Java 为什么会在@PersistenceContext EntityManager上抛出NullPointerException？

我一直在尝试使用@PersistenceContext注释将EntityManager注入到我的Spring应用程序中。然而，当我试图调用它的一个方法时，会抛出一个NullPointerException，这使我相信它不是出于某种原因被注入的。我肯定我在另一个项目中使用了这个，但我没有访问该项目的权限来了解发生了什么变化

注意：我看到的大多数解决方案都提到@EJB或@Stateless，但我在工作项目中没有使用它们

例外情况：

Exception in thread "main" java.lang.NullPointerException
    at dao.CustomerDaoJpaImpl.createCustomer(CustomerDaoJpaImpl.java:41)
    at boot.MainClass.main(MainClass.java:43)

MainClass.java：

public class MainClass {
    public static void main(String[] args) {
        ICustomerDao customerDao = new CustomerDaoJpaImpl();
        customerDao.createCustomer("john@email.com", "John", "Doe");
    }
}

CustomerDaoJpaImpl.java：

@Repository
@Transactional
public class CustomerDaoJpaImpl implements ICustomerDao {
    @PersistenceContext
    private EntityManager entityManager;

    @Override
    public Customer createCustomer(String email, String firstName, String lastName) {
        EntityTransaction transaction = entityManager.getTransaction();
        transaction.begin();
        Customer customer = new Customer(email, firstName, lastName);
        entityManager.persist(customer);
        transaction.commit();
        return customer;
    }
}

WebMvcConfig.java：

@Configuration
@EnableWebMvc
@EnableTransactionManagement
@ComponentScan({ "dao", "service", "controller", "com.nkosi.example" })
public class WebMvcConfig implements WebMvcConfigurer {

    @Bean
    public LocalEntityManagerFactoryBean getEntityManagerFactoryBean() {
        LocalEntityManagerFactoryBean factoryBean = new LocalEntityManagerFactoryBean();
        factoryBean.setPersistenceUnitName("critter-tourney");
        return factoryBean;
    }

    @Bean
    public JpaTransactionManager getTransactionBean() {
        JpaTransactionManager transactionManager = new JpaTransactionManager(getEntityManagerFactoryBean().getObject());
        return transactionManager;
    }
}

persistence.xml：

<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"
    version="2.1">
    <persistence-unit name="critter-tourney">
        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
        <properties>
            <property name="javax.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver"/>
            <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/critter_tourney"/>
            <property name="javax.persistence.jdbc.user" value="root"/>
            <property name="javax.persistence.jdbc.password" value="*"/>        
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.hbm2ddl.auto" value="create"/>
            <property name="hibernate.format_sql" value="true"/>
            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL8Dialect"/>
        </properties>       
    </persistence-unit>
</persistence>

pom.xml：

<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.nkosi.example</groupId>
    <artifactId>critter-tourney</artifactId>
    <packaging>war</packaging>
    <version>0.0.1-SNAPSHOT</version>
    <name>critter-tourney Maven Webapp</name>
    <url>http://maven.apache.org</url>

    <properties>
        <spring.groupId>org.springframework</spring.groupId>
        <spring.version>5.2.4.RELEASE</spring.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>3.0.1</version>
            <!-- <scope>provided</scope> -->
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.19</version>
        </dependency>
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>5.3.15.Final</version>
        </dependency>
        <dependency>
            <groupId>${spring.groupId}</groupId>
            <artifactId>spring-core</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>${spring.groupId}</groupId>
            <artifactId>spring-context</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>${spring.groupId}</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>${spring.groupId}</groupId>
            <artifactId>spring-orm</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/org.springframework.data/spring-data-jpa -->
        <dependency>
            <groupId>${spring.groupId}.data</groupId>
            <artifactId>spring-data-jpa</artifactId>
            <version>2.2.0.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>
    </dependencies>
    <build>
        <finalName>critter-tourney</finalName>

        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.7.0</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

---

当您使用新操作创建这样的对象时，它只是普通类

ICustomerDao customerDao = new CustomerDaoJpaImpl();

不会有任何依赖注入，也不会有任何PersistenceContext。因此，您的entityManager为空

您需要从应用程序上下文中获取Bean，类似这样的内容

注意：这是示例代码，仅供参考，可能有效，也可能无效。因为Bean名称可以是其他的

public static void main(String[] args) {
        AnnotationConfigApplicationContext context =
                new AnnotationConfigApplicationContext(MainClass.class);

         BeanFactory factory = context;
        ICustomerDao  customerDao = (CustomerDaoJpaImpl) factory.getBean("customerDaoJpaImpl");
        customerDao.createCustomer("john@email.com", "John", "Doe");

}

---

您需要首先以某种方式初始化Spring上下文。当使用new关键字创建customerDao实例时，它不是Spring管理的bean。在这种情况下，entityManager的自动接线不会发生，预计会出现NPE。