保存Spring实体时的java.lang.NullPonterException

我尝试读取Excel文件并将数据保存在postgres数据库中。在某些文件中，我收到以下错误消息：

java.lang.NullPointerException:null 在de.hspf.讲师服务.semestertock.semestertock.hashCode（semestertock.java:51）~[classes/：na] 在java.lang.Object.toString（Object.java:236）~[na:1.8.0_161] 在java.lang.String.valueOf（String.java:2994）~[na:1.8.0_161] 在java.lang.StringBuilder.append（StringBuilder.java:131）~[na:1.8.0_161] 在org.hibernate.pretty.MessageHelper.infoString（MessageHelper.java:57）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.ununiqueobjectexception.getMessage（ununiqueobjectexception.java:61）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.internal.ExceptionConverterImpl.convert（ExceptionConverterImpl.java:123）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.internal.ExceptionConverterImpl.convert（ExceptionConverterImpl.java:181）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.internal.ExceptionConverterImpl.convert（ExceptionConverterImpl.java:188）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.internal.SessionImpl.fireMerge（SessionImpl.java:933）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.internal.SessionImpl.merge（SessionImpl.java:892）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.engine.spi.CascadingActions$6.cascade（CascadingActions.java:261）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.engine.internal.Cascade.cascadeToOne（Cascade.java:490）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.engine.internal.Cascade.cascadeAssociation（Cascade.java:415）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.engine.internal.Cascade.cascadeProperty（Cascade.java:216）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.engine.internal.Cascade.cascadeCollectionElements（Cascade.java:523）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.engine.internal.Cascade.cascadeCollection（Cascade.java:455）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.engine.internal.Cascade.cascadeAssociation（Cascade.java:418）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.engine.internal.Cascade.cascadeProperty（Cascade.java:216）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.engine.internal.Cascade.Cascade（Cascade.java:149）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.event.internal.DefaultMergeEventListener.cascadeOnMerge（DefaultMergeEventListener.java:460）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.event.internal.DefaultMergeEventListener.entityIsPersistent（DefaultMergeEventListener.java:202）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.event.internal.DefaultMergeEventListener.onMerge（DefaultMergeEventListener.java:176）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.event.internal.DefaultMergeEventListener.onMerge（DefaultMergeEventListener.java:69）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.internal.SessionImpl.fireMerge（SessionImpl.java:900）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] 在org.hibernate.internal.SessionImpl.merge（SessionImpl.java:886）~[hibernate-core-5.3.9.Final.jar:5.3.9.Final] at sun.reflect.GeneratedMethodAccessor88.invoke（未知源）~[na:na] 在sun.reflect.DelegatingMethodAccessorImpl.invoke（DelegatingMethodAccessorImpl.java:43）~[na:1.8.0¡] 在java.lang.reflect.Method.invoke（Method.java:498）~[na:1.8.0_161] 在org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke（SharedEntityManagerCreator.java:308）~[spring-orm-5.1.6.RELEASE.jar:5.1.6.RELEASE] 在com.sun.proxy.$Proxy116.merge（未知源）~[na:na] 在org.springframework.data.jpa.repository.support.SimpleJpaRepository.save（SimpleJpaRepository.java:492）~[spring-data-jpa-2.1.6.RELEASE.jar:2.1.6.RELEASE] at sun.reflect.GeneratedMethodAccessor86.invoke（未知源）~[na:na] 在sun.reflect.DelegatingMethodAccessorImpl.invoke（DelegatingMethodAccessorImpl.java:43）~[na:1.8.0¡] 在java.lang.reflect.Method.invoke（Method.java:498）~[na:1.8.0_161] 在org.springframework.data.repository.core.support.RepositoryComposition$RepositoryFragments.invoke（RepositoryComposition.java:359）~[spring-data-commons-2.1.6.RELEASE.jar:2.1.6.RELEASE] 在org.springframework.data.repository.core.support.RepositoryComposition.invoke（RepositoryComposition.java:200）~[spring-data-commons-2.1.6.RELEASE.jar:2.1.6.RELEASE] 在org.springframework.data.repository.core.support.RepositoryFactorySupport$ImplementationMethodExecutionInterceptor.invoke（RepositoryFactorySupport.java:644）~[spring-data-commons-2.1.6.RELEASE.jar:2.1.6.RELEASE] 在org.springframework.aop.framework.ReflectiveMethodInvocation.procedue（ReflectiveMethodInvocation.java:186）~[spring-aop-5.1.6.RELEASE.jar:5.1.6.RELEASE] 在org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.doInvoke（RepositoryFactorySupport.java:608）~[spring-data-commons-2.1.6.RELEASE.jar:2.1.6.RELEASE] 在org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.lambda$invoke$3（RepositoryFactorySupport.java:595）~[spring-data-commons-2.1.6.RELEASE.jar:2.1.6.RELEASE] 在org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.invoke（RepositoryFactorySupport.java:595）~[spring-data-commons-2.1.6.RELEASE.jar:2.1.6.RELEASE] 在org.springframework.aop.framework.ReflectiveMethodInvocation.procedue（ReflectiveMethodInvocation.java:186）~[spring-aop-5.1.6.RELEASE.jar:5.1.6

