我一直在尝试使用@PersistenceContext注释将EntityManager注入到我的Spring应用程序中。但是,当我尝试调用其方法之一时,会引发NullPointerException,这使我认为由于某种原因未注入该对象。我确定我可以在另一个项目中使用它,但是我无权访问该项目以了解发生了什么变化。
注:我见过的大多数解决方案都提到了@EJB或@Stateless,但在我的工作项目中并未使用它们。
例外:
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("[email protected]", "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();
将不会有任何依赖项注入,也不会有任何PersistenceContext
。因此,您的entityManager
为空。
您需要从Application上下文中获取类似这样的Bean。
注:这是示例代码,仅供参考,可能会或可能不会起作用。由于Bean Name可以是其他名称。
public static void main(String[] args) {
AnnotationConfigApplicationContext context =
new AnnotationConfigApplicationContext(MainClass.class);
BeanFactory factory = context;
ICustomerDao customerDao = (CustomerDaoJpaImpl) factory.getBean("customerDaoJpaImpl");
customerDao.createCustomer("[email protected]", "John", "Doe");
}