存储库实现中的java.lang.NullPointerException（JPA + Hibernate）

Question

Project正在编译，但在浏览器发出请求时会出错：

ava.lang.NullPointerException
    at org.belarusgo.repository.jpa.JpaNewsRepositoryImpl.findById(JpaNewsRepositoryImpl.java:24)
    at org.belarusgo.repository.jpa.JpaNewsRepositoryImpl.findById(JpaNewsRepositoryImpl.java:12)
    at org.belarusgo.service.BelarusGoServiceImpl.findNewsById(BelarusGoServiceImpl.java:48)

还有一个日志：

   16:13:29,455 DEBUG JpaTransactionManager:371 - Opened new EntityManager [org.hibernate.jpa.internal.EntityManagerImpl@4e54a245] for JPA transaction
    16:13:29,459 DEBUG DataSourceUtils:153 - Setting JDBC Connection [Pooled connection wrapping physical connection org.postgresql.jdbc.PgConnection@5d7ad29] read-only
    16:13:29,460 DEBUG TransactionImpl:51 - begin
    16:13:29,461 DEBUG JpaTransactionManager:403 - Exposing JPA transaction as JDBC transaction [org.springframework.orm.jpa.vendor.HibernateJpaDialect$HibernateConnectionHandle@4f50eb69]
    16:13:29,462 DEBUG BelarusGoServiceImpl:47 - From findNewsById(3)
    16:13:29,463 DEBUG JpaTransactionManager:851 - Initiating transaction rollback
    16:13:29,463 DEBUG JpaTransactionManager:538 - Rolling back JPA transaction on EntityManager [org.hibernate.jpa.internal.EntityManagerImpl@4e54a245]
    16:13:29,463 DEBUG TransactionImpl:86 - rolling back
    16:13:29,464 DEBUG DataSourceUtils:222 - Resetting read-only flag of JDBC Connection [Pooled connection wrapping physical connection org.postgresql.jdbc.PgConnection@5d7ad29]
    16:13:29,465 DEBUG JpaTransactionManager:600 - Closing JPA EntityManager [org.hibernate.jpa.internal.EntityManagerImpl@4e54a245] after transaction
    16:13:29,466 DEBUG EntityManagerFactoryUtils:435 - Closing JPA EntityManager
    16:13:29,468 DEBUG ExceptionHandlerExceptionResolver:133 - Resolving exception from handler [public org.springframework.web.servlet.ModelAndView org.belarusgo.web.controller.NewsController.findNews(int)]: java.lang.NullPointerException
    16:13:29,469 DEBUG ResponseStatusExceptionResolver:133 - Resolving exception from handler [public org.springframework.web.servlet.ModelAndView org.belarusgo.web.controller.NewsController.findNews(int)]: java.lang.NullPointerException
    16:13:29,470 DEBUG DefaultHandlerExceptionResolver:133 - Resolving exception from handler [public org.springframework.web.servlet.ModelAndView org.belarusgo.web.controller.NewsController.findNews(int)]: java.lang.NullPointerException
    16:13:29,471 DEBUG DispatcherServlet:993 - Could not complete request
java.lang.NullPointerException
    at org.belarusgo.repository.jpa.JpaNewsRepositoryImpl.findById(JpaNewsRepositoryImpl.java:24)
    at org.belarusgo.

repository.jpa.JpaNewsRepositoryImpl.findById（JpaNewsRepositoryImpl.java:12）

的persistence.xml：

<?xml version="1.0" encoding="UTF-8"?>
    <persistence xmlns="http://java.sun.com/xml/ns/persistence"
                 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                 xsi:schemaLocation="http://java.sun.com/xml/ns/persistence                                 http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"
                 version="1.0">

       <persistence-unit name="belarusgo" transaction-type="JTA">
          <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>

          <properties>
             <property name="hibernate.connection.driver_class"
                       value="org.postgresql.Driver"/>
             <property name="hibernate.connection.url"
                       value="jdbc:postgresql://localhost:5432"/>
             <property name="hibernate.connection.username" value="postgres"/>
             <property name="hibernate.connection.password" value="1234"/>
             <property name="hibernate.dialect"
                       value="org.hibernate.dialect.PostgreSQL9Dialect"/>
             <property name="hibernate.hbm2ddl.auto" value="update"/>
          </properties>
       </persistence-unit>
    </persistence>

WebConfig：

  @Bean
        public DataSource getDataSource() {
            PGPoolingDataSource dataSource = new PGPoolingDataSource();
            dataSource.setDataSourceName("Postgresql datasource");
            dataSource.setServerName("localhost");
            dataSource.setPortNumber(5432);
            dataSource.setDatabaseName("belarusgo");
            dataSource.setUser("postgres");
            dataSource.setPassword("1234");
            dataSource.setMaxConnections(100);
            dataSource.setInitialConnections(5);
            return dataSource;
        }
 @Bean
    public EntityManagerFactory entityManagerFactory() {

        HibernateJpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
        vendorAdapter.setGenerateDdl(true);

        LocalContainerEntityManagerFactoryBean factory = new LocalContainerEntityManagerFactoryBean();
        factory.setJpaVendorAdapter(vendorAdapter);
        factory.setPackagesToScan("org.belarusgo.model.domain");
        factory.setDataSource(getDataSource());
        factory.afterPropertiesSet();

        return factory.getObject();
    }

    @Bean
    public PlatformTransactionManager transactionManager() {

        JpaTransactionManager txManager = new JpaTransactionManager();
        txManager.setEntityManagerFactory(entityManagerFactory());
        return txManager;
    }

新闻的BaseEntity：

@MappedSuperclass
public class BaseEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    protected Integer id;

新闻：

@Entity
@Table(name = "news")
public class News extends BaseEntity {

    @Column(name = "published_date")
    //@Temporal(TemporalType.DATE)
    private Date publishedDate;

    @Column(name = "headline", length = 200)
    @NotNull
    @Size(min=5, max=200)
    private String headline;

    @Column(name = "author", length = 100)
    @NotNull
    @Size(min=5, max=100)
    private String author;

    @Column(name = "text")
    @NotNull
    @Size(min=20)
    private String text;

服务实施：

@Service
@Transactional
public class BelarusGoServiceImpl implements BelarusGoService{

    private PlayerRepository playerRepository;
    private TournamentRepository tournamentRepository;
    private NewsRepository newsRepository;

    @Autowired
    public BelarusGoServiceImpl(PlayerRepository playerRepository,
                         TournamentRepository tournamentRepository,
                         NewsRepository newsRepository) {
        this.playerRepository = playerRepository;
        this.tournamentRepository = tournamentRepository;
        this.newsRepository = newsRepository;
    }

    @Override
    @Transactional(readOnly = true)
    public News findNewsById(int id) {
        logger.debug("From findNewsById({})", id);
        return newsRepository.findById(id);
    }

存储库实现：

public class JpaNewsRepositoryImpl implements NewsRepository {

    @PersistenceContext(unitName = "belarusgo")
    private EntityManager em;

    @Override
    public News findById(Integer id) {
       return this.em.find(News.class, id);
    }