Spring MVC + JPA-空指针异常

Question

我失去了耐心。 我正在尝试首次打开JPA，但仍然会收到Null Pointer Exception（空指针异常）...我不知道下一步该怎么做。 请帮忙。 我尝试像《行动中的春天》​​那样做。

@Configuration
@EnableJpaRepositories(basePackages = {"org.project"})
public class JpaConfig {
private DataSource dataSource;

@Autowired
public void setDataSource(DataSource dataSource) {
    this.dataSource = dataSource;
}

@Bean
public LocalContainerEntityManagerFactoryBean entityManagerFactory() {
    LocalContainerEntityManagerFactoryBean em = new LocalContainerEntityManagerFactoryBean();
    em.setDataSource(dataSource);
    em.setPackagesToScan("org.project.persistance");

    JpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
    em.setJpaVendorAdapter(vendorAdapter);
    em.setJpaProperties(additionalProperties());

    return em;
}

@Bean
public PlatformTransactionManager transactionManager(EntityManagerFactory emf) {
    JpaTransactionManager transactionManager = new JpaTransactionManager();
    transactionManager.setEntityManagerFactory(emf);
    return transactionManager;
}

@Bean
public PersistenceExceptionTranslationPostProcessor exceptionTranslation(){
    return new PersistenceExceptionTranslationPostProcessor();
}

private Properties additionalProperties() {
    Properties properties = new Properties();
    properties.setProperty("hibernate.hbm2ddl.auto", "create-drop");
    properties.setProperty("hibernate.dialect", "org.hibernate.dialect.MySQL5Dialect");
    return properties;
}

仓库：

import javax.persistence.EntityManagerFactory;
import javax.persistence.PersistenceUnit;

@Repository
public class DogRepositoryImpl implements DogRepository {
    @PersistenceUnit
    private EntityManagerFactory entityManager;

    @Override
    public Dog findById(long id) {
        return entityManager.createEntityManager().find(Dog.class, id);
    }

    @Override
    public void persist(Dog dog) {
        entityManager.createEntityManager().persist(dog);
    }
}

和@Entity：

@Entity
@Table(name = "dogs")
public class Dog {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    private String name;

    private String owner;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getOwner() {
        return owner;
    }

    public void setOwner(String owner) {
        this.owner = owner;
    }
}

请帮助:(当我尝试使用Spring Data进行操作时，它是相同的。

PS：**为什么在MySql的控制器表中执行此代码后会“清理”？ **

Answer 1

这应该是由以下行引起的：

properties.setProperty("hibernate.hbm2ddl.auto", "create-drop");

会话工厂关闭后，所有表将被删除。 您可以对其进行更改以进行create以便表能够持久存在。