我正在尝试使用 JPA 的上下文注入(inject)。目前我的代码无法注入(inject)上下文,抛出 NullPointerException:
Exception in thread "main" java.lang.NullPointerException at learn.spring.dao.ListingDao.findById(ListingDao.java:19)
下面是我的代码,有人看到什么是错误/缺少的吗?谢谢。
Spring配置xml:
<beans ....>
<tx:annotation-driven />
<context:annotation-config />
<aop:aspectj-autoproxy />
<bean id="listingDao" class="learn.spring.dao.ListingDao" />
<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="driverClassName" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://xxx.xxx.xxx.xxx/xxxxx" />
<property name="username" value="xxxxxx" />
<property name="password" value="xxxxxx" />
</bean>
<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
<property name="persistenceUnitName" value="learnSpring" />
<property name="dataSource" ref="dataSource" />
<property name="jpaVendorAdapter">
<bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
<property name="databasePlatform" value="org.hibernate.dialect.MySQLDialect" />
<property name="showSql" value="true" />
<property name="generateDdl" value="true" />
</bean>
</property>
</bean>
<bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
<property name="entityManagerFactory" ref="entityManagerFactory" />
</bean>
...
</beans>
DAO 类:
package learn.spring.dao;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;
import org.springframework.transaction.annotation.Transactional;
import learn.spring.model.Registration;
@Transactional
public class ListingDao {
public ListingDao() {}
@PersistenceContext
private EntityManager entityManager;
public Registration findById(int id) {
Query query = entityManager.createQuery("FROM Registration r WHERE r.memberId = ?1");
query.setParameter(1, id);
return (Registration) query.getSingleResult();
}
}
peristence.xml 文件:
<?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="learnSpring" />
</persistence>
带有“main”方法的类:
public class Performance {
public static void main(String[] args){
ApplicationContext context = new ClassPathXmlApplicationContext("learn-spring.xml");
Performer performer = (Performer) context.getBean("musician");
performer.perform();
ListingDao listingDao = new ListingDao();
Registration registration = listingDao.findById(1);
System.out.println("Member id = " + registration.getMemberId() + " Name = " + registration.getFirstName() + " " + registration.getLastName());
}
}
最佳答案
您的问题是您的 ListingDao
不是 spring 上下文的一部分。
所有想要从 spring 的依赖注入(inject)中受益的类都必须由 spring 实例化。当您使用 new
运算符实例化 ListingDao
时,您的对象不是 spring 上下文的一部分,并且不会收到自动 EntityManager
.
因此,解决方案是从上下文中获取 ListingDao
,就像您对表演者所做的那样:
ListingDao listingDao = (ListingDao) context.getBean("listingDao");
作为旁注 - 避免直接访问 spring 上下文。即避免像上面这样的行。使用注释来注入(inject)您的依赖项,而不是请求它们,因为您因此将依赖项注入(inject)框架转变为服务定位器。
关于java - JPA的上下文注入(inject),我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/1989213/