())
                .getActive();

        Work work = new Work();
        work.setContent(RandomString.make(10));
        work.setAttachments(Arrays.asList(new Attachment()));

        Work resultWork = new Work();
        Mockito.when(this.workRepository.save(Mockito.eq(oldWork)))
                .thenReturn(resultWork);

 137    Assertions.assertEquals(resultWork, this.workService.updateOfCurrentStudent(id, work));
        Assertions.assertEquals(oldWork.getContent(), work.getContent());
        Assertions.assertEquals(oldWork.getAttachments(), work.getAttachments());
    }

功能代碼

@Override
    public Work updateOfCurrentStudent(Long id, @NotNull Work work) {
        Assert.notNull(work, "更新的做業實體不能爲null");
        Work oldWork = this.workRepository.findById(id)
                .orElseThrow(() -> new ObjectNotFoundException("未找到ID爲" + id + "的做業"));
 178    if (!oldWork.getStudent().getId().equals(this.studentService.getCurrentStudent().getId())) {
            throw new AccessDeniedException("無權更新其它學生的做業");
        }

        if (!oldWork.getItem().getActive()) {
            throw new ValidationException("禁止提交已關閉的實驗做業");
        }

        oldWork.setContent(work.getContent());
        oldWork.setAttachments(work.getAttachments());
        return this.workRepository.save(oldWork);
    }

報錯信息
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)

根據報錯信息來看，是測試類在調用功能代碼178行時，出現了空指針，
通過分析，在執行this.studentService.getCurrentStudent().getId()時出現的。

而後就來判斷studentService的注入狀況，

//父類的BeforeEach
    public void beforeEach() {
        this.studentService = Mockito.mock(StudentService.class);
        this.currentStudent.setId(this.random.nextLong());
        Mockito.doReturn(currentStudent)
                .when(this.studentService)
                .getCurrentStudent();
    }
//測試類的BeforeEach
    @BeforeEach
    public void beforeEach() {
        super.beforeEach();
        this.itemService = Mockito.mock(ItemService.class);
        this.workRepository = Mockito.mock(WorkRepository.class);
        this.userService = Mockito.mock(UserService.class);
        this.itemRepository = Mockito.mock(ItemRepository.class);
        this.studentService = Mockito.mock(StudentService.class);
        this.studentRepository = Mockito.mock(StudentRepository.class);
        this.workService = Mockito.spy(new WorkServiceImpl(this.workRepository, this.studentService,
                this.userService, this.itemRepository, this.attachmentService, this.studentRepository));
    }

問題就出在這裏，因爲測試類執行了繼承，父類已經Mock了一個studentService而且成功的設定了Moockito的返回值，但測試類又進行了一次賦值，這就使得父類的Mock失效了，因而致使以前原本能經過的單元測試報錯了。

圖片.png

因此本實例的根本問題是，重複注入了對象。

這致使了原有的mock方法被覆蓋，以致於執行了真實的studentService中的方法，返回了空的學生。

解決方法：

  • 在測試類WorkServiceImplTest中刪除studentService的注入，使用父類。
  • 使用子類的studentService，並在全部的報錯位置，加入對應的mock方法

總結

java.lang.NullPointerException直接翻譯過來是空指針，但根本緣由卻不是空對象，必定是因爲某種錯誤的操做（錯誤的注入），致使了空對象。

最多見的狀況，就是在測試時執行了真正的方法，而不是mock方法。
此時的解決方案，就是檢查全部的依賴注入和Mock是否徹底正確，若是正確，就不會出現空指針異常了。

最根本的辦法，仍是去分析，找到誰是那個空對象，問題就迎刃而解。

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