@Entity
public class SemesterLecture implements Serializable {
    @Id
    @ManyToOne
    @JsonIgnore
    @JoinColumn
    private Semester semester;

    @Id
    @ManyToOne
    @JoinColumn
    private Lecture lecture;

    private String grade;

    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
    private List<LectureDateTime> lectureDateTimes;

    public SemesterLecture(Lecture lecture, String grade) {
        this.lecture = lecture;
        this.grade = grade;
    }

    public SemesterLecture() {
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof SemesterLecture)) return false;
        SemesterLecture that = (SemesterLecture) o;
        return Objects.equals(semester.getTitle(), that.semester.getTitle()) &&
                Objects.equals(lecture.getTitle(), that.lecture.getTitle());
    }

@Override
    public int hashCode() {
        return Objects.hash(semester.getTitle(), lecture.getTitle(), grade);
    }

@EventListener(ApplicationReadyEvent.class)
    @Transactional
    public void readExcelFileFolder() throws IOException, InvalidFormatException, NotFoundException {
        LOG.info("Load Excel Files from folder...");
        FileFilter filter = new ExcelFileFilter ();
        Resource[] resourceFiles = resourcePatternResolver.getResources("classpath:excel-files/*.xls");
        for (Resource resource : resourceFiles) {
            readExcelFile(resource.getInputStream(), resource.getFilename());
        }
    }

    public void readExcelFile(InputStream inputStream, String fileName) throws IOException, InvalidFormatException, NotFoundException {
        Workbook workbook = WorkbookFactory.create(inputStream);
        DataFormatter dataFormatter = new DataFormatter();
        LOG.info("Read Excel File: " + fileName.substring(0, fileName.lastIndexOf('.')));

        Course existingCourse = courseRepository.findByCourseNumber(fileName.replace(" ", "").substring(0, fileName.lastIndexOf('.')));
        if(existingCourse == null){
            return;
        }
        LOG.info("Found course with title " + existingCourse.getCourseTitle());

        if(existingCourse.getSemesters().size() < 1){
            for(int i = 1; i < 8; i++){
                Semester semester = new Semester();
                semester.setTitle("Semester " + i);
                semester.setSemesterLectures(new ArrayList<SemesterLecture>());
                semester.setSemesterNumber(i);
                existingCourse.getSemesters().add(semesterRepository.save(semester));
            }
            LOG.info("Added " + existingCourse.getSemesters().size() + " Semesters");
        }

        for(int y = 0; y < 5; y++){
            Sheet sheet = workbook.getSheetAt(y);
            for (Row row: sheet) {
                String title = dataFormatter.formatCellValue(row.getCell(0));
                String modulNumber = dataFormatter.formatCellValue(row.getCell(1)).replace(" ", "");
                String language = dataFormatter.formatCellValue(row.getCell(2));
                int semesterNumber = 1;
                try{
                    semesterNumber = Integer.parseInt(dataFormatter.formatCellValue(row.getCell(3)).replace(".", ""));
                }catch (Exception e){}
                if(title instanceof String && modulNumber instanceof String && !title.isEmpty() && !modulNumber.isEmpty() && !title.startsWith("Module")){
                    try{
                        if(lectureRepository.existsByModulNumber(modulNumber.replace(" ", ""))){
                            Lecture lecture = lectureRepository.findByModulNumber(modulNumber.replace(" ", ""));
                            SemesterLecture sm = new SemesterLecture(lecture, "0");
                            sm.setSemester(existingCourse.getSemesters().get(semesterNumber-1));
                            sm.setLecture(lecture);
                            sm.setGrade("0");
                            existingCourse.getSemesters().get(semesterNumber-1).getSemesterLectures().add(sm);
                            LOG.info(sm.getLecture().getTitle() + sm.getSemester().getTitle());
                        }
                    } catch (NumberFormatException e) {
                        throw new NotFoundException("Lecture does not exist");
                    }
                }
            }
        }

        for(Semester semester : existingCourse.getSemesters()){
            semesterRepository.save(semester);
        }
        courseRepository.save(existingCourse);
        LOG.info("Saved all lectures from course " + existingCourse.getCourseTitle());
    }